2023 was extremely hot. Then came 2024

2023 was a chart-topper. A surge in global temperatures made it the hottest year since record-keeping began in the mid-1800s, producing heat that one scientist called "gobsmackingly bananas."

Then came 2024.

Scientists say this year is almost certain to take over the top spot as the hottest year. The global average temperature could potentially breach a key threshold, reaching 1.5 degrees Celsius (2.7 degrees Fahrenheit) above the pre-industrial average. Countries have agreed to try to limit warming to below that level in order to avoid much more intense storms, rainfall and heat waves.

The record-setting build up of heat has surprised scientists, setting off a climatic whodunit.

Some drivers are clear. A primary cause is the continued burning of fossil fuels, compounded by the natural El Niño climate pattern, which raised temperatures globally.

But those factors alone may not account for the full surge in persistent heat seen this year, scientists say. They're now exploring what other factors may have contributed, from a volcanic eruption in the South Pacific to a lack of cloud cover that normally shields the planet from the sun's heat.

The year was also marked by destructive storms and heat waves, continuing a concerning trend as the Earth's temperatures steadily rise. The past 10 years have been the warmest 10 years since record-keeping began.

"It's getting tiring saying over and over again that it's the warmest, but it's important to know," says Jared Rennie, research meteorologist at the National Oceanic and Atmospheric Administration. "We keep hearing it's warm, warm, warm but there's repercussions. It's affecting all of us one way or another."

Climate detective work

As 2024 began, monthly temperature records began falling, with every month from January through August ranking as the hottest on record globally.

Some of that heat is well understood: Greenhouse gas emissions from burning fossil fuels reached record levels in 2024, trapping heat in the atmosphere.

The year also began with a strong El Niño event, which occurs when the ocean in the Eastern Pacific releases large amounts of heat to the atmosphere, raising global temperatures. About halfway through the year, the El Niño pattern faded, slowly shifting towards La Niña conditions, which are expected to begin early next year. Scientists expected the heat to recede, but temperatures stayed stubbornly high. This autumn was the hottest on record in the U.S.

"Usually under La Niña conditions, we do tend to be not as warm. But we're still hitting records for not only the U.S. but all over the world, every continent pretty much, and so that is surprising," Rennie says.

The persistent heat over the last two years has scientists searching for other culprits, the possible climatic sidekicks to climate change and El Niño.

One factor could be a lack of cloud cover, which helps reflect the sun's energy back into space. Recent changes in the shipping industry could be a factor. International ships recently switched to cleaner fuels to reduce air pollution. But the tiny particles that make up the pollutants can also help form clouds. A reduction in the amount of Saharan dust in the atmosphere may also have meant less reflection of the sun's energy.

In 2022, the Hunga Tonga-Hunga Ha'apai volcano erupted in the South Pacific, sending massive amounts of water vapor into the air. Water vapor helps trap heat and could also have warmed the planet. The Earth is also receiving slightly more solar radiation than normal due to the cyclical increase in the solar cycle.

All of those factors could be playing a combined role in the record heat, each adding a small amount. While the jump in temperatures could be chalked up to that natural variability, some scientists worry it points to an acceleration in climate change, showing the atmosphere is more sensitive than previously thought.

Extremes getting more extreme

2024 also saw a number of life-threatening disasters, from Hurricane Helene, which devastated parts of North Carolina and Florida, to powerful heat waves across the U.S. In June, 1,300 people died in Saudi Arabia when a heat wave hit during the Hajj religious pilgrimage.

If annual average temperatures do reach 1.5 degrees Celsius above pre-industrial times, it would be the first year that benchmark is breached. In the 2015 Paris Climate Agreement, countries agreed to try to limit warming to that level in order to avoid more powerful storms and heat waves.

Passing that threshold in one year doesn't mean countries have failed, however. Temperatures would need to consistently breach 1.5 degrees Celsius, according to scientists, calculated over a 20-year average. While that number is important politically, scientists warn that every tenth of a degree of warming is important to avoid, even if the world passes 1.5 degrees.

As the world faces increasingly extreme weather, research is finding that heat waves are being underestimated in some parts of the world. A recent study found that in a number of hotspots, like Western Europe and the Arctic, heat waves are getting dramatically worse, even beyond what scientific computer models predict.

While the global change in temperature may seem small, the impact on weather can be extreme, says Kai Kornhuber, author of the study and a senior research scholar at The International Institute for Applied Systems Analysis.

"One degree or one and a half degrees, they don't seem so dramatic," Kornhuber says. "But on a local scale, these events lead to really dramatic record temperatures and extreme weather events."

As the Earth's complex and interconnected systems are altered by climate change, scientists warn the impacts could be amplified in unforeseen ways.