Page last updated at 17:22 GMT, Tuesday, 16 December 2008

This year is coolest since 2000

By Richard Black
Environment correspondent, BBC News website

Cows in snowy field
Winter brought unfamiliarly cold weather to large swathes of Europe

The world in 2008 has been cooler than at any time since the turn of the century, scientists say.

Cooling La Nina conditions in the Pacific brought temperatures down to levels last seen in the year 2000.

The World Meteorological Organization (WMO) notes that temperatures remained about 0.3C above the 1961-1990 average.

Computer models suggest that natural cycles may cool the Earth's surface in the next few years, masking the warming impact of rising greenhouse gas levels.

Human influence, particularly emission of greenhouse gases, has greatly increased the chance of having such warm years
Peter Stott, UK Met Office

One recent analysis suggested there may be no warming for about the next decade, though other scientists dispute the conclusion.

What is beyond dispute is that 2008 saw temperatures a shade below preceding years.

Using data from two major monitoring networks, one co-ordinated by the UK's Hadley Centre and University of East Anglia (UEA) and the other by the US National Oceanic and Atmospheric Administration (Noaa), the WMO reports that despite the cooling, 2008 still ranks among the 10 warmest years on record.

At 14.3C, the average temperature for the year was significantly above the 14.0C average for the 1961-1990 period, a commonly used baseline.

Temperatures are about 0.7C above pre-industrial times.

Peter Stott, head of climate monitoring and attribution at the UK Met Office of which the Hadley Centre is a part, suggested that in previous decades 2008 would have stood out as unusually warm.

Weather station
Data is recorded at thousands of weather stations

"Human influence, particularly emission of greenhouse gases, has greatly increased the chance of having such warm years," he said.

Nasa's Goddard Institute for Space Studies (Giss), which produces its own record of atmospheric temperature, agreed that 2008 was the coolest year since the turn of the century.

But Giss still ranks it as the ninth warmest since 1880.

The warmest of all remains 1998, when exceptionally strong El Nino conditions added to rising greenhouse gas levels sent thermometers to an average of about 14.5C.

"The most important component of year-to-year variability in global average temperatures is the phase and amplitude of equatorial sea surface temperatures in the Pacific that lead to La Nina and El Nino events," observed UEA's Dr Phil Jones.

Millennial warmth

John Christy, a scientist noted for taking a cautious approach to the likely impacts of human-induced climate change, agreed that the Earth's atmosphere had warmed by about 0.4C over 30 years.

His own research team at the University of Alabama in Huntsville (UAH) produces global temperature data from satellite readings, and uses weather balloons to verify the satellite record.

Professor Christy suggested that the trend in the immediate future would be decided by whether conditions in the Pacific veer towards El Nino or La Nina.

1998 - 14.52C
2005 - 14.48C
2003 - 14.46C
2002 - 14.46C
2004 - 14.43C
2006 - 14.42C
2007 - 14.40C
2001 - 14.40C
1997 - 14.36C
2008 - 14.31C
Data: Met Office Hadley Centre

"If you look at the 30-year graph of month-to-month temperature anomalies, the most obvious feature is the series of warmer than normal months that followed the major El Nino Pacific Ocean warming event of 1997-1998," he said.

"Right now we are coming out of one La Nina Pacific Ocean cooling event, and we might be heading into another.

"It should be interesting over the next several years to see whether the post La Nina climate 're-sets' to the cooler seasonal norms we saw before 1997, or the warmer levels seen since then."

The effect of El Nino and La Nina conditions are one reason why scientists prefer to average temperatures over 10-year periods, which smoothes out the annual variations and gives a better picture of long-term trends.

On average, the decade from 1990 to 1999 was 0.23C above the 1961-1990 baseline, while in the period 2000-2008 it was 0.40C over, indicating a warming trend.

It is almost certain that the first decade of this century will turn out to have been significantly warmer than the last decade of the last century, notwithstanding the freak El Nino year of 1998.

The question for the next decade or so will be whether natural cycles such as the Atlantic Multidecadal Oscillation continue to moderate the warming effect of rising greenhouse gas concentrations.

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