CO2 only makes up 0.04% of the air around the Earth. Here’s why it has such a big effect

People often ask me how carbon dioxide, which only makes up 0.041 percent of the Earth’s atmosphere, can have such a big effect on the climate of the whole planet when it is so small. And only 32% of that amount is caused by human activities.

I look into how air pollution and climate change are caused by gases in the air. Carbon dioxide has a big effect on the climate because it can soak up heat from the Earth’s surface and keep it from escaping into space.

Early research on greenhouses

In the 1850s, when scientists first found out how important carbon dioxide was to the climate, they were also surprised by its effects. Working on their own, John Tyndall in England and Eunice Foote in the United States found that carbon dioxide, water vapor, and methane all absorbed heat, while more common gases did not.

Scientists had already calculated that the Earth was about 59 degrees Fahrenheit (33 degrees Celsius) warmer than it should be based on how much sunlight hits its surface. The best way to explain this difference was that the planet’s atmosphere kept heat to keep the planet warm.

Tyndall and Foote showed that nitrogen and oxygen, which make up 99 percent of the atmosphere, don’t have much of an effect on the temperature of the Earth because they don’t absorb heat.

Instead, they found that gases in much smaller amounts were entirely responsible for keeping the Earth’s temperatures at a level that made it possible for people to live there. These gases trap heat to create a natural greenhouse effect.

Like a blanket in the air

Earth gets energy from the Sun all the time and sends it back out into space. For the temperature of the planet to stay the same, the net heat it gets from the Sun must be equal to the heat it gives off.

Since the Sun is hot, it gives off energy as shortwave radiation, mostly in the form of ultraviolet and visible light. Since Earth is much cooler, it gives off heat as longer-wavelength infrared radiation.

Carbon dioxide and other gases that hold on to heat have molecular structures that let them take in infrared light. In a molecule, the bonds between the atoms can vibrate in certain ways, like the pitch of a piano string. When the frequency of a molecule is the same as the frequency of a photon, the photon is taken in and its energy is transferred to the molecule.

Carbon dioxide and other gases that trap heat have three or more atoms and have the same frequency as the infrared radiation that Earth gives off. Since oxygen and nitrogen molecules only have two atoms, they don’t take in infrared light.

Most of the shortwave radiation coming from the Sun is not absorbed by the atmosphere.

But most of the infrared radiation that goes out is absorbed by gases in the atmosphere that keep heat in. Then they can let that heat out, or re-radiate it. Some of it goes back to the surface of the Earth, keeping it warmer than it would be if it didn’t.

Research on how heat moves

During the Cold War, a lot of research was done on how different gases absorbed infrared light. The US Air Force led the work. They were making heat-seeking missiles and needed to know how to find heat moving through the air.

Scientists were able to figure out the atmospheres and climates of all planets in the Solar System by looking at their infrared signatures. For example, Venus has a temperature of about 870 F (470 C) because carbon dioxide makes up 96.5 percent of its thick atmosphere.

It also helped weather forecasters and climate modelers figure out how much infrared radiation is kept in the atmosphere and sent back to the surface of the Earth.

People sometimes ask me why carbon dioxide is important for climate when water vapor absorbs more infrared radiation and the two gases absorb at many of the same wavelengths.

The radiation that gets out into space is controlled by the Earth’s upper atmosphere. The upper atmosphere is much less dense and has a lot less water vapor than the lower atmosphere. Because of this, adding more carbon dioxide has a big effect on how much infrared radiation escapes into space.

Taking a look at the greenhouse effect

Have you ever noticed that deserts are often colder at night than forests, even though their average temperatures are the same? Deserts don’t have much water vapor in the air, so the radiation they give off is easy to send into space.

In places with more water vapor in the air, radiation from the surface is held in place. In the same way, nights with clouds tend to be warmer than nights with no clouds because there is more water vapor in the air.

Changes in the weather in the past show that carbon dioxide has an effect. Ice cores from the last million years show that there was a lot of carbon dioxide in the air when it was warm, about 0.028 percent.

During the Ice Ages, when the Earth was about 7 to 13 F (4-7 C) cooler than it was in the 20th century, carbon dioxide made up only about 0.018 percent of the atmosphere.

Even though water vapor is more important for the natural greenhouse effect, changes in carbon dioxide have caused past temperature changes. On the other hand, the amount of water vapor in the air depends on the temperature.

When the Earth gets warmer, its atmosphere can hold more water vapor, which makes the initial warming worse. This is called the “water vapor feedback.” So, changes in carbon dioxide have been the main cause of climate change in the past.

Small changes have big results

It shouldn’t be surprising that a small amount of carbon dioxide in the air can have a big effect. We take pills that are only a small part of the size of our bodies, but we expect them to have an effect on us.

Carbon dioxide levels are higher now than at any other time in human history. Scientists agree that the average surface temperature of Earth has already risen by about 2 F (1 C) since the 1880s. Scientists also agree that the rise in carbon dioxide and other gases that trap heat is almost certainly caused by humans.

If nothing is done to control emissions, carbon dioxide could make up 0.1% of the air by 2100, which is more than three times as much as it was before the Industrial Revolution. This would be a change that would happen more quickly than other big changes in Earth’s history.

If nothing is done, this tiny piece of the atmosphere will cause a lot of trouble.