Drop in water vapour curbs global warming, says study

Water vapour is the atmosphere’s most abundant greenhouse gas helping to trap heat and increase temperatures on the Earth's surface, according to the Geneva-based World Meteorological Organization.

A study published in Science magazine indicates that water vapour sometimes makes wild fluctuations in the upper sections of the Earth’s atmosphere and has fallen by 10 percent since the year 2000.

Scientists reckon the discovery might help explain why surface temperatures have not risen as rapidly over the last decade when compared to the 1980s and 90s.

"It's a thin wedge of the upper atmosphere that packs a wallop from one decade to the next in a way we didn't expect," said Susan Solomon, senior scientist at the United States National Oceanic and Atmospheric Administration (NOAA) and lead author of the study.

Exactly why the shift has taken place isn’t clear. Nonetheless, models show that the decline has put the brakes on surface temperatures, causing them to rise about 25 percent more slowly than otherwise.

Gian-Kasper Plattner of the University of Bern and science director with the Intergovernmental Panel on Climate Change provided the model used in the study. He told Swisster: “Why it fluctuates is a big question amongst scientists. The danger is that we just don’t understand the climate system well enough yet.”

“There isn’t a single smoking gun and it’s probably a combination of factors, just as we have found that it isn’t only greenhouse gases which have caused global warming,” he said in reference to another recent study which showed that steps to limit pollution may have actually contributed towards increasing the speed of climate change.

Another worrying prospect is when atmospheric water vapour might begin increasing again and the runaway effects such a turnaround could apply to surface temperatures.

“That’s difficult,” Plattner said. “Sea-surface temperature in the tropics which send water vapour into the atmosphere could be where it’s coming from, but we only have data from a very limited period. We need to look at more complex models.”

Experts can roughly calculate past sea-surface temperatures from the composition of ocean sediment and its contents of microscopic marine animals, whose tiny shells absorb carbon dioxide.

“If there is a link [between sea-surface temperatures and atmospheric water vapour] you would expect to see it in past records,” said Plattner.

“El Niño and La Niña [temperature changes in areas of the Pacific Ocean] could also have an effect, but we didn’t make that point in the paper. One could go also back and study that,” he pointed out.

Once again the discovery illustrates the vast complexities behind global warming.

“Some of the climate sceptics are using the water vapour argument to say that [anthropomorphic] global warming is a myth,” warned Plattner.

“It’s not,” he stressed. “And we can see that from the overall rise in historical temperatures” which correspond with the onset and development of the industrial revolution.