If we keep burning fossil fuels with reckless abandon, we could trigger a cloud feedback effect that will add 8°C on top of all the warming up to that point. That means the world could warm by more than 14°C above the pre-industrial level.
By Michael Le Page
Needless to say, this would be cataclysmic. For instance, large parts of the tropics would become too hot for warm-blooded animals, including us, to survive.
The good news is that if countries step up their efforts to cut emissions we should avoid finding out if this idea is correct. “I don’t think we will get anywhere close to it,” says Tapio Schneider at the California Institute of Technology, Pasadena, who led the research.
Read more: When crocodiles roamed the Arctic
Schneider’s team modelled stratocumulus clouds over subtropical oceans, which cover around 7 per cent of Earth’s surface and cool the planet by reflecting the sun’s heat back into space. They found there was a sudden transition when CO2 levels reached around 1200 parts per million (ppm) — the stratocumulus clouds broke up and disappeared.
The reason why this finding applies only to subtropical stratocumulus is that these clouds are unusual. The cloud layer is maintained by the cloudtops cooling as they emit infrared radiation — and very high CO2 levels block this process.
The loss of these bright white clouds would have a dramatic warming effect, adding 8°C to the global temperature, Schneider calculates. Since the world would warm around 6°C or more if CO2 levels passed 1200 ppm, this means the average global temperature could exceed 14°C or more.
No need to panic
CO2 levels will pass 410 ppm this year, up from 280 ppm in preindustrial times. If we burned all available fossil fuels, atmospheric CO2 levels could rise as high as 4000 ppm.
However, even in the standard worst case scenario used by climate scientists, which assumes nothing is done to curb emissions, CO2 levels would only pass 1200 ppm decades after 2100.
Other climate scientists say this cloud feedback is plausible. “Conceptually I think it’s sound,” says Helene Muri at the Norwegian University of Science and Technology. But there are some uncertainties about the numbers, so it will be important to try to narrow them down, she says.
The result might hold up, but we already have more than enough reasons to avoid reaching this point, says Kate Marvel at the NASA Goddard Institute for Space Studies.
Emissions are currently growing in line with the worst-case scenario, but the expectation is that countries will eventually do more. “This result isn’t cause for panic,” says Marvel.
The finding could also help solve a longstanding mystery — how the planet got so hot around 50 million years ago that crocodiles thrived in the Arctic. We know that CO2 levels were generally much higher at the time, but they were not high enough to explain the extreme warmth during this period.
The reason why the cloud feedback effect had not been discovered before is that general climate models of the planet have to greatly simplify cloud physics to make the computations manageable.
Schneider’s team instead modelled only a small part of the subtropical atmosphere in great detail.
And if the models are missing major effects like this, there could be more nasty surprises in store as the world warms. “Yes, for sure,” says Muri. “We will certainly see more surprises.”
Journal reference: Nature Geoscience, DOI: 10.1038/s41561-019-0310-1
First published at New Scientist – Monday 25 February 2019