Extreme global warming not likely soon
Oslo, May 20, 2013
By Alister Doyle, Environment Correspondent
Extreme global warming is less likely in coming decades after a slowdown in the pace of temperature rises so far this century, an international team of scientists said.
Warming is still on track, however, to breach a goal set by governments around the world of limiting the increase in temperatures to below 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times, unless tough action is taken to limit rising greenhouse gas emissions.
"The most extreme rates of warming simulated by the current generation of climate models over 50- to 100-year timescales are looking less likely," the University of Oxford wrote about the findings in the journal Nature Geoscience.
The rate of global warming has slowed after strong rises in the 1980s and 1990s, even though all the 10 warmest years since reliable records began in the 1850s have been since 1998.
The slowdown has been a puzzle because emissions of heat-trapping greenhouse gases have continued to rise, led by strong industrial growth in China.
Examining recent temperatures, the experts said that a doubling of carbon dioxide concentrations in the atmosphere above pre-industrial times - possible by mid-century on current trends - would push up temperatures by between 0.9 and 2.0 degrees Celsius (1.6 and 3.6F).
That is below estimates made by the U.N. panel of climate scientists in 2007, of a rise of between 1 and 3 degrees Celsius (1.8-5.4F) as the immediate response to a doubling of carbon concentrations, known as the transient climate response.
The UN panel also estimated that a doubling of carbon dioxide, after accounting for melting of ice and absorption by the oceans that it would cause over hundreds of years, would eventually lead to a temperature rise of between 2 and 4.5 C (3.6-8.1F).
Findings in the new study, by experts in Britain, the United States, Canada, Australia, France, Germany, Switzerland and Norway, broadly matched that range for the long-term response.
But for government policy makers "the transient response over the next 50-100 years is what matters," lead author Alexander Otto of Oxford University said in a statement.
The oceans appear to be taking up more heat in recent years, masking a build-up of carbon dioxide in the atmosphere that passed 400 parts per million this month for the first time in human history, up 40 percent from pre-industrial levels.
Professor Reto Knutti of ETH Zurich, one of the authors, said that the lower numbers for coming decades were welcome.
But "we are still looking at warming well over the two degree goal that countries have agreed upon if current emission trends continue," he said.
Temperatures have already risen by about 0.8 Celsius (1.4F) since the Industrial Revolution and two degrees C is widely viewed as a threshold to dangerous changes such as more floods, heatwaves and rising sea levels.
"The oceans are sequestering heat more rapidly than expected over the last decade," said Professor Steven Sherwood of the University of New South Wales in Australia, who was not involved in the study.
"By assuming that this behaviour will continue, (the scientists) calculate that the climate will warm about 20 percent more slowly than previously expected, although over the long term it may be just as bad, since eventually the ocean will stop taking up heat."
He said findings "need to be taken with a large grain of salt" because of uncertainties about the oceans. - Reuters