Scientists knew 2023’s heat would be historic—but not by this much

The year 2023 was the hottest in recorded human history, Europe’s top climate agency announced Tuesday, with blistering surface temperatures and torrid ocean conditions pushing the planet dangerously close to a long-feared warming threshold.

According to new data from the Copernicus Climate Change Service, Earth’s average temperature last year was 1.48 degrees Celsius (2.66 degrees Fahrenheit) hotter than the preindustrial average, before humans began to warm the planet through fossil fuel burning and other polluting activities. Last year shattered the previous global temperature record by almost two-tenths of a degree—the largest jump scientists have ever observed.

This year is predicted to be even hotter. By the end of January or February, the agency warned, the planet’s 12-month average temperature is likely to exceed 1.5 degrees Celsius (2.7 degrees Fahrenheit) above the preindustrial level—blasting past the world’s most ambitious climate goal.

The announcement of a new temperature record comes as little surprise to scientists who have witnessed the past 12 months of raging wildfires, deadly ocean heat waves, cataclysmic flooding and a worrisome Antarctic thaw. A scorching summer and “gobsmacking” autumn temperature anomalies had all but guaranteed that 2023 would be a year for the history books.

But the amount by which the previous record was broken shocked even climate experts.

“I don’t think anybody was expecting anomalies as large as we have seen,” Copernicus director Carlo Buontempo said. “It was on the edge of what was plausible.”

The staggering new statistics underscore how human-caused climate change has allowed regular planetary fluctuations to push temperatures into uncharted territory. Each of the past eight years was already among the eight warmest ever observed. Then, a complex and still somewhat mysterious host of climatic influences combined with human activities to push 2023 even hotter—ushering in an age of “global boiling,” in the words of United Nations Secretary General António Guterres.

Unless nations transform their economies and rapidly transition away from polluting fuels, experts warn, this level of warming will unravel ecological webs and cause human-built systems to collapse.

A year that ‘doesn’t have an equivalent’

When ominous warmth first appeared in Earth’s oceans last spring, scientists said it was a likely sign that record global heat was imminent – but not until 2024.

But as the planet transitioned into an El Niño climate pattern – characterized by warm Pacific Ocean waters – temperatures took a steeper jump. July and August were the two warmest months in the 173-year record Copernicus examined.

As Antarctic sea ice dwindled and the planet’s hottest places flirted with conditions too extreme for people to survive, scientists speculated that 2023 would not only be the warmest on record – it might well exceed anything seen in the last 100,000 years. Analyses of fossils, ice cores and ocean sediments suggest that global temperatures haven’t been this high since before the last ice age, when Homo sapiens had just begun to migrate out of Africa and hippos roamed in what is now Germany.

Autumn brought even greater departures from the norm. Temperatures in September were almost a full degree Celsius hotter than the average over the past 30 years, making it the most unusually warm month in Copernicus’s data set. And two days in November were, for the first time ever, more than 2 degrees Celsius (3.6 degrees Fahrenheit) hotter than the preindustrial average for those dates.

“What we have seen in 2023 doesn’t have an equivalent,” Buontempo said.

This year’s record-setting conditions were driven in part by unprecedented warmth in the oceans’ surface waters, Copernicus said. The agency measured marine heat waves from the Indian Ocean to the Gulf of Mexico. Parts of the Atlantic Ocean experienced temperatures 4 to 5 degrees Celsius (7.2 to 9 degrees Fahrenheit) above average—a level that the National Oceanic and Atmospheric Administration classifies as “beyond extreme.”

While researchers have not yet determined the impacts on sea life, similar heat waves have caused massive harms to microorganisms at the base of the food web, bleached corals and fueled toxic algae blooms, she added.

Though the oceans cover about two-thirds of Earth’s surface, scientists estimate they have absorbed about 90 percent of the extra warming from humans’ burning of fossil fuels and the greenhouse effect those emissions have in the atmosphere.

“The ocean is our sentinel,” said Karina von Schuckmann, an oceanographer at the nonprofit Mercator Ocean International.

The dramatic warming in the ocean is a clear signal of “how much the Earth is out of energy balance,” she added – with heat continuing to build faster than it can be released from the planet.

What drove the record warmth

Scientists are still disentangling the factors that made this year so unusual.

The largest and most obvious is El Niño, the infamous global climate pattern that emerges a few times a decade and is known to boost average planetary temperatures by a few tenths of a degree Celsius, or as much as half a degree Fahrenheit. El Niño’s signature is a zone of warmer-than-normal waters in the central and eastern equatorial Pacific Ocean, which release vast amounts of heat and water vapor and trigger extreme weather patterns around the world.

But El Niño alone cannot explain the extraordinary heat of the past 12 months, according to Copernicus. Because it wasn’t just the Pacific that exhibited dramatic warmth this year.

Scientists also believe the Atlantic may have warmed as a result of weakened westerly winds, which tend to churn up waters and send surface warmth into deeper ocean layers. It could also have been the product of below-normal Saharan dust in the air; the particles normally act to block some sunlight from reaching the ocean surface.

Around the world, in fact, there has been a decline in sun-blocking particles known as aerosols, in large part because of efforts to reduce air pollution. In recent years, shipping freighters have taken measures to reduce their emissions. Scientists have speculated the decline in aerosols may have allowed more sun to reach the oceans.

And then there is the potential impact of a massive underwater volcanic eruption. When Hunga Tonga-Hunga Ha’apai blasted a plume 36 miles high in January 2022, scientists warned it released so much water vapor into the atmosphere, it could have a lingering effect for months, if not years, to come.

NASA satellite data showed the volcano sent an unprecedented amount of water into the stratosphere—equal to 10 percent of the amount of water that was already contained in the second layer of Earth’s atmosphere. In the stratosphere, water vapor—like human-caused emissions of carbon dioxide—acts as a greenhouse gas, trapping heat like a blanket around the Earth.

But it won’t be clear how much of a role each of those factors played until scientists can test each of those hypotheses.

What is clear, scientists stress, is that this year’s extremes were only possible because they unfolded against the backdrop of human-caused climate change. The concentration of carbon dioxide in the atmosphere hit a record high of 419 parts per million in 2023, Copernicus said. And despite global pledges to cut down on methane—which traps 86 times as much heat as carbon dioxide over a short time scales—levels of that gas also reached new peaks.

Only by reaching “net zero”—the point at which people stop adding additional greenhouse to the atmosphere—can humanity reverse Earth’s long-term warming trend, said Paulo Ceppi, a climate scientist at Imperial College London.

“That is what the physical science tells us that we need to do,” Ceppi said.

What comes next

Almost half of all days in 2023 were 1.5 degrees Celsius warmer than the preindustrial average for that date, Copernicus said—giving the world a dangerous taste of a climate it had pledged to avoid.

At the Paris climate conference in 2015, nations agreed to a stretch goal of “pursuing efforts to limit the temperature increase to 1.5C above preindustrial levels.” Three years later, a special report from the Intergovernmental Panel on Climate Change found that staying within this ambitious threshold could avoid many of the most disastrous consequences of warming—but it would require the world to almost halve greenhouse gas emissions in just over a decade.

But emissions have continued to rise, and now the world appears poised on the brink of surpassing the Paris target.

At least one climate science organization believes the barrier has already been crossed. Berkeley Earth said in December that 2023 is virtually certain to eclipse it, though its estimates of 19th century temperatures are slightly lower than those other climate scientists use.

This doesn’t necessarily mean the world has officially surpassed the limit set in the Paris climate agreement in 2015. That benchmark will only be reached when temperatures remain 1.5 degrees Celsius above average over a period of at least 20 years.

But scientists are already speculating that the planet could set another average temperature record in 2024. Some also say the latest spike in global temperatures is a sign the rate of climate change has accelerated.

Whether or not 2023 surpasses the 1.5 degree limit, the year “has given us a glimpse of what 1.5 may look like,” Buontempo said.

He hoped that the latest record allows that reality to set in—and spurs action.

“As a society, we have to be better at using this knowledge,” Buontempo added, “because the future will not be like our past.”