‘This was not normal’: US scientists grapple with a year of record heat | Extreme heat

Scientists are still puzzling over the reasons behind a streak of unexpected, record heat that scorched 2023 and into this year, sparking fears that the climate crisis could be moving faster than previously thought.

Is it the clouds? Or shipping? Or maybe a huge volcanic eruption?

A parade of climate researchers presented potential reasons for the apparent surge in global heating at the American Geophysical Union (AGU) meeting in Washington on Tuesday, although none were able to claim a full understanding of what has happened with the world’s climate since the start of last year.

“We weren’t expecting 2023 to be so exceptional,” acknowledged Robert Rohde, a climate scientist at the University of California, Berkeley, who said last year’s temperatures, 0.2C above the previous annual global record, were in the words of a colleague “gobsmackingly bananas”.

Humans are causing the world to warm up by the burning of fossil fuels and last year was also influenced by an El Niño event, a naturally reoccurring climate phenomena that typically pushes up global temperatures. “But this was not normal,” said Rohde. “Global warming and El Niño are the biggest factors but something else was going on.”

“Maybe it’s clouds,” Rohde said, referencing new research that found there was record low cloud cover last year, which reduced the amount of reflective surface that would deflect the sun’s energy back out of the atmosphere. “There was extra warmth because Earth was absorbing extra energy because of a deficit of clouds,” he said. “But that just moves the question to ‘why there weren’t as many clouds?’”

Another potential explanation could be found in measures to cut the amount of air pollution emitted by ships, which came into effect in 2020. While the rules greatly cut this type of pollution, by as much as 90% according to another new paper, it thereby reduced the amount of sulfate aerosols being put into the atmosphere that block incoming solar radiation and help cool the planet slightly.

A further contributing factor might be the eruption of the Hunga Tonga volcano, located in the south Pacific, in 2022. Rohde said the event spewed 150m tons of water vapor into the atmosphere, helping trap more heat close to Earth’s surface into 2023.

Among other pondered factors, discussed in front of an audience of several hundred other researchers, was a reduction in the amount of dust whipping off the Sahara desert, which again would normally act as a solar blocking agent. “Also there could be some sort of climate change feedback we don’t understand,” said Rohde. “It’s a possibility.”

The elevated heat has continued throughout much of 2024, with scientists still waiting to see if the unexpected warming will die down. This year is certain to be the hottest on record, the first that will be 1.5C above the pre-industrial era and a signal that the longer-term international effort to keep the temperature rise below this level is probably doomed.

This level of global heating is already fueling dangerous heatwaves, floods and other disasters and scientists say more research is needed to grasp whether the climate system is heating up at an ominous rate beyond previous expectations.

“There is something to explain, there is still work to do,” said Gavin Schmidt, a Nasa climate scientist who co-chaired the session and asked attendees to raise their hands if they thought the heat anomalies had been fully explained. Only a few out of several hundred did so.