Last year was Australia’s hottest. Photo: Anthony Johnson
An Australian-led research team claims to have solved one of the biggest puzzles in climate science – why air temperatures have largely plateaued since 2001 even as greenhouse gas levels have continued to climb.
While most scientists have long accepted that the Earth continues to trap extra heat with the bulk of it ending up in the oceans, they have been uncertain about the precise process slowing the rise in atmospheric temperatures and whether it will persist.
The answer, says Matthew England from the University of NSW, is an unprecedented strengthening of east-west trade winds over the equatorial Pacific since the 1990s.
The winds, as much as twice their normal strength, are altering circulation patterns and driving huge amounts of heat into the western Pacific while cooling eastern regions, Professor England and fellow researchers found.
Significantly, the wind changes had not been factored into 50 climate models analysed by the team, whose findings are published on Monday in the journal Nature Climate Change. Once wind trends are included, the models account for the so-called hiatus in air temperatures “in its entirety”, Professor England said.
The heat uptake in the western Pacific is so great – temperatures are up about 3 degrees in some regions at depths of 100-300 metres over the past two decades – he said “the question might be why hasn’t the world actually cooled over the past 10 years, rather than plateaued?”
Last year was the world’s sixth warmest in records going back to 1850, the World Meteorological Organisation said last week.
Of the 14th warmest years, 13 of them have occurred this century.
Last year was Australia’s hottest on record, with the warming western Pacific likely to be a factor, Professor England said.
After rising rapidly through the 1980s and 1990s, air temperatures at the earth’s surface – the most closely monitored measure – largely levelled off.
Some sceptics have trumpeted the slowdown as proof the climate was less sensitive to higher carbon dioxide levels than previously estimated, reducing the urgency of cutting emissions from fossil-fuel burning and other human activities.
“[P]rojections of global surface air temperature are very much tied to the strength of the trade winds,” said Helen McGregor, an associate research fellow in the school of earth and environmental sciences at the University of Wollongong.
“And that means that if we have tropical Pacific events such as El Nino, which reduce trade wind strength and affect Australian rainfall, we are likely to see rates of global warming increase.’’
The US Climate Prediction Centre last week reported an increasing chance of an El Nino forming during 2014.
While Pacific winds may be creating a balance between the ocean and atmosphere, it is a precarious one.
Previous natural cycles indicate periods of stronger trade winds last 20-30 years before they slow.
“When that occurs, it’s highly likely that air temperature change over the planet will be one of relatively rapid warming, probably exceeding the warming rate of the ’80s and ’90s,” Professor England said.
That is because greenhouse gas levels “are much, much higher than they were even 30 years ago”.