At an average temperature of 14.98°C, the year was not only 0.17°C higher than the previous highest annual value in 2016 — an El Niño year — it also saw the largest year-on-year deviation.

2023 was 0.3°C warmer than 2022.

A record a day

As record after record fell like dominoes, the hottest day turned into the hottest week, which led to the hottest month, and these combined to give the planet its hottest summer. But the temperature rise didn’t stop there, with at least two days in November breaching the 2°C threshold, and December ending as the hottest ever.

"2023 was an exceptional year with climate records tumbling like dominoes,” Samantha Burgess, C3S deputy director, said.

When scientists combine their satellite readings with geological evidence from tree rings and ice cores, 2023 also appears to be among the warmest years in at least 100,000 years.

The year was a dramatic testimony of how far the climate has come from when civilisation developed, said Carlo Buontempo, director of C3S.

“There were simply no cities, no books, agriculture or domesticated animals on this planet the last time the temperature was so high,” he said at a news briefing.

Also read: 2023: The year that broke climate records

Global impact

The effects were felt across the globe, as hot weather baked much of Asia, Europe and the United States.

Canada witnessed its most destructive wildfire season, with more than 45 million acres burned.

The impacts extended beyond surface air temperatures, as the ice around the coasts of Antarctica failed to recuperate, reaching a “mind-blowing low”, with the Arctic sea ice also below average. Glaciers in western North America and the European Alps experienced an extreme melt season, further assisting the sea level rise.

The sea surface continued to boil over, reaching record levels for the time of the year from April through December, as multiple marine heatwaves, in parts of the Mediterranean, Gulf of Mexico and the Caribbean, Indian Ocean and North Pacific, and much of the North Atlantic ravaged ecosystems. The heat in some regions was such that off the coast of Florida, the water temperature reached around 38°C — conditions similar to a hot shower for humans, but fatal for corals.

The reasons

The main drivers for this temperature rise are obviously greenhouse gases — CO2 concentrations increased by 2.4 ppm and methane by 11 ppb — but experts are still looking for clues into why the year was much hotter than predicted.

“Global temperatures in 2023 were really weird. For almost every other year we can pretty reliably predict temperatures (red dot and bars) based on the long-term trend, the prior year, and the El Nino / La Nina conditions at the start. For 2023 this model completely breaks down,” Zeke Hausfather, climate scientist at Berkeley Earth, said on X.

The World Meteorological Organisation announced the onset of an El Niño in July last year.

The recurrent shift in tropical Pacific weather patterns is often linked with record-setting heat globally and a dry monsoon in India. The onset of this weather pattern also usually contains a warning for a potentially worse year to follow, as El Niño reaches its peak about three months after onset and unleashes full effects in the summer to follow.

In recent decades, very warm years have typically been ones that started in an El Niño state. C3S data reaching as far back as 1998 (another El Niño year, and the first that saw a positive deviation in annual average temperatures), and the most recent in 2016 are evidence of this.

But last year, the El Niño didn’t start until midyear — which suggests that El Niño wasn’t the main driver of the abnormal warmth at that point, said Emily J. Becker, a climate scientist at the University of Miami told NYT.

Additionally, the temperature spike started much earlier in June.

“The earliest signs of how unusual 2023 was to become began to emerge in early June when temperature anomalies relative to 1850-1900 pre-industrial level reached 1.5°C for several days in a row,” the C3S release said.

Contributory factors

Another contributing factor was the 2022 eruption of an underwater volcano off the Pacific island nation of Tonga that spewed vast amounts of water vapour into the atmosphere, helping trap more heat near the Earth’s surface.

Experts, however, said it was a non-event for 2023. Early studies neglected the sulfate particles it also sent into the upper atmosphere, which reflected light and cancelled out the warming effect of water vapour, Mark Schoeberl, an atmospheric scientist at the Science and Technology Corporation, told Science. “For 2022, it was a non-event. I have continued my computations into 2023—still a non-event.”

Recent curbs on sulfur pollution from ships brought down levels of aerosols, or tiny airborne particles that reflect solar radiation and help cool the planet.

Malte Meinshausen, a climate scientist from the University of Melbourne, emphasized that approximately 1.3°Celsius of the warming was attributable to greenhouse gases, with smaller contributions from El Nino and other factors.

What’s next?

What really matters now is whether this temperature extreme will be anomalous and specific to 2023 or become the norm.

2024 has begun on an anomalous note: sea surface temperatures were highest for the time of the year, ice in most regions (including the higher reaches of Kashmir) has failed to build back, and the impact of El Niño is still unknown.

While oceans are known to absorb 90% of the heat generated by GHG emissions, the ocean heat will also start escaping into the atmosphere soon enough.

So while 2023 ended as the hottest year on record, it may also be the coldest this generation witnesses.