Wait, the Atmosphere Is Only 0.04% Carbon Dioxide. How Does It Affect Earth’s Climate?

Orbiting Carbon Observatory Satellite

The Orbiting Carbon Observatory satellite makes precise measurements of Earth’s carbon dioxide levels from space. Credit: NASA/JPL

Carbon dioxide makes up 0.04% of the world’s atmosphere. Not 0.4% or 4%, but 0.04%! How can it be so important in global warming if it’s such a small percentage?

I am often asked how carbon dioxide can have an important effect on global climate when its concentration is so small – just 0.041% of Earth’s atmosphere. And human activities are responsible for just 32% of that amount.

I study the importance of atmospheric gases for air pollution and climate change. The key to carbon dioxide’s strong influence on climate is its ability to absorb heat emitted from our planet’s surface, keeping it from escaping out to space.

Keeling Curve CO2

The ‘Keeling Curve,’ named for scientist Charles David Keeling, tracks the accumulation of carbon dioxide in Earth’s atmosphere, measured in parts per million. Credit: Scripps Institution of Oceanography, CC BY

Early greenhouse science

The scientists who first identified carbon dioxide’s importance for climate in the 1850s were also surprised by its influence. Working separately, John Tyndall in England and Eunice Foote in the United States found that carbon dioxide, water vapor, and methane all absorbed heat, while more abundant gases did not.

Scientists had already calculated that the Earth was about 59 degrees Fahrenheit (33 degrees Celsius) warmer than it should be, given the amount of sunlight reaching its surface. The best explanation for that discrepancy was that the atmosphere retained heat to warm the planet.

Tyndall and Foote showed that nitrogen and oxygen, which together account for 99% of the atmosphere, had essentially no influence on Earth’s temperature because they did not absorb heat. Rather, they found that gases present in much smaller concentrations were entirely responsible for maintaining temperatures that made the Earth habitable, by trapping heat to create a natural greenhouse effect.

A blanket in the atmosphere

Earth constantly receives energy from the sun and radiates it back into space. For the planet’s temperature to remain constant, the net heat it receives from the sun must be balanced by outgoing heat that it gives off.

Since the sun is hot, it gives off energy in the form of shortwave radiation at mainly ultraviolet and visible wavelengths. Earth is much cooler, so it emits heat as infrared radiation, which has longer wavelengths.

Electromagnetic Spectrum

The electromagnetic spectrum is the range of all types of EM radiation – energy that travels and spreads out as it goes. The sun is much hotter than the Earth, so it emits radiation at a higher energy level, which has a shorter wavelength. Credit: NASA

Carbon dioxide and other heat-trapping gases have molecular structures that enable them to absorb infrared radiation. The bonds between atoms in a molecule can vibrate in particular ways, like the pitch of a piano string. When the energy of a photon corresponds to the frequency of the molecule, it is absorbed and its energy transfers to the molecule.

Carbon dioxide and other heat-trapping gases have three or more atoms and frequencies that correspond to infrared radiation emitted by Earth. Oxygen and nitrogen, with just two atoms in their molecules, do not absorb infrared radiation.

Most incoming shortwave radiation from the sun passes through the atmosphere without being absorbed. But most outgoing infrared radiation is absorbed by heat-trapping gases in the atmosphere. Then they can release, or re-radiate, that heat. Some returns to Earth’s surface, keeping it warmer than it would be otherwise.

Energy Budget of Earth's Atmosphere

Earth receives solar energy from the sun (yellow), and returns energy back to space by reflecting some incoming light and radiating heat (red). Greenhouse gases trap some of that heat and return it to the planet’s surface. Credit: NASA

Research on heat transmission

During the Cold War, the absorption of infrared radiation by many different gases was studied extensively. The work was led by the U.S. Air Force, which was developing heat-seeking missiles and needed to understand how to detect heat passing through air.

This research enabled scientists to understand the climate and atmospheric composition of all planets in the solar system by observing their infrared signatures. For example, Venus is about 870 F (470 C) because its thick atmosphere is 96.5% carbon dioxide.

It also informed weather forecast and climate models, allowing them to quantify how much infrared radiation is retained in the atmosphere and returned to Earth’s surface.

People sometimes ask me why carbon dioxide is important for climate, given that water vapor absorbs more infrared radiation and the two gases absorb at several of the same wavelengths. The reason is that Earth’s upper atmosphere controls the radiation that escapes to space. The upper atmosphere is much less dense and contains much less water vapor than near the ground, which means that adding more carbon dioxide significantly influences how much infrared radiation escapes to space.

Carbon dioxide levels rise and fall around the world, changing seasonally with plant growth and decay.

Observing the greenhouse effect

Have you ever noticed that deserts are often colder at night than forests, even if their average temperatures are the same? Without much water vapor in the atmosphere over deserts, the radiation they give off escapes readily to space. In more humid regions radiation from the surface is trapped by water vapor in the air. Similarly, cloudy nights tend to be warmer than clear nights because more water vapor is present.

The influence of carbon dioxide can be seen in past changes in climate. Ice cores from over the past million years have shown that carbon dioxide concentrations were high during warm periods – about 0.028%. During ice ages, when the Earth was roughly 7 to 13 F (4-7 C) cooler than in the 20th century, carbon dioxide made up only about 0.018% of the atmosphere.

Even though water vapor is more important for the natural greenhouse effect, changes in carbon dioxide have driven past temperature changes. In contrast, water vapor levels in the atmosphere respond to temperature. As Earth becomes warmer, its atmosphere can hold more water vapor, which amplifies the initial warming in a process called the “water vapor feedback.” Variations in carbon dioxide have therefore been the controlling influence on past climate changes.

Small change, big effects

It shouldn’t be surprising that a small amount of carbon dioxide in the atmosphere can have a big effect. We take pills that are a tiny fraction of our body mass and expect them to affect us.

Today the level of carbon dioxide is higher than at any time in human history. Scientists widely agree that Earth’s average surface temperature has already increased by about 2 F (1 C) since the 1880s, and that human-caused increases in carbon dioxide and other heat-trapping gases are extremely likely to be responsible.

Without action to control emissions, carbon dioxide might reach 0.1% of the atmosphere by 2100, more than triple the level before the Industrial Revolution. This would be a faster change than transitions in Earth’s past that had huge consequences. Without action, this little sliver of the atmosphere will cause big problems.

Written by Jason West, Professor of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill.

Jason West receives funding from the EPA, NASA, NSF, the Donald and Jennifer Holzworth Faculty Acceleration Fund in Climate Change, and the State of North Carolina.

Originally published on The Conversation. The Conversation

9 Comments on "Wait, the Atmosphere Is Only 0.04% Carbon Dioxide. How Does It Affect Earth’s Climate?"

  1. Climate change is a normal phenomenon governed by solar activity, the moon’s gravitational effect,the jet stream and many other complex factors. CO2 had nothing to do with this. Man made climate change does not exist.(4 percent man-made of 0.04 in nature is irrelevant). To spend a trillion dollars a year on a fictitious fraudulent conclusions from IPCC models is insane. We need more CO2 to encourage the greening of the planet and better crop yields. Just follow the IPCC money grants to isolate the beneficiaries of their corrupt modelling. Greta is barking up the wrong tree like Al Gore. Pity David Attenborough is too old to correct his stance. MAN-MADE CLIMATE CHANGE DOES NOT EXIST.

  2. You must have known this as well.

    “Century-scale changes in the Earth’s magnetic field have a signifcant effect on the upper atmosphere (100-500 km altitude). These are at least as important as changes associated with the increase in greenhouse gas concentrations over the past century, which was previously thought to be the main driver of climate change in the upper atmosphere. Simulations with a whole-atmosphere chemistry-climate model, extending from the surface up to 500 km altitude, demonstrate that magnetic field changes even affect the climate in the middle atmosphere (15-100 km altitude), and regionally also in the troposphere, near the surface.
    The Earth’s magnetic field further plays an important role in creating climatic differences between the polar Northern and Southern upper atmosphere.”

    https://gtr.ukri.org/projects?ref=NE%2FJ018058%2F1

  3. First 2 comments by deniers, or maybe nlp bots financed by oil companies or Russia.

    Both have been found to run disinformation bot farms.

  4. I just read a scientific paper that says the upper atmosphere is cooling dramatically. So I guess your analysis is pushing the IPCC stance and nothing more.

  5. The CO2 level of earth during the most prosperous, biodiverse, Green, life-thriving time on earth was 4,000 to 6,000 ppm. There was Far more GREEN then, far more species, and ZERO EVIDENCE of runaway warming, even at 10x-15x today’s meager near-suffocation levels. To the contrary, we slipped into an ice age.

    How are we on the verge of burn-up disaster at a tenth of a tank of ‘gas’, when we fell into ICE AGES at a full tank?

    Ridiculous.

    The CO2 warming models have failed, and this guy is deeply concerned about the end of the world (of grants) that will surely come to an end if we don’t feed the pigs & fill the climate trough! He he he.

    HAPPY THANKSGIVING !

    THANK GOD for the current warming period, the current climate Optimum, and for increased CO2, resulting in a massive 15% increased GREENING of the earth.

    So much to be THANKFUL for!

    If only we could get it from 400 ppm up to an abundant food-supplying 1500 ppm CO2… The gas of life, the food of photosynthesis.

    🙂

  6. Strange, water vapor is more than one full percent of the atmosphere 25 times the amount of CO2 with 20 times the effect. So, how does CO2 overcome the 500 pound gorilla effect of H2O?

  7. Why worry about 2100? According to that deranged girl, we’ll all be dead in ten years. Live it up!

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