At the 2009 meeting of the American Geophysical Union, renowned climate scientist Richard Alley (Penn State) gave a keynote address, The Biggest Control Knob: Carbon dioxide in Earth’s Climate History, in which he used a variety of paleoclimatological proxy data to show how CO2 changes over much of Earth history have exerted a strong influence on global temperatures.
In this week’s issue of Science, Andrew Lacis and colleagues published an article, Atmospheric CO2: Principal control knob governing Earth’s temperature (abstract only; subscription required), following up on this theme. Unlike Alley’s talk, which mainly focused on the role of CO2, this team starts by going after water vapor and confronting a widely held perception that it is the dominant greenhouse gas:
It often is stated that water vapor is the chief greenhouse gas (GHG) in the atmosphere. For example, it has been asserted that “about 98% of the natural greenhouse effect is due to water vapour and stratiform clouds with CO2 contributing less than 2%”. If true, this would imply that changes in atmospheric CO2 are not important influences on the natural greenhouse capacity of Earth, and that the continuing increase in CO2 due to human activity is therefore not relevant to climate change. This misunderstanding is resolved through simple examination of the terrestrial greenhouse.
Water vapor is a main reason why the world has a pleasant and life-sustaining average temperature of 16 degrees C. Based on the distance of Earth from the Sun, physics tells us that Earth should be about 0 degrees C—a giant snowball hurling through space. The reason why we are warmer than this is because of the natural envelope of greenhouse gases, including water and CO2 that absorb longwave heat radiating from the surface. This warms the surface of the planet just like a thick blanket keeps your body heat near your skin on a cold night.
In round numbers, water vapor accounts for about 50% of Earth’s greenhouse effect, with clouds contributing 25%, CO2 20%, and the minor GHGs and aerosols accounting for the remaining 5%.
So water vapor and clouds make up about 75% of the greenhouse effect, which sounds like the definition of “the dominant greenhouse gas” to most of us. How does one show that CO2 really is more important than water vapor as a primary greenhouse gas driving temperature change when it looks like water is so important?
First, this research team described the roles of each of these gases in greenhouse warming. When you increase the concentration of CO2 in model experiments, this has the effect of raising temperature. We say that CO2 is therefore a direct driver of temperature change. Water vapor, on the other hand, plays a secondary, but very large, role by amplifying the initial temperature increase cause by CO2 and other greenhouse gases.
Why is this? Think of the ocean. When temperatures warm up, more water vapor evaporates to the atmosphere, and some of this condenses to form clouds. This water vapor and cloud water thus represent a feedback to the climate system:
Because CO2, O3, N2O, CH4, and chlorofluorocarbons (CFCs) do not condense and
precipitate, noncondensing GHGs constitute the key 25% of the radiative forcing that supports and sustains the entire terrestrial greenhouse effect, the remaining 75% coming as fast feedback contributions from water vapor and clouds.
It still looks like water has a stronger greenhouse effect, so is this just a matter of semantics? Who cares?
Here’s why this is cool: If CO2 and other greenhouse gases (excluding water) were dramatically reduced—as in scrubbed out of the atmosphere completely—the world would turn into an ice ball, which is NOT something you would expect if CO2 didn’t matter as a greenhouse gas:
A clear demonstration is needed to show that water vapor and clouds do indeed behave as fast feedback processes and that their atmospheric distributions are regulated by the sustained radiative forcing due to the noncondensing GHGs. To this end, we performed a simple climate experiment with the GISS 2° × 2.5° AR5 version of ModelE, using the Q-flux ocean with a mixedlayer depth of 250 m, zeroing out all the noncondensing GHGs and aerosols. The results…show unequivocally that the radiative forcing by noncondensing GHGs is essential to sustain the atmospheric temperatures that are needed for significant levels of water vapor and cloud feedback. Without this noncondensable GHG forcing, the physics of this model send the climate of Earth plunging rapidly and irrevocably to an icebound state, though perhaps not to total ocean freezeover.
This is a great demonstration of the feedback in the other direction—cooler conditions brought about by lower CO2 leads to less water evaporation and clouds, which feeds back on the cooling by causing even more cooling now that the water component of the greenhouse effect is rapidly diminishes following the decrease in CO2:
The scope of the climate impact becomes apparent in just 10 years. During the first year alone, global mean surface temperature falls by 4.6°C. After 50 years, the global temperature stands at –21°C, a decrease of 34.8°C. Atmospheric water vapor is at ~10% of the control climate value (22.6 to 2.2 mm). Global cloud cover increases from its 58% control value to more than 75%, and the global sea ice fraction goes from
4.6% to 46.7%, causing the planetary albedo of Earth to also increase from ~29% to 41.8%. This has the effect of reducing the absorbed solar energy to further exacerbate the global cooling.
After 50 years, a third of the ocean surface still remains ice-free, even though the global surface temperature is colder than –21°C.At tropical latitudes, incident solar radiation is sufficient to keep the ocean from freezing. Although this thermal oasis [exists] within an otherwise icebound Earth…
From the foregoing, it is clear that CO2 is the key atmospheric gas that exerts principal control over the strength of the terrestrial greenhouse effect. Water vapor and clouds are fast-acting feedback effects, and as such are controlled by the radiative forcings supplied by the noncondensing GHGs.
….Earth is unique among terrestrial planets in having a greenhouse effect in which water vapor provides strong amplification of the heat-trapping action of the CO2 greenhouse.
….The anthropogenic radiative forcings that fuel the growing terrestrial greenhouse effect continue unabated. The continuing high rate of atmospheric CO2 increase is particularly worrisome, because the present CO2 level of 390 ppm is far in
excess of the 280 ppm that is more typical for the interglacial maximum, and still the atmospheric CO2 control knob is now being turned faster than at any time in the geological record.
….Furthermore, the atmospheric residence time of CO2 is exceedingly long, being measured in thousands of years. This makes the reduction and control of atmospheric CO2 a serious and pressing issue, worthy of real-time attention.
Lacis, A., Schmidt, G., Rind, D., & Ruedy, R. (2010). Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature Science, 330 (6002), 356-359 DOI: 10.1126/science.1190653
Photo credit: kamshots
You must be logged in to post a comment.