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The National Oceanic and Atmospheric Association (NOAA), NASA and Copernicus—the EU’s meteorological association—released their annual global temperature analyzes (Friday). They all found that The Earth has warmed by about 1.5 degrees Celsius
NOAA Reports 1.46 degrees C of warming, NASA, 1.47; and the EU Copernicus, 1.6.
The 1.5C number gained prominence a decade ago. In the 2015 Paris Climate Agreement, most countries committed to trying to limit global warming below 2 degrees 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.
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 ways, the heat was surprising, because it was even more extreme than he and many other scientists expected and than the models had predicted.
It was in mid-2023 that scientists began looking at temperature data with concern.
“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 broke even more records. “And then September was, as I said at the time, gobsmacking bananas—” nearly a full degree Fahrenheit above previous records, an enormous margin.
That number sounds small, Hausfather says. But “that’s the amount the planet typically warms in about a decade,” he says, which is far from insignificant.
El Niño cycles
The first hypothesis? Maybe the heat was an outcome of El Niño.
“So it’s hard to blame El Niño for events that happened before El Niño even really started,” says Schmidt.
A home is engulfed in flames during the Eaton fire in Southern California in January 2025. Multiple wildfires are raging across the Los Angeles region. Climate change has likely made conditions suitable for these fires more likely.
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JOSH EDELSON/AFP via Getty Images/AFP
Volcanoes?
Typically, flares throw gases and particles into the air that reflect sunlight back into space and help cool the Earth. But the . So it shot tons of water vapor high into the atmosphere, which can trap heat.
Ships and clouds
The following ideas focused on what’s in the air: clouds. They can have a huge impact on global temperatures.
The white clouds reflect incoming sunlight, thereby cooling the planet. But cold clouds also act as a blanket, trapping heat from Earth’s surface. So changes in the type of clouds, or in their behavior or presence, can impact Earth’s temperatures.
In 2020, international rules governing fuels for the maritime sector 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. Since the climate system does not respond instantly, pollution reductions started in 2020 could have started to have an impact in 2023, to the tune of about 0.1°C, or about half of the total mystery heat.
The scale isn’t enormous, compared to the overall impact of human-driven global warming, says Andrew Gettelman, a scientist at the Pacific Northwest National Laboratory. But it’s not nothing. “That’s probably about 10 percent of the global warming we expect over the next decade,” Gettelman says.
A December study in Science took an even wider look at the clouds. Overall, according to the study, cloud cover – and the bright white reflectivity it often brings – has declined in several key regions of the globe over the past decade, and particularly sharply in 2023. The overall effect, the authors calculated, could amount to about 0.2°C of additional warming, almost exactly the size of the gap between climate models and actual average global temperatures.
It’s not yet clear what might be causing this change in cloud behavior, Goessling says. Cloud changes related to shipping are likely a part of this.
But other researchers are exploring another possibility focused on pollution. Sulfate pollution levels in China have motivated by the new air pollution policy in the country. With less pollution, there are fewer cores on which water droplets aggregate to form clouds – and therefore fewer clouds, both over land and downwind from the ocean, emit the researchers’ hypothesis.
The key question, Schmidt says, is understanding whether the changes in clouds are part of a natural variation – something like El Niño, an effect that will reverse itself – or a deeper change and fundamental caused by human-caused climate change.
But regardless, the heat impacts, while significant, pale in comparison to the climate damage caused by burning fossil fuels, Dessler says.
“Don’t get distracted by year-to-year variability,” he says. “As long as we put greenhouse gases into the atmosphere, the climate will warm. And that will have enormous impacts on people’s lives.”