The Surprising Things That Helped Make 2023 the Hottest Year Ever

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.