(The author is a Reuters market analyst. The views expressed are his own.)
By Gerard Wynn
LONDON, April 12 A recent slowdown in the rise of the Earth's surface temperatures has led some commentators to question whether scientists have exaggerated the global warming problem.
Scientific advances have removed any reasonable doubt that carbon emissions are warming the planet, especially since 1970. Most scientists agree that natural cycles of the sun and oceans explain some variation in temperatures, but they cannot explain much of the 0.7 degree Celsius warming since the beginning of the 20th century.
There is less consensus over exactly how much warming will occur in the future. Scientists have provided a wide range of views on how the planet could respond to a doubling of carbon dioxide levels, a measure known as climate sensitivity.
The trend line for global warming has slowed over the past decade or so, with 1998, 2005 and 2010 closely tied for the hottest year in the global temperature record. ****************************************************** Chart of global temperatures: link.reuters.com/sev37t ******************************************************
This slowdown may indicate that the more extreme estimates for climate sensitivity are less likely to prove true. It is not yet an argument for changing the central estimates.
For policymakers, in any case, the uncertainty in the scientific debate over the Earth's climate sensitivity has meant they continue to pursue existing policies based on political, economic and social goals.
The Earth's climate sensitivity includes the direct impact of manmade carbon dioxide, which forces the Earth to warm up by trapping infra-red radiation, and knock-on or feedback effects.
For example, each degree of global warming increases the concentration in the atmosphere of water vapour, which has a strong greenhouse effect, by about 6 percent.
Also the melting of sea ice in the Arctic has increased the expanse of darker, open water, which reflects less heat and light and so speeds up warming.
One area of uncertainty is whether the net effect of more clouds will lead to warming or cooling.
The record for observed surface temperatures (there are few observations in the deep oceans) in the past decade have found no evidence for an extreme response to greenhouse gases.
As a result, some experts have suggested that the extreme upper limit for climate sensitivity can now be narrowed to around 6 degrees Celsius from earlier estimates of up to 10 degrees.
The lower limit is about 1.1 degrees Celsius after excluding all feedback effects, most of which tend to increase warming.
Little evidence has emerged for changing the central estimate of around 3 degrees. In fact, recent analysis of climate change over the past 65 million years has broadly supported it.
There are plenty of potential causes for a blip in the warming trend besides a revised assessment of the impact of greenhouse gases.
Scientists in the larger, specialist institutes such as Britain's Met Office Hadley Centre and Germany's Potsdam Institute for Climate Impact Research have preferred to look for such causes.
Candidates include manmade aerosols from a huge increase in coal burn in the past 10 years in China (which forms hazy clouds that can reflect back sunlight and cool the Earth); changes in the El Nino Pacific Ocean weather pattern (which warms the Earth and whose last big maximum was 1998); an observed fall in stratospheric water vapour (a strong greenhouse gas); and the role of the deep oceans in masking surface warming by sequestering heat.
In a study of the latter, Kevin Trenberth from the U.S. National Center for Atmospheric Research co-authored a paper published last month in the journal Geophysical Research Letters, "Distinctive climate signals in reanalysis of global ocean heat content".
"The differences in recent trends among the different ocean layers are profound. The small warming in the upper 300 metres is belied by the continuing warming for the ocean as a whole, with considerable warming occurring below 700m," the paper said.
Oxford University's Myles Allen argues that all the uncertainty makes climate sensitivity less relevant for policymakers, given that all estimates forecast higher temperatures unless there are cuts in greenhouse gas emissions, which are still rising.
Others say that eliminating the extreme scenarios would be helpful to add information about the risks from building infrastructure and housing on flood plains or near sea level.
Without scientific certainty, governments will probably maintain their default positions regarding international action: to curb emissions knowing that the world is warming and greenhouse gases are largely responsible (the European Union); largely to disregard the issue (Russia); or to maintain a stand-off while acknowledging the problem (the United States, China and India).
Meanwhile, the U.S. government's National Oceanic and Atmospheric Administration reported that February marked the 336th consecutive month, spanning 28 years, when global temperatures were above the 20th century average. (editing by Jane Baird)