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On the Solar Trail

By: Dr. Ricky Rood, 5:12 PM GMT on September 16, 2008

The Sun (3): This is the third of a series on the Sun in the Earth-Sun climate system. The first two entries are linked at the end. I am, really, heading towards the original question of the solar cycle and how solar variability is included in climate predictions. It is, however, a difficult subject and there is work to be done to get us there.

The first two blogs talked about the need for there to be mechanisms to “amplify” the change in heating associated with solar variability to understand the impact of solar variability on the Earth’s climate. One of the easiest ways to visualize how a small signal might be amplified is to think of ice. At the foundation of this reasoning is the assumption, based on observations, that the climate and weather are in some near “equilibrium” balance. This is a balance where energy coming in from the Sun is absorbed or reflected and what comes in, ultimately, goes out. In this ice example, if it got a little colder because of a decrease of solar energy, it could make more ice, which would reflect more solar energy and lead to more cooling. This interaction between cooling and ice leading to more cooling, a positive feedback, is easy to conceive.

The importance of greenhouse gases to the climate and the habitability of the Earth cannot be overstated. Without greenhouse gases the surface of the Earth would be about -18 degrees C (~zero degrees F). That is, without an atmosphere with greenhouse gases, the Sun’s radiation would just be re-emitted to space.

Careful consideration of Figure 1 demonstrates the importance of greenhouse gases. 161 watts/m^2 of solar energy is absorbed at the surface. In the absence of an atmosphere, we would expect 161 watts/m^2 to be emitted back to space. But let’s count how much energy is emitted from the surface. 17 watts/m^2 leave the surface due to thermals; this is essentially heating at the surface making hot air that rises. 80 watts/m^2 leaves the surface through evaporation and condensation of water. And 396 watts/m^2 are emitted from the Earth’s surface as infrared radiation. We have the situation where 161 watts/m^2 come to the surface and 396 + 80 + 17 = 493 watts/m^2 leave the Earth’s surface! How is this possible?

The answer is greenhouse gases. Let’s try to follow the energy. The Sun provides 161 watts/m^2, and after this visible radiation is absorbed at the surface, the surface has 161 watts/m^2 that it can give up. So this 161 watts/m^2 starts to return to space as infrared radiation. The greenhouse gases absorb and store much of this infrared radiation in the atmosphere and returns a large part of it to the surface. Ultimately, more energy, 333 watts/m^2, returns to the surface from this atmospheric store than is provided directly by the Sun. The greenhouse gases are like a blanket; they hold the heat close to the surface for a while. This exchange of heat between the atmosphere and the surface evolves to equilibrium. This 333 watts/m^2 is essentially the amount of energy that is cycled back and forth between the atmosphere and the surface. Subtract 333 from that 493 in the previous paragraph, you get 160, and that just about balances the 161 that comes from the Sun.

The point of this tour through the radiation balance is that greenhouse gases greatly alter the flow of energy from the Sun and its return to space. The temperature at the surface of the Earth is directly determined by both the Sun and the greenhouse gases. Given that, ultimately, there is more flow of energy to the Earth’s surface from the atmosphere than comes directly from the Sun, the greenhouse gases can be viewed in the spirit of an amplifier. With this idea, how the energy at the surface changes is expected to be as, or perhaps more, sensitive to greenhouse gas changes as to changes in the incoming solar energy … assuming, really, that both are incremental changes from the established equilibrium.

What are the greenhouse gases? Water vapor is the most important greenhouse gas. A lot of you ask, what is the contribution of water vapor to the greenhouse effect? It’s not a completely trivial question to answer because water changes phases and makes clouds, and clouds then contribute to the greenhouse effect even more powerfully. I think it is reasonable to say that water and clouds together account for about two thirds, say 65%, of the greenhouse warming of the surface. (I link a dense and classic paper by Ramanathan and Coakley on the subject.) Carbon dioxide is a little less than 10%.

I want to return to the figure again. On the figure there is a 40 watts/m^2 that is returned from the surface of the Earth directly to space. If you consider only water vapor, there are some wavelengths of radiation that are, essentially, completely absorbed. But there are windows where radiation goes directly to space. The greenhouse gases like carbon dioxide and methane have a big impact partly because they start to fill up those windows. And as we all know having a crack in a window can have a big impact on the temperature in the bedroom. Little things can have big effects.


Blogs on the Sun.

The Sun (1)

The Sun (2)

Blogs on radiative balance

Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm

Figure 1: This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.