Free Astronomy Magazine January-February 2018

43 JANUARY-FEBRUARY 2018 SPACE CHRONICLES quire readings spanning 30 years or more for analysing trends accu- rately. Ozone in particular varies depend- ing on the season or solar activity, so decades are necessary to sepa- rate this natural variability from human-influenced change. To tackle this, scientists working under ESA’s Climate Change Initia- tive are harmonising measurements from different satellites to give us a long-term view of ozone variability. The readings date back to 1995 for ‘total columns’ – meaning they show the total ozone of all layers of the atmosphere – and to 2001 for ozone profiles with high vertical resolution, meaning the separate layers can be identified. The record relies on five satellite sensors that provide measurements with high vertical resolution. These include the ones on ESA’s former Envisat mission, along with current information from Canada’s SciSat and Sweden’s Odin. In a new development, measure- ments from NASA’s Earth Radiation Budget Satellite and Suomi NPP satellite have been folded in, fur- ther extending coverage back to 1984. “By merging the Climate Change Initiative’s data with NASA’s, we clearly see negative ozone trends in the upper atmo- sphere before 1997 and positive trends after,” said Viktoria Sofieva, Senior Research Scientist at the Finnish Meteorological Institute. “The upper stratospheric trends be- yond the tropics are statistically sig- nificant and indicate an onset of ozone recovery.” Ozone measure- ments with high vertical resolution are essential to identify these ozone trends. The future Altius mission for ESA’s Earth Watch initiative will provide continuing measurements for long- term climate monitoring. !