By Tim Radford

Researchers now know the difference half a degree can make. They can tell you why 1.5°C warming would be better than a 2°C climb in average global temperatures, because even half a degree Celsius could mean greater extremes of heat, more overwhelming rainfall and longer spells of warm weather.

And they know all this because they've seen it happen in the recent past. There is enough evidence, they say, in the observational record for the last half century to underline the importance of even half a degree.

Scientists from Germany and Switzerland outline the argument and identify the evidence in the journal Nature Climate Change.

In the last two centuries, the ratio of carbon dioxide, the result of extravagant fossil fuel use since the Industrial Revolution, has risen from around 280 parts per million to 400 ppm, and average global temperatures have risen around 1°C during that time.

The researchers matched temperature and climate records for the years 1960 to 1979 and 1991 to 2010, a period when the thermometer averages climbed by a whole half a degree.

Significant changes

They found that the intensity of extreme rainfall had increased by nine percent over that period. The coldest winters were measurably less cold, and half of the global land mass had experienced changes of what they called "warm spell duration" of more than six days.

It is not that perceptible global warming made these extremes happen—extremes happen anyway—but the researchers think it made them more likely. By raising the temperature, humans loaded the climate dice.

"The hottest summer temperatures increased by more than 1°C in a quarter of global land areas, while the coldest winter temperatures warmed by more than 2.5°C," said Peter Pfleiderer, a scientist with the Potsdam Institute for Climate Impact Research and for Climate Analytics.

And his colleague Carl-Friedrich Schleussner said, "As we're moving increasingly outside of the range of natural climate variability, we have to expect that impacts on agriculture, human and biological systems will be more pronounced."

The Intergovernmental Panel on Climate Change has begun to prepare a special report on the impacts of a warming of 1.5°C, the Paris agreement target set by the world's nations in 2015.

There is no certainty that the world's nations can meet the Paris target and contain global warming, and limit climate change, because enough carbon dioxide has been emitted to take air temperatures over land to that level already.

The next decade could be critical, which is why researchers feel they need the evidence in as clear a form as possible.

"One of the pressing questions for scientists today is whether we know that limiting warming to 1.5°C instead of 2°C would make a difference in the future. We have to rely on climate models to predict the future, but given we now have observational evidence of around 1˚C warming, we can also look at the real life impacts this warming has brought," Schleussner said.

And the third signatory, Erich Fischer of ETH Zurich, said, "Communicating abstract quantities like differences in global mean temperature is difficult.

"With the warming the world has already experienced, we have an actual record of warming to study, and we can see very clearly that a difference of 0.5˚C of warming really does matter."

Reposted with permission from our media associate Climate News Network.