Why Half a Degree Matters: Higher Seas, Longer Heat Waves, Dead Reefs

Limiting global warming to 1.5 degrees Celsius could dramatically curb water scarcity in the Mediterranean region, help save tropical reefs, allow more wheat to grow in West Africa, and significantly shorten heat waves, according to a new study by European researchers.

As global leaders have wrangled over how to prevent warming the planet more than 2 degrees, leaders in vulnerable Pacific Island and West African nations have argued that this goal was too lax. Last December, climate negotiators meeting in Paris agreed that the world should seek to limit warming to 1.5 degrees, although that goal would be tough to reach and the benefits were difficult to quantify. On Friday, heads of states of about 160 nations will sign the climate agreement.

The new research published Thursday offers what may be the most detailed glimpse yet of the difference half a degree can make.

Using multiple sets of climate models, the scientists examined nearly a dozen climate indicators–including sea-level rise, rice and soy production, and extreme weather events–to analyze how the two temperature scenarios would affect 25 regions around the globe. While the models don't agree on precisely what will happen, the researchers reported consistent trends suggesting that capping warming at the lower range could substantially reduce harm from climate change in many places. (See all of our Earth Day coverage here.)

"We have a bit of uncertainty at all levels of this–we can't tell you exactly what's going to happen at 2 degrees," co-author Jacob Schewe, at the Potsdam Institute for Climate Research in Germany. The study was published in Earth Systems Dynamics, a journal of the European Geosciences Union. "This is about testing whether or not there is a significant difference between the two goals. And the answer is yes, some robust differences emerge."

This new research was underway before the Paris talks. It's among some of the earliest to attempt to parse how different the world might look if global leaders hit the more stringent target.

"Two degrees comes with a lot of impact, but there was limited information about what happens at 1.5 degrees," said lead author Carl-Friedrich Schleussner, a science advisor to Climate Analytics, a policy institute in Berlin. "We hoped to help fill this gap."

What scientists found were global "hot spots," where substantial trouble could be minimized by limiting warming to 1.5 degrees.

For instance, in parts of Australia, Central America, northern Africa and southern Europe, supplies of fresh water could decline substantially if mean temperatures rise 2 degrees above what they were at the dawn of the industrial revolution. Curbing that temperature rise by half a degree could reduce the loss of water that feeds rivers and streams in all of those regions–by almost half in the region surrounding the Mediterranean Sea. These places already are struggling with water shortages.

"Even a small reduction in water in a country that already has a problem with water management is a pretty big deal," Schewe says.

Soybean yields in the Amazon region are projected to decline as temperatures rise to 1.5 degrees but the declines would be far worse at 2 degrees. Meanwhile, wheat production is likely to grow under both scenarios in high latitude regions, but plummet far more at 2 degrees in many equatorial regions, further exacerbating food scarcity.

Some scientists not affiliated with the research agreed the study revealed huge contrasts.

"Half a degree C may not sound like much, but it makes a world of difference in climate impacts," says Stanford University earth scientist Rob Jackson. "We'd have fewer and less severe droughts, floods, and heatwaves. Arctic systems and coral reefs would be healthier, too."

Between warming temperatures and ocean acidification almost all tropical coral reefs face the likelihood of severe degradation if temperatures rise 2 degrees. But a significant minority of these fish nurseries, which help support a quarter of the world's fisheries and provide food for millions of people, would stand a better shot at survival with a 1.5 degree temperature rise.

Meanwhile, significant sea level rise–which some fear may be almost unavoidable, given recent research about the instability of the West Antarctic ice sheet–could at least be slowed this century if warming is reduced to 1.5 degrees, allowing society more time to prepare for the changes to come.

"For me the relevant question really is: Is it worth it to go for 1.5 degrees instead of 2 degrees?" Schewe says. "And for me this study is one step toward answering that question."

Finding a way to reach that 1.5 degree target, however, is another issue entirely. Many scientists have suggested that capping warming at 2 degrees would require a monumental effort, including removing CO2 from the atmosphere. Research by Jackson suggests that the threshold to prevent a rise beyond 1.5 degrees may be reached within a decade.

"The policy window for limiting warming to 1.5 C is closing," Jackson says. "The Paris agreement is a good start, but there's a lot more work to be done, and it needs to happen quickly."

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