Following a summer and autumn of planetary extremes—the hottest September by a wide margin, supercharged hurricanes, self-perpetuating heat domes—scientists have now declared 2023 the warmest year on record.

Today the National Oceanic and Atmospheric Administration (NOAA) released its 2023 report, finding that last year was 1.35 degrees Celsius above the preindustrial average. Berkeley Earth, a nonprofit climate research group, released its own 2023 report showing that global average temperatures last year were 1.54 degrees C above preindustrial levels. That smashes the previous record year of 2016, which was 1.37 degrees warmer. Earlier this week the European Union’s Copernicus program pegged it at 1.48 degrees C.

The slight variation between all these analyses isn’t so much disagreement around measuring temperatures now, but different estimates of preindustrial temperatures. Scientists weren’t always so lucky as to have satellites and weather stations and sophisticated oceanographic instruments. Regardless, Berkeley Earth, Copernicus, and the NOAA all agree 2023 was the warmest year on record and that it beat 2016 in a big way.

“The margins by which this year broke previous records, I found astounding,” says Sarah Kapnick, chief scientist of the NOAA. “In the fall, we broke records by more than we ever had broken before. Usually you break records by a thousandth of a degree. We were breaking records by tenths of a degree.”

By Berkeley Earth’s calculations at least, it’s the first year that humanity has passed the Paris Agreement’s goal of limiting warming to 1.5 degrees. But that doesn’t mean that goal is lost—it refers to temperatures averaged over longer timescales, whereas this was a single year. Still, like you can see in the graph above, global temperatures continue to march upward as humanity fails to stop growing its greenhouse gas emissions. The last nine years have been the hottest nine years on record.

“There’s a lot of really interesting discussion in the community around what drove 2023’s extreme warmth and what it implies for future years,” says Zeke Hausfather, a research scientist at Berkeley Earth. “And why we saw temperatures so exceptional, particularly in months like September, where we beat the prior record by a half a degree C.”

Berkeley Earth notes that last year, 17 percent of the planet notched a record warm annual average. “By chance, these areas coincided with a number of major population centers,” the report says. “We estimate that 2.3 billion people—29 percent of Earth’s population—experienced a locally record warm annual average in 2023.”

Out at sea, 2023 was extra brutal: Since last March, global sea surface temperatures have soared to record highs and stayed there. The NOAA’s report notes that the amount of heat stored in the upper 2,000 meters of the oceans reached a record high last year.

In this graph, the orange line shows the global sea surface temperature throughout 2023. The other squiggles are previous years, with the uppermost dashed black line being the average between 1982 and 2011. The dark black line at upper left is where we’re starting out 2024. Notice it’s already at a sky-high level several months before temperatures typically peak. Even the record-breaking year of 2023 didn’t see these kinds of temperatures until late March and early April.

The 2023 climate reports also note that Antarctic sea ice extent reached record lows this year. As we reported back in May, scientists are scrambling to figure out whether the southern continent is in the midst of a regime switch—that is, if these record minimums are going to continue for the foreseeable future. This sea ice is critical because it protects Antarctica’s massive ice shelves from wind and waves. Losing more and more of it could hasten the decline of the continent’s ice, which would add many feet to global sea levels.

Losing sea ice also changes the reflectivity of the waters around Antarctica. That threatens to initiate a gnarly feedback loop of warming. “Instead of having that ice there to reflect the sunlight back to space,” says Kapnick, “you now actually have open ocean, which is a lot darker, which means it is going to warm up the ocean faster.”

The drivers of extreme ocean heat are likely both natural and human-caused. For one, the oceans have absorbed around 90 percent of the extra heat that humanity has added to the atmosphere. And two, last year the equatorial Pacific Ocean’s warming and cooling cycle switched from its cooler phase, known as La Niña, to its warmer one, El Niño. That has not only raised ocean temperatures but added heat to the atmosphere and influenced weather all over the world. (It has also created extreme drought in the nearby Amazon.) “El Niño has been very strange this year,” says Hausfather. Typically, there’s a lag of about three months between El Niño conditions peaking and temperatures peaking. “That doesn’t really seem to have happened in 2023. We saw a lot of warmth pretty early on in the El Niño cycle.”

The sea surface temperature anomalies have been particularly acute in the North Atlantic. That’s probably due to less Saharan dust in 2023, which usually blows clear across the ocean into the Americas. That meant less shading for the Atlantic, allowing the sun to heat it more.

Similarly, new shipping regulations have reduced the amount of sulfur in fuels, so ships are producing fewer aerosols. These typically brighten clouds, bouncing some of the sun’s energy back into space, an effect so pronounced that you can actually track ships with satellites by the streaks of white they leave behind. In general, the loss of aerosols is an unfortunate and unavoidable consequence of burning less fossil fuels going forward: With less sulfur going into the atmosphere, we’ll lose some of the cooling effect that’s kept global temperatures from soaring even higher.

Working in the opposite way, the huge eruption of the Hunga Tonga volcano in 2022 might have bumped up global temperatures slightly. Volcanoes can substantially cool the climate by spewing plumes of aerosols into the atmosphere, like ship emissions do, but this one instead fired 146 trillion grams of water vapor into the sky, equivalent to 58,000 Olympic-size swimming pools. “Stratospheric water vapor is a greenhouse gas and hangs around for a few years before it falls back out,” says Hausfather. “And so that might have contributed a bit of additional warming.”

Above you can see the average temperatures for land and sea over time—notice the huge spikes in 2023 for both. While the oceans are heating quickly, they’re lagging behind because it’s a lot harder to warm up so much water.

Looking ahead, Berkeley Earth estimates a 58 percent chance of 2024 being warmer than 2023 and a 97 percent chance of beating 2016’s mark of 1.37 degrees above preindustrial levels. “2024 is definitely going to be an exceptionally warm year,” says Hausfather. “It’s either gonna be the warmest or second-warmest year on record. There’s almost no chance that it’s going to be anything else at this point, barring a big volcanic eruption.”