Global warming… only slower (Image: NASA)

Humanity has a second chance to stop dangerous climate change. Temperature data from the last decade offers an unexpected opportunity to stay below the agreed international target of 2 °C of global warming.

A new analysis took temperature rise in the most recent decades, and worked out what this means for the coming ones. It suggests that Earth will warm more slowly over this century than we thought it would, buying us a little more time to cut our greenhouse gas emissions and prevent dangerous climate change.

Climate scientists caution that this does not mean climate change is not real. Temperatures are currently rising faster than they have been for 11,000 years.


Dangerous climate change

Governments have promised to limit the world to 2 °C of warming – the agreed threshold for dangerous climate change. On previous estimates, that meant global emissions had to peak by 2020 and then fall. With emissions shooting up, this seemed hopelessly unrealistic.

“If previous estimates [of how the climate will warm] were true, keeping the world below 2 °C would have been almost impossible however big our emission cuts,” says Piers Forster of the University of Leeds in the UK, who contributed to the new study. “Now it looks like we have a chance, so we should take it.”

“Prior to this, a lot of us were feeling quite gloomy that whatever we did, we’ll go over 2 °C,” says Forster’s colleague Myles Allen of the University of Oxford, UK. “It’s not a foregone conclusion any more.” That means the UN climate negotiations could still succeed. If a deal comes into force in 2020, and leads to rapid emissions cuts, “there remains a good chance we could hit the 2 °C target”, says Allen.

Slowdown

After heating rapidly in the late 20th century, Earth warmed only slowly in the last decade, partly as a result of natural cycles in the climate system.

Alexander Otto of the University of Oxford in the UK and colleagues have now taken this latest data into account to calculate how much fossil fuel emissions have warmed the Earth so far. They then looked at what that meant for the temperature rise over the coming few decades, and found that global warming this century will indeed be slower than thought.

The team focused on how much hotter the planet will be in the year that carbon dioxide concentrations reach double their pre-industrial value. On current trends, that will happen between 2050 and 2070. Previous studies had suggested temperatures would rise up to 1.6 °C, but Otto found a temperature increase of 1.3 °C.

“In the short term, there is maybe a bit less warming than expected,” says Otto. “It might buy us five or ten years,” agrees Chris Forest of Penn State University in University Park, although he cautions that the problem hasn’t gone away.

Not as hot?

So much for the next few decades. How hot the planet gets in the long term depends on how sensitive the climate is to CO 2 accumulating in the atmosphere.

Lags in the climate system mean that temperatures will continue to rise after CO 2 concentrations double, even if greenhouse gas emissions stop. The 2007 report of the Intergovernmental Panel on Climate Change (IPCC) estimated that temperatures would eventually stabilise between 2 °C and 4.5 °C above pre-industrial temperatures – with a best estimate of about 3 °C – a factor known as climate sensitivity.

The large error bars on that number inject uncertainty into our projections of the effects of climate change – from changing storm patterns to sea level rise.

A growing body of climatologists think that the climate is less sensitive to CO 2 than the IPCC’s best estimate, so temperatures will not rise as much as feared. “I’ve been arguing this for a few years,” says James Annan of the JAMSTEC Yokohama Institute for Earth Sciences in Yokohama, Japan.

When Otto calculated the climate sensitivity from his data, he found it was about 2 °C – with a range of 0.9 to 5 °C – well below the IPCC’s best estimate of 3 °C.

“The observations are telling us one thing and the climate models are telling us another,” says Forest. He thinks the most likely range is between 2.5 and 3 °C.

Saving the world

For the last few years, governments have been planning to sign a deal in 2015 that will come into force in 2020. On previous estimates of the climate sensitivity, that is far too late. But if the sensitivity really is below 3 °C, we might have a shot.

“If we are lucky and the climate sensitivity is at the low end, and we have a strong agreement in 2015, then I think we stand a chance to limit climate change to 2 °C,” says Corinne Le Quéré of the Tyndall Centre for Climate Change Research in Norwich, UK. “But there’s a lot of ifs.”

Even if it is technically possible to limit warming to 2 °C, that does not mean it will actually happen. “I suppose it means that 2 °C isn’t quite as unattainable as it was previously thought to be, but I’m not exactly holding my breath on climate negotiations,” says Annan.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1836