News Story - Ozone Holes North and South
Date: 09 May 2011
This year we almost had an ozone hole over Cambridge. Normally the Arctic ozone layer is about 10° warmer than that over the Antarctic, and this means that polar stratospheric clouds (PSCs) are rare in the north. Not so in 2011. The strong wind system that forms during the polar winter, known as the polar vortex, was unusually stable this year. Parts of the Arctic ozone layer were therefore able to cool below the critical temperature of around -78°C at which PSCs can form. With clouds present, chemical reactions could take place which allowed over 40% of the Arctic ozone to be destroyed. Whilst this level of depletion would class as an ozone hole in the Antarctic, ozone amounts start at a higher level in the north. A correspondingly greater amount of depletion is thus required to hit the 220 Dobson Unit threshold that defines an ozone hole. For the UK, lowest ozone levels were reached around March 29, but by the end of the month the stratosphere was warming and ozone amounts increasing.
When ozone amounts fall, UV levels rise, but in late March the Sun was still relatively low in the sky, and the risk of sunburn was low. By early April, although the ozone amount was up, the Sun was also higher in the sky and UV exposures exceeded the level at which protection is advised. Sunblock is needed to afford protection from the Sun and this not only reduces the risk of sunburn, but also reduces the risk of a future skin cancer.
Recent published research has indicated that the Antarctic Ozone Hole is recovering, but this is wrong. What the research does show is that the amount of chemical ozone depletion is decreasing. This is exactly what is expected, as the Montreal Protocol, now signed by all UN Member states, is working and the amount of ozone destroying substances in the atmosphere is going down. The depth of the Antarctic ozone hole is however also controlled by the 'weather' of the stratosphere, as well as by external events such as massive volcanic eruptions, or even meteorite explosions. These near-random events could still give us an exceptionally deep ozone hole, so it is therefore still too soon to say that the Ozone Hole is on the mend.
Head of Meteorology and Ozone Monitoring Unit