1896

Greenhouse Effect

Joseph Fourier (1768–1830), Svante Arrhenius (1859–1927)

We often think of our home planet as a natural “Goldilocks” world, not the hellish inferno of closer-to-the-Sun Venus or the frozen ice world of farther-from-the-Sun Mars. It wasn’t until the end of the nineteenth century, however, that scientists realized that Earth is a habitable oceanic world only because of the influence of two relatively minor, but critically important, atmospheric gases: water vapor (H₂O) and carbon dioxide (CO₂). Without them, Earth’s oceans would freeze solid, and life on our planet would likely be very different, if indeed any life had developed at all.

In the 1820s the French mathematician Joseph Fourier was the first scientist to realize that the Earth’s equilibrium temperature—how warm the surface would be if it were heated by sunlight alone—was actually well below freezing. So why are the oceans liquid? Fourier speculated that the atmosphere might act as an insulator, perhaps trapping heat like the panes of glass in a greenhouse. But Fourier wasn’t sure.

It was the Swedish physicist and chemist Svante Arrhenius who provided the answer, showing that gases in our atmosphere are indeed warming the surface, by more than 30 degrees—thus keeping our planet above freezing. The specific gases responsible are mostly water and carbon dioxide. These gases are transparent and thus let sunlight reach the surface, but they absorb a large part of the outgoing infrared heat energy emitted by the planet, thereby warming the atmosphere. Even though the warming is different from that in a closed glass box, it’s still called the greenhouse effect, partly because of the earlier ideas and experiments discussed by Fourier.

Arrhenius knew that greenhouse warming was a simple—and fortunate—consequence of Earth’s natural abundance of water and carbon dioxide, and he speculated that past decreases in carbon dioxide, especially, could explain the ice ages. He was the first to further speculate that future burning of fossil fuels could enhance the carbon dioxide abundance and lead to global warming. The Earth’s climate is more complex than Arrhenius envisioned, but still his concern over the role that people might have on changing the Earth’s environment has turned out to be prescient.

Greenhouses stay warm even in cold weather because the glass walls and ceiling block the wind and prevent some infrared heat energy from escaping. Gases like water vapor and CO₂ in the Earth’s atmosphere also prevent some infrared heat energy from escaping, producing extra warming at the surface of our planet.