just what is missing from the model we’ve constructed so far). Up to this point, recall, we’ve been treating the Earth as if it is a naked point: the only feature we’ve added thus far is planetary albedo, which can be thought of as just preventing some energy from reaching the planet in the first place. This is reflected (no pun intended) in the fact that our albedo factor α modifies the value of the solar radiance term So directly: albedo comes in on the left side of the equation on our model. What we’re looking for now, remember, is something that modifies the value on the right side of the equation. In order to do that, we have to tinker with the energy not before it is received, but as it is released back into space. This is what the greenhouse effect does.
But how? Departing from our ZDEBM for a moment, consider the way the atmosphere of the Earth is actually structured. The Earth’s atmosphere is highly non-uniform in several different ways. Most importantly for us right now, the atmosphere is an extremely heterogeneous mixture, containing significant amounts of several gasses, trace amounts of many more, and small airborne solids (e.g. specks of dust and soot) collectively called “aerosols.” Ignoring aerosols for the moment (which are far more relevant to albedo calculation than to the greenhouse effect), the composition of the atmosphere looks like this:
- Aerosols like dust, soot, and sulfate aerosols (which are a byproduct of fossil fuel combustion) modify the albedo directly and indirectly. Direct modification comes as a result of radiation scattering (increasing the albedo of the atmosphere in which they are suspended, providing a kind of “miniature shade”). Indirect modification comes as a result of their action as nuclei of cloud condensation: they make it easier for clouds to form in the atmosphere by acting as “seeds” around which water vapor can condense into clouds. This leads to increased cloud formation and average cloud lifespan (increasing albedo), but also reduced precipitation efficiency (since less water vapor is needed to form clouds, so clouds that do form are less moisture-dense). Aerosols thus play an important (and complicated) role in climate forcing: a role which is beyond the scope of our current discussion. They will be discussed in more detail when we consider feedback mechanisms in Section 4.2.
- Source for figures: Carbon dioxide: NOAA (2012), Methane: IPCC AR4 (2007).