Stories of Other Worlds
How to Build Climate Zones for a World: Greenhouse Warming--Text
Earth Science Essentials
by Russ Colson
How Greenhouse Warming Works
The idea of greenhouse warming is related to two ideas that we've already talked about--heat balance and black body radiation.
For average world temperature to stay the same, the amount of heat coming in must equal the amount of heat going out. Most of Earth's energy comes from the Sun (with a small amount coming from radioactive decay and crystallization within the Earth's interior). This means that for the average temperature of Earth to stay constant, energy coming in from the Sun must equal energy leaving the Earth by blackbody radiation.
Energy arriving from the Sun is mostly visible light. Earth's atmosphere does not absorb visible light (otherwise we couldn't see anything), so the light is largely able to reach Earth's surface, with some absorption by dust in the atmosphere, some reflection by clouds, and so forth. Much of the light that reaches the surface is absorbed, making the ground warmer.
The energy radiated by the Earth (black body radiation) is mostly as infrared light. Infrared energy can be absorbed by some gases in the atmosphere, such as carbon dioxide, methane, and other greenhouse gases. Thus, as the radiation streams out toward space, the more carbon dioxide and other greenhouse gases are present, the more of the infrared is absorbed, raising the atmospheric temperature. As the temperature rises, the atmosphere 'glows' more (black body radiation) until the atmosphere again reaches a balance where energy in and energy out are equal--but with more greenhouse gases, this balance occurs at a higher average temperature.
Understanding what you've read
Climatic Feedback Loops :
We don't know for sure what natural variations in climate we might expect in the future, although evaluation of past climate variations can give us clues. Nor do we know exactly how human activity might alter future climate, but we can make some predictions. We know that the potential exists for a "runaway" greenhouse effect, where feedback loops cause a warming trend to get amplified, as we know that this has happened at several times in the geological past.
Scientists are working toward understanding our climate in the methodical way of science. This can be frustrating for policy makers who want concrete answers now. The ambiguity and uncertainty of science as it explores a new thing, which are a part of the scientific process, can often confuse people about the purpose and nature of science. But there are things we really do know. The key to understanding science is to identify what we do know, and how we know it, and what we don't know, and how we might learn it.
Below, I give some example feedback loops that might be important in our atmosphere. These are simplified, and so no one of them tells the whole story of what affects climate. However, I give them so that you can get a feel for how challenging it is to try to predict how climate will change in the future.
Examples of positive feedback loop (strengthening the original cause of change)
Suppose that you get a warming trend on Earth (by whatever means, human activity, changes in the Sun, whatever). How will this affect the Earth's energy balance? Well, one change in Earth might be that the warmer conditions melt some of the ice at the poles. With less ice, more sunlight is absorbed by darker ground. With more absorption, there is more warming, that causes even more ice to melt, which causes more warming, and so on.
Here's a less obvious example. Let's do a thought experiment with a can of pop. CO2 gas is the carbonation in pop, what gives it its fizz. Will pop go flat faster if it's warm or if it's cold?
Warm, of course. This is because CO2 dissolves easier into cold water than warm water. Earth's oceans hold more than twice as much CO2 as the atmosphere, most in the cold, deeper water.
Now, let's think about what will happen if the Earth warms. A warming atmosphere causes warming water, which causes CO2 to bubble out of the oceans into the atmosphere. More CO2 in the atmosphere causes more greenhouse warming, which causes more CO2 to bubble from the ocean, and so on.
Example of a limiting feedback (weakening the original cause of change)
Warmer conditions might change the rate of plant growth. For example, warmer conditions in the arctic might extend the region of planet growth farther north. To grow, plants extract CO2 from the atmosphere. This can decrease the greenhouse effect and limit the warming trend. However, this process is itself limited because eventually new plant growth will die and the CO2 will be returned to the atmosphere by decay.
Also, it's possible that warmer conditions might cause more rapid decay, especially in arctic areas where ice is melting. This could cause a positive feedback loop like those above.
Example of an uncertain feedback loop :
Clouds reflect sunlight back into space before it ever gets absorbed by the Earth. So, more clouds can cause cooler conditions. However, water vapor in the air is a greenhouse gas like carbon dioxide, and so higher humidity can cause additional greenhouse warming.
Based on what we learned in previous units, we expect that, if the Earth gets warmer, the capacity of the atmosphere to hold water vapor will increase. Thus, warmer conditions could cause more evaporation into the atmosphere, which could cause more greenhouse warming, and even more evaporation.
But, here's the complexity. More water vapor in the air might also result in more clouds when condensation occurs. More clouds could result in more reflection of sunlight which moderates the warming trend.
Interesting side note: Venus, which has 3000 times more CO2 in its atmosphere than Earth, has an average surface temperature of 460ºC (hot enough to melt the lead in your deer cartridges, presuming you don't hunt in a lead-free state).
Understanding what you've read
Last updated Oct 16, 2015. All text and pictures are the property of Russ Colson..
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