It was the evidence that climate change sceptics loved to cite. While the scientific community’s warnings about global warming had become ever more convincing, the critics pointed time and again to graphs showing the rise in the world’s average surface temperatures has slowed down since 1998 – a fact extensively interpreted by many vocal opponents as a fundamental failure in the basic science of climate change.
Now the scientists appear to have come up with an explanation. That much-vaunted “pause” in global warming can be largely explained by a failure to record an unprecedented rise in Arctic temperatures over the past 15 years, a study has found.
Two independent scientists have found that global temperatures over the past decade have almost certainly risen two-and-half times faster than Met Office scientists had conservatively assumed when they estimated Arctic warming because of a lack of surface temperature records in the remote region. Moreover, when the latest estimates of Arctic temperatures are included in the global temperatures, the so-called “pause” in global warming all but disappears and temperatures over the past 15 or so years continue to increase as they have done since the 1980s, the scientists said.
Surface temperatures are effectively measured over only 84 per cent of the Earth, and a lack of weather stations in the Arctic in particular has long been recognised as a major gap in the Met Office database of global temperatures.
This led to the Met Office and the Climatic Research Unit at the University of East Anglia to assume for the purpose of their calculations that the Arctic was warming as fast as the rest of the world, which they realised was probably an underestimate of the true position. But now scientists have worked out a way of estimating these surface temperatures in the Arctic from satellite readings of atmospheric temperatures. The study confirmed that the Arctic is one of the fastest-warming places on Earth – and its rapidly rising temperatures easily offset the “pause” in global warming when its temperatures are included in calculations of average global temperatures.
The study, by two researchers who are not climate scientists, is seen as one of the most important insights into the apparent flatlining of global average temperatures over the past 15 years, which has allowed climate sceptics to claim that global warming has “stopped” and that climate change will not be as bad as predicted.
Kevin Cowtan of the University of York and Robert Way of the University of Ottawa used a specialised statistical technique known as “kriging” to extrapolate from satellite temperature data to the ground, in order to circumvent the problem of there being too few weather stations distributed around the Arctic.
Their study, to be published in the Quarterly Journal of the Royal Meteorological Society, found that if these Arctic surface temperatures were included in global temperature estimates, then the average global increase went from 0.05C per decade to 0.12C per decade – effectively eliminating the “pause”.
“We have developed a method for using satellite data to fill in the gaps in the Met Office data. Our global record suggests that surface temperatures have been warming two and a half times faster than Met Office estimates over the past 16 years,” Dr Cowtan said.
“The temperature change for any individual year is not very large but together they make a significant difference to recent temperature trends,” he said.
Stefan Rahmstorf, a leading climate scientist at the Potsdam Institute for Climate Impact Research in Germany, said that the new study is “excellent” and is a very convincing explanation for something that has puzzled other researchers for many years.
“The problem with the polar areas lacking data coverage has been known for a long time, but I think this study has basically solved it. People will argue about the details, but I think this will hold up to scrutiny,” Dr Rahmstorf said.
Tim Osborn, of the UEA’s Climatic Research Unit, said that it is clear from the rapidly melting ice in the Arctic, as well as the limited temperature readings from northern Canada and Russia, that the Arctic is warming faster than other places in the world.
“This may explain part of the recent slowdown in warming over the Earth’s surface. The real warming may not have slowed as much as out data showed,” Dr Osborn said.
However, he cautioned against stating that this explains everything. “The slowdown in warming over the last decade may still be there, even if not as pronounced as we previously thought,” he added.
Other research has for instance shown that the deeper layers of the ocean are warming far faster than scientists had thought, indicating that huge amounts of heat from the atmosphere are being stored in the deep sea.
Climate scientists also emphasised that the last 16 years is too short a time frame to judge long-term trends and that there have been similar short-term “pauses” in the past which have not lasted.
Gavin Schmidt of Nasa’s Goddard Institute for Space Studies in New York said that there is unlikely to be a single, simple explanation for the apparent “pause”.
The sceptics: what they say
Lord Lawson: “Global warming appears to have ceased: there has been no increase in officially recorded global mean temperature for 15 years... Models all predicted an acceleration in the warming trend throughout the 21st century, as global carbon dioxide emissions rose apace. In fact, there has been a standstill.”
Peter Lilley: “Since the beginning of this century, the average global temperature has flatlined; indeed, over the past 18 months it has fallen back and, according to the satellite measurements of temperature, it is now basically back at the level it was in 1979, when such measurements started to be taken.”
Christopher Booker: “After several years flatlining, global temperatures have dropped sharply enough to cancel out much of their net rise in the 20th century.”