By Tim Radford, Climate News Network
This piece first appeared at Climate News Network.
LONDON—Australian and US scientists think they know where a lot of global warming has been concentrated: it has been tucked away below the surface waters of the western Pacific Ocean. And the agency that took the heat out of the atmosphere and transferred it into a liquid form could have been the equatorial trade winds.
Matthew England from the Australian Centre of Excellence for Climate System Science and colleagues report in Nature Climate Change that a dramatic acceleration in the winds has drawn heat from the atmosphere and transferred it to the ocean: cooler waters have risen to the surface to mask the transaction.
Climate sceptics – and some climate scientists – talk about a slowdown, or a pause, or a hiatus in global warming. In fact, temperatures have gone on rising and 13 of the 14 warmest years ever recorded have all occurred since 2000. But the rate of rise in global average temperatures since 2000 has not been as fast as the rate during the 1980s and 1990s.
Since greenhouse gas levels have continued to rise, and since scientists are sure of their atmospheric physics, then there was some “missing heat” to be accounted for.
Researchers have variously suggested that a puzzling increase in deep ocean temperatures could be one explanation or that perhaps the unevenness of temperature measurements around the planet might be another. But both suggestions were hypotheses: nobody had an answer that could be tested by any kind of experiment.
Professor England and colleagues worked with observed winds, surface air temperatures, and a set of ocean climate models to calculate what may have happened.
The global warming story has always been one of fits and starts: a warming that ought to have been observed 70 years ago stalled between 1940 and 1970, and when it resumed, did so in fits and starts. The overall trend continued upward, but the rate of rise slowed noticeably during the last decade.
Ocean circulation loops are driven by winds, and speed up as the winds intensify. Cool waters well up, warm waters descend. And intensify is just what the trade winds have done. They began strengthening during the 1990s, a process which continues today. Once the researchers added the trade winds to their calculations, the global average temperatures looked very like the observations during the hiatus.
They also found that four-fifths of the surface temperature cooling occurred after 2000, which confirmed that wind acceleration was the key contributor.
“Scientists have long suspected that extra ocean heat uptake has slowed the rise of global average temperatures, but the mechanism behind the hiatus remained unclear”, said Professor England.
Rapid rise on the way
“But the heat uptake is by no means permanent: when the trade wind strength returns to normal – as it inevitably will – our research suggests heat will quickly accumulate in the atmosphere. So global temperatures look set to rise rapidly out of the hiatus, returning to the levels projected within as little as a decade.”
The same mechanism could explain the slowdown between 1940 and 1970. In 1938, the British scientist G S Callendar argued that rising carbon dioxide levels should mean global warming but the evidence proved elusive, perhaps because the trade winds accelerated during those decades.
Richard Allan, professor of climate science at the University of Reading in the UK, said the current slowdown was only a temporary reprieve.
“Measurements from satellites and ocean buoys show that the planet is absorbing more heat than it is radiating out to space and the heat is building up in the oceans.
“This new research suggests that when the trade winds weaken again, the planet can expect rapid warming of the surface to resume, as greenhouse gas concentrations continue to rise.”