Download Name: Period: ______ Global Warming, Clouds, and Albedo

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Period: _______
Global Warming, Clouds, and Albedo: Feedback Loops
Water plays a crucial role in many processes that determine our climate. 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.
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
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.
Now let's look at the outgoing infrared or "long wave" radiation.
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 long wave 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 long wave radiation. So
what is 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, evapotranspiration (total evaporation from the earth- soil, ocean, plants) 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. Latent heat is heat absorbed or released as the result of a phase
change. 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 effect on climate. However,
water vapor and clouds play numerous roles in the climate system, and the net effect of increased evaporation rates caused by
global warming are difficult to predict.
Questions  
Analysis:
1. Name 2 abiotic factors mentioned in the article that affect the climate of an area.
2. Explain the difference between a positive feedback loop and a negative feedback loop without using examples.
3. Give 1 example of a positive feedback loop regarding water and climate.
4. Give 1 example of a negative feedback loop regarding water and climate.
Go to: http://d3tt741pwxqwm0.cloudfront.net/WGBH/ipy07/ipy07_int_albedo/ipy07_int_albedo.html
Go through the animation, paying close attention to the analysis questions that you just answered.
5. Explain the feedback system shown in this animation and explain if it is a positive feedback system or a negative
feedback system.