Why 2023 and 2024 Experienced Record Heat: It Wasn’t Just El Niño

Back-to-back, the planet just lived through the two hottest years ever recorded. Climate scientists have long pointed to greenhouse gas emissions as the engine of global warming, but the extreme heat of 2023 and 2024 left even experts searching for a fuller explanation. That temperature spike ran so far above anything seen in recent years that researchers began asking whether something else, or several things at once, had piled onto the warming humans have driven for more than a century.

A recent study published in Earth System Dynamics offers one of the most detailed attempts yet to pin down what actually happened. Scientists used a climate model trained on 170 years of temperature data and ran it through 160,000 different simulated scenarios to untangle the web of forces behind those record-breaking years. What they found was not one smoking gun, but a collision of natural and human-caused factors that together pushed global temperatures into historic territory.

One of the more telling threads runs through the shipping industry. Starting in January 2020, international maritime rules sharply cut the amount of sulfur allowed in ship fuel. That sounds like an environmental win, and for air quality, it was. But sulfur particles in the atmosphere act like a partial sunshade, reflecting some sunlight back into space, so fewer of them means less cooling. Researchers found that this rule change added a measurable, though relatively modest, amount of warming to global temperatures.

Record Heat and an Accidental Shipping Effect

For decades, massive cargo ships burned some of the dirtiest fuel on earth. Sulfur in that fuel spewed particles into the atmosphere that, while terrible for human lungs, had an unintended cooling side effect. When the International Maritime Organization capped the sulfur content allowed in ship fuel at the start of 2020, air quality improved. But the reflective aerosol shield those particles had been providing weakened at the same time.

Rather than treat the size of that effect as settled, the study’s authors evaluated two plausible estimates for how much the sulfur cut increased the energy Earth’s surface absorbs, equivalent to an increase of either 0.1 or 0.15 watts per square meter, a measure of energy flux across the planet’s surface. Under those two scenarios, the shipping rule change raised global average temperatures by either 0.028 or 0.043 degrees Celsius by the end of 2024.

Those fractions of a degree may sound trivial, yet in the context of global temperature records they are far from nothing. That amount of warming from the shipping regulations alone is roughly comparable to a few years’ worth of human-caused warming at the pace the planet heated between 1975 and 2014. Put simply, cleaning up ship fuel briefly nudged the thermostat in the same direction that fossil fuels have been pushing it for decades.

El Niño Got the Headlines, But It Wasn’t the Whole Story

When 2023’s heat made news, the weather pattern known as El Niño, a periodic natural warming of surface waters in the Pacific Ocean, was the explanation most often cited. El Niño does drive up global temperatures, and this study confirms it played a real role. Shifting from the preceding cooler pattern to El Niño conditions accounted for roughly 0.092 degrees Celsius of the rise in global average temperature from 2022 to 2023, about a third of the total increase between those two years.

But El Niño alone does not explain the full picture, and that is where the research breaks new ground

Indian Ocean Pattern Behind the Record Heat

Scientists have known for years that a climate pattern in the Indian Ocean can influence weather and rainfall across parts of Africa, Asia, and Australia. What they had not fully appreciated, according to this study, is that it can also nudge global average temperatures.

A strong version of that pattern, formally called the Indian Ocean Dipole and sometimes nicknamed the “Indian Niño,” emerged in 2023. Researchers found it contributed to a rise in global average temperature of 0.075 degrees Celsius in 2023 relative to 2022. That effect carried into 2024 as well, adding an estimated 0.053 degrees Celsius that year. Authors describe their work as the first study to identify a meaningful contribution from this Indian Ocean pattern to the abnormally high global temperatures seen in 2023 and 2024.

That contribution is nearly as large as El Niño’s in 2023. Because two earlier high-profile assessments of the record heat left the Indian Ocean Dipole out entirely, this finding suggests scientists may need to weigh it more heavily when explaining unusually warm years.

The Sun, Volcanoes, and What’s Still Unknown

Sea surface temperatures in the North Atlantic were also unusually elevated. Warm waters there pushed global average temperatures up by about 0.070 degrees Celsius in 2023 and 0.069 degrees Celsius in 2024, relative to 2022. Here the picture gets murkier. When researchers filtered out short-term temperature swings in the North Atlantic and left only longer-term trends, their model’s estimated temperatures came in lower than what was actually observed. That gap points to short-term variability as a real player in the 2023 and 2024 heat, though the authors are candid that its exact causes, whether natural, human-driven, or a mix, remain undetermined.

Earth’s roughly 11-year solar cycle was near its peak in 2023 and 2024, meaning the sun put out slightly more energy than in 2022. Researchers estimate this added about 0.025 degrees Celsius in 2023 and 0.029 degrees Celsius in 2024, a real but small contribution.

An underwater volcano called Hunga, which erupted in early 2022 and blasted an unusual amount of water vapor into the upper atmosphere, was weighed as well. Scientific debate continues over whether Hunga ended up warming or cooling the surface overall. Authors used atmospheric particle measurements as a stand-in for the volcanic effect, an approach they acknowledge is a simplification, and estimated a small net cooling from the eruption.

Even after adding up every one of these factors, some uncertainty remains. Authors point to imprecise knowledge of how much cooling man-made particles have historically provided, and how sensitive the climate system is to changes in that balance, as the main sources of the remaining gap. Ongoing research continues to untangle these interactions between natural variability and human-driven forces.

What the research shows is that 2023 and 2024 were not the product of any single dramatic cause. Long-term human-caused warming set the elevated baseline. El Niño and the Indian Ocean pattern added heat from natural variability. The sun sat near its cyclical peak. Warm North Atlantic waters contributed. And the unintended warming from cleaner ship fuel arrived at roughly the same moment. Record heat years tend to get explained in the press with one dominant storyline. Reality, this research shows, is far messier, and making sense of that messiness may be the most important work in climate science right now.