For over a century, scientists have been measuring the effects of greenhouse gasses on the temperature of the planet. Earth naturally experiences temperature fluctuations, but data tracing back to the 1800s reveals an atmospheric warming trend throughout the 20th and 21st centuries, which can be tied to the increase in anthropogenic greenhouse gas emissions. Carbon dioxide is often discussed as one of the main contributors to this warming. But why is a gas accounting for just a little over 0.04% of the atmosphere so dangerous?
Approximately 70% of the sunlight that reaches Earth’s is used to heat the planet through absorption by the oceans, land, and atmosphere, and the remaining 30% is reflected back into space. A small fraction of this absorbed heat gets radiated back into space through something called infrared radiation or IR. IR is invisible but felt through heat.
A bigger portion of the heat radiating from the earth is actually absorbed by certain gasses in the atmosphere like methane and carbon dioxide, keeping 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 leaves Earth’s surface but is trapped in the atmosphere by what are called greenhouse gasses. Because greenhouse gasses are so efficient at heat capture, they manage to trap around 90% of the heat coming from the earth.
The greenhouse effect takes its name from the comparison of gasses in the atmosphere to the glass walls and ceiling of a greenhouse. These walls are effective at letting sunlight in, but keep most of the heat in the interior at night and during the winter, encouraging plant growth in less favorable outdoor conditions. Gasses like carbon dioxide and methane create a similar effect for life on the planet, warming Earth and the troposphere, the lower part of the atmosphere.
So how do greenhouse gasses actually absorb the radiation and insulate the earth? Molecules such as carbon dioxide and methane have three or more atoms, meaning they have the ability to twist and vibrate unlike, say, N2 and O2, which only contain two atoms and make up about 99% of the atmosphere. Temperature is simply a measure of the energy within molecular motion in matter. IR energy, when absorbed by molecules, results in increased vibration.
This vibration then leads to more collisions among energized particles. In effect, these collisions trap IR in the atmosphere as it tries to escape. It’s almost like a game of pinball, but the ball is IR, the flippers are greenhouse gasses, and the game is even more challenging because the flippers are constantly moving and banging into each other. As humans put more metaphorical flippers into the atmosphere, it becomes that much more difficult for IR to actually escape, ultimately raising the temperature.
Not all gasses with three or more atoms behave this way, though. Carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride behave this way, as does water vapor (at much shorter timescales), but many gasses with three or more atoms do not. Greenhouse gasses also have a wide range in how potent they are, with some having a warming effect several thousand times larger than that of carbon dioxide. These differences largely have to do with the geometry and electrical charges of the atoms within the molecules, among other factors.
This phenomenon explains why solely cutting greenhouse gas emissions to zero, or close to it, wouldn’t “solve” climate change. The greenhouse gasses that humans have already emitted would continue to trap IR in the atmosphere, which would actually continue to have a warming effect. Simultaneously, oceans and forests would be sucking carbon out of the atmosphere, leading to a cooling effect. Recent research demonstrates that these two effects essentially cancel each other out, and the climate stabilizes. It does not, however, return to the preindustrial climate. Doing that would actually require sucking over a century of carbon emissions back out of the atmosphere, which can be done through plants, enhanced weathering, or technologies such as direct air capture which pull carbon dioxide out of thin air and bury it deep underground.
That said, a return to a preindustrial climate is not required for the Earth to comfortably sustain human life. The Earth is about 2°F (1.1°C) warmer today than it was back then, and while this warming has led to a lot of issues, they are largely issues where humans could adapt to them. But that conversation is entirely 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.
But despite the major concern of the greenhouse effect running amuck today, the greenhouse effect is, inherently, a good thing. Greenhouse gasses have kept the planet at an average temperature of 58°F. Without it, scientists estimate that the planet’s temperature would be closer to an average of 0°F. That would not be good. As such, the greenhouse effect is essential to maintaining life on Earth, and if humans cut greenhouse gas emissions, the greenhouse effect can return to its original job description: keeping an otherwise very cold planet comfortable and livable for all.