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Scientists debunk conservative myth of global warming ‘pause’

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A new paper shows that global warming has continued over the past decade, and been manifested in different ways

New research by Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research investigates how the warming of the Earth’s climate has behaved over the past 15 years compared with the previous few decades. They conclude that while the rate of increase of average global surface temperatures has slowed since 1998, melting of Arctic ice, rising sea levels, and warming oceans have continued apace.

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The widespread mainstream media focus on the slowed global surface warming has led some climate scientists like Trenberth and Fasullo to investigate its causes and how much various factors have contributed to the so-called ‘pause’ or ‘hiatus.’ However, the authors note that while the increase in global temperatures has slowed, the oceans have taken up heat at a faster rate since the turn of the century. Over 90 percent of the overall extra heat goes into the oceans, with only about 2 percent heating the Earth’s atmosphere. The myth of the ‘pause’ is based on ignoring 98 percent of global warming and focusing exclusively on the one bit that’s slowed.

Nevertheless, the causes of the slowed global surface temperature increase present an interesting scientific question. In examining changes in the activity of the sun and volcanoes, Trenberth and Fasullo estimated that they can account for no more than a 20 percent reduction in the Earth’s energy imbalance, which is what causes global warming. Thus the cause of the slowed surface warming must primarily lie elsewhere, and ocean cycles are the most likely culprit.

Trenberth and Fasullo found that after the massive El Niño event in 1998, the Pacific Ocean appears to have shifted into a new mode of operation. Since that time, Trenberth’s research has shown that the deep oceans have absorbed more heat than at any other time in the past 50 years.

As a recent paper published in the journal Nature showed, the Pacific Ocean in particular appears to be the key component of the climate’s natural internal variability, and the main culprit behind the slowed global surface warming over the past 15 years. However, another important recent paper by Kevin Cowtan and Robert Way showed that the global surface temperature rise has not slowed as much as some previously thought; in fact, the surface warming since 1997 happened more than twice as fast as previous estimates.

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Trenberth and Fasullo’s new paper also casts doubt on the conclusions a few recent studies that estimated the Earth’s climate is less sensitive to the increased greenhouse effect than previously thought. These studies have been based on measurements of recent climate change, including the warming of the oceans. Climate contrarians like Matt Ridley have of course emphasized their results, because these few papers seem to suggest the climate won’t warm quite as much over the next century as climate scientists previously thought.

However, the type of approach taken by these studies suffers from some significant drawbacks. Mainly the size of the cooling effect due to human aerosol pollution remains highly uncertain, and while the oceans have been warming rapidly, just how rapidly is another unsettled question.

Previous estimates put the amount of heat accumulated by the world’s oceans over the past decade equivalent to about 4 Hiroshima atomic bomb detonations per second, on average, but Trenberth’s research puts the estimate equivalent to more than 6 detonations per second. Trenberth and Fasullo note that using their ocean heating estimate by itself would increase the equilibrium climate sensitivity estimate in the paper referenced by Ridley from 2°C to 2.5°C average global surface warming in response to a doubling of atmospheric carbon dioxide, and using other more widespread accepted values would bring the estimate in line with the standard value of 3°C. They thus note,

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“Using short records with uncertain forcings of the Earth system that is not in equilibrium does not (yet) produce reliable estimates of climate sensitivity.”

In any case, the main point of the paper is that global warming is stuck on fast forward. Ice continues to melt, sea levels continue to rise, and the oceans continue to warm rapidly. While the warming of global surface temperatures has slowed somewhat, that appears to primarily be due to changing ocean cycles, particularly in the Pacific. However, these changes are mostly just causing the oceans to absorb more heat, leaving less for the atmosphere. As Trenberth and Fasullo conclude,

“[Global warming] is very much alive but being manifested in somewhat different ways than a simple increase in global mean surface temperature.”

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guardian.co.uk © Guardian News and Media 2013


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