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.
The slowdown has been a puzzle because emissions of heat-trapping greenhouse gases have continued to rise
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˚C above pre-industrial times, unless 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˚C and 2˚C.
That is below estimates made by the UN panel of climate scientists in 2007, of a rise of between 1˚C and 3˚C 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˚C and 4.5˚C.
Findings in the new study by experts in Britain, the US, Canada, Australia, France, Germany, Switzerland and Norway, broadly matched that range for the long-term response.
But for government policymakers “the transient response over the next 50 to 100 years is what matters,” lead author Alexander Otto of Oxford University said.
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 per cent 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 2˚C goal that countries have agreed upon if current emission trends continue,” he said.
Temperatures have already risen by about 0.8˚C since the Industrial Revolution and 2˚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 Prof. Steven Sherwood from the University of New South Wales in Australia, who was not involved in the study.
He said findings “need to be taken with a large grain of salt” because of uncertainties about the oceans.