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The current network of Arctic ground-based UV measurements in shown in Figure 3. Only those installations operated on a regular basis are shown. The instruments are classified to three types: spectroradiometers, multifilter instruments, and broadband instruments. Spectroradiometers scan radiation in small wavelength bands with a typical resolution of 1 nm or less [Seckmeyer et al.

They provide the highest measurement accuracy and the spectral information enables versatile data use. However, they are costly and require trained personnel for maintenance and operation. Multichannel or multifilter instruments typically consist of several filtered photodetectors that measure radiation in selected wavelength bands [Dahlback, ; Harrison et al. They provide much faster sampling than the conventional scanning spectroradiometers and allow evaluation of both long- and short-term changes in UV. Methods have also been developed to reconstruct the high resolution spectrum from these multifilter instrument measurements [Dahlback, ; Fuenzalida, ; Min and Harrison, ].

Broadband instruments collect radiation over a portion of the spectrum and impose a weighting function. Many of these instruments are designed to measure the erythemal irradiance. Broadband instruments are comparatively inexpensive and easy to maintain. However, their spectral response often deviates from the one that they are mimicking. Thus, the absolute calibration of the broadband instruments depends on solar zenith angle and total ozone column, and both determining the absolute calibration [Mayer et al.

In addition to the above-described instruments, the UV dose, or biological dose rate integrated over time, can be measured using biological UV dosimeters, which directly quantify the biologically effective solar irradiance for certain processes by allowing biological systems to act as UV sensors. Fairly good agreement with weighted spectroradiometric measurements has been shown for some of these systems Furusawa et al. Monitoring at these sites started in , , , and , respectively.

Increased UV-B levels have been reported in the Arctic during six winters in the s Fioletov et al. These values are relatively small compared to the UV indices measured at the other Canadian monitoring sites. However, the maximum observed UV index may not be the best indicator for the UV doses received in the Arctic because it does not take into account the length of the day nor the receptor orientation.

Lakkaka et al. No statistically significant changes were observed for any months for the year period One reason is that the period is rather short in comparison to the high natural interannual variability of UV. The lack of a distinct trend over this particular period could be also expected from the corresponding ozone changes, characterized by a decrease in the early s with an increase toward more normal values in recent years, with the smallest value occurring in the middle of the period.

Arola et al. The results, shown in Figure 3. Multiple scattering between ground and cloud base interconnects the effects of surface albedo and cloud cover. Attempts to separate the two factors e. The relative enhancement in monthly erythemal doses due to the presence of clouds green , and albedo from snow cover blue. The figure is adopted from Kylling et al. Enhanced UV levels were observed in , which were likely due to low ozone amounts related to the injections of aerosols into the stratosphere from the eruption of Mt. Pinatubo [Gurney, ]. Photochemically induced ozone depletion events at Barrow have mostly occurred during March and April.

This increase is still small compared to spikes seen in UV measurements at Antarctic sites affected by the ozone hole. By comparing measured and modeled spectra, Bernhard et al. It should be noted that the observed ozone reductions have been much stronger in the European and Russian sector of the Arctic than over Alaska. UV increases at Barrow are therefore less pronounced compared to the other Arctic sites. These methods are usually based on total ozone or other commonly available weather data, including global radiation.

Bodeker and McKenzie [], for instance, presented a model based on total ozone, broadband measurements, and radiative transfer calculations. Similar methods were developed and used by McArthur et al. Gantner et al. The results obtained with reconstruction methods generally compare well with measurements. There are, however, some sources of uncertainties that are worth mentioning. The importance of examining the homogeneity of the data cannot be overestimated when using long time series of measurements as input data for estimating past UV levels.

Likewise, it is of great importance to properly validate the methods against independent measurement data. Fioletov et al. The model is based on previous work to estimate the UV-A irradiance from pyranometer measurements McArthur et al. This increase follows a reported decline in the absolute UV levels from to The UV increase from to is more than twice the trend that would be expected from the observed decline in total ozone, because concurrent changes in the surface albedo and cloud conditions have enhanced the increase in surface UV over this period.

Another reconstruction by Diaz et al. They conducted only a brief analysis of the produced time series of reconstructed UV. However, their results implied increases in the springtime UV irradiance over the period Lindfors et al. The results show a statistically significant increasing trend of 3. April also shows an increasing trend, which is very close to being statistically significant.

For both March and April, the trend is more pronounced during the latter part of the period , suggesting a connection to stratospheric ozone depletion. For July, a significant decreasing trend of 3. June and August exhibit decreasing trends, although not statistically significant, and trends for May and September are negligible. In addition to the atmospheric observations, biological proxies can be used to estimate past UV radiation.

For instance, Leavitt et al.

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The method is based on the fact that some algae or other aquatic organisms produce photoprotective pigments when exposed to UV radiation. Long-term variation of the underwater UV is primarily controlled by the amount of dissolved organic matter, however, which unfortunately limits the use of fossil pigment sediments to reconstruct surface UV [Leavitt et al. In addition to total ozone, satellite-based information on clouds, aerosols, and surface albedo is needed to accurate model these parameters, and various sources of data have been used for that purpose [Herman et al.

The advantages of the satellite retrieval methods are that satellite instruments provide global or near-global spatial coverage as well as long-term continuity. However, satellite retrieval methods have their limitations. Satellite instruments also have difficulty probing the lower atmosphere, where UV-absorbing aerosols or tropospheric ozone can significantly affect surface UV radiation.

Fortunately, neither absorbing aerosols nor tropospheric ozone can be considered major factors affecting UV radiation in the Arctic, with the exception of air masses occasionally transported from lower latitudes. Several studies have validated satellite-based UV estimates against ground-based data Kalliskota et al, ; Wang et al, ; McKenzie ; Slusser et al ; Arola et al, ; Chubarova et al, ; Fioletov et al, Estimating the surface UV radiation in the Arctic is more uncertain than that at lower latitudes: during snow cover, the satellite-derived UV estimates have been systematically lower than the ground-based measurements.


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The bias has originated from the underestimated surface albedo, which is a crucial parameter for accurate estimating surface UV radiation in the Arctic. Additionally, snow or ice cover complicates the determination of the cloud optical depth properties Krotkov et al.

Because of these limitations, the accuracy of the currently used satellite retrieval algorithms cannot be considered adequate for monitoring Arctic surface UV, although some information on UV variation over time can be inferred. The satellite-derived erythemal daily dose over the Arctic on March 21, , is shown in Figure 3. The analyses were performed for clear sky estimates, that is, the modulation of surface UV radiation by clouds or aerosols was not considered. The trend analysis methods applied are similar to those used to study long-term changes in total column ozone. Moreover, the winter and spring trends were found to be the largest.

Ziemke et al. Arctic UV dose distribution derived from satellite data for the 21st of March, Total column ozone over the Arctic in winter and spring is usually higher than that over the equator and northern mid-latitudes. Ozone amounts over the Arctic are marked by a strong annual cycle, with a peak occurring during the springtime, and a decrease in late summer and throughout the autumn months.

Cold temperatures provide the potential for substantial depletion in the winter and early spring, reducing ozone amounts at a time when they would normally be high and when reproduction and new growth leave ecosystems particularly vulnerable. The same physical and chemical processes govern ozone levels and ozone depletion over both the Arctic and Antarctic.

However, a stronger polar vortex over Antarctica has resulted in greater percentage loss over the past two decades. In the years where dynamical conditions allow for similarly cold stratospheric temperatures in the Arctic, significant ozone loss can be observed at northern high latitudes as well. Globally, the Montreal Protocol and its amendments have already resulted in a decrease in the concentrations of some ozone-depleting substances Montzka et al. Detecting the onset of this ozone recovery is likely to require some time because of natural variability Weatherhead et al.

In polar regions, the projections of recovery are complicated by the effects of dynamical contributions and climate change. Ozone recovery is likely to occur in stages. The first signs of recovery are likely to include a reduction in the downward trend followed by an increase in ozone levels. Several models have been used to predict future ozone levels WMO ; An assessment of various models discusses the advantages offered by the two-dimensional and three-dimensional models.

Two-dimensional models have simplified dynamics and advanced chemistry. Three-dimensional models have improved dynamics but simplified chemistry. Because of the importance of both chemistry and dynamics in the Arctic, each model type has specific strengths that complement the other. Both types of models are run at various institutions. Intercomparison of these models WMO, shows qualitative agreement in the projections, although specific projections of the recovery rates can disagree significantly. The three-dimensional models can provide multi-year time slice simulations Austin et al.

These simulations have the advantage that several realizations are available for a single year, allowing a better assessment of the projections. Three- dimensional models are also able to address dynamical changes in well-mixed greenhouse gases and offer a more detailed evolution of ozone, based on the same mechanisms that are likely to occur in the atmosphere. These models can provide information on the expected range of interannual variability. Austin et al. The models were compared based on their ability to predict ozone climatologies for the current atmosphere.

In the northern hemisphere, all models tended to overestimate the area-weighted hemispheric ozone, by an average of 7. Within the Arctic vortex, the models are unable to simulate the observed loss rates Bregman et al. Uncertainties in the model projections include temperature biases, leading to the so-called "cold pole problem" Pawson et al. These biases are worse in some models than others. In the northern hemisphere, the biases are sometimes positive at certain levels, resulting in insufficient ozone depletion in early winter, but excess depletion in the spring.

Other uncertainties include the models' ability to accurately simulate polar stratospheric clouds PSCs and to account for all aspects of constituent transport, including processes occurring at the model's upper boundary. Changes in planetary waves and heat flux also pose uncertainties, and are discussed in greater detail below.

Introduction

Over the Arctic, however, ozone depletion processes are often much more complicated and depend greatly on climate conditions and climate changes. For instance, when potential climate-induced increases in stratospheric water vapor are included in models, the resulting mid-latitude ozone change in the s surpasses that due to halogens Shindell and Grewe, At high latitudes, the effects are anticipated to be even larger due to the impact of PSCs.

Modeling efforts e. Shindell et al. Depletion in the Arctic is governed strongly by the dynamics of the polar atmosphere. Changes in circulation, and particularly changes that affect air temperatures in the polar region, can substantially impact the levels of depletion that are experienced.

For instance, establishment of a strong polar vortex results in decreasing stratospheric temperatures, which act as a positive feedback to further strengthen the polar vortex. This feedback effect contributes to increased ozone depletion, and will likely be exacerbated by stratospheric cooling expected from future climate change. The Montreal Protocol and its amendments have already been observed to reduce levels of chlorine in the atmosphere, and concentrations of ozone- depleting halogens are expected to continue to decrease over the period to The decreases were first reported in the troposphere Montzka et al.

Bromine, a part of many currently used CFC substitutes, is particularly effective in destroying ozone, and overall levels may be highly impacted by shorter-lived substances, such as bromoform Dvortsov et al. As stated earlier, however, the magnitude of the ozone loss can depend greatly on dynamical and climate conditions, with cold temperatures contributing to the formation and persistence of polar stratospheric clouds.

Over the polar regions, heterogeneous chemistry in or on these clouds converts stable chlorine and bromine reservoirs to more active forms that can deplete ozone. Future volcanic eruptions could also change stratospheric ozone levels worldwide for at least several years, and could have a large effect in the Arctic as long as halogen loading remains large Tabazedeh et al. In the Arctic, this cooling could be responsible for increased ozone destruction, as lower temperatures are likely to result in the formation and persistence of polar stratospheric clouds.

Polar stratospheric clouds aid in activating ozone-depleting compounds, and can therefore accelerate ozone destruction. Stratospheric cooling due to global change may therefore result in an increased likelihood of larger and longer-lasting ozone holes in the Antarctic and the formation of an ozone hole over the Arctic Dameris et al.

On the other hand, climate changes could trigger a possible increase in planetary waves, speeding up the transport of warm, ozone- rich air to the Arctic Schnadt et al. This increased transport would counter the effects of heterogeneous chemistry and potentially lessen the time required for an ozone recovery. If radiative effects dominate, planetary wave activity would be more likely to decrease, resulting in more ozone loss at Arctic latitudes.

Another climate feedback affecting ozone is an increase in stratospheric water vapor due to changes in tropopause temperatures Evans et al. Few long-term data sets of water vapor concentrations are available, but previous observations have suggested that stratospheric water vapor is indeed increasing Oltmans and Hofmann, ; Randel et al. Analyses of 45 years of data by Rosenlof et al. Analyses of satellite data, however, have been less supportive of a water vapor increase Randel et al. Increased water vapor may contribute to increased ozone destruction by affecting the radiation balance of the stratosphere Shindell, Water vapor concentrations in the stratosphere affect the threshold temperatures for activating heterogeneous chemical reactions on PSCs, and can also influence the temperature of the Arctic vortex itself Kirk-Davidoff et al.

Ozone itself is central to climate change science. Ozone is an important greenhouse gas in the infrared part of spectrum and is the primary absorber of solar UV radiation. It is responsible for the vertical structure of the atmosphere, as it is the absorption of solar radiation by ozone that causes the stratosphere to exist. Ozone is critical to the radiation balance of the atmosphere, and to the dynamics of the stratosphere. Observations show that the strengths of the polar vortices, which are strongly influenced by stratospheric ozone levels, affect surface temperatures over polar regions and at midlatitudes in both hemispheres.

These feedback processes in the stratosphere can alter weather processes in the troposphere, with an effect whose magnitude is comparable to that of ENSO Gillett and Thompson, ; Hartmann et al. The expected future changes in ozone in polar regions differ significantly from the expected changes over the rest of the globe, where stratospheric temperatures do not reach the cold polar thresholds. Model simulations suggest that a strong polar vortex may warm Northern Hemisphere continents and could substantially delay the Arctic ozone recovery Shindell, The delay in recovery at polar regions means a longer term, and perhaps more severe, threat of ecosystem damage due to increased UV.

Projected changes in ozone amounts A number of two-dimensional 2D models using specified halocarbon scenarios were used to estimate future ozone levels for the recent World Meteorological Organization Ozone Assessment WMO, These models include AER Wiesenstein et al. In Figure 3. The spring period is interesting because the accumulated ozone depletion is at its highest, and the UV radiation is relatively high during what is also the beginning of the growth period for many biological systems.

Generally, the models indicate local minimums in Arctic ozone in the late s, followed by a slow, gradual increase. Ozone values in are significantly lower than the values for all models. The dashed lines represent the results from ten 2-D models. Because the 2D models are unable to fully incorporate dynamical effects their results are considered very rough estimates for the polar regions, where ozone amounts are strongly influenced by dynamics.

The model simulations used in the assessment WMO, differed from those in the prior WMO assessments in that they predict a more rapid recovery than what was presented in the Ozone assessment. About half of the models indicate recovery to levels by The results from the 2D models for the Arctic indicate a range of recovery rates, from about 0. These models offer greater insight into dynamical factors affecting current and future Arctic ozone levels.

In general, the 3D models estimate larger ozone depletion for the Arctic during than the 2D models. The different 3D model results indicate quite different future ozone levels. UMETRAC makes predictions through these predictions indicate slow recovery of a few percent between and This large decrease from to has not been seen in measurements.

The same rate of decrease is seen from to The results show improvement to levels above but lower than The predictions from this model indicate further depletion between and , with only modest recovery in The symbols represent the results from three 3-D models. The results indicate large differences in the modeled total ozone column amounts, with most models underestimating the measurements.

All of the models indicate that ozone levels are expected to remain significantly depleted for at least the next couple of decades. The 3D models can offer insight into the geographic distribution of ozone depletion and recovery. As may be seen in Figure 3. These differences indicate, to some extent, the uncertainty in the geographical distribution of future ozone levels.

Zonally symmetric dynamics in the GISS model result in near zonally symmetric ozone loss and recovery. Some of the most important uncertainties for the Arctic include the fact that most models have temperature biases in the Arctic winter and that the models include less than half of the observed trend in stratospheric water vapor. Differences in gravity wave and planetary wave simulations as well as model resolution can lead to very different results regarding polar temperatures and transport of ozone to the poles. It is also known that Arctic ozone depletion undergoes large natural variability, complicating the ability to determine definitively how ozone levels will evolve WMO, ; Austin et al.

As the text above suggests, modeling past and future ozone levels, particularly in the Arctic, is challenging. One of the main reasons for this challenge is the difficulty in simulating correct polar temperatures, which are essential for determining the severity of ozone depletion. The current chemistry-climate models do not reproduce the observed amounts of PSCs or the large increase since the s WMO, ; Austin et al. The models are also unable to accurately reproduce the observed ozone loss rates within the Arctic vortex.

The difficulties in simulating Arctic stratospheric temperatures stem partly from the strong influence of polar dynamics, and until these processes are better understood, future changes in Arctic dynamics and ultimately in Arctic ozone amounts will be difficult to predict. Because of the effects of the Montreal protocol and its amendments, an increase in Arctic ozone is expected eventually. However, any quantitative statements concerning the timing and magnitude of an Arctic ozone recovery are highly uncertain. Scientists primarily concerned with chemical contributions may be interested in the earlier signs of ozone recovery for example, a reduction in the downward trend , while those studying UV and its effects are likely to focus most on an overall recovery from depleted values.

Delayed recovery of total column ozone in the polar regions means there is the potential for increased UV in at these latitudes for many years. Although many model uncertainties exist, current estimates indicate that ozone depletion over the Arctic region may continue for 50 or more years WMO, ; WMO, Because ozone depletion at polar latitudes is expected to persist, and possibly worsen, for some years Shindell et al. By comparison, the annual UV dose increases for the entire Northern Hemisphere are estimated to be 14 percent for , and 2 percent for Reuder et al.

Their results indicate a further slight increase in springtime UV levels until These increased UV levels could have profound effects on human health. Slaper et al. While these results were based on analyses at mid-latitudes, they illustrate the long-ranged effects of increased UV levels on human health. If UV levels in the Arctic remain high beyond , the relative incidence and timeframe of human health effects would also be extended.

Ozone and future ozone changes are not the only factors affecting anticipated UV levels in the Arctic see Section 4. Changes in clouds will be important. Aerosol concentrations can also affect UV. These factors may change, at least on a regional basis, in the future. Changes in cloud cover can also affect UV levels reaching the surface. Whether clouds act to increase or decrease surface UV amounts depends mostly on cloud type.

A more active hydrologic cycle in the Arctic, as is expected from climate change IPCC, , could result in changes in cloud cover, therefore impacting surface UV. Uncertainties in the anticipated future cloud changes and in our understanding of cloud- UV interactions complicate predictions of surface UV amounts resulting from changes in clouds. Sea ice and snow cover are also likely to be affected by other climate changes, and can have major effects on incident UV radiation, both by reflecting radiation upward toward surfaces and by protecting organisms buried beneath it.

Projections indicate that overall temperatures in the Arctic may be warmer, suggesting that for late spring through early fall, much of the precipitation increase may be in the form of rain, or rain on snow. The extent and duration of snow cover in the Arctic is important, however, in part because of its relation UV doses.


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In the polar regions, UV doses affecting biological organisms depend greatly on the local surface albedo. These amplified doses can be particularly pronounced at low solar elevations, or in the presence of increased multiple scattering by non-absorbing aerosol. Any shift in the extent or duration of snow cover, particularly during the critical spring months, could amplify biologically effective UV doses to ecosystems potentially already stressed by climate change.

In areas normally covered by snow, early spring snowmelt, such as has been observed by Stone et al. Human health concerns include skin cancers, corneal damage, immune suppression, and aging of the skin. UV can also have deleterious effects on both terrestrial and aquatic ecosystems, and is known to affect infrastructure through damage to plastics, wood, and other materials.

Many of these effects and UV linkages are topics of increased study, while others have received considerable attention and are generally better understood. These effects and some of their expected consequences are discussed in greater detail in Chapters 6, 7, 8, 12, 13, 14, 15, and 16 of this assessment. All four of these areas are important both to improve our scientific understanding and to offer relevant information for policy decisions. There continue to be a number of unanswered scientific questions concerning the sources of variability of both ozone and UV in the Arctic.

Improved knowledge is needed to quantify the roles of trace gases, dynamics and temperature on Arctic ozone levels. The influence of climate change on both ozone and UV needs to be better understood. Understanding the controls and interactions of various processes will greatly improve our ability to predict future ozone levels. The interactions and overall influence of these factors are still the subject of much uncertainty, but future changes in any one parameter—for instance, in cloudiness or snow melt data--could substantially affect UV levels in the Arctic.

Quantifying these factors across the Arctic will provide opportunities to more realistically assess UV changes and their effects. Although many of the questions regarding the cause of ozone depletion have been addressed and confirmed in a number of studies both within and outside of the Arctic, questions still remain concerning the future of ozone and UV in the Arctic.

Because of these uncertainties, continued monitoring of both ozone and UV in the Arctic is important. Monitoring efforts are necessary both to document the evolution of ozone and UV over time and to validate model predictions. Satellite monitoring of the Arctic for most times of the year has been available for the last several decades, but can be problematic because of the difficulty distinguishing clouds from snow cover. Furthermore, some of the current satellite instruments are experiencing difficulties and the lifetime of any satellite monitoring is impossible to predict because of unexpected and irreparable failures.

The continuation of ground-based monitoring of ozone depends on available funding and is highly uncertain at this time. Improvements in coordination and calibration among the countries currently monitoring ozone and UV from the surface in the Arctic would allow for a more accurate assessment of the occurring changes.

Adding UV monitoring in the Russian Arctic and coordinating existing ozone monitoring throughout the Arctic would allow for additional improvements in our current assessment of the Arctic. Analyses of emerging ozone and UV data continue to reveal new information concerning the relative importance of trace gases, dynamics and temperatures in the Arctic.

Thermal Infrared Remote Sensing

Continued studies will likely add to our understanding of UV levels in the Arctic. Campaigns with intensive measurements of a variety of parameters as well as detailed monitoring of trace gases and vertically resolved ozone are key to advancing our understanding. The analyses of these measurements as well as the merging of available information into models offer our best insight into future ozone and UV levels and the impacts of specific changes in these levels. Recent studies support the idea that advanced three-dimensional models will be fundamental for obtaining improved estimates of future ozone levels over the Arctic.

One of the most important issues to address in the Arctic is determining the impacts of increased UV, particularly as levels are likely to remain higher than normal in coming decades. This work requires coordinated cooperation of the UV scientific community and the biological and impacts communities. Few cross-disciplinary efforts have been implemented so far, but the collaboration that has taken place has resulted in many of the impacts studies cited in other chapters of this assessment. Many questions remain concerning the impacts of UV on individual species, ecosystems, and human health.

Because the Arctic is likely to be an area with elevated springtime UV levels for some time, understanding the magnitude of potential impacts will be critical for future policy decisions. Because changes in various trace gases are expected, future Arctic ozone levels are highly uncertain, not only for the coming few decades but throughout the rest of the century. Policy decisions regarding trace gas emissions are likely to directly influence ozone levels in the Arctic. These decisions will be best based on not only improved understanding of how ozone and UV levels are likely to evolve, but also on increased knowledge of how the ecosystems, infrastructures, industries and people of the Arctic will be affected.

Most developed countries have ratified the Montreal Protocol and its amendments, but economic and political pressures have led some countries to suggest that they cannot meet their obligations under the current agreements. The concentrations of ozone- depleting substances are decreasing due to compliance with the Protocol and its amendments, but future compliance is uncertain and requires vigilance and cooperation for continued support. In addition, because the Arctic ozone and UV levels are strongly influenced by climatic impacts, including the effects of changes in temperature, trace gases and dynamics, international legislation regarding climate change is likely to directly affect Arctic ozone levels in the coming decades.

Any climate change policies need to be considered in light of the impacts both on Arctic climate and on Arctic ozone and UV levels. Sandvik, and E. Saksnaug, Spectral properties and UV-attenuation in Arctic marine waters, p. Hessen ed. Anderson, J. Russell III, S. Solomon, and L. Deaver, Halogen Occultation Experiment confirmation of stratospheric chlorine decreases in accordance with the Montreal Protocol, J.

Brune, and M. Proffitt, Ozone destruction by chlorine radicals within the Antarctic vortex—the spatial and temporal evolution of ClO-O3 anticorrelation based on in situ ER-2 data, J. Appenzeller, C. Weiss, and J. Staehelin, North Atlantic Oscillation modulates total ozone winter trends, Geophys.

Arola, A S. Kalliskota, P. N den Outer, K. Edvardsen, G. Hansen, T. Koskela, T. Martin, J. Matthijsen, R. Meerkoetter, P. Peeters, G. Seckmeyer, P. Simon, H. Slaper, P. Taalas and J. Verdebout, Assessment of four methods to estimate surface UV radiation using satellite data, by comparison with ground measurements from four stations in Europe, J. Res, D16 , Arola, A. Kaurola, L. Koskinen, A. Tanskanen, T. Tikkanen, P. Taales, J. Herman, N. Krotkov, and V. Fioletov, A new approach to estimating the albedo for snow- covered surfaces in the satellite UV method, J.

Austin J. Butchart, A three-dimensional modelling study of the influence of planetary wave dynamics on polar ozone photochemistry, J. Austin, J. A three-dimensional coupled chemistry-climate model simulation of past stratospheric trends, J. The influence of climate change and the timing of stratospheric warming on ozone depletion, J.

Shindell, S. Beagley, C. Bruhl, M. Dameris, E. Manzini, T. Nagashima, P. Newman, S. Pawson, G. Pitari, E. Rozanov, C. Schnadt, and T. Shepherd, Uncertainties and assessments of chemistry-climate models of the stratosphere, Atmos. Hofmann, N. Butchart, S. Oltmans, Mid stratospheric ozone minima in polar regions, Geophysical Research Letters, 22 18 , Knight, and N.

Butchart, Three-dimensional chemical model simulations of the ozone layer, Q. Journal Royal Meteorol. Bais, A. Zerefos, C. Gardiner, H. Slaper, et al. Baldwin, M. Thompson, E. Norton, N. Gillett, Weather from the stratosphere? Becker, G. McKenna, M. Rex, and K. Rex, K. Carslaw, and H. Fekete, P. Rettberg, G. Horneck and G. B: Biol. Bernhard, G. Booth, and R. Bigelow, D. Slusser, A. Beaubien, and J. Blumthaler M. Webb, G. Seckmeyer, A. Bais, M. Huber, and B.

Keywords/Phrases

Mayer, Simultaneous spectroradiometry: a study of solar UV irradiance at two altitudes, Geophys. Ambach, and M. Salzgeber, Effects of cloudiness on global and diffuse UV irradiance in a high-mountain area, Theoretical and Applied Climatology, 50, , a. Thermal infrared TIR data is acquired by a multitude of ground-based, airborne, and spaceborne remote sensing instruments. A broad variety of fields apply thermal infrared remote sensing, for example to assess general land- or sea-surface temperature dynamics, detect forest, coal and peat fires, map urban heat islands or thermal water pollution, differentiate geologic surfaces, analyze soil moisture, or even to test materials, to name only a few applications.

As thermal infrared data has to be analyzed slightly differently than reflective data, this chapter contains the relevant theoretical background. The thermal domain of the electromagnetic spectrum, the laws of Planck, Stefan-Boltzmann, Wien, and Kirchhoff, as well as important parameters such as kinetic and radiance temperature, emissivity, and thermal inertia are briefly explained. The chapter thus provides readers with a common understanding before proceeding to subsequent chapters.

This chapter presents an overview of thermal imaging sensors for photogrammetric close-range applications. In particular, it presents results of the geometric calibration of thermographic cameras as they are used for building inspection and material testing. Geometric calibration becomes evident for all precise geometric image operations, e. Two different test fields have been designed providing point targets that are visible in the thermal spectral band of the cameras. Five different cameras have been investigated.

One camera is working in scanning mode. The lenses for thermographic cameras are made of Germanium, which is, in contrast to glass, transparent to thermal radiation. Conventional imaging configurations typically 20 images have been used for camera calibration. Standard parameters for principal distance, principal point, radial distortion, decentring distortion, affinity and shear have been introduced into the self-calibrating bundle adjustment. All measured points are introduced as weighted control points. Image coordinates have been measured either in the professional software package AICON 3D Studio ellipse operators , or in the software system Stereomess least-squares template matching , developed by the Institute for Applied Photogrammetry and Geoinformatics of the Jade University of Applied Sciences Oldenburg.

The calibration results differ significantly from camera to camera. All lenses show relatively large decentring distortion and deviations from orthogonality of the image coordinate axes. Using a plane test field with heated lamps, the average image precision is 0. Thermal infrared TIR spectra of Earth surface materials are used in a wide variety of applications. Unlike visible-near infrared VNIR and shortwave infrared SWIR instruments, TIR spectroscopy instrumentation often requires customization in order to acquire reliable and reproducible data, making thermal spectroscopy a potentially complex process.

Within this chapter we intend to provide a simple starting point for the new user of thermal infrared spectroscopy, and a synoptic overview of the technique for the more experienced practitioner. We discuss the theoretical background, give examples of instrument setups and provide typical measurement scenarios for a number of land applications. Christoph A.

Hecker, Thomas E. UAV-borne thermal imaging involves the determination of ground surface temperature from thermal infrared measurements deploying an unmanned airborne vehicle UAV. A large variety of UAVs is available and applied for different military and civil tasks. UAV-borne thermal imaging provides spatially distributed information of the ground surface temperature. In contrast to satellite or ground based measurement, the usage of a UAV allows us to obtain spatially distributed and geometrically highly resolved information on the ground surface temperature without the need to access the ground.

The area can be flat or hilly, and steep walls and hillsides can be investigated easily. However, some problems, especially tasks related to mosaicking of the images, are not fully resolved to date. We address the detection of the anomalies in ground surface temperature induced by underground burning coal seams as example and describe the challenges and opportunities of UAV-borne thermal imaging, based on our experiences in this field. The HyTES pushbroom design has spatial pixels over a degree field of view and contiguous spectral bands between 7.

HyTES includes many key enabling state-of-the-art technologies including a high performance concave diffraction grating, a quantum well infrared photodetector QWIP focal plane array, and a compact Dyson-based optical design. It also minimizes cooling requirements due to the fact it has a single monolithic prism-like grating design which allows baffling for stray light suppression. The monolithic configuration eases mechanical tolerancing requirements which are a concern since the complete optical assembly is operated at cryogenic temperatures.

The system uses two mechanical cryocoolers to maintain instrument temperature. The first cooler holds the focal plane array at 40 K and the second cooler holds the remainder of the cryovacuum system at K. Assembly of the system is now complete and the system is undergoing alignment and laboratory testing. Once laboratory testing is complete the system will be used to acquire airborne data from a Twin Otter aircraft over the southwestern USA in late HyspIRI will fly two instruments: a hyperspectral visible to short wave infrared imaging spectrometer, and a multispectral thermal infrared TIR imager.

In this study we discuss the expected performance and use of the TIR instrument. The TIR instrument will have a swath width of km, and pixel size of 60 m. HyspIRI TIR will provide two visits every 5 days one day and one night at the equator, and more frequently at higher latitudes.

The TIR instrument will always be on and full resolution 60 m data will be downlinked for the entire land surface including the coastal oceans shallower than 50 m depth. Data over the deeper ocean will also be downlinked but at a reduced spatial resolution of 1 km. In response to the Decadal Survey, HyspIRI has been designed to answer important science questions in the areas of coastal, ocean and inland aquatic environments; wildfires; volcanoes; ecosystem function and diversity; land surface composition and change; and human health and urbanization. In addition a direct broadcast capability will allow users to capture and process a subset of HyspIRI data in near real time.

This chapter presents an overview of the most commonly used spaceborne sensors for thermal infrared research applications. There is a large fleet of international sensors available which allow for the acquisition of data in the thermal infrared. Depending on spatial coverage, some sensors are more suitable for mapping large areas, while others support observations at a local scale.

Temporal resolution defines whether temperature patterns or phenomena can be monitored on a daily, weekly, monthly, or even only an annual basis. A wide variety of thermal sensors will be introduced in overview tables. A comprehensive overview of typical thermal infrared application studies and the sensors particularly favored rounds off this chapter.

For example, enhancement of surface UV irradiance by multiple scattering depends both on surface albedo and cloud conditions. These features make polar regions, including the Arctic, unique and complex in terms of their UV radiation environments. In the last century the most distinctive variation has been the year solar cycle, which is reflected by the average number of sunspots. The variation of the solar irradiance has a strong wavelength dependence: the variability is higher at smaller wavelengths [Solanki and Unruh, ].

According to Rozema et al. Thus, while the year solar cycle has only a small effect on the surface UV-B irradiances, very long-term fluctuations of the solar activity have a potential to affect future UV levels. UV amounts reaching the Earth and its atmosphere also depend somewhat on the Earth- Sun distance. The Earth is closest to the Sun on January 3 perihelion and farthest away in July 4 aphelion. At large SZAs, when the Sun appears low in the sky, the absorption of UV radiation is increased by atmospheric gases and absorbing aerosol along the longer path length that photons must travel.

The differences between summer and winter UV levels are higher in the Arctic than at lower latitudes, while diurnal variations in SZA are smaller when moving closer to the poles. However, when daily integrated doses are compared, the length of Arctic summer days somewhat compensates for the effect of large SZAs. The variation in erythemal dose is caused solely by the seasonal variation of SZA. A decrease in total column ozone leads to an increase in UV levels, which has been demonstrated repeatedly [WMO, ].

The relationship depends somewhat on the vertical distribution, or profile, of ozone in the atmosphere. Lapeta et al. The cut-off of the solar spectrum in the UV-B is primarily a consequence of the sharp increase in the ozone absorption cross section toward shorter wavelengths. Thus, the change in surface UV irradiance as a result of change in total column ozone depends highly on the wavelength of the radiation. Traditionally, radiation amplification factors RAFs have been used to quantify the change in biologically effective irradiances as a result of change in the total column ozone [e.

The RAF can also be used to indicate the sensitivity of a particular UV effect to a change in total ozone. RAF values depend largely on the biological effect and vary between 0. For large changes in total ozone, this relationship is no longer linear; instead, an advanced power formulation is required to estimate the corresponding changes in UV Booth and Madronich, In the Arctic, where solar zenith angles are often large, radiation amplification factors should be used with caution due to their pronounced dependence on the SZA and on total ozone at large SZAs Micheletti et al.

For example, the erythemal RAF is reduced to approximately 0. The erythemal radiation amplification factor of a function of total column ozone and solar zenith angle. However, surface UV irradiance can be locally increased if clouds are not obstructing the disk of the Sun, and additional radiation is reflected from the side of a broken cloud field toward the ground [Nack and Green, ; Mims and Frederick, ]. In meteorology, cloud conditions are traditionally measured in octas. The sky is divided into eight sectors and the octa number, from 0 to 8, is based on the number of observed sectors containing clouds.

Bais et al. Thiel et al. Cloud transmittance of UV radiation depends on wavelength [Seckmeyer et al. The maximum transmittance occurs at approximately nm; the actual location of the turning point depends on the cloud optical depth, and on the amount of tropospheric ozone and the solar zenith angle [Mayer et al. In general, clouds in the Arctic tend to be optically thinner than clouds at lower latitudes due to reduced atmospheric water vapor content. When the ground is covered by snow, attenuation by clouds is further diminished due to multiple scattering between the surface and cloud base Nichol et al.

The attenuation of the surface UV irradiance by aerosols, including black carbon, depends on the aerosol optical depth AOD , single scattering albedo, asymmetry factor and aerosol profile. The single scattering albedo is the ratio of the scattering cross section of the aerosol to the extinction cross section and is typically larger than 0.

Episodes of long-range transport of pollutants have been observed to occur in the Arctic, and combined with lowered rates of particle and gas removal in cold and stable Arctic atmosphere, can lead to a phenomenon called Arctic haze [Shaw and Shaw ]. The Arctic haze events imply increased aerosol concentrations and mostly occur in winter and spring. Wetzel et al. The measured AOD at nm ranged from 0. Herber et al. The mean AOD value at nm during Arctic haze conditions was about 0.

Quinn et al. They found that the SO4 concentrations were highest at Barrow and decreased with latitude from Poker Flat to Denali to Homer, suggesting a north to south gradient. Ricard et al. For the range of aerosol sampled at Poker Flat, Alaska, Wetzel et al. The decrease of erythemal UV irradiance as a function of aerosol optical depth is illustrated in Figure 3. The graph is based on theoretical calculations with a radiative transfer model Mayer et al. The decrease of erythemal UV irradiance as a function of aerosol optical depth, based on theoretical calculations with a radiative transfer model.

Clouds are more effective at scattering UV radiation than air molecules. This amplification exhibits a distinct wavelength dependency [Lenoble ; Nichol et al. Surface albedo in the UV range is generally low, except in the presence of snow cover. Blumthaler and Ambach measured erythemally weighted surface albedos for various snow-free surfaces and reported values ranging between 1 and 11 percent.

For snow-covered surfaces, the measurements suggest much higher values, with albedo from snow ranging from 50 to 98 percent. The albedo of a snow-covered surface depends not only on snow depth and condition, but also on terrain and landscape Fioletov et al. Albedo is an important factor for the Arctic, where the ground is covered by snow during extensive periods of the year. As scattering in the atmosphere may occur far away from the location of interest, the ground properties of a large area around the measurement site are relevant.

The regional averaged albedo is often referred to as an effective albedo [Kylling et al. At higher locations, the atmosphere is thinner, and therefore fewer particles exist to absorb or scatter radiation. Higher locations also experience a reduced influence from tropospheric ozone or aerosols in the atmospheric boundary layer.

In the Arctic or in mountainous regions, the ground is more likely covered by snow at higher altitudes, which leads to higher albedo and increased UV. Clouds below a mountain summit have an effect similar to snow covered ground, and will therefore lead to an increase of radiation at the summit. In contrast, the same cloud may cause a decrease of radiation in a valley below the mountain.

Because the variation of UV with altitude depends on several factors, all of which have different wavelength dependences, the increase of UV with height cannot be expressed by a simple relationship. The transmission of the UV radiation through snow or ice depends on wavelength, the thickness of the cover, and the optical properties of the snow or ice. In general, radiation is attenuated by a factor that changes exponentially with the thickness of the snow or ice cover. Shorter wavelengths are more strongly attenuated than longer wavelengths. Field measurements in Alert, Canada, suggest that a modest snow cover of 10 cm reduces the amount of transmitted nm UV radiation by two orders of magnitude [King et al.

According to Perovich , a similar attenuation of transmitted nm UV radiation would require some 1. Snow and ice cover are likely to have the most unpredictable effect on the UV exposures of the terrestrial and aquatic lifeforms in the Arctic. The observed and projected changes in the ice and snow cover seasons are covered in ACIA chapters 5. Lawrence, Canada. UV-A radiation reaches even greater depths. Measurements made in Arctic waters typically fall into the lower-to- middle part of this range Aas et al. Thus, organisms residing in the near-surface layer can be exposed to UV radiation.

Ten percent depth penetrations at selected locations in the estuary and Gulf of St. In shallower water, this depth may be on order of only a meter, but organisms living within this 1-meter layer would still be at risk. While this approach has sound physical merit, it does not represent the amounts reaching many biological receptors. The amount of UV incident on a vertical, as opposed to a horizontal, surface has important biological implications, particularly in terms of effects to the eye Sliney , ; Meyer-Rochow, Because of these effects, recent studies have sought to explore both the effect of high snow reflectivity e.

Schmucki et al. Some investigators have measured the amounts of UV to a surface oriented normal to the Sun e. Philipona et al. As reported by Webb et al. For wavelengths less than nm, Webb et al. For cloudless, snow-free conditions, the maximum ratios ranged from 1. Snow cover, which increases surface albedo, may substantially increase these ratios. For instance, in the presence of fresh snow cover and at solar zenith angles greater than 60 degrees, Philipona et al. Similar results were obtained by Jokela et al. The results indicated a snow albedo of 0.

The ratios of vertical to horizontal dose rates varied from about 0. In general, the observations indicated that springtime ozone depletion could contribute greatly to ocular UV doses because of the significant effect of snow reflection. The measurements by Jokela et al. These high doses suggest that the amount of UV received when looking toward the horizon can be equivalent or greater than the amount of UV received when looking directly upward. For many biological systems the actinic flux, that is the radiation incident at a point, is a more relevant quantity than the horizontal irradiance.

Only recently, have measurements of the spectral actinic flux become more common [Hofzumahaus et al. Webb et al. The ratio was found to depend on wavelength, SZA, and the optical properties of the atmosphere. Long-term changes and variability of UV radiation Several instruments and methods have been used to determine UV levels in the Arctic. However, quality-controlled measurements of UV irradiance have been available for no more than a decade in the Arctic and provide only limited spatial coverage.

In addition to the direct ground-based UV irradiance measurements, surface UV irradiance can be reconstructed using satellite data, or by using the measured total ozone combined with commonly available meteorological data McArthur et al. In addition, historic UV-B levels can be reconstructed using biological proxies. Studies by Lantz et al. The length of reliable data records covers only the past ten years, however, which is not adequate for long-term trend analyses [Weatherhead et al. Nevertheless, the measured surface UV time series illustrate the variability of UV radiation and the role of different UV-affecting factors at each of the monitoring sites.

The ground-based UV records are also crucial for validating the indirect methods for estimating UV. The current network of Arctic ground-based UV measurements in shown in Figure 3. Only those installations operated on a regular basis are shown. The instruments are classified to three types: spectroradiometers, multifilter instruments, and broadband instruments. Spectroradiometers scan radiation in small wavelength bands with a typical resolution of 1 nm or less [Seckmeyer et al. They provide the highest measurement accuracy and the spectral information enables versatile data use.

However, they are costly and require trained personnel for maintenance and operation. Multichannel or multifilter instruments typically consist of several filtered photodetectors that measure radiation in selected wavelength bands [Dahlback, ; Harrison et al. They provide much faster sampling than the conventional scanning spectroradiometers and allow evaluation of both long- and short-term changes in UV. Methods have also been developed to reconstruct the high resolution spectrum from these multifilter instrument measurements [Dahlback, ; Fuenzalida, ; Min and Harrison, ].

Broadband instruments collect radiation over a portion of the spectrum and impose a weighting function. Many of these instruments are designed to measure the erythemal irradiance. Broadband instruments are comparatively inexpensive and easy to maintain. However, their spectral response often deviates from the one that they are mimicking.

Thus, the absolute calibration of the broadband instruments depends on solar zenith angle and total ozone column, and both determining the absolute calibration [Mayer et al. In addition to the above-described instruments, the UV dose, or biological dose rate integrated over time, can be measured using biological UV dosimeters, which directly quantify the biologically effective solar irradiance for certain processes by allowing biological systems to act as UV sensors.

Fairly good agreement with weighted spectroradiometric measurements has been shown for some of these systems Furusawa et al. Monitoring at these sites started in , , , and , respectively. Increased UV-B levels have been reported in the Arctic during six winters in the s Fioletov et al. These values are relatively small compared to the UV indices measured at the other Canadian monitoring sites.

However, the maximum observed UV index may not be the best indicator for the UV doses received in the Arctic because it does not take into account the length of the day nor the receptor orientation. Lakkaka et al. No statistically significant changes were observed for any months for the year period One reason is that the period is rather short in comparison to the high natural interannual variability of UV. The lack of a distinct trend over this particular period could be also expected from the corresponding ozone changes, characterized by a decrease in the early s with an increase toward more normal values in recent years, with the smallest value occurring in the middle of the period.

Arola et al. The results, shown in Figure 3. Multiple scattering between ground and cloud base interconnects the effects of surface albedo and cloud cover. Attempts to separate the two factors e. The relative enhancement in monthly erythemal doses due to the presence of clouds green , and albedo from snow cover blue. The figure is adopted from Kylling et al. Enhanced UV levels were observed in , which were likely due to low ozone amounts related to the injections of aerosols into the stratosphere from the eruption of Mt.

Pinatubo [Gurney, ]. Photochemically induced ozone depletion events at Barrow have mostly occurred during March and April. This increase is still small compared to spikes seen in UV measurements at Antarctic sites affected by the ozone hole. By comparing measured and modeled spectra, Bernhard et al. It should be noted that the observed ozone reductions have been much stronger in the European and Russian sector of the Arctic than over Alaska.

UV increases at Barrow are therefore less pronounced compared to the other Arctic sites. These methods are usually based on total ozone or other commonly available weather data, including global radiation. Bodeker and McKenzie [], for instance, presented a model based on total ozone, broadband measurements, and radiative transfer calculations. Similar methods were developed and used by McArthur et al. Gantner et al. The results obtained with reconstruction methods generally compare well with measurements.

There are, however, some sources of uncertainties that are worth mentioning. The importance of examining the homogeneity of the data cannot be overestimated when using long time series of measurements as input data for estimating past UV levels. Likewise, it is of great importance to properly validate the methods against independent measurement data. Fioletov et al. The model is based on previous work to estimate the UV-A irradiance from pyranometer measurements McArthur et al.

This increase follows a reported decline in the absolute UV levels from to The UV increase from to is more than twice the trend that would be expected from the observed decline in total ozone, because concurrent changes in the surface albedo and cloud conditions have enhanced the increase in surface UV over this period. Another reconstruction by Diaz et al. They conducted only a brief analysis of the produced time series of reconstructed UV. However, their results implied increases in the springtime UV irradiance over the period Lindfors et al. The results show a statistically significant increasing trend of 3.

April also shows an increasing trend, which is very close to being statistically significant. For both March and April, the trend is more pronounced during the latter part of the period , suggesting a connection to stratospheric ozone depletion. For July, a significant decreasing trend of 3. June and August exhibit decreasing trends, although not statistically significant, and trends for May and September are negligible. In addition to the atmospheric observations, biological proxies can be used to estimate past UV radiation.

For instance, Leavitt et al. The method is based on the fact that some algae or other aquatic organisms produce photoprotective pigments when exposed to UV radiation. Long-term variation of the underwater UV is primarily controlled by the amount of dissolved organic matter, however, which unfortunately limits the use of fossil pigment sediments to reconstruct surface UV [Leavitt et al. In addition to total ozone, satellite-based information on clouds, aerosols, and surface albedo is needed to accurate model these parameters, and various sources of data have been used for that purpose [Herman et al.

The advantages of the satellite retrieval methods are that satellite instruments provide global or near-global spatial coverage as well as long-term continuity. However, satellite retrieval methods have their limitations. Satellite instruments also have difficulty probing the lower atmosphere, where UV-absorbing aerosols or tropospheric ozone can significantly affect surface UV radiation.

Fortunately, neither absorbing aerosols nor tropospheric ozone can be considered major factors affecting UV radiation in the Arctic, with the exception of air masses occasionally transported from lower latitudes. Several studies have validated satellite-based UV estimates against ground-based data Kalliskota et al, ; Wang et al, ; McKenzie ; Slusser et al ; Arola et al, ; Chubarova et al, ; Fioletov et al, Estimating the surface UV radiation in the Arctic is more uncertain than that at lower latitudes: during snow cover, the satellite-derived UV estimates have been systematically lower than the ground-based measurements.

The bias has originated from the underestimated surface albedo, which is a crucial parameter for accurate estimating surface UV radiation in the Arctic. Additionally, snow or ice cover complicates the determination of the cloud optical depth properties Krotkov et al. Because of these limitations, the accuracy of the currently used satellite retrieval algorithms cannot be considered adequate for monitoring Arctic surface UV, although some information on UV variation over time can be inferred.

The satellite-derived erythemal daily dose over the Arctic on March 21, , is shown in Figure 3. The analyses were performed for clear sky estimates, that is, the modulation of surface UV radiation by clouds or aerosols was not considered. The trend analysis methods applied are similar to those used to study long-term changes in total column ozone.

Moreover, the winter and spring trends were found to be the largest. Ziemke et al. Arctic UV dose distribution derived from satellite data for the 21st of March, Total column ozone over the Arctic in winter and spring is usually higher than that over the equator and northern mid-latitudes. Ozone amounts over the Arctic are marked by a strong annual cycle, with a peak occurring during the springtime, and a decrease in late summer and throughout the autumn months.

Cold temperatures provide the potential for substantial depletion in the winter and early spring, reducing ozone amounts at a time when they would normally be high and when reproduction and new growth leave ecosystems particularly vulnerable. The same physical and chemical processes govern ozone levels and ozone depletion over both the Arctic and Antarctic. However, a stronger polar vortex over Antarctica has resulted in greater percentage loss over the past two decades.

In the years where dynamical conditions allow for similarly cold stratospheric temperatures in the Arctic, significant ozone loss can be observed at northern high latitudes as well. Globally, the Montreal Protocol and its amendments have already resulted in a decrease in the concentrations of some ozone-depleting substances Montzka et al.

Detecting the onset of this ozone recovery is likely to require some time because of natural variability Weatherhead et al. In polar regions, the projections of recovery are complicated by the effects of dynamical contributions and climate change. Ozone recovery is likely to occur in stages. The first signs of recovery are likely to include a reduction in the downward trend followed by an increase in ozone levels. Several models have been used to predict future ozone levels WMO ; An assessment of various models discusses the advantages offered by the two-dimensional and three-dimensional models.

Two-dimensional models have simplified dynamics and advanced chemistry. Three-dimensional models have improved dynamics but simplified chemistry. Because of the importance of both chemistry and dynamics in the Arctic, each model type has specific strengths that complement the other.

Thermal Infrared Remote Sensing | zopusalawyky.ga

Both types of models are run at various institutions. Intercomparison of these models WMO, shows qualitative agreement in the projections, although specific projections of the recovery rates can disagree significantly. The three-dimensional models can provide multi-year time slice simulations Austin et al. These simulations have the advantage that several realizations are available for a single year, allowing a better assessment of the projections. Three- dimensional models are also able to address dynamical changes in well-mixed greenhouse gases and offer a more detailed evolution of ozone, based on the same mechanisms that are likely to occur in the atmosphere.

These models can provide information on the expected range of interannual variability. Austin et al. The models were compared based on their ability to predict ozone climatologies for the current atmosphere. In the northern hemisphere, all models tended to overestimate the area-weighted hemispheric ozone, by an average of 7. Within the Arctic vortex, the models are unable to simulate the observed loss rates Bregman et al. Uncertainties in the model projections include temperature biases, leading to the so-called "cold pole problem" Pawson et al.

These biases are worse in some models than others. In the northern hemisphere, the biases are sometimes positive at certain levels, resulting in insufficient ozone depletion in early winter, but excess depletion in the spring. Other uncertainties include the models' ability to accurately simulate polar stratospheric clouds PSCs and to account for all aspects of constituent transport, including processes occurring at the model's upper boundary.

Changes in planetary waves and heat flux also pose uncertainties, and are discussed in greater detail below. Over the Arctic, however, ozone depletion processes are often much more complicated and depend greatly on climate conditions and climate changes. For instance, when potential climate-induced increases in stratospheric water vapor are included in models, the resulting mid-latitude ozone change in the s surpasses that due to halogens Shindell and Grewe, At high latitudes, the effects are anticipated to be even larger due to the impact of PSCs.

Modeling efforts e. Shindell et al. Depletion in the Arctic is governed strongly by the dynamics of the polar atmosphere. Changes in circulation, and particularly changes that affect air temperatures in the polar region, can substantially impact the levels of depletion that are experienced. For instance, establishment of a strong polar vortex results in decreasing stratospheric temperatures, which act as a positive feedback to further strengthen the polar vortex.

This feedback effect contributes to increased ozone depletion, and will likely be exacerbated by stratospheric cooling expected from future climate change. The Montreal Protocol and its amendments have already been observed to reduce levels of chlorine in the atmosphere, and concentrations of ozone- depleting halogens are expected to continue to decrease over the period to The decreases were first reported in the troposphere Montzka et al. Bromine, a part of many currently used CFC substitutes, is particularly effective in destroying ozone, and overall levels may be highly impacted by shorter-lived substances, such as bromoform Dvortsov et al.

As stated earlier, however, the magnitude of the ozone loss can depend greatly on dynamical and climate conditions, with cold temperatures contributing to the formation and persistence of polar stratospheric clouds. Over the polar regions, heterogeneous chemistry in or on these clouds converts stable chlorine and bromine reservoirs to more active forms that can deplete ozone. Future volcanic eruptions could also change stratospheric ozone levels worldwide for at least several years, and could have a large effect in the Arctic as long as halogen loading remains large Tabazedeh et al.

In the Arctic, this cooling could be responsible for increased ozone destruction, as lower temperatures are likely to result in the formation and persistence of polar stratospheric clouds. Polar stratospheric clouds aid in activating ozone-depleting compounds, and can therefore accelerate ozone destruction. Stratospheric cooling due to global change may therefore result in an increased likelihood of larger and longer-lasting ozone holes in the Antarctic and the formation of an ozone hole over the Arctic Dameris et al.

On the other hand, climate changes could trigger a possible increase in planetary waves, speeding up the transport of warm, ozone- rich air to the Arctic Schnadt et al. This increased transport would counter the effects of heterogeneous chemistry and potentially lessen the time required for an ozone recovery. If radiative effects dominate, planetary wave activity would be more likely to decrease, resulting in more ozone loss at Arctic latitudes.

Another climate feedback affecting ozone is an increase in stratospheric water vapor due to changes in tropopause temperatures Evans et al. Few long-term data sets of water vapor concentrations are available, but previous observations have suggested that stratospheric water vapor is indeed increasing Oltmans and Hofmann, ; Randel et al.

Analyses of 45 years of data by Rosenlof et al. Analyses of satellite data, however, have been less supportive of a water vapor increase Randel et al. Increased water vapor may contribute to increased ozone destruction by affecting the radiation balance of the stratosphere Shindell, Water vapor concentrations in the stratosphere affect the threshold temperatures for activating heterogeneous chemical reactions on PSCs, and can also influence the temperature of the Arctic vortex itself Kirk-Davidoff et al.

Ozone itself is central to climate change science. Ozone is an important greenhouse gas in the infrared part of spectrum and is the primary absorber of solar UV radiation. It is responsible for the vertical structure of the atmosphere, as it is the absorption of solar radiation by ozone that causes the stratosphere to exist.

Ozone is critical to the radiation balance of the atmosphere, and to the dynamics of the stratosphere. Observations show that the strengths of the polar vortices, which are strongly influenced by stratospheric ozone levels, affect surface temperatures over polar regions and at midlatitudes in both hemispheres. These feedback processes in the stratosphere can alter weather processes in the troposphere, with an effect whose magnitude is comparable to that of ENSO Gillett and Thompson, ; Hartmann et al. The expected future changes in ozone in polar regions differ significantly from the expected changes over the rest of the globe, where stratospheric temperatures do not reach the cold polar thresholds.

Model simulations suggest that a strong polar vortex may warm Northern Hemisphere continents and could substantially delay the Arctic ozone recovery Shindell, The delay in recovery at polar regions means a longer term, and perhaps more severe, threat of ecosystem damage due to increased UV. Projected changes in ozone amounts A number of two-dimensional 2D models using specified halocarbon scenarios were used to estimate future ozone levels for the recent World Meteorological Organization Ozone Assessment WMO, These models include AER Wiesenstein et al.

In Figure 3. The spring period is interesting because the accumulated ozone depletion is at its highest, and the UV radiation is relatively high during what is also the beginning of the growth period for many biological systems. Generally, the models indicate local minimums in Arctic ozone in the late s, followed by a slow, gradual increase. Ozone values in are significantly lower than the values for all models.

The dashed lines represent the results from ten 2-D models. Because the 2D models are unable to fully incorporate dynamical effects their results are considered very rough estimates for the polar regions, where ozone amounts are strongly influenced by dynamics. The model simulations used in the assessment WMO, differed from those in the prior WMO assessments in that they predict a more rapid recovery than what was presented in the Ozone assessment.

About half of the models indicate recovery to levels by The results from the 2D models for the Arctic indicate a range of recovery rates, from about 0. These models offer greater insight into dynamical factors affecting current and future Arctic ozone levels. In general, the 3D models estimate larger ozone depletion for the Arctic during than the 2D models.

The different 3D model results indicate quite different future ozone levels. UMETRAC makes predictions through these predictions indicate slow recovery of a few percent between and This large decrease from to has not been seen in measurements. The same rate of decrease is seen from to The results show improvement to levels above but lower than The predictions from this model indicate further depletion between and , with only modest recovery in The symbols represent the results from three 3-D models. The results indicate large differences in the modeled total ozone column amounts, with most models underestimating the measurements.

All of the models indicate that ozone levels are expected to remain significantly depleted for at least the next couple of decades. The 3D models can offer insight into the geographic distribution of ozone depletion and recovery. As may be seen in Figure 3. These differences indicate, to some extent, the uncertainty in the geographical distribution of future ozone levels.

Zonally symmetric dynamics in the GISS model result in near zonally symmetric ozone loss and recovery. Some of the most important uncertainties for the Arctic include the fact that most models have temperature biases in the Arctic winter and that the models include less than half of the observed trend in stratospheric water vapor. Differences in gravity wave and planetary wave simulations as well as model resolution can lead to very different results regarding polar temperatures and transport of ozone to the poles. It is also known that Arctic ozone depletion undergoes large natural variability, complicating the ability to determine definitively how ozone levels will evolve WMO, ; Austin et al.

As the text above suggests, modeling past and future ozone levels, particularly in the Arctic, is challenging. One of the main reasons for this challenge is the difficulty in simulating correct polar temperatures, which are essential for determining the severity of ozone depletion. The current chemistry-climate models do not reproduce the observed amounts of PSCs or the large increase since the s WMO, ; Austin et al. The models are also unable to accurately reproduce the observed ozone loss rates within the Arctic vortex. The difficulties in simulating Arctic stratospheric temperatures stem partly from the strong influence of polar dynamics, and until these processes are better understood, future changes in Arctic dynamics and ultimately in Arctic ozone amounts will be difficult to predict.

Because of the effects of the Montreal protocol and its amendments, an increase in Arctic ozone is expected eventually. However, any quantitative statements concerning the timing and magnitude of an Arctic ozone recovery are highly uncertain. Scientists primarily concerned with chemical contributions may be interested in the earlier signs of ozone recovery for example, a reduction in the downward trend , while those studying UV and its effects are likely to focus most on an overall recovery from depleted values.

Delayed recovery of total column ozone in the polar regions means there is the potential for increased UV in at these latitudes for many years. Although many model uncertainties exist, current estimates indicate that ozone depletion over the Arctic region may continue for 50 or more years WMO, ; WMO, Because ozone depletion at polar latitudes is expected to persist, and possibly worsen, for some years Shindell et al.

By comparison, the annual UV dose increases for the entire Northern Hemisphere are estimated to be 14 percent for , and 2 percent for Reuder et al. Their results indicate a further slight increase in springtime UV levels until These increased UV levels could have profound effects on human health. Slaper et al. While these results were based on analyses at mid-latitudes, they illustrate the long-ranged effects of increased UV levels on human health. If UV levels in the Arctic remain high beyond , the relative incidence and timeframe of human health effects would also be extended.

Ozone and future ozone changes are not the only factors affecting anticipated UV levels in the Arctic see Section 4. Changes in clouds will be important. Aerosol concentrations can also affect UV. These factors may change, at least on a regional basis, in the future. Changes in cloud cover can also affect UV levels reaching the surface. Whether clouds act to increase or decrease surface UV amounts depends mostly on cloud type.

A more active hydrologic cycle in the Arctic, as is expected from climate change IPCC, , could result in changes in cloud cover, therefore impacting surface UV. Uncertainties in the anticipated future cloud changes and in our understanding of cloud- UV interactions complicate predictions of surface UV amounts resulting from changes in clouds.

Sea ice and snow cover are also likely to be affected by other climate changes, and can have major effects on incident UV radiation, both by reflecting radiation upward toward surfaces and by protecting organisms buried beneath it. Projections indicate that overall temperatures in the Arctic may be warmer, suggesting that for late spring through early fall, much of the precipitation increase may be in the form of rain, or rain on snow.

The extent and duration of snow cover in the Arctic is important, however, in part because of its relation UV doses. In the polar regions, UV doses affecting biological organisms depend greatly on the local surface albedo. These amplified doses can be particularly pronounced at low solar elevations, or in the presence of increased multiple scattering by non-absorbing aerosol.

Any shift in the extent or duration of snow cover, particularly during the critical spring months, could amplify biologically effective UV doses to ecosystems potentially already stressed by climate change. In areas normally covered by snow, early spring snowmelt, such as has been observed by Stone et al. Human health concerns include skin cancers, corneal damage, immune suppression, and aging of the skin. UV can also have deleterious effects on both terrestrial and aquatic ecosystems, and is known to affect infrastructure through damage to plastics, wood, and other materials.

Many of these effects and UV linkages are topics of increased study, while others have received considerable attention and are generally better understood. These effects and some of their expected consequences are discussed in greater detail in Chapters 6, 7, 8, 12, 13, 14, 15, and 16 of this assessment. All four of these areas are important both to improve our scientific understanding and to offer relevant information for policy decisions.

There continue to be a number of unanswered scientific questions concerning the sources of variability of both ozone and UV in the Arctic. Improved knowledge is needed to quantify the roles of trace gases, dynamics and temperature on Arctic ozone levels. The influence of climate change on both ozone and UV needs to be better understood. Understanding the controls and interactions of various processes will greatly improve our ability to predict future ozone levels.

The interactions and overall influence of these factors are still the subject of much uncertainty, but future changes in any one parameter—for instance, in cloudiness or snow melt data--could substantially affect UV levels in the Arctic. Quantifying these factors across the Arctic will provide opportunities to more realistically assess UV changes and their effects.

Although many of the questions regarding the cause of ozone depletion have been addressed and confirmed in a number of studies both within and outside of the Arctic, questions still remain concerning the future of ozone and UV in the Arctic. Because of these uncertainties, continued monitoring of both ozone and UV in the Arctic is important. Monitoring efforts are necessary both to document the evolution of ozone and UV over time and to validate model predictions.

Satellite monitoring of the Arctic for most times of the year has been available for the last several decades, but can be problematic because of the difficulty distinguishing clouds from snow cover. Furthermore, some of the current satellite instruments are experiencing difficulties and the lifetime of any satellite monitoring is impossible to predict because of unexpected and irreparable failures. The continuation of ground-based monitoring of ozone depends on available funding and is highly uncertain at this time. Improvements in coordination and calibration among the countries currently monitoring ozone and UV from the surface in the Arctic would allow for a more accurate assessment of the occurring changes.

Adding UV monitoring in the Russian Arctic and coordinating existing ozone monitoring throughout the Arctic would allow for additional improvements in our current assessment of the Arctic. Analyses of emerging ozone and UV data continue to reveal new information concerning the relative importance of trace gases, dynamics and temperatures in the Arctic.

Continued studies will likely add to our understanding of UV levels in the Arctic.