Analysis of difference between 1.5C and 2C of warming finds extra 0.5C would mean longer heatwaves, greater droughts and threats to crops and coral reefs

A difference of half a degree centigrade may be barely noticeable day to day, but the difference between 1.5C and 2C of global warming is a shift into a new, more dangerous climate regime, according to the first comprehensive analysis of the issue.

The scientists found the additional 0.5C would lead to longer heatwaves, greater droughts and, in the tropics, reduced crop yield and all coral reefs being put in grave danger.

The global climate change deal agreed in Paris in December pledged to “hold the increase in the global average temperature to well below 2C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5C.” Vulnerable countries, such as low-lying islands, have warned that 2C of climate change would wipe their nations from the map.

Understanding the different impacts from 1.5C or 2C of warming has been made more urgent by the recent run of record temperatures, with 2014 and 2015 breaking long-term records and recent months smashing previous highs. In February, the global temperature was 1.34C above the average from 1951-1980, according to Nasa data.

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The new research was published in the journal Earth System Dynamics, and lead author Carl Schleussner, a scientific adviser at Climate Analytics in Germany, said: “We analysed the climate models used by the Intergovernmental Panel on Climate Change [and] considered 11 different indicators including extreme weather events, water availability, crop yields, coral reef degradation and sea-level rise. We found significant differences [between 1.5C and 2C] for all the impacts we considered.”

The researchers found: “For heat-related extremes, the additional 0.5C marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions.”

The analysis found that regional dry spells increased by 7% with 1.5C of warming but by 11% with 2C, while sea level rises by 10cm more in the hotter scenario. Some regions would be more affected than others with, for example, water availability in the Mediterranean falling by 9% under 1.5C of warming but 17% under 2C.

The research found crop yields might rise in some high-latitude regions, but “tropical regions like west Africa, south-east Asia, as well as central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize.”

Coral reefs, which provide vital nurseries for many fish on which people rely on for food, would be particularly affected by an additional 0.5C of warming. “In a [2C] scenario, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards.” This is reduced to 70% by 2100 for the 1.5C scenario, the scientists found.

Jacob Schewe, one of the research team and at the Potsdam Institute for Climate Impact Research in Germany, said: “Some researchers have argued that there is little difference in climate change impacts between 1.5C and 2C. Indeed, it is necessary to account for natural variability, model uncertainties, and other factors that can obscure the picture. We did that in our study, and by focusing on key indicators at the regional level, we clearly show that there are significant differences in impacts.”

Prof Nigel Arnell, at the University of Reading, UK, who was not involved in the research, welcomed the new study: “This study demonstrates that the impacts in 2100 are lower under a 1.5C world than under a 2C world and that the difference is greater for some sectors than for others. Impacts on heat extremes are most affected.

“The study also shows that the rate of change over time is really important for future impacts, so in order to really understand the differences between a 1.5C and a 2C world we need to run more comprehensive global climate models with lower rates of [warming] than used so far to see how the climate system responds.”