Guest Commentary by Drew Shindell (NASA GISS)
The current winter and early spring have been extremely cold in the Arctic stratosphere, leading to the potential for substantial ozone depletion there. This has been alluded to recently in the press (Sitnews, Seattle Post Intelligencer), but what’s the likely outcome, and why is it happening?
First, let’s go over some background. The recipe for massive springtime ozone loss in the polar regions, such as the annual ozone hole seen over Antarctica during the past two decades, is fairly simple. The two main ingredients are reactive halogen gases such as chlorine or bromine and sunlight. To prepare, keep the halogens at extremely cold temperatures, typically below –78 C (195 K). Use a strong polar vortex to mix the halogens to help achieve the required temperatures. When the mixture has been properly chilled, add sunlight and you’ll get rapid ozone destruction.
In the real world, both chlorine and bromine are readily available in the stratosphere worldwide, especially chlorine from chlorofluorocarbons which are well mixed in the lower atmosphere (where they are stable), before entering the stratosphere where they are photochemically decomposed. The halogens that are released generally end up in fairly unreactive forms where they have only a small effect on ozone. Under extremely cold conditions, however, ice and supercooled liquid droplets (so-called Polar Stratospheric Clouds – PSCs) can form even at the low densities present in the lower stratosphere (~15-25 km altitude). Chemical reactions on the surfaces of these particles can rapidly convert halogens into very reactive forms. Ozone depletion is therefore extremely sensitive to small changes in temperature when the stratosphere is near this freezing point. The temperatures themselves are greatly influenced by the strength of the polar vortex, a wind that swirls around the pole and when strong, can keep air confined throughout the winter in the polar night, allowing it to cool dramatically. The primary reason ozone depletion has been weaker over the Arctic than over Antarctica is than Arctic temperatures are typically about 10 degrees warmer as the Arctic vortex is generally weaker than its Antarctic counterpart. This is because of the differences in layout of the continents in the two hemispheres affects the dynamics of stratospheric circulation.
The other key factor is that even if chemical conversion into reactive forms occurs during the cold, dark polar winter, the reactive chlorine must stick around until sunlight returns to the polar region for ozone destruction to take place. This is why ozone depletion over the poles is a springtime phenomenon. Even following a very cold winter, if temperatures warm quickly during spring very little ozone loss may take place. Alternatively, a milder winter, provided it was still cold enough to lead to chemical processing of halogens, could be followed by greater springtime ozone losses if temperatures stayed cold longer. Thus temperatures during the period when a lot of sunlight first returns to the polar areas following winter, March in the Arctic and September in the Antarctic, are crucial.
This year has seen an exceptionally strong polar vortex over the Arctic (see the red line in the figure). Undoubtedly, chemical processing of halogens into reactive forms has taken place and the Arctic is primed for ozone depletion. Now that we’re in March, sufficient sunlight is available to cause sizeable ozone losses. Should cold temperature persist for another couple weeks, ozone depletion could reach record levels for the Arctic. While the vortex was weakened and pushed to the side of the Arctic during the last week of February, temperatures below the critical freezing point are still present as of March 9 (see figure). The displacement off the pole also pushes the colder air into latitudes with more sunlight, enhancing ozone depletion in the short term. Ozone measurements from the first week of March already show a region over the North Atlantic with very low ozone levels (<250 Dobson units, versus minimum values of ~300 in the early 1980s).
There is much debate over whether this has anything to do with climate change. Some climate models suggest that increasing greenhouse gases may be leading to a gradual strengthening of the Arctic vortex and hence increasing ozone losses, while others do not. Observations show that the vortex was typically more stable in the 1990s than during the 1980s, but the present decade has been mixed thus far. Temperature during the winter as a whole have generally decreased over the past two decades, likely as a result of climate change, but the sensitivity of ozone loss to the exact timing of March warming events makes ozone depletion a much more variable quantity. With only one winter vortex per year, it will take many more years to determine if the exceptionally cold 2004-2005 winter is part of a trend or simply a single cold event. Prepare to see a lot of press coverage if this ends up being a big year though….
Update: Indeed it was.