'Slowdown' in warming exposed as fallacy

Increased carbon dioxide emissions have had other effects, such as increasing the acidity of the oceans.

Increased carbon dioxide emissions have had other effects, such as increasing the acidity of the oceans.

Published Jun 5, 2015

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London - A major review of global temperatures by a US government agency has failed to find support for the view - regularly propagated by climate-change sceptics - that global warming has slowed down since 1998.

The US National Oceanic and Atmospheric Administration (NOAA) has re-evaluated its surface temperature records over land and sea and concluded that the rate of global warming has been just as fast at the start of this century as it was at the end of the last.

NOAA scientists believe the global warming “hiatus” highlighted in the last report of the UN's Intergovernmental Panel on Climate Change (IPCC) - and exploited by sceptics to undermine climate change policy - is nothing more than an illusion resulting from flaws in the way the data was collected.

In a study published in the journal Science, the scientists said that previous measurements of surface temperatures on land and sea have underestimated the rate of warming over the past 15 years. When the figures are properly calibrated it becomes clear that the warming has continued as fast or even faster than before.

“These results do not support the notion of a 'slowdown' in the increase of global surface temperatures,” says the NOAA study, led by Thomas Karl, director of the US National Climate Data Centre, which runs the largest climate archive in the world.

Climate sceptics such as former Chancellor Lord Lawson and ex-minister Peter Lilley have repeatedly claimed that global warming has either stopped or slowed down. Even the IPCC's last report two years ago said that average global surface temperature “has shown a much smaller increased linear trend over the past 15 years than over the past 30 to 60 years”.

However, the NOAA study reviewed the way measurements have been collected for sea-surface temperatures over the decades, as well as including fresh data for land-surface temperatures, and concluded that the IPCC's statement “is no longer valid”. Jay Lawrimore, an NOAA scientist and co-author of the study, said the improvements in temperature recording and analysis, and the fact that the Arctic is under-represented in the dataset even though the region is known to be warming much faster than the global average, means that, if anything, the latest calculation of the rate of global warming is an underestimate.

“It's clear there is no hiatus and temperatures are continuing to warm - 2014 was a record warm year and we expect 2015 to be if not the warmest, then close to it. The evidence we see is that temperatures are continuing to rise,” Dr Lawrimore said.

“We are likely to be under-estimating the true trend in warming because we're not including the Arctic. If we did, then we'd probably get even higher warming trends,” he told The Independent.

The study found that between 2000 and 2014, average global surface temperatures increased at a rate of 0.116C per decade, compared with 0.113C per decade between 1950 and 1999. “There is no discernible (statistical or otherwise) decrease in the rate of warming between the second half of the 20th century and the first 15 years of the 21st century,” the study's authors concluded.

Changes in the way sea-surface temperatures have been collected, with a movement towards taking measurements from buoys in the ocean rather than from water drawn up in buckets, meant the data had to be better calibrated, NOAA said. The increasing number of weather stations recording surface temperatures over land has also led to the need to correct inherent biases.

“This reassessment of global temperatures, which finds that there has been no pause or slowdown in surface warming since 1998, is very important as it comes from an extremely well regarded group at a US government laboratory,” said Professor Sir Brian Hoskins, a distinguished British meteorologist and chair of the Grantham Institute at Imperial College London.

However, other scientists pointed out that NOAA is one of four independent organisations gathering and analysing global temperatures, and other groups have detected a slight slowdown in the rate of global warming, which is why the IPCC report mentioned a “hiatus”.

“This new study suggests that the slowdown in the rate of warming may be much less pronounced than in the global temperature records that were available for the IPCC to assess,” said Professor Tim Osborn of the University of East Anglia, which handles the UK dataset with the Met Office Hadley Centre.

“The IPCC's assessment wasn't wrong, but perhaps the emphasis would be slightly different if the assessments were carried out afresh with the new studies since 2013 that could now be considered,” he said.

“I would caution against dismissing the slowdown in surface warming on the basis of this study … There are other datasets that still support a slowdown over some recent period of time, and there are intriguing geographical patterns such as cooling in large parts of the Pacific Ocean that were used to support explanations for the warming slowdown,” he added.

The Independent

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