Greenhouse gases are molecules that absorb thermal radiation emitted by a planet and emit it in all directions, including back to the planet's surface. Unlike the various types of light that a planet can reflect, this emitted planetary radiation is primarily a smaller subset of the electromagnetic spectrum (e.g., infrared). By acting as a blanket that prevents some of this energy from escaping to space, a planet warms in a phenomenon called the greenhouse effect.
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Molecules vibrate and rotate with different frequencies based on their elemental composition and shape, which can produce uneven distributions of electrons. The resulting unbalanced electric field can interact with electromagnetic waves of the same frequency, causing the molecule to absorb the radiation.
Molecules that absorb and emit infrared radiation from the planet's surface effectively trap additional incident solar energy that would otherwise escape to space. Without the greenhouse effect, the planet's average surface temperature would be approximately -18 degrees Celsius (0 degrees Fahrenheit).
Some online experiments involving glass containers of air and carbon dioxide show no temperature differences upon exposure to infrared light because glass blocks that type of radiation. Using plastic bags, carbon dioxide can be demonstrated to absorb infrared radiation, thereby trapping energy and confirming its role as a greenhouse gas.
Investigate how cloud cover, surface albedo—a measure of a surface's reflectiveness—greenhouse concentrations, and atmospheric layers affect the energy balance on a simulated planet. Using either the wave or particle models of light, users can also explore how changes in solar intensity affect planetary surface temperature.
Although Irish physicist John Tyndall showed in 1859 that water vapor and carbon dioxide could absorb and radiate infrared radiation, experiments by American scientist Eunice Foote three years earlier identified the same phenomenon for light in general. Historically overshadowed, her work was rediscovered by female historians over a century later.
Although carbon dioxide, methane, nitrous oxide, ozone, and most fluorinated gases account for only a minority of the planet's greenhouse effect due to their lower concentrations, they are more effective at trapping thermal radiation. While water vapor condenses within two weeks, other greenhouse gases linger in the atmosphere for years on average.
Each type of greenhouse gas absorbs a distinct set of wavelengths of light, which serve as a unique spectral signature. By shining light through an atmospheric sample and measuring the light on the other side, the amount of each gas can be calculated from the amount of missing light.
Although greenhouse gases include methane, nitrous oxide, and most fluorinated compounds, their warming potential is often measured relative to carbon dioxide. In these units, greenhouse gas emissions have gone from 4.29 billion metric tons of carbon dioxide equivalent in 1850 to 54.43 billion metric tons in 2024.
As of 2023, the energy sector, including electricity and heating, accounts for the largest share of greenhouse gas emissions. The 10 largest greenhouse gas-emitting nations account for two-thirds of global emissions, with the world's top three—China, the US, and India—contributing 15 times more than the bottom 100.
These include turning biomass into charcoal for storage in soil before it decays and releases carbon dioxide, crushing CO₂-absorbing rocks—increasing exposed surface area—and spreading them across land to accelerate GHG uptake, and restoring degraded peatlands, which can store twice the amount of carbon as the average forest.
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