World's climate could cool first, warm later

Fred Pearce, New Scientist 4 Sep 09;

Forecasts of climate change are about to go seriously out of kilter. One of the world's top climate modellers said Thursday we could be about to enter "one or even two decades during which temperatures cool.

"People will say this is global warming disappearing," he told more than 1500 of the world's top climate scientists gathering in Geneva at the UN's World Climate Conference.

"I am not one of the sceptics," insisted Mojib Latif of the Leibniz Institute of Marine Sciences at Kiel University, Germany. "However, we have to ask the nasty questions ourselves or other people will do it."

Few climate scientists go as far as Latif, an author for the Intergovernmental Panel on Climate Change. But more and more agree that the short-term prognosis for climate change is much less certain than once thought.
Nature vs humans

This is bad timing. The UN's World Meteorological Organization called the conference in order to draft a global plan for providing "climate services" to the world: that is, to deliver climate predictions useful to everyone from farmers worried about the next rainy season to doctors trying to predict malaria epidemics and builders of dams, roads and other infrastructure who need to assess the risk of floods and droughts 30 years hence.

But some of the climate scientists gathered in Geneva to discuss how this might be done admitted that, on such timescales, natural variability is at least as important as the long-term climate changes from global warming. "In many ways we know more about what will happen in the 2050s than next year," said Vicky Pope from the UK Met Office.
Cold Atlantic

Latif predicted that in the next few years a natural cooling trend would dominate over warming caused by humans. The cooling would be down to cyclical changes to ocean currents and temperatures in the North Atlantic, a feature known as the North Atlantic Oscillation (NAO).

Breaking with climate-change orthodoxy, he said NAO cycles were probably responsible for some of the strong global warming seen in the past three decades. "But how much? The jury is still out," he told the conference. The NAO is now moving into a colder phase.

Latif said NAO cycles also explained the recent recovery of the Sahel region of Africa from the droughts of the 1970s and 1980s. James Murphy, head of climate prediction at the Met Office, agreed and linked the NAO to Indian monsoons, Atlantic hurricanes and sea ice in the Arctic. "The oceans are key to decadal natural variability," he said.

Another favourite climate nostrum was upturned when Pope warned that the dramatic Arctic ice loss in recent summers was partly a product of natural cycles rather than global warming. Preliminary reports suggest there has been much less melting this year than in 2007 or 2008.

In candid mood, climate scientists avoided blaming nature for their faltering predictions, however. "Model biases are also still a serious problem. We have a long way to go to get them right. They are hurting our forecasts," said Tim Stockdale of the European Centre for Medium-Range Weather Forecasts in Reading, UK.

The world may badly want reliable forecasts of future climate. But such predictions are proving as elusive as the perfect weather forecast.