Future changes in ozone in the Arctic

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This is Section 5.6 of the Arctic Climate Impact Assessment
Lead Authors: Betsy Weatherhead, Aapo Tanskanen, Amy Stevermer; Contributing Authors: Signe Bech Andersen, Antti Arola, John Austin, Germar Bernhard, Howard Browman,Vitali Fioletov,Volker Grewe, Jay Herman, Weine Josefsson, Arve Kylling, Esko Kyrö, Anders Lindfors, Drew Shindell, Petteri Taalas, David Tarasick; Consulting Authors: Valery Dorokhov, Bjorn Johnsen, Jussi Kaurola, Rigel Kivi, Nikolay Krotkov, Kaisa Lakkala, Jacqueline Lenoble, David Sliney

300px-Two dimensional model projections.JPG Figure 5.14. Two-dimensional model projections of the change in spring (March–May) total column ozone relative to 1980; averaged over the band from 60º to 90º N. TOMS/SBUV observations are shown for comparison. (Source: ACIA)

Stratospheric ozone levels over the polar regions are very different from levels over the mid-latitudes. Total column ozone over the Arctic in winter and spring is usually higher than that over the equator and northern mid-latitudes. Ozone levels over the Arctic are marked by a strong annual cycle, with a peak in the spring, and a decrease in late summer and throughout the autumn. Low stratospheric temperatures provide the potential for substantial ozone depletion in the winter and early spring, reducing ozone levels 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, less disturbed polar vortex over Antarctica and thus uniformly lower stratospheric temperatures, have resulted in greater percentage ozone losses over the past two decades in the Antarctic compared to the Arctic. In the years where dynamic conditions allow for similarly cold stratospheric temperatures in the Arctic, significant ozone losses have been observed at northern high latitudes as well.

The Montreal Protocol and its amendments have already resulted in a decrease in the atmospheric concentrations of some ozone-depleting substances[1]. Although scientific understanding of the dynamics and other factors influencing ozone depletion remains incomplete, most projections suggest that mid-latitude ozone levels will gradually recover over the next 50 years[2]. Confirming either a change in ozone trends or an actual increase in ozone levels is likely to require some time because of natural variability and intrinsic measurement errors[3]. In polar regions, the projections of recovery are complicated by the effects of dynamic processes and climate change.

Recovery of the ozone layer is likely to occur in stages. The first signs of recovery should be a reduction in the downward trend followed by an increase in ozone levels. Final recovery may be defined as an overall return to pre-depletion ozone levels or as the determination that ozone levels are no longer being affected by anthropogenic ozone-depleting substances. Newchurch et al.[4] reported evidence of a reduction in the downward trend in ozone levels based on satellite estimates averaged over 60º S to 60º N. Their analysis indicates that since 1997 there has been a slowdown in mid- and low-latitude stratospheric ozone losses at altitudes of 35 to 45 kilometers (km). These changes in loss rates are consistent with the slowdown in total stratospheric chlorine increases, and, if they continue, will represent the first stage of a mid-latitude ozone recovery. No evidence of this change in loss rates has been reported for polar latitudes, and it is also important to note that ozone at the altitudes where these reduced loss rates are being observed plays a lesser role than total column ozone in absorbing UV radiation.

Several models have been used to project future ozone levels[5]. Intercomparison of these models[6] shows qualitative agreement between the projections, although specific projections of recovery rates can disagree significantly. Two-dimensional models have been used for global ozone level projections, while three-dimensional chemistry–climate models are useful for simulating polar processes[7]. Three-dimensional models can provide multi-year time slice simulations, which 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 dynamic changes in well-mixed greenhouse gases and provide a more detailed evolution of ozone levels based on the mechanisms that are likely to occur in the atmosphere. These models can also provide information on the expected range of interannual variability.

Austin et al.[8] compared several chemistry–climate models used in recent ozone assessments[9]. These include the Unified Model with Eulerian Transport and Chemistry (UMETRAC)[10], the Canadian Middle Atmosphere Model[11], the Middle Atmosphere European Centre Hamburg Model (ECHAM) with chemistry[12], the ECHAM model with chemistry run at Deutschen Zentrum für Luft- und Raumfahrt (E39/C)[13], the University of Illinois at Urbana-Champaign (UIUC) model[14], the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) model[15], the Goddard Institute for Space Studies (GISS) model[16], and the Università degli Studi dell’Aquila (ULAQ) model[17]. The models were compared based on their ability to simulate ozone climatologies for the current atmosphere. For the Northern Hemisphere, all the models tended to overestimate the area-weighted hemispheric total column ozone, by an average of 7.2%. The models were unable to simulate the observed loss rates within the arctic polar vortex[18], so the modeled ozone depletion is less than that observed. Uncertainties in the model projections include temperature biases, leading to the “cold pole problem”[19]; these biases are worse in some models than in others. In the Northern Hemisphere, the biases are sometimes positive at certain altitudes, resulting in projections of insufficient ozone depletion in early winter, but excess depletion in spring. The cold pole problem is due largely to the absence of gravity-wave forcing, which many models now include. Other uncertainties include the inability of the models to accurately simulate PSCs and to account for all aspects of constituent (chemical) transport, including processes occurring at the upper boundary of the model. Changes in planetary waves and heat flux also pose uncertainties, and are discussed in greater detail in Section 5.6.2 (Future changes in ozone in the Arctic).

Considerations for Projecting Future Polar Ozone Levels (5.6.1)

300px-Three dimensional model projections.JPG Figure 5.15. Three-dimensional model projections of the change in spring (March–May) total column ozone relative to 1980; averaged over the band from 60º to 90º N. Error bars represent variability in model projections averaged over 5 years for the GISS model, and twice the standard deviation of the 20 individual years within each model sample for the E39/C model. TOMS/SBUV observations are shown for comparison. (Source: ACIA)

The chemical contributions to ozone depletion are generally understood well enough to describe the annual ozone losses observed over Antarctica as a result of efforts to understand the ozone hole observed there. Over the Arctic, however, ozone depletion processes are often much more complicated and depend greatly on climate conditions and climate change. For example, when potential increases in stratospheric water vapor and corresponding stratospheric cooling resulting from climate change are included in models, the resulting projected mid-latitude ozone decrease in the 2030s surpasses that resulting from the projected amounts of CFC-derived halogens[20]. At high latitudes, the effects of stratospheric water vapor and stratospheric cooling on the ozone column are anticipated to be even larger due to the effects of PSCs. Separating the chemical and dynamic/climate-related contributions to ozone depletion is not a simple task, and many questions concerning the future of ozone over the Arctic remain unanswered.

Most model projections suggest small but continuing ozone losses over the Arctic for at least the next two decades[21]. Ozone depletion in the Arctic is strongly influenced by the dynamics of the polar atmosphere: changes in circulation, and particularly changes that affect air temperatures in the polar region, can have a substantial effect. For example, a strong polar vortex results in decreasing stratospheric temperatures, which further strengthen the polar vortex. This positive feedback effect contributes to increased ozone depletion, and is likely to be exacerbated by the stratospheric cooling projected to occur as a result of future climate change.

While dynamics determine the onset of ozone depletion and also influence the rate and severity of the depletion processes, the main driver for upper stratospheric (~40 km) ozone loss and for the spring losses in the polar stratosphere is the chemistry associated with chlorine and bromine[22]. The Montreal Protocol and its amendments have led to a reduction in atmospheric chlorine concentrations, and concentrations of ozone-depleting halogens are expected to continue to decrease between 2000 and 2050. The decreases were first reported in the troposphere[23], but have also been observed in the upper stratosphere[24]. Bromine is another halogen particularly effective at destroying ozone, and its overall levels may increase or remain high because of shorter-lived substances, such as bromoform[25]. However, the magnitude of ozone loss will depend greatly on dynamic and climate conditions (Section 5.2 (Future changes in ozone in the Arctic)), with low temperatures contributing to the formation and persistence of PSCs. 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 on arctic ozone levels as long as halogen loading remains large[26].

The Role of Climate Change in Arctic Ozone Recovery (5.6.2)

Projections of the recovery of ozone levels in the Arctic depend on projections of global climate change. Understanding long-term changes (natural and anthropogenic), will be essential to improving assessments and projections of the dynamic structure of the stratosphere[27]. Current chemical and dynamics models project that climate change resulting from increased atmospheric concentrations of carbon dioxide and other greenhouse gases will warm the troposphere, but will cool the stratosphere. In the Arctic, this cooling is likely to lead to increased ozone destruction, as lower temperatures are likely to result in the formation and persistence of PSCs, which aid in the activation of ozone-depleting compounds and can therefore accelerate ozone depletion. Stratospheric cooling resulting from climate change is therefore likely to lead to an increased probability of larger and longer-lasting [[ozone hole]s] in the Antarctic and extensive, more severe ozone losses over the Arctic[28]. On the other hand, climate change could possibly trigger an increase in planetary waves, enhancing the transport of warm, ozone-rich air to the Arctic[29]. This increased transport would counter the effects of heterogeneous chemistry and possibly hasten recovery of the ozone layer. Understanding this “dynamic effect on chemistry” requires improved information about the effects of increasing greenhouse gas concentrations so that the balance between dynamics and radiation can be deduced. If radiative effects dominate, planetary wave activity would be more likely to decrease, resulting in more ozone depletion at arctic latitudes.

Another climate feedback affecting ozone is a potential increase in stratospheric water vapor due to changes in tropopause temperatures[30]. Few long-term datasets of water vapor concentrations are available, but previous studies of existing observations have suggested that stratospheric water vapor has been increasing[31]. Analyses of 45 years of data (1954–2000) by Rosenlof et al.[32] found a 1% per year increase in stratospheric water vapor concentrations. Analyses of satellite data, however, have shown less evidence of a water vapor increase[33]. Increased water vapor is likely to contribute to increased ozone destruction by affecting the radiation balance of the stratosphere[34]. Greater water vapor concentrations in the stratosphere can raise the threshold temperatures for activating heterogeneous chemical reactions on PSCs, and can cause a decrease in the temperature of the polar vortex[35].

Ozone itself is central to climate change science: it is an important greenhouse gas in the infrared part of the spectrum and is the primary absorber of solar UV radiation. Ozone is critical to the radiation balance of the atmosphere, and to the dynamics of the stratosphere. Indeed, recent observational findings confirm that “the stratosphere is a major player in determining the memory of the climate system”[36]. Stratospheric ozone levels play a role in determining many properties of the polar atmosphere, including the strength of the polar vortex. Observations show that the strengths of the polar vortices affect surface temperatures in the polar regions and at mid-latitudes in both hemispheres. Connections between ozone levels and other properties of the stratosphere can alter weather processes in the troposphere, with an effect whose magnitude is comparable to that of the El Niño–Southern Oscillation[37].

Projected future changes in ozone levels over the polar regions differ from projected changes over the rest of the globe, where stratospheric temperatures do not reach the low thresholds necessary for the formation of PSCs. In recent years, the arctic polar vortex has increased in strength and has become more persistent[38]. A strong polar vortex can enhance the amount of depletion experienced over the Arctic. If these strong polar vortex conditions continue in future years, arctic ozone recovery is likely to be substantially delayed[39]. For example, in an analysis of approximately 2000 ozonesonde measurements, Rex et al.[40] found that each 1 ºC cooling of the arctic stratosphere resulted in an additional 15 DU of chemical ozone loss. Their findings indicate that over the past four decades, the potential for the formation of PSCs increased by a factor of three, resulting in stratospheric conditions that have become significantly more favorable for large arctic ozone losses. This relationship between potential amounts of PSCs and ozone loss is not well-represented in current chemistry–climate models. If the arctic stratosphere continues to cool as a result of climate change, the region is likely to continue to experience severe ozone depletion until chlorine and bromine loadings have returned to background levels. Any delay in the recovery of the ozone layer over polar regions means a longer-lasting, and perhaps more severe, threat of ecosystem damage due to increased UV irradiance.

Projected Changes in Ozone Amounts (5.6.3)

300px-Two and three dimensional models.JPG Figure 5.16. Two- and three-dimensional model projections of spring (March–May) total column ozone averaged over the band from 60º to 90º N. TOMS/SBUV observations through 2002 are shown for comparison. (Source: ACIA)

A number of two-dimensional models using specified scenarios of atmospheric halocarbon concentrations were used to estimate future ozone levels for the most recent Scientific Assessment of Ozone Depletion[41]. These included the Atmospheric and Environmental Research (AER) model[42], the Max-Planck Institute (MPI) model[43], the Goddard Space Flight Center (GSFC) model[44], GSFC-INT (interactive version of the GSFC model)[45], the National Oceanic and Atmospheric Administration/National Center for Atmospheric Research (NOCAR) model[46], the University of Oslo (OSLO) model[47], the National Institute for Public Health and the Environment (RIVM) model[48], the State University of New York – St. Petersburg (SUNY-SPB) model[49], UIUC[50], and ULAQ[51]. Figure 5.14 shows the spring (March–May) changes in ozone for the latitude band 60º to 90º N (relative to 1980 levels) projected by these two-dimensional models. The spring is interesting because ozone depletion reaches its most severe levels and UV irradiance can also be relatively high during what is the beginning of the growth period for many biological systems. The model results shown are for the greenhouse gas scenario MA2 and baseline halocarbon scenario AB[52]. All the models except RIVM include arctic chemistry, while only the MPI and UIUC models include the 11-year solar cycle. The projected spring changes in arctic ozone levels are about twice as large as those projected for the Northern Hemisphere mid-latitudes and about three times as large as projected changes in the 60º N to 60º S annual average. Generally, the models simulate local minimums in arctic ozone levels in the late 1990s, followed by a gradual increase. The majority of the models project significantly lower ozone levels in 2020 compared to 1980.

Because the two-dimensional models are unable to incorporate dynamic effects, their results are considered very rough projections for the polar regions, where ozone levels are strongly influenced by atmospheric dynamics. The model simulations used in the 2002 assessment[53] differed from those used in prior assessments in that they incorporate a lower level of stratospheric aerosols and thus project a more rapid recovery of the ozone layer. About half of the models project recovery to 1980 levels by 2050. A two-dimensional model simulation with the GSFCINT model by Rosenfield et al.[54] projects that arctic ozone recovery will be slowest in the spring, with total column ozone returning to 1980 levels after 2050, and earliest in the autumn, with total column ozone returning to 1980 levels before 2035. The results from the two-dimensional models project a range of arctic ozone recovery rates, from about 0.5% per decade to about 2% per decade.

Three-dimensional model simulations for the Arctic are also presented in the assessment[55]. These models offer greater insight into the dynamic factors affecting current and future arctic ozone levels. Figure 5.15 shows the spring (March–May) average change in ozone relative to 1980 for the latitude band 60º to 90º N projected by the UMETRAC and GISS models, which are transient simulations, and E39/C, which is a time-slice simulation. In general, the three-dimensional models simulate larger ozone depletion over the Arctic between 1980 and 2000 than do the two-dimensional models. The different three-dimensional models project quite different future ozone levels. The UMETRAC model provides projections through 2020; these projections indicate slow recovery (a few percent) between 2000 and 2020. The E39/C model provides simulations for 1960, 1980, and 1990, and a projection for 2015. The ozone levels simulated by the E39/C model for 1980 are about 6% lower than the 1960 levels, but this large decrease between 1960 and 1980 is not corroborated by observations. The E39/C model simulates the same rate of decrease between 1980 and 1990, while the projections for 2015 show ozone levels above those of 1980 but still lower than 1960. The GISS model is the only three-dimensional model that provides projections beyond 2020; these projections indicate further ozone depletion between 1995 and 2015, with only modest recovery in 2045.

The two- and three-dimensional model projections of spring total column ozone for the 60º to 90º N band are shown in Fig. 5.16. The results indicate large differences in the projected total ozone column amounts, with most models projecting lower ozone levels than have been observed. All of the models project that ozone levels will remain substantially below pre-depletion levels for at least the next two decades.

The three-dimensional models can offer insight into the spatial distribution of ozone depletion and recovery. As Fig. 5.17 shows, both the E39/C and UMETRAC models project greater changes near Greenland than elsewhere in the Arctic, although the UMETRAC model projects continued depletion and the E39/C model projects earlier recovery for this region. These differences indicate the uncertainty in the spatial distribution of future ozone levels. Zonally symmetric dynamics in the GISS model result in near-zonally symmetric ozone loss and recovery.

300px-Percentage change in total column ozone.JPG Figure 5.17. Percentage change in average annual total column ozone projected by (a) the UMETRAC model for the period 2010–2019 (average over the time slice) relative to 1980–1984; (b) the E39/C model for 2015 relative to 1980; and (c) the GISS model for the period 2010–2019 (average over the time slice) relative to 1980. (Source: ACIA)

Austin et al.[56] summarized the uncertainties in many of the chemistry–climate models that are used to project future ozone levels. Some of the most important uncertainties related to arctic projections are due to the cold temperature biases in the arctic winter that most models have, and that the models are forced with less than half 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 projections of polar temperatures and transport of ozone to the poles. Arctic ozone depletion is also subject to large natural variability, complicating definitive projections of how ozone levels will evolve[57].

As this section suggests, modeling past and future ozone levels, particularly in the Arctic, is challenging. One of the primary challenges is the difficulty of simulating observed polar temperatures, which are essential for determining the severity of chemical ozone depletion by anthropogenic chlorine and bromine. Many of the current chemistry–climate models do not reproduce the observed occurrence of PSCs or the large observed increase in PSC occurrence since the 1960s[58]. Some models are also unable to accurately reproduce the observed ozone loss rates within the arctic polar vortex. As reported by Rex et al.[59], the limitations of accurately simulating the relationship between potential amounts of PSCs and ozone depletion may be leading to more optimistic projections of arctic ozone levels than are likely to occur given the influences of climate change. 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 levels will be difficult to project.

Because of the anticipated decline in stratospheric chlorine and bromine concentrations resulting from the Montreal Protocol and its amendments, an increase in arctic ozone levels is expected to occur eventually. However, any quantitative statements concerning the timing and magnitude of arctic ozone layer recovery are highly uncertain.

Chapter 5: Ozone and Ultraviolet Radiation

5.1. Introduction (Future changes in ozone in the Arctic)
5.2. Factors affecting arctic ozone variability
5.3. Long-term change and variability in ozone levels
5.4. Factors affecting surface ultraviolet radiation levels in the Arctic
5.5. Long-term change and variability in surface UV irradiance
5.6. Future changes in ozone
5.7. Future changes in ultraviolet radiation
5.8. Ozone and Ultraviolet Radiation in the Arctic: Gaps in knowledge, future research, and observational needs

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Citation

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