How Exactly Does Carbon Dioxide Cause Global Warming?
CO2 molecules make up only a small percentage of the atmosphere, but their impact on our climate is huge. The reason comes down to physics and chemistry.
“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.
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.
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.’
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?
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.
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.
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!