The Variable Arctic
Although many global scale models agree with each other and with observations on the future of ozone recovery, most regional scale models do not agree. Atmospheric models show that the cooling influence of ozone depletion accounts very well for observed cooling winter-time temperature trends in the Antarctic, but not in the Arctic.
Differences among regions make predictions about complex atmospheric chemistry problematic. The Arctic and Antarctic regions, where low stratospheric ozone amounts are of great concern, differ in significant ways. The complex topography of the high latitude Northern Hemisphere, with its distribution of land masses and oceans, makes the Arctic atmosphere more dynamic and variable.
The Antarctic is colder than the Arctic. Antarctic winds form a relatively stable vortex for long periods of time, and the vortex allows temperatures of the air trapped within it to get extremely low. Drew Shindell: “In the south, air masses just sit over the pole and get colder.” Such stability makes the Antarctic somewhat more predictable than the Arctic. Drew Shindell says, “It’s so variable in the Arctic that we have to have better data to figure out what we should believe and what we can have confidence in for the future.”
Although dramatic ozone depletion did not occur in the Arctic in the 1980s when it occurred in the Antarctic, times are changing. Very large ozone losses have occurred in the Arctic recently, especially in the late 1990s. Ozone chemistry is very sensitive to temperature changes. Since temperatures in the Arctic stratosphere often come within a few degrees of the threshold for forming polar stratospheric clouds, further cooling of the stratosphere could cause these clouds to form more frequently and increase the severity of ozone losses.
The Arctic may be changing in another way that differs from the Antarctic. With stratospheric cooling, the differences in temperature between the stratosphere and the troposphere are increasing. Differences in temperature creates winds, so stratospheric wind speeds have been increasing. (The Antarctic isn’t affected by increasing greenhouse gases like the Arctic is because it’s colder, and the polar wind circulation over the Antarctic is already very strong.)
Drew Shindell says that from both observations and models, he has found increasing wind speeds not only at high altitudes but also near the surface. “That’s a large effect on climate,” he points out. “Changes in stratospheric ozone and winds affect the flow of energy at altitudes just below, which then affect the next lower altitudes, and so on all the way to the ground. That would be the most intriguing aspect of all this, though it’s still controversial.”