The slowdown in global warming is temporary and a return to “substantial warming” is expected in coming decades, scientists have said.
The Intergovernmental Panel on Climate Change’s ( IPCC) latest study on climate has acknowledged that while predictions of climate change matched the real long-term trend, in the last 15 years warming had slowed compared with the models.
The fifth assessment by the IPCC said the reduction in warming in recent years was down to variation in the climate system, along with volcanic eruptions putting cooling ash into the atmosphere and changes in the sun’s strength.
Bill Collins, professor of climate change at the University of Reading and one of the authors of the report, said the computer models for projecting climate change were accurate over the long term, but that there were variations in the real world.
Because the models could not predict when the variations might happen, at times the models underestimated the warming that was happening, and at other times it overestimated it.
"Because the bumps and wriggles are random, the models do produce these changes, the speed-ups and slow-downs, but there's no reason they should produce them between 1998 and 2012," Professor Collins said.
Some of the models did produce a period without much warming over the past 15 years, purely by chance, he added.
Scientists know that elements such as volcanoes, which put ash into the atmosphere that deflects heat, and the sun, which has been weaker in recent years, had played a role in the slowdown, he said.
And they know that changes in the circulation of the oceans have led to more heat being taken down into the deep ocean, although experts are not sure exactly why it has happened and if it is natural or man-made.
Prof Collins said: “The deep ocean hasn’t seen any slowdown in heating. The slowdown is only seen on the surface. The same amount of energy is going into the climate, and it’s still being seen in the ocean, but not so much in the surface.”
“The climate hasn’t stopped warming. We haven’t seen it so much at the surface, but the heat has gone somewhere and it will come back. We haven’t escaped.”
His colleague Dr Richard Allan, of Reading University's department of meteorology, said heat had been accumulating within the oceans since 2000 at a rate equivalent to 250 billion 1KW electric heaters spread across the planet.
“Recent research indicates that natural fluctuations in the ocean have caused this heating to build up at deeper levels below the sea surface, rather than the upper layers which influence surface temperatures.
“This is important since it provides evidence that the slowdown in surface warming is temporary and we can expect a return to substantial surface warming over the coming decades,” he said.