Changes in the flow of heat between the atmosphere and the Pacific Ocean could help to explain the recent "pause" in global warming that has seen a fall in the rate at which global surface temperatures has risen over the past 15 years or so, a study has suggested.
The current global warming hiatus, where the increase in global temperatures has levelled off, can be explained at least in part by natural changes to a cold Pacific Ocean current called La Nina which may have helped to absorb excess heat from the atmosphere, scientists said.
It is further evidence that the deep ocean may be playing a major role in helping to dampen down temperature rises at the surface. A previous study for instance found that the heat being absorbed by the deep ocean is equivalent to the power generated by 150 billion electric kettles.
Although global average temperatures are now higher than they have ever been since modern records began, they have not increased as fast over the past 10 or 15 years as some climate models have predicted, leading climate "sceptics" to claim that global warming has stopped.
Climate scientists have countered by saying that the last decades was still warmer than any previous decade, with 12 of the 14 hottest years on record occurring since 2000, and that periods of natural variability, with temperatures falling temporarily, are always to be expected.
Calculations suggest that the overall heat balance of the Earth is showing a positive trend, with more solar heat coming in than thermal energy that is lost into space. Climate researchers have argued that the deep parts of the ocean are likely to be absorbing this extra heat, rather than it accumulating at the Earth's surface.
The latest study by Yu Kosaka and Shang-Ping Xie of the Scripps Institution of Oceanography in San Diego supports this idea by a theoretical study of the cold La Nina current in the eastern Pacific which they found is capable of taking up huge amounts of heat from the atmosphere over a period of decades.
"Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Nina-like decadal cooling," the researchers said in the journal Nature.
"Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase," they said.
In their computer modelling experiment, the two researchers altered the amount of heat flowing between the east Pacific Ocean and the atmosphere so that the sea surface temperatures in the computer model were forced to agree with the actual observations.
"This results in the realistic simulation of the recent surface warming slowdown globally and some unusual weather patterns such as drought experienced in the southern United States," said Richard Allan, a climate scientist at Reading University, who was not involved with the research.
"This new study adds further evidence that the recent slowdown in the rate of global warming at the Earth's surface is explained by natural fluctuations in the ocean and is therefore likely to be a temporary respite from warming in response to rising concentrations of greenhouse gases," Dr Allan said.
"This is important since it adds to a great body of evidence in continuing to confirm the realism of projected dangerous warming in the future due to human activities such as burning fossil fuels," he said.