Taking the Measure of the Greenhouse Effect
Most of us have heard that the greenhouse effect keeps the planet much warmer than it would be otherwise, and similarly we may have heard that increasing amounts of greenhouse gases are enhancing the natural greenhouse effect. But few of us appreciate what exactly it is in the atmosphere that makes the effect work and why small changes in trace gases such as carbon dioxide (CO2) might make a difference.
It has been understood since the 19th century that some gases absorb infrared radiation (IR) that is emitted by the planet, slowing the rate at which the planet can cool and warming the surface. These so-called greenhouse gases include carbon dioxide and water vapor, as well as ozone and methane among others. Note, however, that the bulk of the atmosphere is made up of nitrogen and oxygen molecules which don't absorb IR at all. Less well appreciated is that clouds (made of ice particles and/or liquid water droplets) also absorb infrared radiation and contribute to the greenhouse effect, too. Clouds, of course, also interfere with incoming sunlight, reflecting it back out to space, making their net effect one of cooling, but their contribution to the greenhouse effect is important.
Outgoing spectral radiance at the top of Earth's atmosphere showing the absorption at specific frequencies and the principle absorber. For comparison, the red curve shows the flux from a classic "blackbody" at 294°K (≈31°C ≈ 69.5°F). (View larger image)
The size of the greenhouse effect is often estimated as being the difference between the actual global surface temperature and the temperature the planet would be without any atmospheric absorption, but with exactly the same planetary albedo, around 33 °C. This is more of a "thought experiment" than an observable state, but it is a useful baseline. Another way of quantifying the effect is to look at the difference between the infrared radiation emitted at the surface of the Earth, and the amount that is emitted to space at the top of the atmosphere. In the absence of the greenhouse effect, this would be zero (in other words, no difference). In actuality the surface emits about 150 Watts per square meter (W/m2) more than goes out to space.
So of all the greenhouse substances in the atmosphere, which of them absorbs what? This is a more complicated issue than it might first appear because of the nature of the absorption and the complex distribution of absorbers both horizontally and vertically. Different substances absorb different frequencies of IR, and the different parts of the planet differ wildly in how much IR is being emitted (based as it is on surface temperature) and how much cloud and water vapor there is at that location (carbon dioxide is very well mixed). Indeed, some wavelengths of IR can be absorbed by both water vapor or clouds, or water vapor and CO2. This "spectral overlap" means that if you remove a substance, the change in how much IR is absorbed will be less than if you only had that substance in the air. Alternately, the impact of all the substances together is less than what you would get if you added up their individual components. This needs to be taken into account in any attribution of the greenhouse effect.
A satellite map of the outgoing longwave radiation emitted by Earth in September 2008 demonstrates not only geographical variations but also those caused by cloud presence. More heat escapes from areas just north and south of the equator, where the surface is warmer and there are fewer clouds. (Image: NASA/Earth Observatory/Robert Simmon from CERES data.)
We use the GISS model of radiative transfer through the global atmosphere to try and break down the attribution using realistic distributions of local temperature, water vapor and clouds. By removing each of the absorbers in turn and calculating the absorption for many different combinations, we can calculate all the overlaps and allocate the absorption fairly. We find that water vapor is the dominant substance — responsible for about 50% of the absorption, with clouds responsible for about 25% — and CO2 responsible for 20% of the effect. The remainder is made up with the other minor greenhouse gases, ozone and methane for instance, and a small amount from particles in the air (dust and other "aerosols").
Given that CO2 has such a major role in the natural greenhouse effect, it makes intuitive sense that changes in its concentration because of human activities might significantly enhance the greenhouse effect. However, calculating the impact of a change in CO2 is very different from calculating the current role with respect to water vapor and clouds. This is because both of these other substances depend on temperatures and atmospheric circulation in ways that CO2 does not. For instance, as temperature rises, the maximum sustainable water vapor concentration increases by about 7% per degree Celsius. Clouds too depend on temperature, pressure, convection and water vapor amounts. So a change in CO2 that affects the greenhouse effect will also change the water vapor and the clouds. Thus, the total greenhouse effect after a change in CO2 needs to account for the consequent changes in the other components as well. If, for instance, CO2 concentrations are doubled, then the absorption would increase by 4 W/m2, but once the water vapor and clouds react, the absorption increases by almost 20 W/m2 — demonstrating that (in the GISS climate model, at least) the "feedbacks" are amplifying the effects of the initial radiative forcing from CO2 alone. Past climate data suggests that this is what happens in the real world as well.
What happens when the trace greenhouse gases are removed? Because of the non-linear impacts of CO2 on absorption, the impact of removing the CO2 is approximately seven times as large as doubling it. If such an event were possible, it would lead to dramatic cooling, both directly and indirectly, as the water vapor and clouds would react. In model experiments where all the trace greenhouse gases are removed the planet cools to a near-Snowball Earth, some 35 °C cooler than today, as water vapor levels decrease to 10% of current values, and planetary reflectivity increases (because of snow and clouds) to further cool the planet.
Despite being a trace gas, there is nothing trivial about the importance of CO2 for today, nor its role in shaping climate change in the future.
Related Links
Science Brief: CO2: The Thermostat that Controls Earth's Temperature
NASA News Release: Carbon Dioxide Controls Earth's Temperature
AGU News Release: Taking Measure of the Greenhouse Effect
References
Schmidt, G.A., R. Ruedy, R.L. Miller, and A.A. Lacis, 2010: The attribution of the present-day total greenhouse effect. J. Geophys. Res., in press.
Lacis, A.A, G.A. Schmidt, D. Rind and R.A. Ruedy, 2010: Atmospheric CO2: Principal control knob governing Earth's temperature Science, 330, 356-359, doi:10.1126/science.1190653
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