The year 2008 was likely the coolest year of the current decade, but it was still the tenth warmest year on record since instrumental climate records began in 1850.
The average global sea-surface and land-surface air temperature from December 2007 to November 2008 was 14.3 °C, the World Meteorological Organization (WMO) has announced. This is slightly lower than for all previous years of the ‘naughties’, but still some 0.3 °C above the 1961-1990 annual average. The warmest year on record is 1998, with an average global temperature just above 14.5 °C.
Left: Global map of surface temperature anomalies in degrees Celsius for the 2008 meteorological year. Right: Annual-mean global-mean anomalies, except 2008, which is the 11-month (Jan-Nov) mean anomaly. Credit: NASA Goddard Institute for Space Studies.
When calculating global average temperatures, climatologists prefer the meteorological year, from December through November, as it is easier to split it into actual seasons than is the calendar year.
The WMO temperature analysis is based on land-based weather stations in 188 countries, complemented by measurements from ships, buoys and satellites. NASA’s Goddard Institute for Space Studies and the UK Met Office, which both contributed their own datasets to the WMO analysis, independently arrive at very similar values.
What seems to have cooled things off is a cool La Niña event in the Pacific Ocean at the beginning of this year.
And, as Gavin Schmidt points out on RealClimate, the current minimum in solar forcing may also have played a role.
Then, of course, there is natural internal climate fluctuation. No-one has ever said that global warming will proceed in a linear fashion, not even numerical climate simulations support that view.
So even in a warming climate there’s always a certain chance that a given year will be cooler than the previous year (or years). But, stochastically, you’d be rather ill-advised to bet money on it unless you’re offered a proper odd. Or think of it like this: You do have a chance to win at roulette, even though the odds are slightly against you. If, however, you keep on gambling infinitely your chances of winning approach zero.
Says Peter Stott of the Met Office: “As a result of climate change, what would once have been an exceptionally unusual year has now become quite normal. Without human influence on climate change we would be more than 50 times less likely of seeing a year as warm as 2008.”
Predictably, there are not few who, deliberately or not, take the message wrongly. Comments to Andrew Revkin’s entry on Dot Earth, for example, range from the scornful to the downright mean.
But fact is that the ten warmest years on record have occurred since 1997, and global temperatures for 2000-2008 stand almost 0.2 °C warmer than the average for the 1990s. Scientists expect that in the 21st century temperatures will continue to rise on average by at least 0.2 °C per decade. Even if global warming takes a short break, as a recent study suggests it might do during the next decade, the long-term upward trend in global temperature is very unlikely to stop at any time soon.