Last December in Paris, 195 nations agreed to slow the planet’s warming trend by cutting their greenhouse gas emissions. Their goal was to prevent more than 2°C of additional warming. But even before the meeting, scientists questioned whether that target was too high. A study presented in Vienna today at the European Geosciences Union’s (EGU's) annual meeting backs up those concerns, providing new evidence that such warming could still lead to catastrophic droughts and sea level rise. But reducing the threshold by just half a degree, to 1.5°C, the scientists say, would make a world of difference.
To conduct the study, Carl Schleussner, a scientific adviser at Climate Analytics in Berlin, and an international team of researchers analyzed 11 different indicators of climate impacts, including extreme weather events, water availability, crop yields, coral reef degradation, and sea level rise for 26 nonpolar areas around the world. To do this, they compiled simulations from a wealth of existing climate models, including those used by the Intergovernmental Panel on Climate Change and an ensemble of general ocean and atmosphere circulation models.
The results of these simulations confirmed that areas already known to be vulnerable to climate change will be at even greater risk if the planet warms by 2°C than by 1.5°C, the team reports in the study, also published today in Earth System Dynamics. For example, warm spells in the tropics would last up to 50% longer—so that virtually all coral reefs in warmer tropical waters would be at risk of severe degradation after 2050. Under a 1.5°C warming scenario, the threat to those reefs wouldn’t be significantly different before 2050—in both situations an expected degradation of the tropical coral ecosystems is likely to begin by 2030 regardless. But more of them—about 30%—would survive until 2100.
“Some researchers have argued that there is little difference in climate change impacts between 1.5°C and 2°C," said study co-author Jacob Schewe, a climate physicist at the Potsdam Institute for Climate Impact Research in Germany. He acknowledges that certain factors such as natural variability and model uncertainties need to be accounted for because they 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 between 1.5°C and 2°C,” he added in a statement to the press.
Certain places are particularly vulnerable, Schewe says. "There are tipping points in the system such as the Greenland ice sheet where if it were to melt, the mechanisms there are irreversible." Scientists can’t say with certainty where that tipping point is exactly, he adds—but they estimate that it’s somewhere between 1.5°C and 2.5°C. “So going for 1.5°C is much safer.”
The paper cites a number of ways to assess the significance of an extra half-degree of warming. Crop yields of maize and wheat in Central America and West Africa, for example, would dwindle by twice as much under 2°C, when compared with 1.5°C, the team found. And sea levels would rise an extra 10 centimeters by 2100 (because of both the physical expansion of ocean waters as they warm, as well as water added from melting ice sheets). “Sea level rise will slow down during the 21st century only under a 1.5°C scenario,” Schleussner explained.
Slowing sea level rise also has a significant impact on regions frequently inundated by storm surges due to tropical storms, such as the Philippines. "Typhoon Haiyan in 2013 set a new benchmark for storm surges," says geologist Janneli Lea Soria of the Earth Observatory of Singapore, who was not involved in the new study. A poster she presented at EGU this week showed that the typhoon pushed beach and ocean sediments inland by nearly 2 kilometers in some locations.
In the Mediterranean, 2°C warming would cut water availability by 20% by the late 21st century, compared with a 10% reduction from 1.5°C warming, the team found. A separate study, also presented today at EGU by paleoclimatologist Joel Guiot of the European Center for Geoscience Research and Education in Aix-en-Provence, France, puts that into historical context. Looking back over the last 10,000 years of pollen data for the Mediterranean, Guiot found that severe drought has changed vegetated areas in the region by 10% to 15%—often pushing margins of the land cover in other directions.
Projecting forward, Guiot found little difference in changes to vegetation cover between 1.5°C and 2°C until 2050. After that, however, a 1.5°C threshold keeps land changes in the realm of droughts seen in the past, Guiot says. NASA identifies the current drought in the Mediterranean as the worst in 900 years. "But if we go to 2°C, we see we are at the maximum range of change seen during the Holocene [dating back about 10,000 years]. And if we go to 3°C we will reach a situation that never existed [in the Holocene] before."
On 22 April—Earth Day—the climate agreement signed in Paris will be available for the signatures of heads of state around the world at the United Nations's headquarters in New York City for 1 year. At least 55 countries representing 55% of the world's emissions need to ratify the final agreement that came out of the December 2015 climate conference in order to see it put into force. That agreement reflects the urging from Pacific island nations and others calling themselves “most vulnerable” to climate change to aim for a more stringent target of 1.5°C, “recognizing that this would significantly reduce the risks and impacts of climate change."
For most of the indicators, “It's not the timing, but the level of warming,” Schewe says. “And as soon as we reach that warming we see the impacts associated with it.”