“That’s a very remarkable result, that all those data sets agree,” he added. “It’s the clearest evidence in one place from a range of different indices.”
Currently 1998 is the hottest year on record. Two combined land and sea surface temperature records from Nasa’s Goddard Institute for Space Studies (GISS) and the US National Climatic Data Centre (NCDC) both calculate that the first six months of 2010 were the hottest on record. According to GISS, four of the six months also individually showed record highs.
A third leading monitoring programme, by the Met Office, shows this period was the second hottest on record, after 1998, with two months this year – January and March – being hotter than their equivalents 12 years ago.
The Met Office said the variations between the figures published by the different organisations are because the Met Office uses only temperature observations, Nasa makes estimates for gaps in recorded data such as the polar regions, and the NCDC uses a mixture of the two approaches. The latest figures will give weight to predictions that this year could become the hottest on record.
Despite annual fluctuations, the figures also highlight the clear trend for the 2000s to be hotter than the 1990s, which in turn were clearly warmer than the previous decade, said Stott.
“These numbers are not theory, but fact, indicating that the Earth’s climate is moving into uncharted territory,” said Rafe Pomerance, a senior fellow at Clean Air Cool Planet, a US group dedicated to helping find solutions to global warming.
The Met Office published its full list of global warming indicators, compiled by Hadley Centre researcher John Kennedy. It formed part of the State of the Climate 2009 report published as a special bulletin of the American Meteorological Society by the US National Oceanic and Atmospheric Administration, which runs the NCDC temperature series.
Seven of the indicators rose over the last few decades, indicating “clear warming trends”, although these all included annual fluctuations up and down. One of these was air temperature over land – including data from the Climatic Research Unit at the UEA, whose figures were under scrutiny after hacked emails were posted online in November 2009, but the graphic also included figures from six other research groups all showing the same overall trends despite annual differences.
The other six rising indicators were sea surface temperatures, collected by six groups; ocean heat to 700m depth from seven groups; air temperatures over oceans (five data sets); the tropospheric temperature in the atmosphere up to 1km up (seven); humidity caused by warmer air absorbing more moisture (three); and sea level rise as hotter oceans expand and ice melts (six).
Another four indicators showed declining figures over time, again consistent with global warming: northern hemisphere snow cover (two data sets), Arctic sea ice extent (three); glacier mass loss (four); and the temperature of the stratosphere. This last cooling effect is caused by a decline in ozone in the stratosphere which prevents it absorbing as much ultraviolet radiation from the sun above.
One key data set omitted was sea ice in the Antarctic, because it was increasing in some areas and decreasing in others, due to reduced ozone causing changes in wind patterns and sea-surface circulation. This data set showed no clear trend, said Stott. These figures were also in the last report from the UN’s Intergovernmental Panel on Climate Change (IPCC) in 2007.
“It’s not that the IPCC didn’t look at this data, of course they did, but they didn’t put it all together in one place,” he added.
The cause of the warming was “dominated” by greenhouse gases emitted by human activity, said Stott. “It’s possible there’s some [other] process which can amplify other effects, such as radiation from the sun, [but] the evidence is so clear the chance there’s something we haven’t thought of seems to be getting smaller and smaller,” he said