Despite extremely cold temperatures that ramped up the formation of clouds containing ozone-destroying forms of chlorine and bromine the annual hole in the ozone layer above the Antarctic was still smaller in 2018 than it would have been 20 years ago.
The hole, which peaks every year at the end of the southern winter, topped out at about 22.9 million square kilometres – almost three times the area of the continental US – and was the thirteenth largest out of the past 40.
And while that’s not exactly cause for unrestrained celebration, it is, say scientists at NASA’s Goddard Space Flight Centre in Maryland, which monitors the phenomenon, testament to the tentative success of the 1987 Montreal Protocol, which led to the phasing out of man-made ozone-depleting substances.
“Chlorine levels in the Antarctic stratosphere have fallen about 11% from the peak year in 2000.
“This year’s colder temperatures would have given us a much larger ozone hole if chlorine was still at levels we saw back in the year 2000.”
The size of the ozone hole is directly affected by annual temperatures, with warmer weather restricting its growth. Warm averages in 2016 led to a hole 19.7 million kilometres in area, but it increased in 2018 on the back of the coldest run of temperatures since 1979.
Despite that, however, the relative size of the hole still constituted cause for, well, at least two cheers.
“Even with this year’s optimum conditions, ozone loss was less severe in the upper altitude layers, which is what we would expect given the declining chlorine concentrations we’re seeing in the stratosphere,” Bryan Johnson from the US National Oceanic and Atmospheric Administration explains.