ESA’s Envisat satellite has measured record low levels of ozone over the Euro-Atlantic sector of the northern hemisphere during March. This record low was caused by unusually strong winds, known as the polar vortex, which isolated the atmospheric mass over the North Pole and prevented it from mixing with air in the mid-latitudes. This led to very low temperatures and created conditions similar to those that occur every southern hemisphere winter over the Antarctic. As March sunlight hit this cold air mass it released chlorine and bromine atoms — ozone-destroying gases that originate from chlorofluorocarbons (CFCs) and break ozone down into individual oxygen molecules -- predominantly in the lower stratosphere, around 20 km (12 mi.) above the surface. Ozone is a protective atmospheric layer found at around 25 km (15 mi.) altitude that acts as a sunlight filter shielding life on Earth from harmful ultraviolet rays, which can harm marine life and increase the risk of skin cancer and cataracts. Stratospheric temperatures in the Arctic show strong variations from winter to winter. Last year, temperatures and ozone above the Arctic were very high. The last unusually low stratospheric temperatures over the North Pole were recorded in 1997. Scientists are investigating why the 2011 and 1997 Arctic winters were so cold and whether these random events are statistically linked to global climate change. "In a changing climate, it is expected that on average stratospheric ...