Study explains temperature patterns

Global temperature rises stopped after 1998 because carbon emissions were partly offset by “cooling” pollution from coal power stations, researchers are stating.

The study by scientists in the US and Finland found that as a result of rising sulphur emissions from coal power plants, natural cycles of the sun and weather patterns dominated the impact on temperatures.

The research, published in the Proceedings of the National Academy of Sciences journal, said that global temperatures did not rise between 1998 and 2008, sparking scepticism in some quarters about whether climate change was happening.

While there were temperature increases in 2009 and 2010, there was a lack of clear warming in the decade before that and global temperatures fell 0.2C between 2005 and 2008, the study said.

Although greenhouse gas emissions rose, the scientists said the use of coal-fired power stations also rapidly increased, particularly in China where coal consumption doubled between 2003 and 2007, creating more sulphur emissions which have a cooling effect on the climate.

This impact partly offset greenhouse gas emissions and with declining solar activity on its usual 11-year cycle and a cyclical shift from the “El Nino” weather pattern which warms the global climate to the cooler “La Nina” accounted for the hiatus in warming.

But the researchers from Boston University, the University of Turku and Harvard said the results from their climate modelling, that natural factors were largely driving the current pause in warming, did not undermine the theory of man-made global warming.

While 1998 is, according to some data series, the hottest year on record, the past 10 years are warmer than any previous decade overall, according to the Met Office.

And as China installs technology to clean up its coal-fired power plants to stop sulphur pollution, the impact of greenhouse gases is likely to be felt.

Commenting on the research, Joanna Haigh, professor at Imperial College London, said: “In the period 1998 to 2008 temperature variations were dominated by the El Nino southern oscillation - with large warming events between 1998 and 2000 and mainly cooling events between 2002 and 2005.

“The paper’s authors are making the important point that, underlying these, the warming due to the CO2 released by Chinese industrialisation has been partially masked by cooling due to reflection of solar radiation by associated particle emissions.

“On longer timescales, with cleaner emissions, the warming effect will be more marked.”

Peter Stott, head of climate monitoring and attribution at the Met Office, said: “The ‘noughties’ decade of 2000-2009 was significantly the warmest in the instrumental record, more than 0.15C warmer than the 90s decade, part of a long-term warming pattern dominated by the effects of greenhouse gas emissions.”