OSLO — A natural cooling of the Pacific Ocean explains what is likely to be only a brief slowdown in the pace of global warming so far this century, a study showed on Wednesday.
What many scientists call a hiatus in warming has raised hopes among some governments that it will be easier and cheaper to achieve long-term goals for limiting temperature rises to avert more heatwaves, droughts, floods and rising sea levels.
But scientists said a series of naturally occurring La Nina weather events in the Pacific in recent years, which bring cooler waters to the surface, had masked the global heat-trapping effect of rising emissions of greenhouse gases.
"Our results show that the current hiatus is part of natural climate variability, tied specifically to a La Nina-like decadal cooling," according to the study by Yu Kosaka and Shang-Ping Xie at the University of California, San Diego.
"Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase," they wrote in the journal Nature.
Past studies have linked the slowdown in the pace of warming this century to factors such as a build-up of sun-dimming air pollution in the atmosphere or a decline in the sun's output. Others suggest that the deep oceans may be absorbing more heat.
Nine of the 10 warmest years since records began in the mid-19th century have been since 2000, with 1998 the exception, according to U.N. World Meteorological Organization (WMO).
Even so, the pace of warming has slowed from the 1980s and 1990s even though greenhouse gas emissions have hit record highs.
Four years of La Ninas
There have been four years with La Nina cooling events in the Pacific since 1998 and only two with the opposite, El Nino, when the Pacific waters warm, according to WMO data. The predominance of La Ninas is unusual.
The Nature study said its computer models, based on a reconstruction of Pacific temperatures, also successfully accounted for droughts in the southern United States and winter coolings in northwestern North America this century.
A U.N. panel of scientists, the main authority on global warming, will issue a report on Sept. 26 in Stockholm. Drafts show it is likely to raise the probability that human activities are the main cause of climate change to "extremely likely", or a 95 percent chance.
That is up from 90 percent in the last report in 2007. Its main scenarios also show that temperatures could rise by up to about 5 degrees Celsius (9 degrees Fahrenheit) by 2100. The drafts devote little space to explaining the hiatus in rising temperatures.
Almost 200 governments have agreed to limit a rise in temperatures to less than 2 degrees Celsius above levels before the Industrial Revolution - meaning average world surface temperatures should not rise above about 15.6 Celsius.
Temperatures have so far gained by about 0.8 C and many scientists say that warming is already causing more extreme weather, ranging from heatwaves to downpours.
Alex Sen Gupta, of the University of New South Wales in Australia, said the new study of the Pacific was "compelling evidence" that warming was being masked by the oceans and that the slowdown was tied to natural cycles.
Other experts noted the study did not fully explain the hiatus, especially where extra the heat trapped by greenhouse gases, led by carbon dioxide, was building up in the system.
"An important question that the paper does not address is where this energy has gone. Almost certainly it is in the deep ocean," said Will Hobbs, of Australia's Institute for Marine and Antarctic Studies.