Global warming will slow during the next few years but then speed up again, and that at least half of the years after 2009 will be warmer than 1998, the warmest year on record, scientists predicted in a study published Thursday.
Climate experts have long predicted a general warming trend over the 21st century spurred by the greenhouse effect, but this new study gets more specific about what is likely to happen in the decade that started in 2005.
To make this kind of prediction, researchers at Britain's Met Office — which deals with weather and climate issues — made a computer model that takes into account such natural phenomena as the El Nino pattern in the Pacific Ocean and other fluctuations in ocean circulation and heat content.
A forecast of the next decade is particularly useful, because climate could be dominated over this period by these natural changes, rather than human-caused global warming, study author Douglas Smith said by telephone.
In research published in the journal Science, Smith and his colleagues predicted that the next three or four years would show little warming despite an overall forecast that saw warming over the decade.
"There is ... particular interest in the coming decade, which represents a key planning horizon for infrastructure upgrades, insurance, energy policy and business development," Smith and his co-authors noted.
The real heat will start after 2009, they said.
Until then, the natural forces will offset the expected warming caused by human activities, such as the burning of fossil fuels, which releases the greenhouse gas carbon dioxide.
To check their models, the scientists used a series of "hindcasts" — forecasts that look back in time — going back to 1982, and compared what their models predicted with what actually occurred.
Factoring in the natural variability of ocean currents and temperature fluctuations yielded an accurate picture, the researchers found. This differed from other models which mainly considered human-caused climate change.
"Over the 100-year timescale, the main change is going to come from greenhouse gases that will dominate natural variability, but in the coming 10 years the natural internal variability is comparable," Smith said.
While the Met Office considers 1998 as the warmest year on record, the U.S. National Climate Data Center ranked 2005 in a virtual tie with 1998.