UV in Antarctica Continues its Rise
As reported in a United Nations Assessment ...
A most apposite follow-up from last week’s offering.
Every four years, the United Nations Environment Program (UNEP) reconvenes an “Environmental Effects Assessment Panel” (EEAP), to update our understanding of the environmental impacts of ozone depletion (and its interactions with climate change). And in the intervening years, it provides shorter interim updates. I’ve been involved with this work since the early 1990s: every year except 2020, when I thought I had succeeded in passing on the baton. But the 2021 update has just appeared, and my name is back! I’m gradually weaning myself off it, but I haven’t quite got there yet!
Of course the main environmental effect of ozone depletion is due to the increased UV that can get through the atmosphere to reach the Earth’s surface.
For me the most interesting thing in this year’s update was that UV measurements in Antarctica reached their highest values ever. The peak UVI at the South Pole and Arrival Heights reached 4.1 and 7.5 respectively. That’s not particularly high by global standards - as we saw last week - but still a lot more than used to occur at those sites before ozone depletion began in the 1970s.
Measurements began there in the early 1990s, and the range of maximum UVI each day is shown by the grey shaded areas in the figure below, with the average shown by the white lines. The red and blue lines show the daily progression in the springs of 2019 and 2020. Measurements from 2021 weren’t available at the time the report went to press, but based on the persistent ozone hole, would probably be similar to 2020.
Oddly, UV values just one year earlier were among the lowest in decades at both sites.
What’s going on? Because of the success of the Montreal Protocol, we know that the amount of chlorine that leads to ozone destruction is decreasing. So why was last year’s ozone hole so bad? The answer is that high levels of chlorine aren’t the only thing required for rapid ozone depletion in Antarctica. It also has to be cold enough for ice crystals to form in the stratosphere. It’s reactions on the surface of those ice crystals that cause the much more rapid ozone depletion over Antarctica than elsewhere.
It’s an example of an interaction between Ozone Depletion and Climate Change.
Climate change warms the surface, but cools the stratosphere much more. So, if the air’s circulation pattern over Antarctica is stable, as it was in 2020, the cooler atmosphere allows ice crystals to be formed over larger areas and to persist for longer periods, leading to more complete ozone destruction and more intense UV. But if there are intrusions of warmer air from lower latitudes, as in 2019, the effect is reduced. The net effect for any particular year depends on the wind patterns at those high altitudes. But on average, as the concentrations of chlorine continue to decrease, ozone will continue to recover.
If the worst isn’t already behind us, it soon will be.
Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack.