Mixed-phase clouds slow down global warming, but only up to a certain point

cloud
Credit: CC0 Public Domain

As the ice in the clouds melts into droplets, they reflect more sunlight. But in the end there is no more ice left to melt.


Clouds that contain both water droplets and ice crystals, calledmixed-phase clouds, have a slowing effect on global warming. This is because the clouds reflect more and more sunlight as ice turns to water.

But what happens when all the ice crystals have turned into droplets?

Doctoral Research Fellow Jenny Bjordal and Professor Trude Storelvmo at the Department of Geosciences at the University of Oslo have tried to figure it out.

“What we have found, is exciting, but also scary,” Storelvmo says to Titan.uio.no.

The results were published today in the scientific journal Nature Geoscience.

Clouds of ice and water

The clouds they have studied are in areas of the atmosphere with temperatures between 0 and 38 degrees below zero.

“The area where we find most of them is across the Southern Ocean. They are also found elsewhere, but that is where most of them are,” Bjordal says.

As climate scientists, they do not look at individual clouds and how they change or move. They study how the clouds change in an area over a long period of timeā€”and whether they move in height, for example.

“These clouds consist of a mixture of ice crystals and drops,” Storelvmo says.

“When the climate gets warmer, more of the ice crystals will melt,” Bjordal explains.

Each small ice crystal turns into many even smaller droplets which gives an overall larger surface.

“Then the clouds reflect more sunlight,” Bjordal says.

“In that sense, those clouds have helped us a little. They have curbed the warming so far,” Storelvmo says.

Feedback effect

What happens to these clouds is an example of what is calledfeedback effects. Higher temperatures on Earth lead to changes in the clouds, which in turn affect the temperature.

Feedback effects can be both positive and negative. What the researchers call a positive feedback, means that the temperature will be even higher. Negative for the climate, that is, but positive in the calculations.

Mixed-phase clouds have a negative feedback. When it gets warmer, they reflect more light. They prevent sunlight from hitting the earth. Positive for the climate, but a minus in the figures.

“This means that we get a smaller temperature rise than we would have had without this effect,” Bjordal says.

The hotter it gets, the more ice will melt. This effect will increase as the temperature rise and keep it in check a bit. It does not stop it, but slows it down.

But only up to a certain point.

What happens when all the ice is melted?

At some point, there will be no more ice left. The clouds will still reflect sunlight, but the effect will not be able to increase and become even greater.

“As long as you have ice that can melt, the clouds will constantly reflect more and more. But eventually it will reach a

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Greenhouse effect of clouds instrumental in origin of tropical storms

Greenhouse effect of clouds instrumental in origin of tropical storms
The cloud greenhouse effect accelerates tropical cyclone development. Schematic depiction of how the trapping of infrared radiation by deep convective clouds leads to locally increased warming (red shading), and how this warming promotes the thermally direct transverse circulation (thin arrows) of the tropical cyclone. (A) An incipient storm, characterized by a weak, broad primary circulation. (B) An intensifying hurricane characterized by a well-defined eye and a strong primary circulation. Credit: James H. Ruppert Jr. / Penn State

With the tropical storm season in the Atlantic Ocean underway and already well into the Greek alphabet for naming, better storm track prediction has allowed timely evacuations and preparations. However, the formation and intensification of these storms remains challenging to predict, according to an international team of researchers who are studying the origin of tropical cyclones.


“There are critical questions around the formation and intensification of hurricanes that makes forecasting them extremely difficult,” said James H. Ruppert Jr., assistant research professor of meteorology and atmospheric science, Penn State. “We don’t yet have sufficient understanding of the processes that drive storm formation.”

Tropical depressions are the weak precursors to intense hurricanes, usually identifiable as a disorganized cluster of clouds in a weak low-pressure area, according to Rupert.

“The tropical depression stage is usually the first time that forecasters are able to identify and start tracking a storm,” he said.

Environmental conditions usually provide a narrow window in which these depressions can form into intense tropical cyclones.

“Understanding the transition from this depression stage to an intensifying hurricane is what we are after,” said Ruppert.

To investigate tropical cyclone formation, the researchers looked at storms forming in the Atlantic and in the western Pacific oceans. They considered two storms, Super Typhoon Haiyan, which occurred in 2013, and Hurricane Maria, which occurred in 2017.

Greenhouse effect of clouds instrumental in origin of tropical storms
Satellite image of Hurricane Maria (2017) as the eye was about make landfall in Dominica. Credit: James H. Ruppert Jr. / Penn State

The researchers found that infrared radiative feedback from clouds creates a localized greenhouse effect that traps heat in the area of the tropical depression. Deep clouds that are heavily laden with water droplets and ice crystals trap outgoing infrared radiation and warm the atmosphere. This local warming causes lifting motion in the storm, which helps fully saturate the atmosphere and increase inward flowing winds near the ocean’s surface. As long as the storm is more than a few degrees above or below the equator, the Coriolis Effect causes these inward flowing winds to form a circulation near the surface. This circulation then intensifies with the help of surface evaporation and eventually forms a central eye, taking on the classic appearance of an intense tropical cyclone.

The researchers found that the localized warming created by the cloud greenhouse effect helped accelerate the formation of both Haiyan and Maria. When they removed the effect in the model simulation, the storms either formed more slowly or not at all. The researchers argue that the cloud greenhouse effect is therefore likely instrumental in

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