Striking study reveals a climate tipping point in the clouds

The word "hysteresis" does not seem immediately threatening; it alludes to a background of "history" and "dissertation" – a dense reading, perhaps, but these have never killed anyone. But that's not what the word means. Hysteresis is a profound behavior that some systems can display, crossing a kind of point of no return. Compose things just one notch, and you can push the system through a radical change. To return to normal, you may need to reduce the number of notches by five or six.

The Earth's climate system can provide examples. Take water circulation as a treadmill in the Atlantic Ocean. Looking back, you may find that the traffic seems to have tipped into an alternative pattern regarding the climate consequences around the North Atlantic. Moving from one model to another requires a big boost, but it's hard to reverse it – like climbing to the top of a ridge and down to the next valley.

A new study led by Caltech's Tapio Schneider may have revealed a disturbing hysteresis of the Earth's climate: a change in the structure of clouds in response to warming that could quickly heat up the planet much more. If we continue to emit more and more greenhouse gases, we will end up conducting this experiment for real. (Do not do it, please.)

Cloud services

The center of this drama is a particular type of cloud. Stratocumulus generally cover about one fifth of the ocean at low latitude. Most clouds form when air warmed by the surface of the Earth (or forced over the mountains) cools as it rises, condensing the steam from the atmosphere. water in droplets of clouds.

Stratocumulus are a little different. The convection that raises their humidity is not motivated by the warming of the bottom but by the cooling of the top.

The water of this cloud absorbs much of the infrared radiation emitted by the hot surface. The cloud of clouds re-emits radiation downward and into space. The air above these clouds is drier and absorbs much less outgoing energy flowing through it. This means that you can think of these clouds as the cooling fins of a radiator. They give off more heat than they receive from the atmosphere above them, which allows them to cool down and down. The cold air at the top of the clouds flows, creating a convection loop that brings water vapor from the surface of the sea to the cloud.

So, what happens to this unique process in a warmer world?

It's hard to say. The key processes inside these cloud layers of stratocumulus occur on a much smaller scale than a single grid cell in global climate models, so their physics is not simulated directly. Instead, we have a simplified mathematical substitute for their general behavior. There is good reason to think that this prevents us from understanding in detail how they will react to continued global warming.

Nothing but blue skies

To solve this problem, Schneider and his colleagues reversed the trend. They used a model capable of simulating these clouds in a small part of the atmosphere, from a simplified version of the world around them. Specifically, they simulated a portion of the subtropical ocean with stratocumulus above and an adjacent portion of the tropical ocean reacting to global warming. They did it for different concentrations of greenhouse gases equivalent to 400 parts per million CO₂ (similar to today) at 1,600 parts per million.

Up to about 1000 pieces per million, there were no major surprises. The temperature warmed by 4 ° C and the numbers changed to such parameters as water vapor and cloud altitude. But the cloud of clouds seemed generally familiar.

At around 1200 parts per million, however, the simulated clouds suddenly dissipated. And without this shadow reflecting sunlight, the world has warmed up still 8 ° C.

How is CO₂ operate the switch on these clouds? The researchers found two simple processes working together in their simulation. First, the warmer air carries more water vapor from the sea surface, and when this water vapor condenses, it releases a lot of latent heat. This additional latent heat gives a little lash to the air, increasing turbulent motion capable of mixing dry air from above into the cloud layer. This dries the cloud and makes cloud formation less likely.

Second, the increase of CO₂ (and water vapor) in the air above the cloud layer means that it absorbs more of the outgoing infrared radiation. Instead of staying out of the way and letting the cloud layer heat up the space, the upper layer retains more of it and restores some of it to the clouds.

Both processes weaken the cloud, either by slowing the convection caused by cooling or by mixing dry air. And in the model, the cloud deck can no longer be maintained. It breaks.

From there, the hysteresis is impressive. The warming caused by the loss of these clouds amplifies the processes that first divided them. Fall below 1200 parts per million CO₂ still done do not turn on the clouds again. Instead, the researchers had to reduce it to about 300 parts per million to see the cloud reform and stabilize.

What does it all mean

Are we doomed to see this piece soon? There are good reasons to be, if not optimistic, at least to resist pessimism. It would take about a century of continuous emissions growth to reach the equivalent of 1,200 parts per million CO₂. Even the emissions reduction commitments already made can prevent this.

But in addition to these numbers, the researchers noted that other changes in a warming atmosphere could increase this threshold. Some other anticipated traffic patterns would increase the stability of the cloud, allowing it to persist up to an increase in CO₂ concentrations. The model used in this study, which simplifies the overall vision to zoom in on one aspect, can not represent these processes. This means that the exact numbers are not the important thing here. The main conclusion is that this sudden change in cloud behavior is possible. The researchers will now try to understand better.

It is tempting to think that this could help solve another scientific puzzle. Climate models are often tested against past periods of climate change – and to study the causes of these climate changes. Efforts to simulate very hot climates (like that of the Eocene 50 million years ago) have not generally been hot enough. To match the heat indicated by the geological records, the models required a higher concentration of CO₂ that seems to have been present then. It may be that what is missing from the models: a sharp increase in the temperature produced by a loss of marine cloud cover.

Fortunately, remarkable studies like this one attract a lot of follow-up work. We should therefore have more answers to these questions long before we feel the need to look up and see that these clouds are still there.

Nature Geoscience, 2019. DOI: 10.1038 / s41561-019-0310-1 (About DOIs).