As world leaders gather at COP28, the annual climate change negotiations held in Dubai this year, one number will be front and center: 1.5 degrees Celsius (2.7 degrees Fahrenheit). That's the amount countries have agreed to limit warming to by the end of the century.

The world is already perilously close to that number. Since the Industrial Revolution, the planet has warmed by about 1.2 degrees Celsius, predominantly due to heat-trapping emissions that come from burning fossil fuels. This year is expected to be the hottest on record, with temperatures in September reaching 1.8 degrees Celsius above average.

Currently, the world is on track for just under 3 degrees Celsius of warming (more than 5 degrees Fahrenheit) by the end of the century. While a few degrees of difference may seem small, climate research shows that every tenth of a degree can have a profound effect when it comes to the dangers posed by extreme weather.

"We're not destined for some catastrophic climate," says Deepti Singh, who is an assistant professor at Washington State University. "We know that we can have a future that is more equitable and less volatile if we limit the warming through our actions today."

Here are three climate impacts that get substantially worse in the U.S. if the world exceeds 1.5 degrees Celsius of warming.

1. At 1.5 degrees of warming worldwide, the U.S. will heat up even faster

When scientists use numbers like 1.5 degrees Celsius to measure climate change, it represents an average of all the annual temperatures worldwide. That average masks the fact that some parts of the planet are heating up faster than others.

In fact, the U.S. is warming up at a faster rate than the global average, which means the effects of climate change will be more pronounced. That difference has to do with how the extra heat is absorbed, as well as regional weather patterns. Generally, warming is happening faster on land and in the polar regions.

"The U.S. has already warmed at a rate that's higher than the global average," says Singh. "We're warming at a rate that's 60% higher than that."

2. Rainfall intensifies beyond 1.5 degrees of warming

Hurricanes and tropical storms are getting more intense as the climate warms, but they aren't the only storms affected. Even regular rainstorms are getting more extreme.

"Every time we have a heavy rainfall event, it's more likely to be even heavier than what we're typically used to seeing," says Deanna Hence, assistant professor at the University of Illinois Urbana-Champaign. "So that can mean flooding and other risks that come with those really high rainfall rates."

The air in a hotter atmosphere can hold more water vapor, which can fuel heavier rainfall. That means that beyond 1.5 degrees Celsius of warming, cities in the Midwest and Northeast will be faced with dramatically more water, which can overwhelm storm drains and infrastructure, causing flooding. Today, most cities aren't designing their infrastructure to handle more intense rain.

3. Extreme heat gets worse, meaning more hot days and fewer cold ones

Heat waves take a massive and sometimes hidden toll in the U.S., causing cardiovascular and other health impacts, in addition to deaths. Climate science shows they're already getting longer and more intense.

Nighttime temperatures are also increasing, which exacerbates the effects of a heat wave. Humans, animals and plants need recovery time from extreme daytime temperatures. Without it, health impacts and crop losses are even greater.

As the planet warms, winters will also be affected. The number of days below freezing would shrink past 1.5 degrees Celsius of warming, especially in the Mountain West. That could impair the snowpack that provides a vital water source for millions of people. Warmer winters can also harm crops and increase vector-borne diseases.

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There's a number that will be a focus over the next two weeks - 1.5 degrees Celsius. World leaders have agreed that's the limit for how much the planet can warm before the extremes of climate change become insurmountable. But countries are not on track to meet that limit, and they'll discuss this at negotiations in Dubai that begin tomorrow.

So what would the U.S. look like if warming goes beyond that temperature? Lauren Sommer from NPR's climate desk is here to tell us. Lauren, so if the world goes past 1.5 degrees to 2 or 2.5 degrees Celsius, that difference might seem small on paper, and it sounds small when I just said it, but what would it actually look like on the ground?

LAUREN SOMMER, BYLINE: Right, yeah. I mean, half a degree kind of seems minor, but it makes a massive difference in terms of extreme weather in the U.S. and, you know, as a result, the cost to lives and property - because, you know, that number - 1.5 Celsius, which is 2.7 Fahrenheit - it's an average. It takes into account all the temperatures across the planet all year. But warming doesn't happen evenly, and the U.S. is actually heating up faster than that.

MARTÍNEZ: So does that mean if the planet goes beyond 1.5 degrees of warming, the U.S. would get hotter than that?

SOMMER: Yeah, exactly. So say the world reaches 3 degrees Celsius, which is 5.5 Fahrenheit. Parts of the U.S., like Alaska and northern states, would heat up much more - twice as much in some cases. And when it's hotter, that affects the severity of the weather, like extreme storms.

MARTÍNEZ: Yeah. And the U.S. has seen some very destructive hurricanes in recent years. Would that trend keep getting worse?

SOMMER: Yeah, hurricanes, tropical storms are getting more intense. But, you know, so are storms in general because a hotter atmosphere, it can hold more water vapor. I talked to Deanna Hence, an assistant professor at the University of Illinois Urbana-Champaign, and she says that means clouds can drop more rain.

DEANNA HENCE: Every time we have a heavy rainfall event, it's more likely to be even heavier than what we're typically used to seeing.

SOMMER: Hence says, you know, that could mean 30 to 40% more rain in the eastern U.S. from those extreme storms. And that can overwhelm storm drains and infrastructure, and that causes flooding even if you don't live next to a river.

MARTÍNEZ: Wow. I know the U.S. saw some pretty extreme heat waves this year. How much worse do you think those could get if the Earth warms, say, more than 1.5 degrees Celsius?

SOMMER: Yeah, right. I mean, that trend keeps going. So if the world warms 2 degrees Celsius, the southern U.S. could see more than 30 extra days above 95. That's a month more of days like that. And cold days start disappearing, too. The Mountain West could lose 20 to 30 days where it's below freezing.

MARTÍNEZ: Wow. All right, so world leaders meet this week to negotiate how to avoid a future like this. Is it inevitable, really, at this point that the Earth goes beyond 1.5 degrees Celsius?

SOMMER: Yeah, I mean, if countries don't change course. So if we keep burning fossil fuels at the same rate, it looks like the planet will go beyond 1.5 sometime in the next decade. You know, the window of time to avoid that is shrinking. But Deepti Singh, who is an assistant professor at Washington State University, says, you know, it's not too late.

DEEPTI SINGH: We have control over our future. We're not destined to some catastrophic climate. We know that we can have a future that is more equitable and less volatile if we limit the warming through our actions today.

SOMMER: She says every fraction of a degree matters to limit the impacts of climate change. You know, it's not all or nothing. So 1.6 is just as important as 1.5 degrees when it comes to the planet's future.

MARTÍNEZ: That's Lauren Sommer from NPR's climate desk. Lauren, thank you.

SOMMER: Thanks.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

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