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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
Guest
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

Mary M. Douthwaite
Guest
Mary M. Douthwaite

JLocke asked above, “If CO2 increases follow temperature increases how can it be the cause?” Miki answered, “It’s the initial cause because it’s emitted in the first place.” ? The response not appear to answer the question. How can a rise of CO2 be emitted in the first place if it follows the rise in temperature?

Matt
Guest
Matt

CO2 can be both a primary and secondary cause of warming. CO2 can be the primary cause of rising temperatures, as it has been in the last couple centuries. CO2 can also be release from other sources (e.g. permafrost) in response to rising temperatures. And so, 1) in previous warming periods that were I initially started by non-CO2 causes, CO2 rises predictably follow temperature rises (and then further drive more increase). 2) In other cases (including the current warming period) it looks like CO2 is the primary cause of warming, but similar to the above this warming will cause even… Read more »

Abhaya Thiele
Guest
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
Guest
Uno Hansson
Jonathan
Guest
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
Guest
Neobiognosis

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

Honeybadger
Guest
Honeybadger

Because water vapor has a lower density than CO2, so shouldn’t it be concentrated higher in atmosphere too? Water has more vibrational modes than CO2, so shouldnt CO2 absorb less IR radiation than water and allow more heat escape than water? Have you ever seen an IR spectrum of water compared to co2??? Water is largely ignored as a contributed to global warming because scientists have no good way to accurately measure it’s atmospheric concentration.

No CAGW
Guest
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
Guest
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 »

RJWan
Guest
RJWan

Only half the Earth is being bombarded by the sun at any one time, not the entire Earth. If you get simple things like this wrong, it’s hard to believe you on the more complicated parts.

tired of this
Guest
tired of this

I don’t think Neobiognosis has got this wrong: The energy reflected back is the infrared energy emitted by the earth, which is emitted all the time – all objects emit radiation according to their temperature. So even at night the earth is radiating energy outwards, and some of it is being absorbed by CO2 (or other greenhouse gases) and radiated back to earth.

Rolf E
Guest
Rolf E

What are the formulas to calculate the temperature change from H2O- and the CO2- concentration in the athmosphere given a certain start temperature and a certain time with constant solar radiation.

Helmut Wober
Guest
Helmut Wober

If CO2 is such a greenhouse gas, especially 50 km up, how come that the surface of Mars with an atmosphere that consist of 95% CO2 has an average surface temperature of – 40 degrees Celsius? The 288 K, risen to 288.8K in the last 100 years on the surface of the earth are primarily due to adiabatic compression of the atmosphere, not green house effects. Yes , rises in CO2 follow global warming with a time lag of 600 – 800 years, due to degassing of the Oceans, which cover about 71% of the surface of the earth. Yes,… Read more »

Curt Lightle
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Curt Lightle

That is a good explanation. I hope you are correct about CO2 and i suspect you are. Unfortunately for most of us (me included) without the training to critically discern the data we are left to wonder what the truth is. What bothers me the most is the human caused crowd was caught back in about 2009 blatantly distorting the data. Many scientist are now committed to being right, so they would not admit they were wrong even if they knew they were. This is the biggest flaw in all scientific disciplines: ego, careers and money. Therefore you generally cannot… Read more »

Johnwood
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Johnwood

Because the atmosphere is much thinner and at far lower pressure .

Wayne Hughes
Guest
Wayne Hughes

Mars is further away from the sun so will likely be cooler by distance alone

Lonnie king
Guest
Lonnie king

So if co2 is so high up in the atmosphere how do plants in the ocean and on land absorb it? Do certain densities make it come down ? Is the earth supposed to have ice on it? Have there been periods on this planet where it had no ice? If most of earths heat is released through the hole in the atmosphere in the Antarctic does the size of that hole change? Is earth’s orbit around the sun the exact same every year or is it moving closer and farther over the years? How much have the ocean levels… Read more »

Brian Partrick
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Brian Partrick

WHAT IS THE MOLECULAR MECHANISM THAT ACCOUNTS FOR INFRA-RED TRANSPARENCY ‘WINDOWS’ OF H2O? WHAT IS THE COMPOSITION PROFILE OF CO2 WITH ALTITUDE? HOW WAS CO2 COMPOSITION WITH ALTITUDE OBTAINED? HOW DOES CO2 (MOL.WT 44)ALMOST 3X MOL.WT OF H2O(18) RISE TO 50KM WHEREAS H2O VAPOUR DOES NOT? BEST REGARDS BP

WTaylor
Guest
WTaylor

CO2 is .0004 particles for every 1 particle of the rest of the atmosphere. Saying that CO2 is causing global warming is like saying that 40 people in a stadium can out yell the other 99960. The truth is there for anyone who does not just want to believe it. Specific heat in physics proves the point. Unless CO2 has a specific heat that is a million, which it doesn’t, it cannot cause warming. However if you believe stop driving, heating your house and and any other activity not powered by solar or wind. Put your actions in line with… Read more »

Private Citizen
Guest
Private Citizen

A cooler mass cannot heat a warmer mass. CO2 molecules cannot heat the earth in reverse (back radiation). The earth is the source of energy the CO2 above it in the atmosphere absorbs. Just like the radiators in your home or office cannot heat the boiler water in reverse. Heat flows from hot to cold.

John wood
Guest
John wood

The sun is the source of energy!!

Phil
Guest
Phil

‘Negligibility’ based on volume is not a scientific measure. Just ONE gram (very negligible) if polonium can kill 50 million people.

The issue of climate change (in my opinion) does indeed hinge around just how much of an affect a change from 0.03% to 0.04% (25% more) of CO2 can have on temperature.

It may be a lot more than people assume, as the polonium example indicates, and that’s the area science needs to unequivocally understand.

Johnwood
Guest
Johnwood

It’s not like people in a stadium…..it is more the effect on infrared wavelengths, and the heat reflection in upper atmosphere. Hence greenhouse effect…natural processes have been locking carbon away for millions of years!! since the industrial revolution carbon dioxide levels have risen higher and faster than ever, that combined with massive environmental destruction , population growth ,pollution and micro plastics should give us reason to be cautious . Is the current situation sustainable?

Wayne Hughes
Guest
Wayne Hughes

I like the specific heat property you mention and will research this more as I am highly skeptical of C02 as a driver for any suggested global temp change..however..is that not like saying “how could this tiny bit of snake venom kill you? Its only 0.04% of your body mass”…?

Phil
Guest
Phil

I am unclear on the explanation as to ‘how’ the the absorption and retransmission of carbon dioxide heats the atmosphere, in the sense that you didn’t explain why it doesn’t negate itself because that process should apply in both directions, not just one. This answer seems to say that the molecules allow heat from the sun in but reflect them down when the heat is trying to escape. Why doesn’t the heat from the sun get reflected back in that case and why doesn’t the heat from the atmosphere get absorbed and retransmitted out into space? Is it because the… Read more »

David
Guest
David

CO2 and other greenhouse gases are transparent to visible light (which is why we can’t see them) but opaque to certain frequencies of infrared. So sunlight passes through unimpeded to hit the Earth and warm it. Then the warmed Earth radiates that heat as infrared back out into space. However, some of that infrared is scattered by GHGs, half of it back at the Earth to be reabsorbed and converted back into heat. (The author of this blog doesn’t do a good job of explaining how that works.) This hasn’t always been a bad thing. The greenhouse effect has been… Read more »

Dan Pangburn
Guest

The public has been falsely indoctrinated about causes of climate change. Deeper penetration into science confirms that CO2 follows climate change, not causes it. Water vapor has been accurately measured worldwide by NASA/RSS. It has been increasing faster than POSSIBLE from feedback. The irony is that reducing the use of fossil fuels, which is agenda of many, will have no effect on climate. https://watervaporandwarming.blogspot.com

John wood
Guest
John wood

What science? Real science?

Pawan Ranta
Guest
Pawan Ranta

Co2 is a heat-trapping gas. I agree but how long it can hold the heat?

Zbigniew
Guest
Zbigniew

“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.”

But as we can see here:comment image

Co2 has much more and much bigger “windows” so your argument make no sense.

Andrew
Guest
Andrew

I had a question about CO2 and Venus. Venus has an atmosphere about 95% CO2 and 93 times denser than Earth’s, giving it 256000 more CO2 than Earth. But the temperature is only 2.5 times higher. Why isn’t the increase more if CO2 is a big player in trapping heat?