Earth may be getting darker, which could explain 2023's record heat
Less low cloud cover in the northern mid-latitudes and tropics may have contributed to 2023's record warmth
A new study suggests that there may be a possible explanation to the exceptional warmth the planet experienced in 2023: a reduction in its reflectivity.
By now it's common knowledge that 2023 was the hottest year on record, coming in at 1.48 C warmer than the pre-industrial average from 1850-1900. The monthly records began to break in the middle of the year, with no sign of abating and carrying into 2024. And now, climate scientists are saying that 2024 is likely to beat out 2023.
The frustrating part for these climate scientists is that they don't quite understand why the planet seems to have heated up as much as it has.
Our continuous use of fossil fuels is changing the atmosphere and warming the planet, but the observed temperatures suggested there was also something else at work.
Some potential theories for the abrupt warming were put forth, including the reduction of sulphur dioxide from ships, which began in 2020, the eruption of the Hunga Tonga–Hunga Ha'apai in 2022, the sun's increased activity in its 11-year cycle, and El Niño was present in 2023.
Now, a new paper published in the journal Science, suggests that one explanation could be that Earth's albedo is in decline, making its surface less reflective.
As sunlight hits Earth, some of it gets absorbed and some of it is reflected back into space. The darker a surface is, the more heat is absorbed. The lighter it is, more of it gets reflected back into space.
"2023 [took] us by surprise, and 2024 continues to do so," said Thomas Jung, professor for physics of the Climate System at the University of Bremen, Germany, and co-author of the paper. "And you know, we had some explanations for some of the warming … [but] there was this gap, this 0.2 or so degree of global warming that was unexplained. So the idea was to find out where that was coming from."
Jung said that when they began to investigate, they found that there were unusual sea ice conditions in the Antarctic, mainly the reduction of sea ice, which reflects sunlight. But that only accounted for about 15 per cent of the warming.
Instead, the authors found that low clouds — which are particularly good at reflecting the sun's radiation back into space — seemed to have been reduced by roughly four per cent.
Though that may not seem like a big number, it seems that it may have big implications.
"I think that we see such a clear change in cloud cover and cloud reflectivity is a little surprising. I hadn't looked at that data recently and realized just how big the signal we are starting to see and that certainly is worrying," said Zeke Hausfather, a research scientist at Berkeley Earth, a non-profit climate analysis organization who was not involved in the study.
"I pulled out a little bit of hope that it might just be variability, that we still might get back to just being as bad as we thought. That certainly was concerning to me, that we're seeing such a big signal here."
While it could be a result of new shipping rules that reduced the harmful pollutant sulphur dioxide which creates clouds as the ships cross the oceans, Jung said the location of where the low cloud cover had been reduced — in tropical Atlantic and mid-latitudes — wasn't in high shipping lanes.
No single explanation
As to why there's been such a reduction in low clouds, Jung said that there's no clear answer.
"It could be natural variability, these natural ups and downs of the of the of the climate system," Jung said. "We are a little bit doubtful that this is playing a role, because cloud cover has been remaining very stable until something like 2015, and then there was this rather unusual decline that doesn't really look a lot like what you would expect as a fingerprint of natural variability."
Jung also said the warming of the oceans — which hit a record high in 2023 — likely plays a role, as a warmer ocean can hold more moisture which can affect the clouds above.
Another issue is how climate change — mainly driven by fossil fuel emissions — is also affecting our oceans in general.
One is the changes in the winds and large-scale circulation, Jung said. In fact, there was a reduction of the trade winds in 2023, so that causes less mixing in the oceans, which can lead to changes in these low clouds.
The role of clouds is one of the biggest uncertainties of climate modelling, Hausfather said. Models that show more warming in response to emissions, he said, tend to have larger changes in clouds which lead to more warming.
And it's worrying.
"The big discussion early this year was, is what we're seeing a blip that's going to return to the regular, abnormal conditions of a rapidly warming world, or is this a new phenomenon that's here to stay?" Hausfather said. "And at least this paper suggests that part of it might be a new phenomenon that's here to stay, which I think is worrying, because, as the authors point out, a stronger cloud feedback, would — all things being equal — indicate a higher climate sensitivity and potentially higher levels of future warming."
Jung said overall the new findings are helpful.
"I think we have now sort of a much clearer view of what to look at in order to understand the origin of that extreme year, and probably also 2024."
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