Eight charts illustrate 2023’s extreme weather
RECORD-BREAKING temperatures can fail to impress these days. The past nine years, one after the other, have been the hottest ever recorded. But even against this background, 2023 was remarkable. Climate researchers are calling last year’s events “weird” and say they will be untangling what exactly drove its remarkable extremes for months or even years to come.
Data released this week by international, American and European government agencies and research groups confirmed that 2023 was the hottest year on record by a considerable margin. The World Meteorological Organisation (WMO) said that it was 1.45°C warmer than pre-industrial temperatures, give or take 0.12°C. That is within a sliver of the symbolic warming threshold of 1.5°C. The year brought a rash of regional records and extreme events, including floods, wildfires, drought and heatwaves. Below are eight charts that help make sense of it.
Beginning in the northern-hemisphere spring, researchers saw the early signs of a coming El Niño, a natural climate cycle which can temporarily boost temperatures by 0.1°C to 0.2°C. El Niños are gradual processes that spread heat and energy from the tropical Pacific to other parts of the globe. They typically cause a spike in air temperatures the year after they begin. But the startling rise last year took climate scientists by surprise. Each of the last seven months of 2023 was considerably warmer than the corresponding months in any other year.
Our second chart shows that nearly half of the days in this period were at least 1.5°C hotter than pre-industrial averages. Two days, in November, were more than 2°C warmer. Days above 1.5°C were first recorded in 2015 but had never been so numerous; days above 2°C had never been recorded before.
Each year climate modellers are able to project how warm temperatures will be for the next 12 months (based on the cyclical nature of El Niños and the long-term warming trend of greenhouse-gas emissions). Given uncertainty ranges, these projections are generally pretty good. “For 2023 this model completely breaks down,” says Zeke Hausfather, a modeller at Berkeley Earth. The only other year in which actual temperatures fell outside the error range of projected temperatures was in 1992, after the eruption of Mount Pinatubo in the Philippines injected soot and pulverised-rock particles into the stratosphere, creating a natural sunshade that briefly cooled the planet.
Next consider which regions drove annual average temperatures higher. El Niño is characterised by warmer-than-usual surface waters in the tropical Pacific (see grey area). Its pattern can be seen growing from the second quarter of the year onward. El Niño, however, cannot fully explain the hot weirdness of 2023. There were anomalous temperatures and extremes in the oceans and on land before El Niño was formally declared by the WMO in July, and before its effects had spread from the Pacific to other parts of the world.
Warming in the sub-tropical northern oceans was particularly unusual and notable. So too were warm temperatures over sub-tropical landmasses, which caused regional extremes in populated areas (light-red band).
A closer look at the subtropical oceans shows that the North Atlantic had a particularly strange year. Temperatures at the surface rocketed. There is little doubt that this was a large factor in pushing up the global average for the year. But what caused it is a matter of debate. One possibility is that it was somehow linked to heat that has been accumulating in the deeper layers of the ocean, down to 2,000 metres, for decades. But researchers are unsure for now.
It was also a highly unusual year on the other side of the planet. The ring of sea ice that surrounds Antarctica shrank to record lows for its winter, when sea ice should reach its greatest extent. “What is happening in Antarctica is scary,” says Francesca Guglielmo, a climate scientist at Copernicus Climate Change Service, an EU agency. Some have been concerned that Antarctica entered a new phase in 2016, when its sea ice began to show signs of decline. This too, is up for debate. Things appeared to recover in December; researchers are watching closely to see how things evolve in 2024.
The specific geophysical processes that created all this weirdness will keep climatologists busy for months to come. But the long-term warming trend is clearly being driven by industrial emissions of greenhouse gases. In 2023, as at regular intervals in the past, that warming was amplified by a relatively strong El Niño event, particularly coming on the heels of three consecutive years of La Niña, El Niño’s cooling counterpart.
Three other factors may have marginally contributed to the warming temperatures. First is the 11-year cycle in the sun’s activity and radiation. The solar cycle influences Earth’s climate (though only to a very small degree compared with greenhouse gases). The cycle is currently waxing, nudging temperatures up a bit. It will continue to do so until it peaks and flips into its waning phase sometime around 2025.
Second, on January 15th 2022 an eruption of the Hunga Tonga-Hunga Ha’apai volcano, in the southern Pacific ocean, belched gases, soot and water vapour into the atmosphere. Large volcanic eruptions like this can typically cool the climate for a few months or years. (The soot and pulverised-rock particles produce a shading effect). But because Hunga Tonga is an underwater mound it generated a huge amount of water vapour, a greenhouse gas. One study found that around 146m tonnes of it ended up in the stratosphere, boosting its water content by roughly 10%. Precisely how much warming this caused in 2023 is still in question. The good news is that the vapour will eventually fall back down to Earth and its warming effects will disappear.
A third factor will have more long-term consequences. Regulations that came into force in 2020 have begun to clean up the fuel used by ships. Historically, the particles suspended in those plumes have had a shading, and therefore cooling, effect on Earth. So cleaning up shipping emissions is, perversely, warming the planet. This effect will persist. Some believe it helped make 2023 particularly hot. Others point out that shipping emissions produce a small fraction of the total aerosols generated by other industrial installations, such as factories and power stations. It would also have been expected to warm oceans sooner.
What does all this mean for 2024? Hunga Tonga’s water vapour will continue to warm the climate for some time. Similarly, shipping soot will remain absent. Greenhouse-gas emissions will continue to pile up in the atmosphere. El Niño is expected to peak in the first half of the year, but it is too soon to say whether the cycle will head into a neutral or a cooling La Niña phase.
All things considered, the annual “year-ahead” projections agree that 2024 is likely to be marginally warmer than 2023 (see our eighth and final chart). When asked how last year’s turn of events affects his confidence in the projections, Dr Hausfather chuckles. “I certainly think it makes me a little less confident,” he says. “Maybe what happened in 2023 was a weird combination of factors that is not going to persist into 2024. Or maybe it will. Time will tell.”■
