How Do Clouds Affect Climate?

Clouds are a highly complex feature of our climate set-up. They exert both a warming and a cooling effect on the surface of the planet. We explain the different types and their overall effect on climate change. We also look at a new way of measuring cloud properties from space and outline a new theory that predicts the disintegration of clouds.
How Clouds Affect Climate Change

Clouds Influence Climate in Various Ways

Clouds are an important part of Earth’s climate system that manages solar energy arriving from the sun and keeps Earth’s temperature stable.

Clouds affect climate in various ways. Some prevent heat escaping from the planet’s surface, which has a warming effect. Others reflect solar energy back into space, which helps to keep the planet cool. Some scientists believe that as the planet warms, we may not be able to count on this cooling effect.

This contradictory warming/cooling influence on Earth’s climate is extremely confusing for scientists, not least because cloud behavior is notoriously difficult to record and measure.

Clouds also play an essential role in the global water cycle that works continuously to recycle water from the hydrosphere (ocean and lakes) to the atmosphere (air), and then to the pedosphere (soils and surface rock).

In effect, clouds are the Planet’s water transport system, ferrying water around the atmosphere for the benefit of microorganisms, plants, trees, animals and humans.

This biogeochemical transportation role also helps to regulate climate. For example, without rainfall, there would be no rainforest, or indeed any trees or plants to remove carbon dioxide (CO2) from the air. Without snowfall to freeze into ice, there would be no glaciers or polar ice sheets to maintain Earth’s albedo, so the planet would heat up very quickly. In a nutshell, without a regular supply of water – delivered by clouds – life on Earth would be impossible.

How Do Clouds Form?

To understand how clouds form let’s take a quick look at the three stages of the water cycle.

  • Stage 1. Water evaporates (turns from a liquid into a gas) from the ocean or other bodies of water (or from land sources). 
  • Stage 2. Rising air currents carry the water vapor up into the atmosphere. As it rises, it cools and eventually condenses into liquid water droplets. Water vapor finds it easier to condense into water droplets when it has something to condense upon, like a small particle of dust, soot, pollen, sea salt, or piece of organic matter. These tiny aerosols, are known as condensation nuclei. After a while, enough water vapor condenses to form a cloud. (A cloud is simply a visible accumulation of water droplets.)
  • Stage 3. Either the cloud becomes so heavy that it rains (or snows), or sunshine causes the liquid droplets turn back into vapor and the cloud dissipates. About 505,000 cubic km (121,000 cubic mi) of water falls as rain or snow every year – an average of 990 mm (39 inches) over the entire surface of the Earth.

Why Are Clouds White?

The reason clouds are white is because light from the Sun is white. 1

Ordinarily, when sunlight hits an atmospheric particle in the sky – like a particle of dust or pollen – more blue light is scattered away than any other colour in the light wave, making the sky look blue.

However, when sunlight passes through a cloud, it meets and interacts with water droplets, which are much bigger than the atmospheric particles we just mentioned. These droplets scatter all colours of the light wave equally, meaning that the sunlight continues to remain white. The clouds therefore appear white against the background of the blue sky.

But some clouds look grey. Why is this? The top of the cloud (the part nearest the sun) receives the most light. So, this typically looks pure white. The bottom of the cloud receives the least light – in fact, sometimes, part of it gets almost no light at all – and for this reason may look grey or black. The side of a cloud is not usually as white as the top, and nowhere near as grey or black as the bottom.

The next time you travel by plane, look out the window when you’re above the clouds. All the tops of the clouds will be a brilliant white.

The fact that cloud tops are all bright white in colour is extremely important when it comes to assessing the effect of clouds on Earth’s climate.

NOTE: Regular clouds have nothing in common with phenomena like the so-called “Asian Brown Cloud“, a type of atmospheric haze consisting of airborne particles of soot, black carbon, fly ash, and numerous other toxic compounds. It appears every year during India’s dry season, and is caused (mostly) by emissions from the burning of coal, wood and crop stubble across the region.

Where Are Clouds Located?

Nearly all water vapor in the atmosphere is found in the troposphere, the layer of air closest to the surface of the Earth. It’s here that gravity is strongest, where all greenhouse gases accumulate to trap the radiant heat emitted by the planet, and it’s where nearly all the clouds are. The troposphere is easily the wettest layer of the atmosphere. By comparison, the stratosphere – the layer immediately above the troposphere – is very dry since it contains little water vapor. As a result, almost no clouds are found in this layer, with the exception of polar stratospheric clouds (PSCs) that appear occasionally over the poles when temperatures fall below minus 78°C.

Diagram: Different types of cloud types in stratosphere
Clouds affect climate either by reflecting sunlight back into space (cooling effect) or by absorbing heat escaping from the surface of the Earth (warming effect). Image: Argonne National Library

Cloud Levels In The Sky

Clouds and their appearance in the sky are commonly categorized as high-level, mid-level or low-level. Here are the standard heights for clouds in the mid-latitudes. Cloud heights in tropical zones are significantly higher. Cloud levels tend to be lowest in polar regions. 2 3

  • High-Level Clouds (5-13 km/ 16,000-43,000 ft)
    Cirrocumulus, cirrostratus, cirrus.
  • Mid-Level Clouds (2-7 km/ 7,000 – 23,000 ft)
    Altocumulus, altostratus.
  • Low-Level Clouds (0-2 km/ up to 7,000 ft)
    Nimbostratus, stratocumulus, stratus. 
    Cumulonimbus and cumulus clouds may also appear at low levels but grow vertically up to 13 km/ 40,000 ft. Cumulonimbus clouds are associated with major thunderstorm systems, like supercells, that produce tornadoes across the central/southern United States. These two types of cloud also form much of the cloud structure of hurricanes and cyclones.

How Much Of Earth’s Surface Is Covered By Cloud At Any One Time?

Roughly 70 percent of Earth’s surface is obscured by clouds at any given time. One review of satellite photographs of Earth, taken over a 10-year period, showed average global cloud cover of 67 percent. 4

Oceans are cloudier than continents. Less than 10 percent of the sky over the ocean is completely cloud-free, compared to 30 percent over land.  

Three areas exist where the sky is most likely to be cloudy: a narrow band near the equator and two wider bands in the mid-latitudes. The equatorial band is where the Inter Tropical Convergence Zone (ITCZ) sits. This is a belt of low pressure which circles the Earth close to the thermal equator where the trade winds of the Northern and Southern Hemispheres come together. It is noted for its thunderstorms, especially over land masses.

The two other bands of regular cloud formation lie in the mid-latitudes 60 degrees north and south of the equator, caused by the pattern of wind circulation cells moving to the poles.

The important point about cloud coverage, is that it reflects solar radiation back into space, thus helping to reduce the amount of heat absorbed by Earth’s surface.

How Do Clouds Affect Climate?

Clouds have both a cooling and a warming effect on Earth’s climate.

During the day, the white cloud tops cool the Earth’s surface by reflecting shortwave solar radiation from the sun and reducing the amount of solar energy that is absorbed by the surface. This reflective capacity or “albedo”, stems from their bright white color. For example, cumulonimbus clouds reflect up to 90 percent of the sunlight they receive (the same as fresh snow, traditionally the most reflective), while stratocumulus clouds reflect 60 percent and cirrus clouds 50 percent. As a rule of thumb, lower, thicker clouds tend to reflect the Sun’s heat the most. 5 This reflection of sunlight produces a significant cooling effect.

At night, clouds help to make Earth’s temperature warmer by trapping heat radiation from the surface. (A cloudy night is invariably much warmer than a cloudless night.) The Earth radiates infrared heat which is easily absorbed by water in the clouds. The water reacts by radiating, also in the infrared spectrum, both upward and downward, and the downward radiation results in increased warming at the surface. This is similar to the greenhouse effect produced by CO2 and water vapor.

Thus, clouds both reflect incoming sunlight and prevent the radiation of heat radiation from the Earth’s surface, thereby boosting both sides of the global energy balance equation.

Which clouds cool the planet and which warm it? It depends largely on how high the clouds are in the atmosphere. Clouds up to about 6,000 feet (1 mi) of the surface, tend to cool more than they warm. Whereas clouds high up in the atmosphere have the opposite effect: they tend to warm more than they cool. 6

Climate Variability and Clouds
Regional weather cycles – such as the Indian Ocean Dipole (IOD) and the El Nino-Southern Oscillation (ENSO) in the tropical Pacific, and the travelling Madden-Julian Oscillation (MJO) in the Indo-Pacific, are driven by sea surface temperature and cloud formation. Their mechanisms are not thought to be directly controlled by climate change, but their overall effects probably are.

What Is The Net Effect Of Clouds On Global Warming?

Scientists used to think that clouds had a net cooling effect of about 5°C (9°F). In other words, if Earth was completely cloudless, and there were no clouds to reflect sunshine or trap escaping heat, the world would be 5°C warmer. 7 In 2013, however, the IPCC assessed net cloud feedback as being “between near-zero and moderately positive.” In other words, the warming effect of clouds is likely to be slightly more powerful than its reflective cooling effect. But the matter is not definitively settled and the scientific debate continues.

Will Clouds Increase Or Reduce Global Warming In The Future?

If Earth’s temperature continues to rise, due to the greenhouse effect, the planet’s weather patterns are going to change, as will the planet’s cloud structure and behaviors. The question is, will cloud changes amplify the warming (a positive feedback) or diminish the warming (a negative feedback)? We know, for example, that higher greenhouse gas emissions lead to higher surface temperatures, provided nothing else changes, but what happens if, as part of the climatic response, the clouds themselves increase or diminish? Right now, clouds affect climate, but as global warming increases, the climate will start to affect the clouds. What will happen then?

In a warmer climate, one would expect more water to enter the atmosphere by evaporation at the ocean surface, causing an increase in cloud formation and cover. But in a warmer climate, higher temperatures might also tend to evaporate clouds. 8 Both of these statements are theoretically accurate, and both outcomes, known as cloud feedbacks, feature in today’s climate model calculations. 9

So how will clouds affect climate change? The truth is, we don’t know. No one knows yet whether clouds will accelerate or restrain global warming. In the meantime, researchers are scrambling to increase the sophistication of their climate models in order to simulate clouds’ ever-changing qualities, such as the sizes of the tiny ice particles and water droplets that compose them.

A New Method Of Measuring Cloud Properties From Space

In a new study, scientists have come up with a new way of measuring cloud properties from space more accurately, which could improve climate forecasts. The researchers used two types of remote sensing technology – passive and active sensors – to capture different properties of clouds at high resolution.

Improving on an existing instrument called Tropospheric Water and Cloud ICE (TWICE), the new instrument – known as Earth’s Next-generation ICE (ENTICE) – is able to capture the effective diameter of ice particles within clouds while simultaneously measuring humidity and temperature. Such measurements are vital to fully appreciate the processes that occur within clouds and could enhance both climate models and weather forecasts, the study says. 10

New Study Predicts Disintegration Of Clouds

According to a new study, huge areas of stratocumulus clouds in the Earth’s atmosphere – which normally reflect sunlight away from the planet – could disintegrate, destabilizing Earth’s radiative equilibrium and causing global temperatures to rise by 8°C (14°F). 11

True, the circumstances that might trigger such an event are extreme. Atmospheric carbon dioxide levels would have to reach about 1,200 parts per million, or roughly triple their current levels. But researchers point out that under a high emissions climate scenario, with no agreement to control CO2 emissions in the future, CO2 levels are likely to approach 1,000 ppm towards the end of the century.

The study also implies that the 1,200 ppm level might be a major climate tipping point, when all clouds will start to break up. This point has been queried by some scientists who say that while clouds may indeed start to dissipate in response to high levels of CO2, it is more likely to occur at different times around the world.

Whatever the specific scientific merits of its theoretical conclusions, the study has focused attention on the issue of how clouds affect climate, and how best to model their interactions. Indeed, finding ways to simulate cloud behaviors is a hot topic in climate science and one of today’s fastest-growing priorities among climate scientists.

The new study uses a technique known as a “large-eddy simulation.” It focuses on tracking the behavior of tiny particles that affect the formation of individual clouds over a small area, and then scales up the results to a wider level. It represents a significant improvement over regular models, but global simulations are not yet possible.

Tapio Schneider, lead author of the study, is also the principal investigator on a new project called the Climate Modeling Alliance, a collaborative project to build an earth system model that includes precise behaviors of clouds.

Other researchers have similar goals. Climate scientists from the University of California, Irvine, Columbia University and the Ludwig Maximilian University of Munich are experimenting with ‘deep learning’ — a type of machine learning that uses artificial neural networks – to improve how clouds are represented in large-scale climate models. Known as the “Cloud Brain”, their project, is learning to predict the outcomes of models that specifically simulate clouds. Watch this space.

References

  1. Why are clouds white?” UK Met Office. (1) []
  2. “The types of clouds: everything you need to know.” Tibi Puiu ZME Science. July 8, 2017. (2) []
  3. “Cloud Types.” UCAR Center for Science Education. 2019 (3) []
  4. King et al; “Spatial and Temporal Distribution of Clouds Observed by MODIS Onboard the Terra and Aqua Satellites.” IEEE Transactions on Geoscience and Remote Sensing. (2013) (4) []
  5. “Albedo.” Dr. Timothy Bralower and Dr. David Bice, College of Earth and Mineral Science, The Pennsylvania State University. (5) []
  6. Ackerman, Steven A. (2011). Meteorology: Clouds and the Greenhouse Effect. Jones & Bartlett. (7) []
  7. Cloud Climatology.” NASA Goddard Institute for Space Studies. International Satellite Cloud Climatology Project (ISCCP) []
  8. “A World Without Clouds.” Natalie Wolchover. Quanta Magazine. Feb 25, 2019. []
  9. Randall, D. et al. (2007) “Climate models and their evaluation” in S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, M.Tignor, and H. Miller (eds.) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. (10) []
  10. Simulation of Remote Sensing of Clouds and Humidity from Space Using a Combined Platform of Radar and Multifrequency Microwave Radiometers.” Jonathan H. Jiang, Qing Yue, Hui Su, Pekka Kangaslahti, Matthew Lebsock, Steven Reising, Mark Schoeber, Longtao Wu, Robert L. Herman. Earth and Space Science. Volume 6, Issue 7, Pages 1234-1243, July 2019. (11) []
  11. See for instance: “Possible climate transitions from breakup of stratocumulus decks under greenhouse warming.” Schneider, T., Kaul, C.M. & Pressel, K.G. Nat. Geosci. 12, 163–167 (2019). (8) []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email