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For example, a team of four NASA scientists led by Andrew Lacis and including Gavin Schmidt, found this: “Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere.”
Yes, water vapor and clouds are the major contributors to Earth’s overall greenhouse effect. And, in fact, a companion study led by Schmidt showed that water vapor and clouds together account for 75 percent, with CO2 coming in at 20 percent, and other non-condensing greenhouse gases making up the rest.
So given that CO2 accounts for just a fifth of Earth’s overall greenhouse effect, what supports the claim that it nevertheless is the most important greenhouse gas?
The answer involves different characteristics of greenhouse gases. When the atmosphere cools enough, water vapor condenses and rains out. By contrast, carbon dioxide, methane and other greenhouse gases do not — they are non-condensing.
The researchers found that without these non-condensing greenhouse gases — CO2 foremost among them — there would be nothing to prevent the atmosphere from cooling enough to cause water vapor to rain out. And since it is such a potent greenhouse gas, if water vapor were to rain out, the result would be very dramatic cooling. In this way, CO2 may not be as potent a greenhouse gas as water vapor, but it is actually more important.
“Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state,” the authors of the first study concluded.
Just how much does carbon dioxide contribute? The second study led by Gavin Schmidt concluded that the CO2 in our atmosphere is itself is responsible for 80 percent of the radiative forcing that sustains Earth’s greenhouse effect.
CO2 and Earth's Energy Budget
Scientists have worked out the fine details of how energy flows through Earth’s atmosphere, as seen in this diagram. It shows how energy contained in sunlight warms our planet, and how this energy becomes temporarily trapped as it flows away from Earth’s surface as longwave infrared radiation. This energy trap produces the greenhouse effect, the main driver of global warming. (Source: Kevin Trenberth, John Fasullo and Jeff Kiehl via UCAR)
This brings me to another claim made by some commenters here at ImaGeo. Climate records show that global temperatures drop before CO2 does as Earth enters an ice age, and visa versa too: Temperatures rise before CO2 as we come out of an ice age. So once again, CO2 cannot be the most important factor.
Scientists have actually long known that something something other than CO2 sets things in motion when Earth enters and emerges from ice ages: shifts in solar radiation reaching Earth due to variations in the Earth’s orientation to the Sun. (These are known as Milankovitch cycles). Then other natural feedbacks kick in — most especially changes in carbon dioxide.
Scientists haven’t fully teased out all of the details yet. But in general, the picture looks like this:
As Earth starts to warm at the end of an ice age due to increased solar radiation reaching Earth, ice sheets and snow begin to contract. These surfaces are very reflective. So as they shrink, less sunlight is reflected back into space. This helps to enhance the warming. The warming causes ocean waters to give up CO2 — because CO2 is less soluble in warmer water. This strongly enhances the warming, which reduces the ice and snow, which causes more warming, which increases the CO2, leading to even more warming.
The bottom line is that a change in the amount of solar energy reaching Earth may get things going, but it’s CO2 that plays the dominant role.
This general picture leaves out some important details, such as the role of fresh water flowing into the oceans as ice sheets melt. A 2012 study led by Jeremy Shakun, now a Boston College climatologist, examined some of these details. Skeptical Science posted an excellent explainer about the results here. But the upshot of the study was this: “While the orbital cycles triggered the initial warming, overall, more than 90% of the glacial-interglacial warming occured after that atmospheric CO2 increase.”
I’ll finish with one recent piece of research in which a team of five scientists examined the role of greenhouse gases in temperature anomalies, including the overall warming trend, since the onset of the industrial revolution.
Here, too, commenters on this blog often claim that since recent periods in Earth’s past were almost as warm as it is now, we can’t know for sure that the CO2 we’ve added to the atmosphere is responsible for the observed recent warming.
But in their paper, published in the journal Scientific Reports, the scientists confirmed that our emissions of greenhouse gases, “especially CO2, are the main causal drivers of the recent warming.”
Earth’s climate is clearly an incredibly complex system. And climate scientists have never contended that they’ve understood all the details, or that their current understanding isn’t subject to revision when new evidence comes along. This is why they continue to do their research – to improve our understanding of how one of Earth’s key life support systems works.
They’ve also never contended that CO2 is the sole factor driving climate changes over geologic history. As we’ve seen, however, it plays a key role: Without the CO2 thermostat, Earth would likely be a proverbial snowball.
And now, we humans have turned the thermostat up, with predictable results that we’re already observing — such as changes to permafrost in the Arctic that got me going on this post to begin with.