Are Increasing Greenhouse Gases Creating an Arctic Ozone Hole?
|Figure 1: Past and projected abundances for carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), chlorofluorocarbons (CFCs), and chlorine (Cly) used in the model simulation.|
Ozone holes are caused by chemical reactions that take place primarily on the surface of polar stratospheric clouds, ice particles, or liquid droplets, which form at high altitudes in the extreme cold of the polar regions. The number of particles that form, and therefore the amount of chemical ozone destruction, is extremely sensitive to small changes in stratospheric temperature. Hence, even small amounts of stratospheric cooling can greatly increase ozone depletion. In fact, it is because stratospheric temperatures in the Northern Hemisphere winter/spring are generally slightly warmer than those in the Southern Hemisphere that ozone losses over the Arctic have been much smaller than over the Antarctic during the 1980s and early 1990s. However, the Arctic stratosphere has gradually cooled over the past few decades. Very large ozone losses have been observed there recently, especially in 1997. Why might this be happening, and what is likely to happen in the future?
The well-known increase in greenhouse gas emissions is one possible cause of the observed trends. We have investigated the coupled chemistry-climate response to projected emissions of greenhouse gases and ozone-depleting halogens over time, using the NASA GISS general circulation model, incorporating simple chemistry. While the buildup of greenhouse gases leads to global warming at Earth's surface, it also cools the stratosphere, which increases the amount of ozone depletion in the polar regions. In addition, the Arctic stratosphere has historically been warmer than the Antarctic stratosphere, because more energy is deposited there by planetary waves. These are large-scale waves, which travel through Earth's atmosphere similar to waves in the oceans (see "Why Does the Ozone Hole Vary in Size?"). While a very stable, isolated vortex of cold air forms over Antarctica, the corresponding vortex over the Arctic has historically been much less stable. It frequently breaks up during the winter, allowing warmer air from lower latitudes to mix with the Arctic air.
|Figure 2: Total column ozone loss relative to pre-ozone hole conditions in the 1970s in percent (left) and total number of molecules (right) (1 DU = 2.69 molecules / cm2) averaged over 2010-2019, during September for the Southern Hemisphere and March for the Northern Hemisphere.|
In our simulations, temperature and wind changes induced by increasing greenhouse gases alter the propagation of planetary waves. As the abundance of greenhouse gases gradually increases, planetary waves no longer break up the Arctic vortex as often. The frequency of Northern Hemisphere sudden stratospheric warmings is reduced. The more stable Arctic vortex leads to significantly colder lower stratospheric temperatures, adding to the greenhouse cooling of the stratosphere. The combination of these two cooling effects causes dramatically increased ozone depletion so that ozone loss in the Arctic by the year 2020 is roughly double what it would be without greenhouse gas increases. Increasing greenhouse gases may therefore be at least partly responsible for the very large Arctic ozone losses in recent winters, and the situation may worsen in the future.
International controls on the emission of ozone-depleting halogens are now in place, so that their abundance in the stratosphere is expected to peak around the year 2000. In our model, Arctic ozone depletion is greatest during the decade 2010-2019, showing a delay of 10-15 years after the maximum stratospheric chlorine levels are reached. During this decade, the average ozone loss is roughly equal to losses over Antarctica during the early 1990s. In the worst years, two-thirds of the Arctic ozone column (the total amount of ozone between the ground and the top of the atmosphere) is destroyed in the most severely depleted regions. The severity and duration of the Antarctic ozone hole also increases due to greenhouse gas-induced stratospheric cooling over the coming decades, although ozone there is already so depleted that any additional losses are relatively small.
Shindell, D.T., D. Rind, and P. Lonergan. 1998. Increased polar stratospheric ozone losses and delayed eventual recovery due to increasing greenhouse gas concentrations. Nature 392, 589-592.