THE NUMBERS ARE in: 2020 tied with 2016 for the hottest year on record, a clear sign of a planet in distress.

The results are not surprising. Despite a seven percent drop in carbon emissions from fossil fuels in 2020, driven by COVID-19-related economic disruptions, humans still added some 40 billion metric tons of carbon dioxide to the atmosphere, on top of the trillions of tons we’ve put there already since the 19th century. Under that sustained pressure, global average temperatures continued to rise.

Incoming U.S. president Joe Biden has pledged to tackle the climate crisis head-on as soon as soon as he takes office on January 20. He has promised to rejoin the Paris Agreement, cancel the Keystone XL pipeline, and adopt an ambitious program to set U.S. emissions on a steadily declining path. As a new era in American climate policy begins, here are six numbers that define the challenge.

1.25 °C DEGREES THE EARTH HAS WARMED

Or 2.25°F. That’s how much higher Earth’s average temperature was in 2020 compared to the late 1800s, according to a comprehensive European analysis. Different estimates, from NASA, NOAA, the UK Met Office, and others, say it might even be as high as 1.29°C (2.32°F).

That 2020 was as hot as 2016 was particularly alarming because in 2016 the planet was in the grips of a powerful El Niño event, which is known to temporarily bump global temperatures up by a few degrees. In 2020, a mild La Niña (El Niño’s counterpart, which usually cools the planet slightly) developed late in the year—but even that couldn’t offset the extra greenhouse effect of additional carbon.

“We’ve added a strong El Niño’s-worth of warmth since 2016,” says Zeke Hausfather, a climate scientist at the Breakthrough Institute and who worked on the Berkeley Earth annual temperature estimate.

In 2015, nearly all of the world’s nations signed the Paris Agreement, pledging to keep planetary warming “well under” 2°C (3.6°F) above pre-industrial levels and aiming to keep it below 1.5ºC (2.7º F). At the rate we’re going—currently about 0.2ºC per decade—Earth is on track to exceed both those goals, not just for a single year but consistently, within a few decades.

945 Gigatons - CARBON EMISSIONS LEFT IN THE BUDGET TO STAY UNDER 2C

That’s how much carbon dioxide remains in the “carbon budget” that would give Earth a 66 percent chance of warming less than 2ºC above pre-industrial temperatures. Last year, even with the effect of COVID-19 lockdowns, the world emitted 34 gigatons (billion tons) of CO2 from fossil fuels, down from just over 36 gigatons in 2019. Another six gigatons or so from land use change (deforestation, for example) brought the total to about 40 gigatons. At that rate, we’d run through the entire remaining budget in less than 25 years.

And the budget is even tighter to keep to the 1.5-degree target. For a 50 percent chance of hitting it, the emitted number plummets to 355 gigatons. For a 66 percent chance, it’s only 195 gigatons—meaning the world would have to reduce all emissions to net-zero within years, or quickly develop technologies that can effectively pull carbon dioxide back out of the air.

The budget is calculated for the whole planet, but just a few countries have been responsible for most of the emissions that have warmed the world up so far. The United States, for instance, has pumped out a quarter of all carbon dioxide emitted since 1751. There is no international consensus about how much of the remaining global budget each country can use up, but experts stress that developed countries need to get to net-zero emissions as quickly as possible. Biden’s climate plan aims to put the U.S. on track to hit that goal before 2050.

22 NUMBER OF BILLION-DOLLAR CLIMATE-RELATED DISASTERS IN U.S. IN 2020

The number of major weather-and-climate disasters set a record in the U.S. in 2020: 22. That’s much higher than the long-term average of seven per year and even more than 16 that occurred during the previous record-setting years, 2017 and 2011.

Climate change is already changing the character, intensity, and risk from many different types of natural hazards. In many cases, scientists can pinpoint its fingerprint. In 2020, for instance, it primed four million acres of California to burn and contributed to a historic hurricane season.

Climate change tends to slow hurricanes down and supercharge them with moisture, causing them to dump unprecedented amounts of rain— storms like Hurricane Harvey, for example, which flooded Houston and helped make 2017 the record year for total climate- and weather-related damages in the U.S. Recent research shows that climate change-induced shifts in rain and snowfall contributed about one-third of the total flood damages in the U.S. between 1988 and 2017, over $70 billion worth.

“That’s billions of dollars a year in damages over the last three decades that have been caused by these changes in precipitation,” says Noah Diffenbaugh, a climate scientist at Stanford University—and that number is only going to increase until climate change is controlled.

2035 YEAR WHEN ARCTIC SEA ICE MAY VANISH IN THE SUMMER

Arctic sea ice has been dwindling and thinning for decades. In the not-too-distant future the Arctic Ocean could be ice free in summer for the first time in 2 million years or so—a potential “tipping point” in the region that could reshape the entire planet.

What happens in the Arctic doesn’t stay in the Arctic: a warmer pole can affect weather as far away as Antarctica by influencing major atmospheric and oceanic circulation patterns. It can also trigger feedback loops that hasten warming. In one, warm, snowless summers can speed permafrost defrosting, which releases potent greenhouse gases into the atmosphere.

The Arctic has warmed more quickly than anywhere else on the planet—about 3ºC since 1900—and it has accelerated in recent decades to four times the global average. 2020 fit all too neatly into that dramatic warming trend. Siberia baked and burned in a nearly six-month-long heat wave that, at its height, drove air temperatures above 100 degrees Fahrenheit. Summer Arctic sea ice dropped to its second-lowest coverage ever.

Other “tipping points,” like the collapse of the Greenland or West Antarctic ice sheets or the failure of plants to suck up more carbon than they release, hover uncomfortably close in our future if global warming continues unabated.

28 % ELECTRICITY THE WORLD GETS FROM RENEWABLES

Analysts at the International Energy Agency project that renewables will soon outstrip coal and natural gas as electricity sources, becoming the largest source of energy globally by 2025. Contrary to predictions early in the COVID-19 pandemic, installation of renewable power projects—primarily solar and wind—soared in 2020, and the IEA forecasts continued growth.

Will it come fast enough? To meet net-zero emissions targets, renewable electricity will have to almost completely supplant coal and gas by mid-century—and also gasoline, as we switch to electric cars. Many countries, from China to South Korea to Spain, are ramping up their renewable power capabilities—and their ambitions—dramatically. The U.S. will need to as well: In 2019, it still generated only 17.6 percent of its electricity from renewables, less than the global average of 28 percent.

51 % AMERICANS UNDER 45 WHO SAY CLIMATE CHANGE HAS INFLUENCED THEIR DECISION ABOUT WHERE TO LIVE

That’s the percentage of Americans under the age of 45 who have taken climate change into account when deciding where to live, according to a recent National Geographic and Morning Consult poll. Men are more likely to consider climate change than women, liberals more than conservatives, and young people more than old.

Recognizing that climate change is real, happening now, and affecting peoples’ lives personally, enough to influence their choice about where they live—that’s a big deal, says Jennifer Marlon, a researcher at the Yale Center for Climate Communication. Her team has conducted surveys since 2010 asking Americans about their understanding of and opinions about climate change.

“In the most recent five years, things have really started to move,” she says. Most notably, she says, in her team’s polls, the number of people who are “alarmed” by climate change—those who are likely to take the issue seriously, organize politically, and push policy changes forward—has doubled since 2015.

Changes like those, Marlon thinks, will propel climate action forward—and that, in turn, will help bring along many Americans who are still only slightly or moderately interested in climate change. As a new president takes office with an ambitious climate agenda, we may soon learn whether she’s right.