2024 is officially the hottest year ever recorded on Planet Earth, at least while humans have been around, according to the official tallies from meteorological organizations around the world.
The National Oceanic and Atmospheric Association (NOAA), NASA and Copernicus—the EU's meteorological association—released their annual global temperature analyses [Friday]. They all found that Earth has warmed roughly 1.5 degrees Celsius (2.7 Fahrenheit) above temperatures in the 1800s, before people began burning vast reserves of fossil fuels.
The numbers vary slightly. NOAA reports 1.46 degrees C of warming, NASA, 1.47; and the EU's Copernicus, 1.6.
"The real punchline is, it was another really warm year," says Russell Vose, a climate scientist at NOAA's National Center for Environmental Information, the group that produces the temperature estimates.
The 1.5C number gained prominence a decade ago. In the 2015 Paris Climate Agreement, most countries pledged to try to limit global warming to below 2 Celsius, and ideally no more than 1.5. Then, in 2018, scientists published a major report warning that surpassing that 1.5 C level of warming would greatly increase the risks of longer and more intense heat waves, more destructive hurricanes, and drastic losses of biodiversity.
Scientists use a long-term average to determine total warming, so this single year beyond the 1.5 level doesn't signal that the Paris Agreement numbers have been breached—but they say it's an ominous sign. The planet is projected to warm to about 3 degrees Celsius (5.4 Fahrenheit) above pre-industrial temperatures by the end of the century without further, major action to cut planet-warming emissions, according to a recent international report on climate change.
"This isn't even the new normal—this is halfway to the new normal," says Clair Barnes, a climate scientist at Imperial College London.
The 2024 record comes on the heels of 2023, which was itself a record-smasher.
In some ways, the extreme hot temperatures from the past two years are not surprising at all, says Gavin Schmidt, a climate scientist at NASA's Goddard Institute for Space Studies: an increasingly hot planet is the well-forecasted outcome of burning vast amounts of fossil fuels.
But in other respects, the heat was surprising, because it was even more extreme than he and many other scientists expected and models had predicted.
A double whammy of warming and a mystery brewing
It was partway through 2023 when scientists started looking at temperature data with alarm.
"When it started getting weird was around June and July of the summer," says Zeke Hausfather, a climate scientist with the organization Berkeley Earth. July 2023 crushed all previous heat records from that month. Then August beat records by even more. "And then September was, as I said at the time, gobsmacking bananas—" nearly a full degree Fahrenheit above previous records, an enormous margin.
In total, 2023 ended up at least nearly 1.5 C (2.7 F) degrees above pre-industrial temperatures.
"That was not only a record, it was a record by a record-breaking margin," Schmidt says.
Climate models suggest Earth should have heated up about 1.3 C by now, because of fossil fuel burning and other human-driven disruptions to the planet. That leaves about 0.2 degrees of warming, beyond the models' predictions, unaccounted for.
That number sounds small, Hausfather says. But "that's the amount that the world usually warms in about a decade," he says—far from insignificant.
El Niño cycles
So scientists went digging for answers to the mystery heat, tapping ideas from volcanoes to the sun to the clouds floating overhead.
The first hypothesis? Maybe the heat was an outcome of El Niño.
El Niño and La Niña events are part of a natural climate cycle that can influence weather across a broad swath of the planet. During an El Niño phase, which occurs every few years on average, global temperatures tend to be higher overall, while cooler global temperatures generally prevail during the La Niña phase.
Earth switched into an El Niño phase in mid-2023 and stayed there through spring of 2024. So, Schmidt says, it likely contributed to 2023's record. But it couldn't explain all of it: the timing was wrong. 2023 started off in a La Niña state and was still in that cooler phase when the heat began breaking records in June of that year.
"So it's hard to blame the El Niño for things that happened before the El Niño even really started," Schmidt says.
It likely did contribute to the heat in 2024, even as the El Niño effect faded later in the year, Schmidt says.
Volcanoes?
Another hypothesis involved a volcanic eruption.
Typically, eruptions shoot gases and particles into the air that reflect sunlight back into space and help cool the Earth. But the Hunga Tonga-Hunga Ha'apai volcano, which erupted in 2022, was underwater. So it shot tons of water vapor high into the atmosphere, which can trap heat.
Some scientists hypothesized that the warming effect could have contributed to some of the mystery heat. But after close study, scientists realized the impact was probably minimal.
"People talked about that a lot but our best guess is that it had an impact of zero," says Andrew Dessler, a climate scientist at Texas A&M University.
Ships—and clouds
The next ideas focused on what's up in the air: clouds. They can have an enormous impact on global temperatures.
White clouds reflect away incoming sunlight, cooling the planet. But cold clouds also act like a blanket, trapping heat from to the Earth's surface. So changes in cloud type, or cloud behavior or presence, can have an impact on Earth's temperatures.
In 2020, international rules governing the fuels for the shipping industry changed. The old fuel was heavy in sulfur; once in the atmosphere, sulfate pollution attracted water droplets, causing visible cloud plumes to trail behind a ship chugging across the ocean.
The newer, cleaner fuel produces less sulfate pollution—and fewer, smaller cloud plumes. When scientists did the math, they realized those ship trails had been common and reflective enough to cool down the planet. Because the climate system doesn't respond instantaneously, the reductions in pollution set in motion in 2020 could have started having an impact in 2023—to the tune of roughly 0.1 C, or about half the total mystery heat.
The scale isn't enormous, compared to the overall impact of human-driven global warming, says Andrew Gettleman, a scientist at the Pacific Northwest National Laboratory. But it's not nothing. "It's probably about 10% of the global warming that we would expect over the next decade," Gettleman says.
A December study in Science took an even wider look at clouds. Overall, it found, cloud cover—and the bright white reflectivity they often bring—has dropped in several key parts of the globe over the past decade, and particularly strongly in 2023. The overall effect, the authors calculated, could add up to about 0.2 C of extra warming—almost exactly the size of the gap between climate models and actual average global temperatures.
"It's very clear that the clouds and specifically the low level clouds are playing the dominant role," says Helge Goesseling, a climate scientist at the Alfred Wegner Institute in Germany and the lead author of the new study.
It's not yet clear what might be causing the shift in cloud behavior, Goessling says. The shipping-related cloud changes are likely a component.
But some other researchers are poking around another pollution-focused possibility. Sulfate pollution levels in China have dropped precipitously since 2013, driven by new air pollution policy in the country. With less pollution, there are fewer nuclei on which water droplets aggregate to form clouds—and therefore fewer clouds, both over land and the ocean downwind, researchers hypothesize.
The key question, Schmidt says, is understanding whether the cloud changes are part of natural variation—something like El Niño, an effect that will revert on its own—or a deeper, fundamental change brought on by human-caused climate change..
But either way, the heat impacts, though meaningful, pale in comparison to the climate damage done by burning fossil fuels, Dessler says.
"Don't get distracted by year-to-year variability," he says. "As long as we're dumping greenhouse gas into the atmosphere, the climate's going to get warmer. And that's going to have enormous impacts on people's lives."
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