Cloud loss could add 8 degrees to global warming

PETM does not only provide a past example of CO₂climate change caused by climate; Scientists say this also indicates an unknown factor that has a disproportionate influence on the Earth's climate. When the planet was hot, it was very hot. The episodes of ancient warming such as PETM were still much more extreme than suggested by theoretical models of climate. Even after taking into account the differences in geography, ocean currents and vegetation during these past episodes, paleoclimatologists find that something big seems to be missing from their models: a factor X whose wild swings leave no trace in the fossil record.

The evidence accumulates in favor of the answer that the experts suspected for a long time but which has only recently been able to explore in detail. "It's quite clear at this point that the solution is to create clouds," said Matt Huber, paleoclimate modeller at Purdue University.

Clouds currently cover about two-thirds of the planet at any time. But cloud computer simulations have begun to suggest that as the Earth heats up, clouds are becoming scarce. With fewer white surfaces reflecting sunlight in space, the Earth warms up even more, resulting in greater cloud loss. This feedback loop causes an uncontrollable heating of the warming.

For decades, rough calculations have suggested that cloud loss could have a significant impact on the climate, but this concern has remained hypothetical until recent years, when cloud observations and simulations are have been improved to such an extent that researchers have been able to gather convincing evidence.

Now, new discoveries reported today in the newspaper Nature Geoscience show that the effects of cloud loss are dramatic enough to explain past warming episodes such as PETM – and to precipitate a future disaster. The California Institute of Technology's climate physicists conducted a high-level simulation of stratocumulus, the low-lying cloud that has by far the largest cooling effect on the planet. The simulation revealed a critical point: a warming level during which the stratocumulus breaks completely. The disappearance occurs when the concentration of CO₂ in the simulated atmosphere, it reaches 1,200 parts per million – a level that fossil fuel combustion could take us to in about a century, in "normal" emission scenarios. In the simulation, when the tipping point is exceeded, the Earth's temperature rises 8 degrees Celsius, in addition to CO's 4 degrees or more warming₂ directly.

Once the clouds are gone, the simulated climate "passes over a cliff," said Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology. Emanuel, an authority on atmospheric physics, described the new discoveries as "very plausible," although, as he noted, scientists now have to strive to replicate the work independently.

To imagine a warming of 12 degrees, think of the crocodiles swimming in the Arctic and the equatorial regions burned and almost lifeless during the PETM. If carbon emissions are not sufficiently reduced and the tipping point is exceeded, "this would lead to a truly devastating climate change," said Caltech's Tapio Schneider, who did the new simulation with Colleen Kaul and Kyle Pressel.

Huber said the tipping point of the stratocumulus helps explain the obvious volatility of the paleoclimate. He thinks that this could be one of the many unknown instabilities of Earth's climate. "Schneider and his co-authors revealed Pandora's climate surprise," he said, adding that as the mechanisms behind the disappearing clouds become clear, "all of a sudden, Suddenly, this enormous sensitivity that emerges from past climates is no longer a problem. it's just in the past. It becomes a vision of the future. "

The question of the cloud

The clouds come in different forms – stratus filling the heavens, cumulus popcorn, wispy cirrus, anvil-shaped nimbus and their hybrids – and cover many physical scales. Composed of microscopic droplets, they measure kilometers and collectively cover most of the Earth's surface. By preventing sunlight from reaching the surface, the clouds cool the planet by several crucial degrees. And yet they are without substance, woven into greatness by complicated physics. If the white and fragmented cloud of the planet descends to the ground, it would not make an aqueous shine thicker than a hair.

The clouds seem simple at first: they form when warm, moist air rises and cools. The water vapor in the air condenses around grains of dust, sea salt or other particles, thus forming droplets of liquid water or ice – "droplets of cloud". But the situation is getting more and more complicated by the effects of heat, evaporation, turbulence, radiation, wind, geography and many others. other factors come into play.

Physicists have been striving since the 1960s to understand how global warming will affect the many types of clouds, and how this will affect global warming. For decades, clouds have been considered by far the greatest source of uncertainty about the severity of global warming, except what society will do to reduce carbon emissions.

Kate Marvel looks at the cloud issue at NASA's Goddard Institute of Space Studies in New York. Last spring, in his multi-story office above Tom's Restaurant in the Upper West Side, Marvel, dressed in a cloud-patterned scarf, revealed a graph showing the range of weather forecasts. different global climate models. The approximately 30 models, managed by climate research centers around the world, take into account all known factors to predict the Earth's temperature, which will increase as CO₂ the level is set up.

Each climate model solves a set of equations on a spherical grid representing the atmosphere of the Earth. A supercomputer is used to evolve the grid of solutions over time, indicating how air and heat flow through each cell of the grid and circulate around the planet. By adding carbon dioxide and other greenhouse gases that hold heat to the simulated atmosphere and observing what is happening, scientists can predict the response of the Earth's climate. All climate models include the ocean and wind currents of the Earth and incorporate most climate feedback loops, such as melting polar ice and rising humidity, which exacerbate global warming. Models agree on most factors but differ greatly in the way they try to represent clouds.

The least sensitive climate models, predicting the most moderate response to increasing CO₂, finds that the Earth will warm up by 2 degrees Celsius if the atmospheric CO₂ concentration is double that of pre-industrial times, which is expected to be about 2050 (the CO₂ The concentration was 280 parts per million before the start of fossil fuel combustion, now it exceeds 410 ppm. Until now, the average global temperature has increased by 1 degree Celsius.) But the 2-degree forecast is the best of the scenarios. "What really scares people is this upper limit here," Marvel said, indicating 4 or 5-degree warming projections in response to doubling of CO₂. "To put this in context, the difference between today and the last ice age was 4.5 degrees."

The enormous range of model predictions comes down to the fact that they see clouds blocking more or less sunlight in the future. As Marvel says, "You can say with enough confidence that the climate sensitivity model is essentially a model of the type of cloud behavior."

The problem is that in computer simulations of the global climate, today's supercomputers can not solve grid cells that are smaller than about 100 km by 100 km. But clouds often do not exceed a few kilometers. Physicists must therefore simplify or "parameterize" the clouds in their global models, assigning an overall level of cloudiness to each cell of the grid according to other properties, such as temperature and humidity. .

But clouds involve the interaction of so many mechanisms that it is not clear how to best parameterize them. The warming of the Earth and the sky strengthens some of the mechanisms involved in cloud formation, while feeding other forces that dispel them. Global climate models predict 2-degree warming in response to doubling of CO2₂ generally also see little or no change in cloudiness. Models predicting a rise of 4 degrees or more predict fewer clouds in the coming decades.

Climate scientist Michael Mann, director of the Earth System Science Center at Pennsylvania State University, said even two-degree warming would result in "significant loss of life and suffering." He also said it would kill the coral reefs that feed millions of fish, while raising the risk of causing floods, forest fires, droughts, heat waves and hurricanes, as well as causing " several meters of sea level rise and threaten the islands and low-cost coastal cities of the world ".

At the limit of 4 degrees, we would see not only "the destruction of the world's coral reefs, the massive loss of animal species and catastrophic extreme weather events," Mann said, but also "meters of water." rising sea level that would challenge our ability to adapt. It would mean the end of human civilization in its present form. "

It is hard to imagine what would happen if, in a century or more, the stratocumulus disappeared suddenly, causing an 8 degree jump over the warming that has already occurred. "I hope we will never get there," said Tapio Schneider in his Pasadena office last year.

The simulated sky

Over the past decade, advances in computing power and new observations of real clouds have attracted dozens of researchers like Schneider on the problem of the factor X of global warming. Researchers are now able to model high-resolution cloud dynamics by generating simulated cloud patches that closely match actual clouds. This allowed them to see what happens when they increase the level of CO₂.

First, physicists have begun to attack the highest clouds – icy, vaporous clouds like cirrus clouds several miles in height. In 2010, the work of Mark Zelinka of the national laboratory Lawrence Livermore and others has convincingly shown that as the Earth heats up, the highest clouds move higher in the sky and also move towards the higher latitudes, where they do not block as much direct sunlight as they do closer to the Ecuador. This should slightly exacerbate global warming and all global climate models have incorporated this effect.

But the low, thick and turbulent clouds, especially the stratocumulus, are much larger and more difficult than the high clouds. Stratocumulus layers of bright white cover a quarter of the ocean, reflecting 30 to 70% of the sunlight that would otherwise be absorbed by the dark waves below. The simulation of stratocumulus clouds requires considerable computing power because they contain turbulent eddies of all sizes.