Greenhouse Effect and Global Warming

Why it is called the "greenhouse" effect

The term borrows its name from an actual greenhouse — a small glass structure used to grow plants, especially through winter. The glass lets sunlight in freely but does not let the resulting heat escape easily, so the enclosed space warms up — much the way the inside of a car left parked in the sun heats up over a few hours.

The natural greenhouse effect

The greenhouse effect is itself a naturally occurring process, and it is what keeps Earth's surface and atmosphere warm enough to support life as we know it. Without it, Earth's average surface temperature would be a frigid about –18°C, rather than the present average of roughly 15°C.

To understand how it works, consider what happens to sunlight once it reaches the outer atmosphere. Clouds and gases reflect roughly a quarter of the incoming solar radiation straight back to space and absorb a further portion, but almost half of it reaches and warms Earth's surface directly, with only a small remaining fraction reflected away. The warmed surface then re-radiates this energy as infrared (heat) radiation, but much of this outgoing heat never escapes into space, because certain atmospheric gases — carbon dioxide, methane and others — absorb a large share of it. These gas molecules then re-radiate that absorbed heat in all directions, and a substantial portion of it travels back down to the surface, warming it again. This absorb-and-re-emit cycle repeats many times over, trapping heat that would otherwise escape, which is why carbon dioxide, methane and similar gases are called greenhouse gases.

• cover: 16.6 Sunlight energy at the outermost atmosphere — a cross-section of Earth and its atmosphere viewed from space shows sunlight arriving from the Sun and passing through a "greenhouse gases" layer in the atmosphere to reach the surface; outgoing infrared radiation from the surface is repeatedly absorbed and re-emitted by this layer, both back toward the surface and further out, until eventually the surface has no more long-wave radiation left to give up.
• cover: 16.7 Relative contribution of various greenhouse gases to total global warming — a pie chart divides the greenhouse effect's warming contribution among carbon dioxide (60%, by far the largest single share), methane (20%), CFCs (14%) and nitrous oxide, N2O (6%).

From natural warming to global warming …