What Would a Global Warming Increase of 1.5 Degrees Be Like?

How ambitious is the world?  The Paris climate conference last December astounded many by pledging not just to keep warming “well below two degrees Celsius,” but also to "pursue efforts" to limit warming to 1.5 degrees C.  That raised a hugely important question: What's the difference between a two-degree world and a 1.5-degree world? 

Given we are already at one degree above pre-industrial levels, halting at 1.5 would look to be at least twice as hard as the two-degree option.  

So would it be worth it?  And is it even remotely achievable? 

In Paris, delegates called on the U.N.’s Intergovernmental Panel on Climate Change (IPCC) to report on the implications of a 1.5 target.  They want the job done by 2018, in time to inform renewed talks on toughening emissions targets beyond those agreed upon in Paris. 

But the truth is that scientists are only now getting out of the blocks to address what a 1.5-degree world would look like, because until recently it sounded like a political and technological impossibility.  As a commentary published online in Nature Climate Change last week warned, there is "a paucity of scientific analysis” about the consequences of pursuing a 1.5-degree target. 

To remedy this, the paper’s researchers, led by Daniel Mitchell and others at Oxford University, called for a dedicated program of research to help inform what they described as "arguably one of the most momentous [decisions] to be made in the coming decade."  And they are on the case, with their own dedicated website and a major conference planned at Oxford in the fall. 

So what is at stake?  There are two issues to address.  First, what would be gained by going the extra mile for 1.5?  And second, what would it take to deliver? 

First, the gains.  According to available research, says the Oxford group, the biggest boost will not be measured in average temperatures.  On its own, the difference between 1.5 degrees and 2 degrees is marginal.  But it would have a much greater effect on the probability of extreme and destructive weather events like floods, droughts, storms, and heat waves. 

We know extreme weather is happening more often.  A study last year by Erich Fischer of the Institute for Atmospheric and Climate Science in Zurich found that the risk of what was “once in a thousand days” hot weather has already increased fivefold.  His modelling suggests that it will double again at 1.5 degrees and double once more as we go from 1.5 to 2 degrees.  The probability of even more extreme events increases even faster.

The same will be true for droughts, says Carl-Friedrich Schleussner of the Potsdam Institute for Climate Impact Research in Germany.  Last year, he reported that the extra half-degree would produce dramatic increases in the likely length of dry spells over wide areas of the globe, including the Mediterranean, Central America, the Amazon basin, and southern Africa, with resulting declines in river flows from a third to a half.  Schleussner concluded that going from 1.5 to 2 degrees “marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions.”

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