The last time I wrote about the role of reflection in the balance of energy of the planet. This time I will write about absorption. First a word about feedbacks--in the last blog I mentioned what is called the ice-albedo feedback. That is, ice melts, less solar energy is reflected, Earth warms, more ice melts. A lot of the comments were about other feedback mechanisms, especially the feedbacks associated with clouds and how aerosols impact clouds. Before I get to feedbacks, I want to make it through absorption, as clouds and aerosols are important in both absorption and reflection.
I will use the same figure as last time and show the Sun with the Earth divided into four major components--the atmosphere, the ocean, land, and ice. Since clouds are so important to the climate, I have also explicitly labeled, surrounding the atmosphere, "cloud-world." To emphasize that we are concerned about the surface, I have placed a thin blue atmospheric layer on the top of the ocean, land, and ice. Climate modelers derive and develop the budget equations for each of these components. Figure 1:
Schematic of the Earth System, which shows the component models of a comprehensive climate model as well as the places that are most important to the absorption of radiative energy.
When talking about the absorption of radiative energy we have to divide the radiative energy into two major pieces. The first is the solar radiative energy which is mostly visible light. At the Earth's surface, when solar energy is not reflected it is absorbed. Hence the darker parts of the surface, for instance, the ocean, red rocks, and black-top parking lots absorb energy. (What about trees?) Solar energy is also absorbed in the atmosphere. There is solar energy in the ultraviolet portion (shortwave) of the spectrum; most of this energy is absorbed in the stratosphere by ozone. There is also solar radiation in the infrared portion (longwave) of the spectrum. In the atmosphere water vapor and carbon dioxide absorb some of this energy. If we look at the total amount of energy that comes from the Sun, about 22% is reflected back to space before it reaches the surface, about 9% is reflected by the surface, about 20% is absorbed in the atmosphere, and, therefore, 49% is absorbed at the surface.
Much of the energy that is absorbed at the surface is ultimately emitted as infrared radiation. This is the energy that is important for keeping the surface warm. Just as the infrared part of the solar spectrum is absorbed by water and carbon dioxide, the terrestrial radiation is absorbed by the atmosphere. After it is absorbed, some of it is emitted back down to the surface, and some is radiated upwards, towards space. Water and carbon dioxide are the most important infrared energy absorbers. Other infrared absorbers are methane, nitrous oxide, and the chlorofluorocarbons. The emission of infrared radiation from the atmosphere back to the surface holds energy near the surface and keeps it warmer than it would be if there was no atmosphere. This is the greenhouse effect and has been known about for more than 200 years. ("Spencer Weart's Carbon Dioxide Greenhouse Effect"
) If you use the idea that energy is absorbed and re-emitted a number of times close to the surface, slowly making its way back up to space, then you can imagine a certain average amount of time that the energy is held close to the surface. When we add greenhouse gases we increase the amount of time that energy stays close to the surface.
Returning to the figure, humans are changing the climate system in two places most importantly. The first and most important is by adding greenhouse gases to the atmosphere. This slows down how fast the planet cools; hence, the surface becomes warmer. The second place we are directly changing the planet is at the land surface. These changes contribute to the greenhouse gas increase. Also, there are changes in the absorption of solar radiation. The changes in the amount and the distribution of energy leads to changes in the motion of the atmosphere and the ocean which, as pointed out by many previous comments to the blog, are trying to move the system towards an equilibrium state.
If you want to find out more about coupled climate models, get model data, and even download a model here are links to two of the United States models used in the "IPCC's Climate Change 2007."
. National Center for Atmospheric Research Community Climate System Model Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Climate Models
ricky (See my new blog at climatepolicy.org