Greenhouse gases are the gases in our atmosphere that absorb heat. By itself, the thermal energy we receive from the sun would not lead to dangerous global warming. While much of the sun’s thermal energy is absorbed by the Earth, a significant amount reflects back into the air. Nitrogen and oxygen, the predominant gases in the atmosphere, allow that reflected heat to escape freely into space.
But greenhouse gases are larger, more complex molecules that bounce some of that naturally escaping heat back again to the Earth’s surface—hence the “greenhouse effect.”
Greenhouse gases present an existential problem for humanity. The most abundant type, carbon dioxide, is released whenever we burn coal, oil, or gas for energy. Once emitted from a tailpipe or smokestack, CO2 remains in the atmosphere for centuries.
By contrast, methane lasts far more briefly in the atmosphere, but traps 25 times more solar radiation over a 100-year-period, pound for pound, than carbon dioxide. The primary ingredient in natural gas, methane is burned to generate electricity, heat our homes, and fuel our stoves. Agriculture and food waste are other major sources of methane; cows release it in their digestion.
Other gases heat the planet too. They include nitrous oxide, a fertilizer by-product, as well as some common refrigerants. All of these greenhouse gases can be calibrated with a single measure: carbon dioxide equivalents, or CO2e. This umbrella metric accounts for uneven warming impacts and makes for more meaningful comparisons among various greenhouse gases.
In the preindustrial era, every million molecules of air contained around 283 molecules of CO2e. In 2018, the Intergovernmental Panel on Climate change warned that needed to keep CO2e below 485 parts per million. The problem is that we've already crossed that threshold and are now at more than 500 parts per million. (This data comes from eighty collection sites around the world and is rigorously measured by the National Oceanic and Atmosphere Administration.)
To stave off a climate catastrophe, our goal must be to prevent any additional greenhouse gas accumulation, drive CO2e back under 430 parts per million, and keep it there.