2008 will be slightly cooler than recent years globally but will still be among the top 10 warmest years on record since 1850 and should not be seen as a sign global warming was on the wane, British forecasters said.
The Met Office and experts at the University of East Anglia on Thursday said global average temperatures this year would be 0.37 of a degree Celsius above the long-term 1961-1990 average of 14 degrees and be the coolest since 2000.
They said the forecast took into account the annual Pacific Ocean La Nina weather phenomenon which was expected to be particularly strong this year and which would limit the warming trend.
It also took account of rising atmospheric concentrations of so-called greenhouse gases, solar variations and natural changes in the ocean currents.
“The fact that 2008 is forecast to be cooler than any of the last seven years does not mean that global warming has gone away,” said Phil Jones, director of climate research at UEA.
“What matters is the underlying rate of warming – the period 2001-2007 with an average of 0.44 degree C above the 1961-90 average was 0.21 degree C warmer than corresponding values for the period 1991-2000.”
La Nina and its opposite El Nino ocean-atmosphere phenomenon have strong influences on global temperatures. La Nina reduces the sea surface temperature by around 0.5 degrees Celsius while El Nino has the opposite effect.
“Phenomena such as El Nino and La Nina have a significant influence on global surface temperature and the current strong La Nina will act to limit temperatures in 2008,” said Chris Folland from the Met Office Hadley Centre.
“However mean temperature is still expected to be significantly warmer than in 2000, when a similar strength La Nina pegged temperatures to 0.24 degree C above the 1961-90 average. Sharply renewed warming is likely once La Nina declines,” he added. The current La Nina is now the strongest since 1999-2000. The lag between La Nina and the full global surface temperature response means that the cooling effect is expected to be a little greater in 2008 than it was during 2007.
The World Meteorological Organisation said last month there were indications that the 10 years from 1998 to 2007 were the hottest decade on record.
The Met Office Hadley Centre said the top 11 warmest years have all occurred in the last 13.