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Here's what happens if the world warms more than 1.5 degrees Celsius

ARI SHAPIRO, HOST:

There's one number you hear a lot at the international climate talks in Glasgow, Scotland.

(SOUNDBITE OF MONTAGE)

UNIDENTIFIED PERSON #1: One-point-five degrees.

PRESIDENT JOE BIDEN: To just 1.5 degrees Celsius.

UNIDENTIFIED PERSON #2: To 1.5 degrees.

SHAPIRO: Nations have agreed to limit warming to 1.5 degrees Celsius by the end of the century, but right now they're not close to reaching that goal. So why is that number so important, and what happens if the world gets hotter than that? Well, NPR's Lauren Sommer is here to explain. Hey, Lauren.

LAUREN SOMMER, BYLINE: Hi, Ari.

SHAPIRO: I remember covering the Paris Climate Summit in 2015, people chanting 1.5 to stay alive. How did scientists and the world's nations settle on that particular number?

SOMMER: Yeah, what the science shows is if there's warming above that level, the planet sees impacts that become, you know, catastrophic for communities. So, yeah, back in 2015, in Paris, countries agreed to limit their heat-trapping emissions to that goal, which is also 2.7 Fahrenheit. Before that, the goal was actually 2 degrees Celsius, but developing countries really pushed for the lower number because they're the most vulnerable to the impacts of climate change. And the climate has already warmed about 1.1 degrees Celsius, so there's not much wiggle room left.

SHAPIRO: Let's talk about what that means in practical terms. World leaders have described irreversible damage when they meet in Scotland. So if the planet does get hotter than 1.5 degrees Celsius, what does irreversible look like?

SOMMER: Yeah, coral reefs are probably the clearest example of that kind of risk. I spoke to Ove Hoegh-Guldberg, who studies coral at the University of Queensland. He's watched as the Great Barrier Reef has been hit by repeated marine heatwaves in recent years.

OVE HOEGH-GULDBERG: Something around 50% of the shallow-water corals were killed literally over a couple of months, and in some cases, over a couple of weeks.

SOMMER: Those heat waves will get more intense as temperatures rise. At 1.5 degrees Celsius, it's likely that 70- to 90% of coral reefs will die worldwide. But at 2 degrees of warming, 99% are lost.

HOEGH-GULDBERG: We really are going down a pathway where there will be no return.

SOMMER: And, you know, coral reefs aren't just vacation spots. You know, around 500 million people depend on them for both food and livelihoods.

SHAPIRO: Another impact we've seen in the U.S. this year is extreme storms. Dozens of people lost their lives during floods in Tennessee and New Jersey. Do those storms get even more extreme and more common above 1.5 degrees?

SOMMER: They do, and sometimes the hard thing to understand is just how much worse they get because a hotter atmosphere holds more water. So hurricanes get more intense and storms can dump more rain. But it's not that twice the warming makes things only twice as bad. Gabriel Vecchi, who studies extreme weather at Princeton University, says it'll be a lot more than that.

GABRIEL VECCHI: Now more and more things that are unheard of or have been unheard of will become relatively commonplace, and our built environment is not adapted to those events.

SOMMER: Heat waves are actually a good example of that. You know, at 2 degrees Celsius of warming, an extreme heat wave is around 14 times more likely.

SHAPIRO: So countries have been making a lot of promises this week about new cuts to greenhouse gas emissions. How close are they to that 1.5-degree target?

SOMMER: Yeah, so far, the plans don't get there. The world is still on track for around 2 degrees Celsius of warming, and that's if countries actually follow through. And there's a new report out today that shows if the world keeps emitting greenhouse gases at the current rates, there's only 11 years left until we've added enough to the atmosphere to cause that 1.5 degrees of warming. But it's important to say, you know, 1.5 isn't a magic number. There still will be major impacts at that level of warming. But the flip side of that is that every fraction of a degree of warming that the world can avoid also really matters in preventing some of these really destructive impacts.

SHAPIRO: That's NPR climate correspondent Lauren Sommer. Thank you.

SOMMER: Thanks. Transcript provided by NPR, Copyright NPR.