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You Asked

You Asked: If CO2 Is Only 0.04% of the Atmosphere, How Does it Drive Global Warming?

by |July 30, 2019

Got a burning question about climate change? Feeling curious about conservation? “You Asked” is a series where Earth Institute experts tackle reader questions on science and sustainability. To submit a question, drop a comment below, message us on Instagram, or email us here.

A reader named Paul submitted this question on one of our previous posts:

Why does CO2 drive global warming when there is only 0.04% of it in the atmosphere? And why isn’t water vapor the major driving factor?

Yochanan Kushnir

Yochanan Kushnir is a research professor at Lamont-Doherty Earth Observatory, in the Division of Oceans and Climate Physics

Answer provided by Yochanan Kushnir

Earth absorbs energy from sunlight, but as the surface warms, it also emits energy in the form of infrared radiation (which we know of as heat) out into space. Water vapor and CO2, however, act like a cap, making it more difficult for Earth to get rid of this energy. Without gases like these to absorb the energy, our planet’s average surface temperature would have been near zero degrees Fahrenheit.

About 99 percent of the atmosphere is made of oxygen and nitrogen, which cannot absorb the infrared radiation the Earth emits. Of the remaining 1 percent, the main molecules that can absorb infrared radiation are CO2 and water vapor, because their atoms are able to vibrate in just the right way to absorb the energy that the Earth gives off. After these gases absorb the energy, they emit half of it back to Earth and half of it into space, trapping some of the heat within the atmosphere. This trapping of heat is what we call the greenhouse effect. Because of the greenhouse effect created by these trace gases, the average temperature of the Earth is around 15˚C, or 59˚F, which allows for life to exist.

CO2 makes up only about 0.04% of the atmosphere, and water vapor can vary from 0 to 4%. But while water vapor is the dominant greenhouse gas in our atmosphere, it has “windows” that allow some of the infrared energy to escape without being absorbed. In addition, water vapor is concentrated lower in the atmosphere, whereas CO2 mixes well all the way to about 50 kilometers up. The higher the greenhouse gas, the more effective it is at trapping heat from the Earth’s surface.

The burning of fossil fuels affects the concentration of CO2 in the atmosphere. Before the industrial revolution, the amount of CO2 in the atmosphere was about 288 ppm. We have now reached about 414 ppm, so we are on the way to doubling the amount of CO2 in the atmosphere by the end of this century. Scientists say that if CO2 doubles, it could raise the average global temperature of the Earth between two and five degrees Celsius. We are already increasing the amount of energy that bounces back to the Earth. Because of the greenhouse effect, this is causing global warming with its many destructive impacts.

Both water vapor and CO2 are responsible for global warming, and once we increase the CO2 in the atmosphere, the oceans warm up, which inevitably triggers an increase in water vapor. But while we have no way to control water vapor, we can control CO2. And because we are increasing the amount of CO2 in the atmosphere by continuing to burn fossil fuels, even in relatively small amounts compared to the entire mass of the atmosphere, we are disturbing the entire heat balance of the planet.

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JLocke
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If CO2 increases follow temperature increases how can it be the cause? It makes sense that it could exacerbate the rise but how can it be the cause of the initial rise?

Miki
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Miki

It’s the initial cause because it’s emitted in the first place
And that emission rises the amount of greenhouse gas and therefore the amount of heat trapped

Abhaya Thiele
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Abhaya Thiele

Thank you for this. To all those doing climate research or reporting . . . As many Americans think in terms of Fahrenheit, might you always accompany a Celsius temperature with its Fahrenheit equivalent?

Uno Hansson
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Uno Hansson
Jonathan
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Jonathan

Wow, that was a weak answer, with a lot of “scientists say” type references. Not convinced at all. I was hoping for so much better.

Neobiognosis
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Neobiognosis

Can you articulate why you remain unconvinced and clarify the reasons you made this comment.

No CAGW
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No CAGW

Water vapor has a window? It’s 5x the ghg and 10x more than co2. Co2 has “windows” too, it only absorbs in three small bands leaving 92% transparent to IR.

So, when the IR is “back radiating” down (not heat btw) it is more likely to be intercepted by N2, o2 which is 99% of the atmosphere, and repeat the process back to space. This happens at the speed of light. Chances of the same IR making it back to the surface from the TOA is unlikely.

Neobiognosis
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Neobiognosis

It is not unlikely, it is certain that a proportion of it reaches the earth’s surface, and the earth’s heat energy balance is then lost. The energy reflected back to the earth’s surface is 2.9 Wm-2. You may examine this, and decide it is a small increment on the radiation already reaching earth, you may not understand the significance of the unit, so I’ll add some clarification. The unit represents the amount of energy reaching every square meter of the surface of earth every second. A simple extrapolation takes you to the amount of energy we are incementing our budget… Read more »