Global Warming, Clouds, and Albedo: Feedback Loops
Water plays a crucial role in many processes that determine our climate. Water also defies our human desire to neatly classify things. When does a water molecule in the air that becomes a droplet in a cloud and then a raindrop that falls into the ocean make its transition from being part of the atmosphere to being part of the hydrosphere?
Water vapor and clouds play several important roles in controlling Earth's climate. There are two important and competing feedback loops involving water vapor and clouds. Predicting the net influences these feedback loops produce is possibly the greatest challenge facing modern climate scientists who are trying to determine our future climate.
This diagram illustrates the water cycle (or hydrological cycle). Global warming accelerates evaporation, placing more water vapor in the air. More water vapor generates a a stronger greenhouse effect, since water vapor is a greenhouse gas. However, it may also lead to the formation of more clouds, which might help cool the planet by increasing Earth's albedo.
Rising global temperatures are expected to cause greater evaporation of water vapor into the atmosphere, primarily from the oceans. On one hand, we know that water vapor is a powerful greenhouse gas, so an increase in water vapor might be expected to produce yet more warming through an enhanced greenhouse effect. This warming should further enhance evaporation, producing more water vapor, and leading to a "vicious cycle" (or "positive feedback loop") of more and more warming... and eventually to a "runaway greenhouse effect".
On the other hand, more water vapor in the air is likely to cause more clouds to form. The presence of clouds dramatically increases Earth's overall albedo, reflecting a lot of the incoming sunlight back into space. Increased cloudiness would be expected to further reduce the amount of sunlight reaching our planet's surface, thus providing a net cooling effect. Thus an increase in water vapor, and hence cloudiness, might actually serve as a "self correcting" mechanism (or "negative feedback loop") that would "put the brakes on" global warming; or possibly induce a period of "global cooling".
Which of these two effects will "win out"? Scientists are not entirely certain, and much of the research in climate modeling involves improvements designed to better predict the roles that water vapor and clouds will play in Earth's future climate. Let's take a closer look at some aspects of this puzzle.
Earth's Energy Budget Revisited
The diagram below shows Earth's energy budget. Of the incoming 342 W/m2 of solar energy (sunlight), 77 W/m2 or 23% is reflected back into space by clouds and the atmosphere. Another 30 W/m2 is reflected back into space from Earth's surface. Of the 107 W/m2 that is reflected into space, the portion reflected by clouds and the atmosphere is 72%. Clouds are responsible for about 55% of the sunlight that is reflected into space. Clouds alone roughly double Earth's albedo, from 0.15 (no clouds) to 0.31 (including clouds). In short, clouds are the predominant means by which incoming sunlight is reflected back out into space.
Earth's energy budget diagram. Incoming sunlight is on the left; outgoing infrared or "longwave" radiation is on the right.
Now let's look at the outgoing infrared or "longwave" radiation.
|Major Greenhouse Gas||% of Greenhouse Effect|
36% to 66%
|Water vapor & Cloud droplets||
66% to 85%
9% to 26%
4% to 9%
3% to 7%
The table at the right shows the percent of Earth's greenhouse effect attributeable to each of the listed greenhouse gases. We can see that water vapor alone produces somewhere between about a third and two-thirds of the greenhouse effect. Subtracting, we can also deduce that cloud droplets apparently produce somewhere between 19% and 30% of the greenhouse effect. These are large values in comparison to the effect produced by even the next most important greenhouse gas, carbon dioxide.
Referring once again to the energy budget diagram, note especially that only a fairly small fraction (40 W/m2 or 10.3%) of the 390 W/m2 of infrared radiation emitted from Earth's surface makes it directly into space without first being trapped by various greenhouse gases in the atmosphere. In other words, about 89.7% of the outgoing infrared radiation is affected by the greenhouse effect. Let's see how much of the emitted longwave radiation may be affected by clouds and water vapor. Water vapor "intercepts" about 32% to 59% (36% to 66% times 89.7%) of the outgoing infrared. Clouds "intercept" about 17% to 27% (19% to 30% times 89.7%) of the outgoing infrared. Water vapor plus cloud droplets combine to "intercept" about 59% to 76% (66% to 85% times 89.7%) of the outgoing longwave radiation. The bottom line? Water vapor and clouds are important contributors to the greenhouse effect, and an increase in the amount of water vapor in the air or of the amount of cloud coverage will exert a powerful influence on climate.
Finally, note also in the energy budget diagram that evapotranspiration carries 78 W/m2 of heat upward from Earth's surface to the atmosphere. As the water vapor cools and condenses to form clouds, it releases this "latent heat" into the atmosphere, where it sheds the heat as infrared radiation. Water vapor and clouds, once again, contribute to Earth's energy budget and hence its climate balance.
So what's the bottom line? All these numbers combine to show that water vapor and clouds play a huge role, and in several different ways, in determining the flow of energy and heat within Earth's climate system. About one sixth (17%) of incoming sunlight is reflected back into space by clouds. Water vapor and clouds combine to "intercept" 59% to 76% of the outgoing infrared radiation. Changes to the amount of water vapor of the number of clouds seem certain to have an important affect on climate. However, water vapor and clouds play numerous roles in the climate system, and the net affect of increased evaporation rates caused by global warming are difficult to predict.
There are many different types of clouds. Different types of clouds vary in terms of their effectiveness at reflecting incoming sunlight and their ability to trap outgoing infrared radiation. Effective climate modeling and prediction needs to go beyond determining merely whether there will be more or fewer clouds; the types and locations of clouds matter a lot.
Here is a list of some of the features of clouds that affect their roles in the water vapor/albedo/greenhouse effect feedback loops:
- Altitude: thin, high-altitude cirrus clouds made up of ice particles are nearly transparent to incoming sunlight in visible wavelengths, so they let light in. They are, however, very effective at trapping outgoing long wavelength infrared radiation. An increase in the occurrence of this type of cloud exerts a net warming influence on the atmosphere. Conversely, dense low-altitude cumulus clouds tend to have a high albedo and thus reflect away much of the incoming visible light that strikes them from above. Although they are also good at trapping IR radiation from below, their high reflectance of incoming visible light is such a powerful influence that an increase in this type of cloud tends to exert a net cooling effect on the atmosphere. Subtropical oceans are often covered by huge areas of marine stratocumulus clouds that blanket vast areas with such a high albedo covering. The bottom line: more water vapor in the atmosphere because of increased global temperatures is likely to generate more clouds, but whether this leads to further warming (a runaway positive feedback loop) or to partial cooling (a moderating negative feedback effect) depends on what type of clouds form and where in the atmosphere increased cloudiness occurs.
- Ice versus water clouds: some clouds are made of water droplets, others are composed of ice crystals. Water droplets and ice crystals have different properties in terms of their transparency to visible light and to infrared radiation. So water vs. ice clouds behave differently in terms of both the greenhouse effect and albedo. Altitude and composition (ice vs. water) of clouds are usually closely related.
- Cloud droplet sizes: the water droplets inside clouds can vary in size. Small droplets make clouds better at reflecting incoming sunlight and at trapping outgoing infrared. The specific aerosols that serve as cloud condensation nuclei partially determine droplet sizes; altitude, ambient temperature, and other factors play a role as well.
- Geographic location: sunlight beats down most directly in the tropics, and much less so at high latitudes. Since there is more direct sunlight near the equator, low latitude clouds have the potential to reflect away a lot of incoming energy. Also, polar regions are often covered with snow or ice, so light that gets by the cloud cover is often reflected away by high-albedo ground cover anyways. Much of the tropics is covered by rainforests or oceans, both of which have low albedo; so cloud cover can make an especially large impact on the amount of sunlight received. Deserts often, of course, have clear skies overhead; and generally have relatively high albedos. Improving climate models that can better predict regional variations take such factors into account.
- Seasons: the seasons combine with latitude to determine where sunlight is most intense, and also where surface temperatures generate the highest evaporation rates. Polar regions have periods of perpetual night or day, impacting the potential role of cloud albedo in opposite directions.
- Time of day (or night): you've probably watched summer thunderstorms build over the course of an afternoon, and so intuitively realize that some types of clouds are more prevalent at certain times of day or night. Daytime clouds contribute both albedo and greenhouse effects; nighttime clouds only help to trap outgoing longwave radiation.
Other factors also contribute to the complexities of the water vapor/albedo/clouds/greenhouse effect feedback loops. Here are a few:
- Transport of water vapor: major global atmospheric circulation patterns and weather systems often carry water vapor far from where it evaporated and clouds far from where they formed. Successful climate models must take this transport into account.
- Climate change vs. global warming: though important, the flow of energy and heat and global warming are not the entire story in our changing climate. Clouds do influence the flow of sunlight and infrared radiation, but they also produce precipitation. Changes to clouds generated by changing patterns of energy flow can profoundly influence precipitation patterns. Rain belts or deserts may gradually shift position, in latitude or longitude or both. Some areas may experience more frequent droughts, others more frequent flooding, and others still may experience both! Some climate models predict greater rainfall as global warming induced evaporation increases, but primarily rainfall over oceans. Some models predict wetter coastal regions and drier climates near the centers of continents. Most models agree that disruptions to the water cycle are likely as climate changes.
- Varying time scales of change: different phenomena vary with different characteristic time scales. Bright sunshine on water can increase the evaporation rate within minutes. Clouds persist for hours to days. Water cycles into and out of the atmosphere; the average "residence time" of a water molecule in the atmosphere is around nine days. Increased levels of carbon dioxide in the atmosphere are expected to take 200 years or more to return to prior levels even after increased emissions are halted (if they are). The worlds oceans warm very slowly as heat is added to them; but they also cool back down very slowly. All this means that various changes to climate may not be "in sync" with one another. A change in albedo or greenhouse effect that reduces or increases the amount Earth is heated might take many years to show up in sea surface temperatures, for instance. A major concern, from the human perspective, that results from this lag time is that intentional changes we make to mitigate problems introduced by a changing climate might also take many years to come to fruition. Some problems could easily continue to get worse for many years after we had ceased the behaviors that led to those problems.