Major climate changes seen in the Northern Hemisphere over the past 50 years have been driven by a progressive warming of tropical oceans, probably caused by the man-made build up of greenhouse gases in the atmosphere, scientists said yesterday.
An atmospheric pressure seesaw between Iceland at one end and Spain and Portugal at the other - known as the North Atlantic Oscillation (NAO) - is the key player, experts said in research published in the journal Science.
"Until recently, scientists believed the NAO was entirely chaotic, random and unpredictable," Mr James Hurrell, of the National Center for Atmospheric Research, a Boulder, Colorado-based laboratory managed by a consortium of 66 universities, said in a statement. "No one paid much attention to it."
However, he discovered that the NAO's variations from winter to winter were masking an underlying pattern unfolding over several decades.
The trend has been linked to changes in weather, agriculture and wildlife from Canada to Siberia and from the Arctic to northern Africa, the researchers said.
The NAO controls winter weather in Europe and over much of the Northern Hemisphere, the experts said.
They said they believe the link between tropical ocean warming and the Northern Hemisphere climate trend may be a symptom of human-induced climate change which has begun to emerge in the past 50 years.
The findings are the latest to focus attention on global climate changes apparently wrought by the man-made build up of so-called greenhouse gases.
Many scientists believe emissions of certain pollutants from industry, power plants and vehicles threaten to disrupt global climate and ecosystems by causing the Earth's atmosphere to trap more of the sun's energy, triggering global warming.
Studies examining the likely early effects of increased greenhouse gases in the atmosphere have pointed to a warming trend in the tropical oceans, and observations have demonstrated such a trend beginning around 1950, the researchers said.
Mr Hurrell and Mr Martin Hoerling, of the National Oceanic and Atmospheric Administration (NOAA), an agency of the US Commerce Department, analysed the results of a number of experiments using global climate models.
They found a correlation between the warmer sea-surface temperatures and climate changes in the Northern Hemisphere winter.
The researchers said warmer waters, especially in the Pacific and Indian Oceans, produce more equatorial rain, which heats the tropical atmosphere.
Northern Hemisphere surface temperatures have displayed a warming trend over the past several decades which have not been seen in the past 1,000 years, they added, noting that the NAO change has greatly influenced this.
The warmer waters exert a strong influence on the atmospheric pressure pattern and winds over the north Atlantic and north Pacific, Mr Hoerling said.
Resulting changes in circulation have warmed land surfaces and shifted storm tracks farther north.
Over the past two decades, the NAO has been characterised by stronger westerly winds across the middle latitudes of the Atlantic Ocean and into Europe, southerly flow over the eastern US, and northerly flow across western Greenland, the Canadian Arctic and the Mediterranean.
Mr Hoerling and Mr Hurrell said the warming tropical oceans have driven this phase.