Climate: Why a half-degree matters

Study shows why Paris climate goals are important

global temperature map March 2016
The average global temperature spiked to yet another record in March 2016.

By Bob Berwyn

VIENNA — One-half degree Celsius may barely register on a backyard thermometer, but when it comes to temperatures on a global scale, it can make all the difference in the world, according to a new study that examined the relative impacts of 1.5 degrees Celsius global warming versus 2 degrees Celsius.

At issue is the worldwide climate-change target set late last year in Paris under the COP21 agreement. The deal, now signed by more than 175 countries, aims to cap global warming somewhere between 1.5 and 2 degrees Celsius above the pre-industrial level by drastically cutting greenhouse gas emissions. The new research shows why many scientists are pushing for the lower target.

A temperature increase of 2 degrees Celsius will have a much bigger impacts to the world’s food and water supplies, as well as to ecosystems like tropical reefs, the researchers said in late April, releasing their findings during a press conference at the European Geosciences Union conference in Vienna. Austria. The study examined 11 different indicators, including extreme weather events, water availability, crop yields, coral reef degradation and sea-level rise.

“We found significant differences for all the impacts we considered,” said lead author Carl Schleussner, a scientific advisor at Climate Analytics in Germany. Climbing temperatures won’t play out evenly around the globe. It will get warmer faster over land, in the Arctic and around the equator, Schleussner said.

“Our study shows that tropical regions, mostly developing countries that are already highly vulnerable to climate change, face the biggest rise in impacts,” said Climate Analytics senior scientist and CEO William Hare. The findings support assertions by least developed countries that limiting warming to 1.5 degrees Celsius is critical, Hare said.

In some regional hotspots, the difference between 1.5 and 2 degrees Celsius would mark the difference between conditions that (barely) within the margins of historical observations to completely new climate regimes.

In the tropics, heat waves will be affected at twice the global rate, lasting up to three months in a world with 2 degrees Celsius of warming, compared with two months at 1.5 degrees. That will affect regional food supplies. Yields of important food crops like maize and wheat would be reduced twice as much with an increase of 2 degrees Celsius compared to a 1.5 degree increase.

“There is a big change in annual water availability for irrigation and residential sectors, especially for dry subtropical regions like Central America and South Africa,” said Michiel Schaeffer, also a researcher with Climate Analytics. “And if you look at heavy precipitation days, we see … a significant increase between 1.5 and 2 degrees for high latitudes as well as monsoon regions regions,” Schaeffer said.

In the Mediterranean region, which is already drying up due to climate change, water availability would decrease by as much as 17 percent with 2 degrees Celsius warming; with 1.5 degrees warming, the decline in water supplies would be limited to about 10 percent by 2100.

And the difference will be decisive for coral reefs. With a 2 degree Celsius rise in the average global temperature by 2100, nearly all tropical reefs will be devastated by coral bleaching as soon as 2050. If warming is limited to 1.5 degrees Celsius, 30 percent of reefs could have a fighting chance to adapt to climate change.

“It’s a very bleak picture, where 90 percent of coral reefs would already be at risk under 1.5 degrees by 2050 and almost 100 percent at 2 degrees,” Schaeffer said.

But limiting warming to 1.5 degrees Celsius “offers some kind of a window for adaptation. If resiliency is at the upper edge of current estimates, than coral reefs might hang out at many locations to the end of the century and beyond,” he said.

The scientists involved in the study said their findings highlight why the discussions about limiting global warming are important. A 2 degree Celsius increase is too risky, according to Joeri Rogelj, a researcher with the International Institute for Applied Systems Analysis in Vienna who focused on coral reefs and sea level rise for the study.

“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,” Rogelj said. The findings will help policy makers decide on the priority and urgency of climate action, he said, adding that limiting warming to 1.5 degrees Celsius would also slow the rate of sea level rise by 30 percent after 2100.

Climate tipping points?

For now, the world is not even close to being on pace to achieving the 1.5 degree Celsius goal, University of Massachusetts climatologist Rob DeConto said at the conference. Based on on the various national goals now on the table, the outcome is more like 2 to 2.7 degrees Celsius warming—and that could spell disaster for Antarctica’s ice sheets, which could melt fast enough to raise sea level 1 meter by 2100, DeConto said.

But if the world lives up to its stated intent to limit global temperature rise to 1.5 degrees, it would make a huge difference. Under that scenario, the Antarctic ice shelves would remain fairly stable, DeConto said.

“So that’s the good news. You could think of this as policy still having a very, very critical role to play in all of this,” he said, contrasting the new findings with earlier IPCC reports that likely underestimated Antarctica’s potential contribution to sea level rise.

“The problem is, the 2.7 degree increase, where things stand now, we get a very big response, it’s 80 centimeters or so of potential sea level rise. So there’s threshold behavior in the Antarctic ice sheet system. We don’t know exactly where that line is is, but somewhere between 2 and 2.7 degrees makes a very big difference in terms of future sea level trajectories,” he said, adding that it would take millennia for the ice to heal itself.

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2 thoughts on “Climate: Why a half-degree matters

  1. Reblogged this on GarryRogers Nature Conservation and commented:
    Such temperature increases have drastic consequences for terrestrial species. Because of the natural decline of temperature with altitude, a rise of 1.5 degrees Celsius is equal to a 500 ft increase in altitude. The change increases the optimum altitude for many plant and animal species. Making the climb in decades or even centuries is not possible for most plants and the animals that depend on them.

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