英文摘要: | The projected large increases in damaging ultraviolet radiation as a result of global emissions of ozone-depleting substances have been forestalled by the success of the Montreal Protocol. New challenges are now arising in relation to climate change. We highlight the complex interactions between the drivers of climate change and those of stratospheric ozone depletion, and the positive and negative feedbacks among climate, ozone and ultraviolet radiation. These will result in both risks and benefits of exposure to ultraviolet radiation for the environment and human welfare. This Review synthesizes these new insights and their relevance in a world where changes in climate as well as in stratospheric ozone are altering exposure to ultraviolet radiation with largely unknown consequences for the biosphere.
In the early 1970s, Molina and Rowland proposed that chlorofluorocarbons, widely used as refrigerants and propellants, would reach the stratosphere and catalyze the destruction of ozone molecules there1. In 1985 evidence of an 'ozone hole' over Antarctica was first published2 and its progression over the ensuing years has been captured in images that have become symbols of human influences on the global environment. Large-scale depletion of stratospheric ozone and high levels of ultraviolet (UV) radiation have been avoided by the unprecedented success of the Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987. The Montreal Protocol remains the only treaty ever ratified by all members of the United Nations. This unusual consensus on an environmental issue was driven by concerns that life on Earth was at risk, a concern that is supported by recent analyses of the 'world avoided' scenario of what could have happened without the Montreal Protocol3, 4. The actions taken under the protocol have also made the single largest contribution to the mitigation of climate change so far, because many of the ozone-depleting substances (ODS) are also greenhouse gases (GHGs)5.
Solar radiation is essential to life on Earth, but its UV component may also damage both living organisms and non-living matter. UV radiation is usually divided into three wavelength bands: UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (100–280 nm). UV-C radiation is potentially the most damaging, but is completely filtered out by the Earth's atmosphere and does not reach the surface. The Earth's surface is also largely protected from the most damaging short wavelength UV-B radiation due to absorption by stratospheric ozone. UV-A radiation passes through the atmosphere with little attenuation and thus is the largest component of ground-level solar UV radiation. Although generally less harmful than UV-B radiation, UV-A radiation has important effects on tropospheric chemistry, air quality, and aquatic and soil processes, as well as being mutagenic and causing immune suppression in humans6. Implementation of the Montreal Protocol has drastically curtailed production of chlorofluorocarbons and other ODS7. It has thus successfully reduced depletion of stratospheric ozone and associated increases in ground-level UV-B radiation. However, the long lifetimes of many ODS in the atmosphere mean that substantial ozone depletion still occurs over the Antarctic, and is expected to continue for several more decades8. Stratospheric ozone loss has also been observed over the Arctic9, with 2011 showing the largest depletion ever recorded10. This major depletion event was caused by a combination of unusually low stratospheric temperatures, ODS-derived chlorine in the stratosphere and a change in circulation patterns that delayed the seasonal transport of ozone from the tropics10. During the twenty-first century, upper stratospheric ozone is projected to increase due to the reduction in ODS and continued cooling from the increasing concentrations of GHGs. In the lower stratosphere, ozone is projected to decrease11, offsetting the effect of upper stratospheric cooling. The net effect of these changes on terrestrial UV radiation is complex, as additional factors, such as increasing concentrations of carbon dioxide (CO2) and other GHGs, begin to play an ever-increasing role in determining levels of stratospheric ozone and cloud cover. For example, by 2100, models predict that UV radiation will have increased in the tropics (where the current UV radiation is already intense), and to have decreased at polar latitudes (where the current UV radiation is generally less intense)12.
A different world has evolved after 26 years of the Montreal Protocol. The phase-out of ODS is projected to lead to recovery of stratospheric ozone. However, additional climate-related changes in the incident UV radiation at Earth's surface may result from changes in cloud, snow and ice cover, land-use, and atmospheric and oceanic circulation, and will vary regionally. Circulation patterns, such as the North Atlantic Oscillation, account for a high proportion of the variability in the total ozone column13. Such patterns are predicted to be altered by the accumulation of GHGs with subsequent changes in UV-B radiation levels at Earth's surface. These changes will, in turn, alter sinks and sources of CO2 and other trace gases that will affect future climate warming. The unequivocal warming of the climate system14 may have important impacts on future stratospheric ozone depletion independently of the concentration of ODS in the atmosphere. Increasing concentrations of GHGs cause a radiative cooling in the stratosphere, and extremely cold polar stratospheric winters are responsible, in part, for the Antarctic and Arctic spring ozone depletions15, 16. Denitrification of the chlorine reservoir (chlorine nitrate, ClONO2) occurs on the surfaces of polar stratospheric clouds and this process is a major reason for the observed 2011 Arctic spring ozone loss10, 16. The response to global warming is particularly rapid in the Arctic17. Moreover, global warming may also affect stratospheric ozone by increasing the atmospheric water content and its rate of transport through the cold tropopause (the troposphere–stratosphere boundary)18. Water vapour is a key component of stratospheric chemistry and may influence stratospheric temperatures and winds. It is involved in ozone destruction by accelerating the gas-phase hydrogen oxides (HOx) catalytic cycle, and by increasing the surface area of stratospheric aerosol particles on which ozone-depleting halogen molecules can be activated. Models suggest that in the first half of the twenty-first century, levels of UV radiation at Earth's surface will be determined by the recovery of stratospheric ozone, while in the second half, changes in UV radiation will be dominated by changes in clouds and GHG-induced transport of ozone12. These climate-driven changes are projected to markedly influence the amount of UV radiation received at Earth's surface. For example, by 2050, sunburning or erythemal UV irradiance (primarily in the UV-B region of the spectrum) is projected to decrease by 2–10% at mid-latitudes, and by up to 20% at northern and 50% at southern high latitudes, relative to 1980 levels. By the end of the twenty-first century, erythemal UV irradiance is projected to remain below 1960 levels at mid-latitudes, be reduced at high latitudes (particularly in the Arctic) by 5–10% due to increases in clouds19, but to increase in the tropics by between 3 and 8% due to decreases in clouds and ozone, caused by increasing GHGs12 (Fig. 1). Improvements in air quality, especially reductions of aerosols, may in the future result in higher UV radiation levels at Earth's surface. In the Arctic, there may be increases in sea-salt aerosols from the larger open-ocean area, as well as reductions in surface albedo due to the loss of sea ice20, 21, resulting in lower surface UV irradiance.
The extent and duration of periods of ice and snow cover on oceanic and inland waters have been decreasing in recent decades, altering the underwater light environment and potentially resulting in direct exposure of the aquatic environment to higher UV radiation49. The Arctic Ocean is expected to be ice-free during the summer within the next 30 years20, URL: | http://www.nature.com/nclimate/journal/v4/n6/full/nclimate2225.html
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