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How Exactly Does Carbon Dioxide Cause Global Warming?
Comments“You Asked” is a series where Earth Institute experts tackle reader questions on science and sustainability. Over the past few years, we’ve received a lot of questions about carbon dioxide — how it traps heat, how it can have such a big effect if it only makes up a tiny percentage of the atmosphere, and more. With the help of Jason Smerdon, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory, we answer several of those questions here.
How does carbon dioxide trap heat?
You’ve probably already read that carbon dioxide and other greenhouse gases act like a blanket or a cap, trapping some of the heat that Earth might have otherwise radiated out into space. That’s the simple answer. But how exactly do certain molecules trap heat? The answer there requires diving into physics and chemistry.
Simplified diagram showing how Earth transforms sunlight into infrared energy. Greenhouse gases like carbon dioxide and methane absorb the infrared energy, re-emitting some of it back toward Earth and some of it out into space. Credit: A loose necktie on Wikimedia Commons
When sunlight reaches Earth, the surface absorbs some of the light’s energy and reradiates it as infrared waves, which we feel as heat. (Hold your hand over a dark rock on a warm sunny day and you can feel this phenomenon for yourself.) These infrared waves travel up into the atmosphere and will escape back into space if unimpeded.
Oxygen and nitrogen don’t interfere with infrared waves in the atmosphere. That’s because molecules are picky about the range of wavelengths that they interact with, Smerdon explained. For example, oxygen and nitrogen absorb energy that has tightly packed wavelengths of around 200 nanometers or less, whereas infrared energy travels at wider and lazier wavelengths of 700 to 1,000,000 nanometers. Those ranges don’t overlap, so to oxygen and nitrogen, it’s as if the infrared waves don’t even exist; they let the waves (and heat) pass freely through the atmosphere.
A diagram showing the wavelengths of different types of energy. Energy from the Sun reaches Earth as mostly visible light. Earth reradiates that energy as infrared energy, which has a longer, slower wavelength. Whereas oxygen and nitrogen do not respond to infrared waves, greenhouse gases do. Credit: NASA
With CO2 and other greenhouse gases, it’s different. Carbon dioxide, for example, absorbs energy at a variety of wavelengths between 2,000 and 15,000 nanometers — a range that overlaps with that of infrared energy. As CO2 soaks up this infrared energy, it vibrates and re-emits the infrared energy back in all directions. About half of that energy goes out into space, and about half of it returns to Earth as heat, contributing to the ‘greenhouse effect.’
By measuring the wavelengths of infrared radiation that reaches the surface, scientists know that carbon dioxide, ozone, and methane are significantly contributing to rising global temperatures. Credit: Evans 2006 via Skeptical Science
Smerdon says that the reason why some molecules absorb infrared waves and some don’t “depends on their geometry and their composition.” He explained that oxygen and nitrogen molecules are simple — they’re each made up of only two atoms of the same element — which narrows their movements and the variety of wavelengths they can interact with. But greenhouse gases like CO2 and methane are made up of three or more atoms, which gives them a larger variety of ways to stretch and bend and twist. That means they can absorb a wider range of wavelengths — including infrared waves.
How can I see for myself that CO2 absorbs heat?
As an experiment that can be done in the home or the classroom, Smerdon recommends filling one soda bottle with CO2 (perhaps from a soda machine) and filling a second bottle with ambient air. “If you expose them both to a heat lamp, the CO2 bottle will warm up much more than the bottle with just ambient air,” he says. He recommends checking the bottle temperatures with a no-touch infrared thermometer. You’ll also want to make sure that you use the same style of bottle for each, and that both bottles receive the same amount of light from the lamp. Here’s a video of a similar experiment:
A more logistically challenging experiment that Smerdon recommends involves putting an infrared camera and a candle at opposite ends of a closed tube. When the tube is filled with ambient air, the camera picks up the infrared heat from the candle clearly. But once the tube is filled with carbon dioxide, the infrared image of the flame disappears, because the CO2 in the tube absorbs and scatters the heat from the candle in all directions, and therefore blurs out the image of the candle. There are several videos of the experiment online, including this one:
Why does carbon dioxide let heat in, but not out?
Energy enters our atmosphere as visible light, whereas it tries to leave as infrared energy. In other words, “energy coming into our planet from the Sun arrives as one currency, and it leaves in another,” said Smerdon.
CO2 molecules don’t really interact with sunlight’s wavelengths. Only after the Earth absorbs sunlight and reemits the energy as infrared waves can the CO2 and other greenhouse gases absorb the energy.
How can CO2 trap so much heat if it only makes up 0.04% of the atmosphere? Aren’t the molecules spaced too far apart?
Before humans began burning fossil fuels, naturally occurring greenhouse gases helped to make Earth’s climate habitable. Without them, the planet’s average temperature would be below freezing. So we know that even very low, natural levels of carbon dioxide and other greenhouse gases can make a huge difference in Earth’s climate.
Today, CO2 levels are higher than they have been in at least 3 million years. And although they still account for only 0.04% of the atmosphere, that still adds up to billions upon billions of tons of heat-trapping gas. For example, in 2019 alone, humans dumped 36.44 billion tonnes of CO2 into the atmosphere, where it will linger for hundreds of years. So there are plenty of CO2 molecules to provide a heat-trapping blanket across the entire atmosphere.
In addition, “trace amounts of a substance can have a large impact on a system,” explains Smerdon. Borrowing an analogy from Penn State meteorology professor David Titley, Smerdon said that “If someone my size drinks two beers, my blood alcohol content will be about 0.04 percent. That is right when the human body starts to feel the effects of alcohol.” Commercial drivers with a blood alcohol content of 0.04% can be convicted for driving under the influence.
“Similarly, it doesn’t take that much cyanide to poison a person,” adds Smerdon. “It has to do with how that specific substance interacts with the larger system and what it does to influence that system.”
In the case of greenhouse gases, the planet’s temperature is a balance between how much energy comes in versus how much energy goes out. Ultimately, any increase in the amount of heat-trapping means that the Earth’s surface gets hotter. (For a more advanced discussion of the thermodynamics involved, check out this NASA page.)
If there’s more water than CO2 in the atmosphere, how do we know that water isn’t to blame for climate change?
Water is indeed a greenhouse gas. It absorbs and re-emits infrared radiation, and thus makes the planet warmer. However, Smerdon says the amount of water vapor in the atmosphere is a consequence of warming rather than a driving force, because warmer air holds more water.
“We know this on a seasonal level,” he explains. “It’s generally drier in the winter when our local atmosphere is colder, and it’s more humid in the summer when it’s warmer.”
As carbon dioxide and other greenhouse gases heat up the planet, more water evaporates into the atmosphere, which in turn raises the temperature further. However, a hypothetical villain would not be able to exacerbate climate change by trying to pump more water vapor into the atmosphere, says Smerdon. “It would all rain out because temperature determines how much moisture can actually be held by the atmosphere.”
Similarly, it makes no sense to try to remove water vapor from the atmosphere, because natural, temperature-driven evaporation from plants and bodies of water would immediately replace it. To reduce water vapor in the atmosphere, we must lower global temperatures by reducing other greenhouse gases.
If Venus has an atmosphere that’s 95% CO2, shouldn’t it be a lot hotter than Earth?
Thick clouds of sulfuric acid surround Venus and prevent 75% of sunlight from reaching the planet’s surface. Without these clouds, Venus would be even hotter than it already is. Credit: NASA
The concentration of CO2 in Venus’ atmosphere is about 2,400 times higher than that of Earth. Yet the average temperature of Venus is only about 15 times higher. What gives?
Interestingly enough, part of the answer has to do with water vapor. According to Smerdon, scientists think that long ago, Venus experienced a runaway greenhouse effect that boiled away almost all of the planet’s water — and water vapor, remember, is also a heat-trapping gas.
“It doesn’t have water vapor in its atmosphere, which is an important factor,” says Smerdon. “And then the other important factor is Venus has all these crazy sulfuric acid clouds.”
High up in Venus’ atmosphere, he explained, clouds of sulfuric acid block about 75% of incoming sunlight. That means the vast majority of sunlight never gets a chance to reach the planet’s surface, return to the atmosphere as infrared energy, and get trapped by all that CO2 in the atmosphere.
Won’t the plants, ocean, and soil just absorb all the excess CO2?
Eventually … in several thousand years or so.
Plants, the oceans, and soil are natural carbon sinks — they remove some carbon dioxide from the atmosphere and store it underground, underwater, or in roots and tree trunks. Without human activity, the vast amounts of carbon in coal, oil, and natural gas deposits would have remained stored underground and mostly separate from the rest of the carbon cycle. But by burning these fossil fuels, humans are adding a lot more carbon into the atmosphere and ocean, and the carbon sinks don’t work fast enough to clean up our mess.
A simplified diagram showing the carbon cycle. Credit: Jack Cook/Woods Hole Oceanographic Institution
It’s like watering your garden with a firehose. Even though plants absorb water, they can only do so at a set rate, and if you keep running the firehose, your yard is going to flood. Currently our atmosphere and ocean are flooded with CO2, and we can see that the carbon sinks can’t keep up because the concentrations of CO2 in the atmosphere and oceans are rising quickly.
The amount of carbon dioxide in the atmosphere (raspberry line) has increased along with human emissions (blue line) since the start of the Industrial Revolution in 1750. Credit: NOAA Climate.gov
Unfortunately, we don’t have thousands of years to wait for nature to absorb the flood of CO2. By then, billions of people would have suffered and died from the impacts of climate change; there would be mass extinctions, and our beautiful planet would become unrecognizable. We can avoid much of that damage and suffering through a combination of decarbonizing our energy supply, pulling CO2 out the atmosphere, and developing more sustainable ways of thriving.
Editor’s note (March 17, 2021): This post was updated with additional links to Youtube videos with experiments showing the effects of carbon dioxide. Enjoy!
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I’m writing a paper for my environmental class How does the cooler atmosphere transport heat Q to the warmer surface?
Q = sigma•(Ts^4 -Ta^4)
If the air’s cooler than the surface, it wouldn’t.
GREAT question Joe! The short answer: It doesn’t. Heat always flows from higher temperature to lower temperature. If it didn’t, the 2nd Law of Thermodynamics would be violated, and entropy would decrease (as enthalpy increased). The longer answer: The “greenhouse gas” mechanism does not exist. The atmosphere (including CO2) provides convection cooling to the Earth’s surface. Cloud cover does *temporarily* prevent radiational cooling by reflecting radiation back to the surface. This is distinctly different than the fictitious greenhouse gas model of absorption-and-reemission. The atmosphere also distributes heat more evenly around the planet through convection heat transfer in general; this is… Read more »
Hi Daniel, The obvious retort here is that many actions essential if mainstream theories are correct make sense regardless of the nature, extent, cause and direction if climate change. They would help cope with a volcanic winter (e.g. that in 1816 after the Tambora eruption) and also the collapse of a major food crop. Examples include less waste, combining conservation with careful use, restoring fish stocks, growing fewer cash crops, regenerative agriculture, silviculture and reducing the impact per head and probably numbers of conventional livestock.. Instead of shovelling grain and soya down cattle they can be fed on crop residues,… Read more »
The greenhouse gas mechanism definitely does exist. Colder objects still radiate, unless they are at absolute zero. The atmosphere is well above that at 255 K or so on average, and it radiates plenty of infrared light. When that infrared light strikes the ground, what do you think happens?
There are 3 ways that energy can be transferred: conduction, convection, and radiation. What John LK and Daniel H have described are 2 of those, and both are certainly at play in distributing the sun’s energy that the surface (and some atmospheric constituents) absorb. What they both have not addressed is radiation. This is how greenhouse gasses work in our atmosphere, and incidentally, how the sun’s energy reaches Earth. If there were no greenhouse gasses in the atmosphere, heat energy radiated from the surface would almost entirely radiate back to space, leaving the surface at a very very cold -18C… Read more »
Hi Lisa,
The intelligent and accurate retort as opposed to my less intellectual bypassing the whole argument – see previous post.
Regards,.
Iain
Thanks,
I’m after the physics describing the greenhouse effect/mechanism of heat transfer.
The equation is a radiative heat transfer equation the units are expressed in power per unit area, not energy. To get energy integrate power per unit area over time then multiply by area
The equation is for heat transfer between two surfaces, earth and the atmosphere.
Suppose the sun is delivering power to the surface over time transferring energy
generating surface temperature Ts.
Q=sigma(Ts^4-Ta^4)
When is Q negative? K
Thanks, In my paper I’m after the physics describing the greenhouse effect/mechanism of heat transfer. The equation used is a radiative heat transfer equation applied between two surfaces, earth and the atmosphere. The equation has units of power not energy. For simplicity epsilon is 1. Applying the equation to a single layer atmospheric model we know heat from the sun (Qs), and can find atmosphere temperature (Ta), earth surface temperature (Ts)…… The term “back radiation” is used to describe the heat transfer mechanism. Using the radiative heat transfer equation and applying it to a single layer model with the known… Read more »
Joe, Your equation is set up to always give the answer that the atmosphere can’t warm the surface, which is wrong. You need to compare the situation with a warm atmosphere to one with no atmosphere at all. For the present situation, Ts = 288, Ta = 255, so Q = 5.670373e-8 (288^4 – 255^4) = 150 W m^-2. Now try Ts = 288, Ta = 2.7 K (the temperature of interstellar space). You get Q = 390 W m^-2. In other words, with the warm atmosphere there, net radiation leaving the surface is 150 W m^-2, but without the… Read more »
If 97 to 98% of the co2 in the atmosphere comes from natural sources how much impact can industrial sources have based on the small % of co2 in air. Isn’t it true that during the jurrasic period co2 levels were 10 times what they are today. Seems to me like a futile effort, nature rules in this case.
The atmosphere is not 2-3% artificial CO2 but 33% artificial CO2. You are confusing the fraction of emissions with the fraction of build-up. All the natural sources are matched by natural SINKS. The artificial production is not, so that’s where the increase comes from.
yes, remove all the CO2, and all the plants die, and the human race is not far behind. you can kiss your ass goodbye if all the plants die.
No one is saying we should remove all the CO2. It’s about returning CO2 to reasonable levels.
CO2 is at the Optimal level right now
Yeah I guess if you like extra droughts and wildfires and deadlier hurricanes? Not my idea of optimal.
What is a reasonable level in ppm.?
Climate scientist James Hansen has suggested that we should try to limit CO2 to 350ppm, although for thousands of years, natural cycles didn’t bring it above 300ppm: https://climate.nasa.gov/vital-signs/carbon-dioxide/
All human civilization and agriculture developed when the CO2 level was about 280 ppmv and the (mean global annual surface) temperature was 286-287 K. Serious deviations from that either way have the potential to badly disrupt our agriculture and our civilization.
So, what is your opinion of what a “reasonable level” of CO₂ is? Do you think that during the Ordovician period when the CO₂ level was at 2,240 ppm and the Earth survived that was a “reasonable level”?
Yeah, the Earth has survived a lot of things. For millions of years, the surface of the planet was molten from being struck by so many asteroids and other space debris, and the Earth survived. So I guess it’s ok to return to those conditions, too? Just because the Earth has survived hell, doesn’t necessarily mean the human species can or will. Climate change is already causing a lot of human suffering, and it could get worse if we let it — does that just not matter to you? Do the profits of fossil fuel companies matter more than human… Read more »
I am strugling to find a percentage, or range of percentages showing the proven human activity responsible for the global warming. This is a question I get stumped with by sceptics. Is there unquestionable data and science to support that, say, 70% to say 90% of the increase in temperature is proven to be a result of human activity? While CO2 modelling I appreciate is complex, does the science (at a molecular modelled level) show without question that the increase in CO2 in our atmosphere causes the associated increase in termperature we measure. While I can see the data graphs… Read more »
Hi John, The obvious retort to sceptics is that many ideas essential if mainstream views are correct make sense even if climate change were a damp squib or temperatures fell e.g following a major volcanic eruption like Tambora in 1815. For that matter they work if a major food crop collapses. Typical actions include reducing waste, silviculture, regenerative agriculture, alternatives to fossil fuels (whose extraction can be polluting or destructive), fewer cash crops, combining conservation with careful use and cutting the impact per head and probably numbers of conventional livestock. These win-win options are effective no matter what. Instead the… Read more »
All the recent warming can be attributed to human activity. If you add up all the natural forcings, the Earth should be slowly cooling. It’s only when you add the artificial ones that you get warming.
I have some question, When the sunlight hit the ground it transforms into infrared light, when the infrared light hit the CO2, shouldn’t the wavelength changed too?
My understanding is that when sunlight hits the ground, it heats the ground. Because the surface of the sun is so hot, the radiation is mainly in the visual, i.e. at relatively short wavelengths. The ground is radiating back, but because the ground is so much less hot, it radiates at longer wavelengths, i.e. in infrared. The sunlight is not transformed directly. It is the net result of absorption and emission by the ground. If CO2 is then heated by infrared radiation, and the temperature is not much different, it should re-emit the radiation in about the same wavelength.
So does Co2 absorb and emit radiation or does it block it ? The article talks about radiating, but the experiment you show seems to show blocking. Shouldn’t we see the Co2 absorb the heat and re radiate it ? Of course the experiment is faked anyway. That is a laboratory FLIR. Camera. It can show temperatures in at least 4000 colors . But the only thing it shows at all is the candle flame. Therefore the sensitivity on the expensive FLIR camera is cranked down so low it only registers if something is on fire. Then he fills the… Read more »
The CO2 scatters the infrared by absorbing it and reemitting in all directions — which is exactly what the video claims to show. Since some of the infrared is bounced back to the source, it is often characterized as “blocking.”
But it doesn’t show that at all. It only shows that the cold gas blocks infrared for few seconds, to a badly adjusted FLIR camera.
Unfortunately, neither you nor I know the exact conditions of the experiment and what temperature the CO2 gas was at. However, climate scientist Jason Smerdon says that even if the gas was cold, the IR from the candle would still transmit directly to the camera if the gas were not interacting with the IR radiation. So, the experiment shows that the CO2 is scattering the IR, regardless of the gas’s temperature. It’s also worth noting that even if there were a problem with the experiment, scientists know from many other lines of evidence that CO2 absorbs and scatters infrared energy… Read more »
Hi James, here’s a slightly experiment where cold CO2 is definitely not an issue, and it shows the same results: https://youtu.be/Rt6gLt6G5Kc?t=107 Hope this helps
If Mars is 95%co2 how come it is not hotter. My last question is what happened to the sunspots. Did the industrial revolution cause that too?
Most of Earth’s greenhouse effect comes from water vapor and clouds, which together account for about 25 K of the Earth’s 33 K difference from the radiative equilibrium temperature (CO2 accounts for most of the rest). Mars has a very dry atmosphere. In addition, its atmospheric pressure is very low, so the absorption lines are not pressure-broadened the way they are on Earth, and the greenhouse effect is less effective. Lastly, Mars receives much less sunlight than Earth. Despite all this, Mars does wind up with a greenhouse effect of about 4 K (radiative equilibrium temperature is 210, emission temperature… Read more »
Indeed the climate is changing and CO2 certainly seems to be playing a role. However, I find the statement “Unfortunately, we don’t have thousands of years to wait for nature to absorb the flood of CO2. By then, billions of people would have suffered and died from the impacts of climate change; there would be mass extinctions, and our beautiful planet would become unrecognizable” to be coming out of thin air. The climate has changed in human history (medieval warm period, ice ages) and humans have always been able to adapt. Why would this climate change be different? “Billions dead”… Read more »
Droughts, wildfires, extreme heat, hurricanes, sea level rise, infectious disease — climate change makes all of these things worse, and the climate is changing faster and more dramatically than in all of human history. Surely we can and will adapt, and a big part of adapting means moving away from fossil fuels.
Recently, I became embroiled in an online debate on the subject of anthropogenic global warming (“Claim”) originated by a talk radio host, who was hostile to the claim of anthropogenic global warming. Some responders were outright abusive, but one at least posed the following counter-arguments to the Claim: (1) “So one of you educated climate alarmists please the explanation of how CO2 in the atmosphere is capable of increasing its fingerprint absorption wavelengths of 2.7, 4.3, and 15 microns so that it can absorb more than 8% of the infrared spectrum that it already does” “Don’t give me the ‘broadens… Read more »
https://blogs.ei.columbia.edu/2021/02/25/carbon-dioxide-cause-global-warming/
Dr. Fecht:
I did read the article, which was very informative. I was informed by someone else that the Stefan-Boltzmann constant is not used in the more current, detailed models, which accounts for “Challenge” 3. Challenge 2 seems a bit absurd, and is a thermal transfer issue. The one that kind of confounded me was Challenge 1, an atmospheric chemistry issue. Would have something specific to say about this one, say if it was posed directly to you?
Thank You