By Alex Kirby, Climate News Network
This piece first appeared at Climate News Network.
LONDON—Scientists believe they have made significant progress towards explaining why global average surface temperatures have risen more slowly this century than previously.
They say cooling waters in the tropical Pacific Ocean have played a large part in slowing recent warming, a finding which challenges those who argue that the slowdown means climate change is not as serious a problem as most climate scientists are convinced it is.
Before 2000 global temperatures had risen at a rate of 0.13ºC per decade since 1950. The hiatus has occurred while levels of carbon dioxide, the main greenhouse gas from human activities, continued a steady rise, reaching 400 parts per million for the first time in human history in May this year.
The eastern tropical Pacific has been distinctly cooler in the last few years, thanks to the influence of one of the world’s biggest ocean circulation systems, the Pacific decadal oscillation (PDO).
The better-known El Niño and La Niña weather systems, which also originate in the Pacific and can affect the weather thousands of miles away, occur just a few years apart. Both are parts of the much bigger PDO, which comes and goes over decade-long timescales. It is now in a cooling phase which could last for years – the last one stretched from the 1940s to the 1970s when warmer, drier weather dominated in the midwestern US.
In such a phase the temperature of the eastern Pacific’s waters falls while those in the west warm. In the oscillation’s warming phases this is reversed. In winter the PDO’s cooler phase lowers northern hemisphere temperatures slightly, but in the summer this cooling has less impact.
The scientists are from the Scripps Institution of Oceanography in California. Their study is published in the journal Nature. Dan Barrie, programme manager at the US National Oceanic and Atmospheric Administration (NOAA), which supported their research, called it “compelling” and said: “[It] provides a powerful illustration of how the remote eastern tropical Pacific guides the behaviour of the global ocean-atmosphere system, in this case exhibiting a discernible influence on the recent hiatus in global warming.”
The Scripps team, using computer models, compared their results with observations and concluded that global average annual temperatures have been lower than they would otherwise have been because of the oscillation.
But they say the observed recent higher summer temperatures show more of the true effects of global warming. Global average temperatures are calculated over the whole year, blurring the effect of this seasonal variation.
Shang-Ping Xie, professor of environmental science at Scripps and co-author of the study, said: “In summer, the equatorial Pacific’s grip on the northern hemisphere loosens, and the increased greenhouse gases continue to warm temperatures, causing record heat waves and unprecedented Arctic sea ice retreat.”
Oceans’ key role
Dr Alex Sen Gupta, of the Climate Change Research Centre at the University of New South Wales, who was not part of the study team, told the London Guardian: “The authors have set up some elegant experiments using a climate model to test whether a natural oscillation that has gone through a large swing in the tropical Pacific Ocean over the last decade can explain the recent halt in surface global warming…
“…[T]he new simulation accurately reproduces the timing and pattern of changes that have occurred over the last four decades with remarkable skill. This clearly shows that the recent slowdown is a consequence of a natural oscillation.”
Research shows that much of the heat caused by global warming has been absorbed by the oceans, and about a third of the extra carbon dioxide emitted since the Industrial Revolution.
Scientists also think the heat is not staying near the ocean surface but is now penetrating to deeper water, and that this may be another factor which can create the impression of a slowdown in global warming. In any case, they say, the slower pace of recent warming is easily explained by natural climate variability – such as the PDO.
The Scripps scientists say that when the PDO’s cooling phase ends the growth of global average temperatures is likely to resume, perhaps faster than before as greenhouse gas emission rates will be higher.
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