Why Do Greenhouse Gases Warm the Planet?

By Hallie Cordingley

For over a century, scientists have been measuring the effects of greenhouse gases on the temperature of the planet. Earth naturally experiences temperature fluctuations, but data tracing back to the 1800s reveals a severe atmospheric warming trend throughout the 20th and 21st centuries, traceable to the increase in anthropogenic greenhouse gas emissions. Carbon dioxide is often discussed as one of the main contributors to this warming. But why would a gas accounting for just over 0.04% of the atmosphere pose such a danger?

Of the sunlight that reaches Earth, approximately 70% heats the planet through absorption by the oceans, land, and atmosphere. The remaining 30% is reflected back into space. A small fraction of the absorbed heat also gets radiated back into space through something called infrared radiation, or IR. Though IR is invisible, it is detectable by temperature.

A larger proportion of the heat radiating back from the Earth is actually absorbed by certain gases in the atmosphere that keep the planet warm and insulated when it otherwise would be too cold for existing life forms. Sunlight warms the planet during the day. At night, heat leaving the Earth’s surface is trapped in the atmosphere by what are called greenhouse gases. These gases, which include methane and carbon dioxide, efficiently capture 90% of the heat radiating from the Earth.

The greenhouse effect takes its name from the comparison of gases in the atmosphere to the glass walls and ceiling of a greenhouse. These walls admit sunlight during the day, and retain heat in the interior at night and during the winter, encouraging plant growth in colder conditions. Gases like carbon dioxide and methane create a similar effect for life on the planet, retaining heat within the Earth’s troposphere, the lower layer of atmosphere.

So how do greenhouse gases actually absorb radiation and insulate the earth? Molecules such as carbon dioxide and methane have three or more atoms, allowing them to twist and vibrate unlike, say, N2 and O2, which contain only two atoms and make up about 99% of the atmosphere. Temperature is simply a measure of the energy within molecular motion in matter. Infrared radiation energy, when absorbed by molecules, results in increased vibration.

Animation by Isabel Plower

This vibration then leads to more collisions among energized particles. In effect, these collisions trap infrared radiation (IR) in the atmosphere as it tries to escape. It’s almost like a game of pinball, where the ball is IR and the flippers are greenhouse gases. And the game is made more challenging because the flippers are constantly moving and colliding with one another. As humans put more metaphorical flippers into the atmosphere, it becomes that much more difficult for IR to escape, ultimately raising the temperature.

Animation by Isabel Plower

Carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride behave this way, as does water vapor. But many gases with three or more atoms do not. Greenhouse gases also have a wide range of potency. Some exert a warming effect several thousand times greater even than that of carbon dioxide. These differences have to do with the geometry and electrical charges of the atoms within the molecules, among other factors.

This phenomenon explains why cutting greenhouse gas emissions to zero wouldn’t be enough to “solve” climate change. The greenhouse gases that humans have already emitted would continue to trap infrared radiation (IR) in the atmosphere. Simultaneously, oceans and forests would be sucking carbon out of the atmosphere, leading to a cooling effect. Recent research demonstrates that these two effects could essentially cancel each other out and stabilize the climate, though not to the extent of a return to preindustrial conditions. To do that would require sucking over a century of carbon emissions back out of the atmosphere. This could be done through plants, enhanced weathering, or such technologies as direct air capture, which pulls carbon dioxide out of thin air and buries it deep underground.

Animation by Isabel Plower

That said, a return to a preindustrial climate is not required for the Earth to sustain human life. The Earth is about 2°F (1.1°C) warmer today than before the modern age. These are conditions to which humans are still able to adapt. But this conclusion is largely theoretical. Humans have a long way to go to cut greenhouse gas emissions before talking about the pros and cons of working to become carbon negative.

Over time, greenhouse gases have kept the planet at an average temperature of 58°F. Without the greenhouse effect, scientists estimate that the planet’s temperature would be closer to an average of 0°F. Thus the greenhouse effect is essential to maintaining life on Earth. If humans can just learn to limit greenhouse gas emissions, these gases can return to their original job description: keeping an otherwise very cold planet temperate and livable for all.