This is the conclusion from the first study to compare and contrast the consequences of 1.5C world compared to a 2C world, published today in Earth System Dynamics.
Both 2C and 1.5C are explicitly mentioned in the Paris agreement as potential upper limits for global warming since the preindustrial era, but details from scientists on how the temperature thresholds compare have been sparse.
For example, an extra 0.5C could see global sea levels rise 10cm more by 2100, water shortages in the Mediterranean double and tropical heatwaves last up to a month longer. The difference between 2C and 1.5C is also “likely to be decisive for the future of coral reefs”, with virtually all coral reefs at high risk of bleaching with 2C warming.
The authors presented their research today at the European Geosciences Union, an annual major gathering of geoscientists taking place this week in Vienna.
The Paris agreement – adopted in December 2015 and due to be officially signed by more than 150 countries on Friday – codified what the authors of today’s study call a “two-headed” temperature goal.
It pledged to keep the average global surface temperature “well below 2C” and “pursue efforts” to limit the increase since preindustrial times to 1.5C.
The nod to 1.5C recognised that many low lying island nations are already feeling the impacts of climate change and that coral reef and Arctic ecosystems face high risks well below 2C.
But the specific reference to 1.5C as well as 2C caught the scientific community somewhat off-guard. Today’s paper says:
“Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing.”
A recent commentary in Nature by Prof Simon Lewis, professor of global change at University College London, is a little stronger on this point. As he puts it:
“The emergence of 1.5 C as a serious policy position comes with important lessons for scientists. The global research community has shockingly little to say on the probable impacts of a 1.5 C rise.”
The scientific community now, at least, seems to be rising to the challenge. Last week, the IPCC confirmed it will dedicate one of its special reports to the 1.5C goal. This is due to be published in 2018.
Work on today’s paper began in 2014, long before the Paris conference. The United Nations Framework Convention on Climate Change called on scientists to explore the difference between a 1.5C and 2C long term goal, as part of its 2013-2015 review.
Prof Carl-Friedrich Schleussner, scientific advisor at Climate Analytics in Germany and lead author of today’s study, tells Carbon Brief:
“[The review] concluded last year that, while 2C cannot be considered safe and 1.5C would clearly be a safer limit, the science on 1.5C is less robust than on 2C. So clearly, there’s a research gap here.”
A 1.5C vs 2C world
The study compared how extreme weather, water availability, crop yields, sea level rise and risks to coral reefs differ in a world where global temperature rises 1.5C, compared to if it rises 2C.
Using 11 climate models, the authors looked at how each of the impacts plays out globally, as well as in 26 different regions. This is important since the world won’t warm at the same pace everywhere, the paper notes.
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