21 April 2016

1.5°C vs 2°C: Why half a degree matters

An average global temperature rise of 1.5°C or 2°C over the coming century would have substantially different impacts on the planet, according to a new study.

Coral bleaching is one of the projected impacts of climate change. © ead72 - Fotolia | Adobe Stock Photo

Coral bleaching is one of the projected impacts of climate change. © ead72 - Fotolia | Adobe Stock Photo

The additional 0.5°C would mean a 10-cm-higher global sea-level rise by 2100, longer heat waves, and greater risk of killing off tropical coral reefs. The research was published today in the open access journal Earth System Dynamics

“We found significant differences for all the impacts we considered,” says the study’s lead author Carl Schleussner, a scientific advisor at Climate Analytics in Germany. The researchers analyzed the climate models used in the IPCC Fifth Assessment Report, focusing on the projected impacts at 1.5°C and 2°C warming at the regional level. They considered 11 different indicators including extreme weather events, water availability, crop yields, coral reef degradation and sea-level rise. 

Average global temperature rise of 1.5°C and 2°C does not mean that temperature will rise equally around the globe. Rather, different regions are expected to see different levels of warming, with temperatures rising faster over land and in the Arctic.   

“At the Paris Climate Summit the world decided to try to limit warming to below 1.5°C, in part because many considered climate impacts at 2°C to be too risky. Our new study provides essential new information about the risks under these two warming levels, and will feed into the ongoing international climate talks, helping policymakers deciding on the priority and urgency of climate action,” says IIASA researcher Joeri Rogelj, who contributed emissions scenarios for the assessment of sea-level rise and global coral reef risks to the study. He says, “Until recently, it was impossible to find any systematic information on the impact of an additional half degree of warming. Our study helps fill that information gap.” 

The team, with researchers from Germany, Switzerland, Austria and the Netherlands, identified a number of hotspots around the globe where projected climate impacts at 2°C are significantly more severe than at 1.5°C. One of these is the Mediterranean region, which is already suffering from climate change-induced drying. With a global temperature increase of 1.5°C, the availability of fresh water in the region would be about 10% lower than in the late 20th century. In a 2°C world, the researchers project water availability to decrease as much as 17%.  

In tropical regions, the half-degree difference in global temperature could have detrimental consequences for crop yields, particularly in Central America and West Africa. On average, local tropical maize and wheat yields would reduce twice as much at 2°C compared to a 1.5°C temperature increase. In other regions and at a global scale, these numbers can differ for various crops and aggregated differences can become both smaller and larger.  

Tropical regions would bear the brunt of the impacts of an additional 0.5°C of global warming by the end of the century. Heat waves in the tropics are projected to be affected at about twice the global rate, and could last up to three months in a 2°C world, compared to two months at 1.5°C, the researchers say.  

Average change in extremely high maximum temperatures (left) and long warm periods (right), at 1.5 and 2 degrees average global temperature rise. The bottom panel shows the difference between the two scenarios. © Schleussner et al (2016)


The additional warming would also affect tropical coral reefs. Limiting warming to 1.5°C would provide a window of opportunity for some tropical coral reefs to adapt to climate change. In contrast, a 2°C temperature increase by 2100 would put virtually all of these ecosystems at risk of severe degradation due to pervasive and persistent coral bleaching.  

On a global scale, the researchers anticipate sea level to rise about 50 cm by 2100 in a 2°C warmer world, 10 cm more than for 1.5°C warming. More importantly, however, is the rate at which sea level continues to rise in 2100. In a 1.5°C warmer world, this rate is about 30% lower than in a 2°C world, reducing our commitment to long-term sea-level rise.  

This study is being presented at a press conference at the European Geophysical Union General Assembly in Vienna today. 

Reference
Schleussner C, Lissner TK, Fischer EM, Wohland J, Mahe P, Antonius G, Rogelj J et al. (2016). Differential climate impacts for policy-relevant limits to global warming: The case of 1.5°C and  2°C. Earth System Dynamics


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Last edited: 21 April 2016

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