By Peter de Kruijff, ABC environment reporter
All clouds have some level of cooling effect on the planet. Photo: Bruce Rolff
A drop in the number of low-altitude clouds was behind unexplained warming that contributed to the world's hottest year on record, a new study suggests.
The record-breaking heat of 2023, which saw the planet warm an average 1.45 degrees Celsius over pre-industrial surface temperatures, took many climate scientists by surprise.
Their closest predictions, which simulated the effects of human-created warming and other known drivers, were around 0.2C lower than observed temperatures.
Experts could not fully explain the extra heat source at the time.
Now research published in Science today suggests the missing warming mechanism from the prediction models was a low in "planetary albedo".
That's the amount of solar radiation, including heat, that's reflected into space. The less radiation is reflected, the more it reaches Earth and warms the atmosphere.
Study lead author Helge Goeslling, a climate physicist at the Alfred Wegener Institute in Germany, said 2023 appeared to have the lowest level of albedo since 1940.
Earth's albedo, a Latin word meaning "whiteness", has been declining for decades, with much of that drop linked to a reduction in large reflective areas such as sea ice and snow cover in polar regions.
But Goeslling said the study suggested a decline in a different white substance was mostly responsible for 0.22C of warming in 2023.
"What we find is it is quite obviously linked to cloud changes and, in particular, low cloud changes," he said.
But what this decline in low cloud cover means for the future is uncertain.
What's clouds got to do with it?
All clouds have some level of cooling effect on the planet as they reflect the Sun's rays away from the planet.
But high-level clouds, which form in cold atmospheric layers, also act like a blanket, trapping warmth from below.
Low-level clouds, which form within 2 kilometres of the Earth's surface, reflect radiation, but don't trap as much heat.
To assess how low cloud cover changed in 2023, the new study used data from the European Centre for Medium-Range Weather Forecasts as well as radiation readings from NASA's Clouds and the Earth's Radiant Energy System.
The analysis suggested in 2023, there were far fewer low clouds over the northern mid-latitude and tropical oceans, particularly in the Atlantic, and this could account for the 0.2C of warming.
But the reasons for this drop in low cloud cover are unclear.
Goeslling said changes in aerosol use by people may affect low cloud cover. Climate change may also change how low clouds form.
A third reason could be natural regional variability, which are variations in the climate system outside of human influence.
"We are not daring to put numbers on which is contributing how much to do this," Dr Goeslling said.
"I consider our study just another piece of the puzzle."
NASA's Goddard Institute for Space Studies director and climatologist Gavin Schmidt, who was not involved in the study, said the research "goes some way into explaining the process of recent warming.
"But we still aren't able to say why the albedo has been changing so much, and so there is still more to do before we can say what this means going forward."
Climate research scientist Zeke Hausfather from the not-for-profit data science group Berkeley Earth, and who was also not involved in the study, thought the study provided a useful assessment of changes in cloud cover.
"Though it raises as many questions as it provides answers," he said.
"We still do not know for sure that these changes in cloud behaviour are not due to short-term variability - which would return to more normal conditions with time - or if they represent a new ongoing change to the climate system.
"If they do represent an ongoing change, it remains difficult to disentangle how much might be due to changing human aerosol emissions versus a feedback from human greenhouse gas emissions."
Hausfather said either case was not good news as they both suggested a warmer future.
What can we expect?
If global warming produces fewer low clouds, the world may more quickly surpass the agreement to try to keep long-term annual warming averages below 1.5C.
But further research is needed to figure out what exactly is behind the cloud decline.
Goeslling and his colleagues are also keen to analyse what kind of impact albedo has had in 2024.
In the next few years, Goeslling said, climate scientists should have enough information to provide better climate predictions and reduce uncertainty.
In the meantime, 2024 looks all but guaranteed to take the crown from 2023 as the hottest year on record.
- ABC
