Greenhouse gas concentrations
Annual average global carbon dioxide concentrations in 2015 reached 400 parts per million (ppm) for the first time. Initial observations indicate new records in 2016. At Cape Grim (Australia), CO2 levels in August averaged 401.42 ppm, compared with 398.13 ppm in August 2015. At Mauna Loa (Hawaii), mean weekly concentrations of CO2 as of 23 October were 402.07 ppm, compared with 398.50 ppm at the same time in 2015, whilst the May 2016 value of 407.7 ppm was the highest monthly value on record.
Ice and snow cover
Arctic sea ice extent was well below normal throughout the year. The seasonal minimum in September was 4.14 million square kilometres, the equal-second (with 2007) lowest extent on record after 2012. The winter maximum in March was the lowest on record. The autumn freeze-up has also been much slower than normal; the sea ice extent as of the end of October is the lowest on record for the time of year.
After several years of well-above-normal values, Antarctic sea ice extent fell to near normal by the start of 2016. It reached a seasonal maximum nearly a month earlier than usual. It was still well below normal as of the end of October.
Summer melting on the Greenland ice sheet was substantially above the 1990-2013 average, with especially strong melting in July, but was less than in the record melting year of 2012.