The Sun powers Earth’s climate, radiating energy at very short wavelengths, predominately
in the visible or near-visible (e.g., ultraviolet) part of the spectrum. Roughly one-third
of the solar energy that reaches the top of Earth’s atmosphere is reflected directly back
to space. The remaining two-thirds is absorbed by the surface and, to a lesser extent, by
the atmosphere. To balance the absorbed incoming energy, the Earth must, on average, radiate
the same amount of energy back to space. Because the Earth is much colder than the Sun, it
radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see
Figure 1). Much of this thermal radiation emitted by the land and ocean is absorbed by the
atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse
effect. The glass walls in a greenhouse reduce airflow and increase the temperature of the
air inside. Analogously, but through a different physical process, the Earth’s greenhouse
effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at Earth’s surface would be below the freezing
point of water. Thus, Earth’s natural greenhouse effect makes life as we know it possible. However, human activities,
primarily the burning of fossil fuels and clearing of forests, have greatly intensified the
natural greenhouse effect, causing global warming.
The two most abundant gases in the atmosphere, nitrogen (comprising 78% of the dry atmosphere)
and oxygen (comprising 21%), exert almost no greenhouse effect. Instead, the greenhouse effect
comes from molecules that are more complex and much less common. Water vapour is the most
important greenhouse gas, and carbon dioxide (CO2) is the second-most
important one. Methane, nitrous oxide, ozone and several other gases present in the atmosphere in
small amounts also contribute to the greenhouse effect. In the humid equatorial regions, where
there is so much water vapour in the air that the greenhouse effect is very large, adding a small
additional amount of CO2 or water vapour has only a small direct
impact on downward infrared radiation. However, in the cold, dry polar regions, the effect of a
small increase in CO2 or
water vapour is much greater. The same is true for the cold, dry upper atmosphere where a small
increase in water vapour has a greater influence on the greenhouse effect than the same change
in water vapour would have near the surface.
Several components of the climate system, notably the oceans and living things, affect
atmospheric concentrations of greenhouse gases. A prime example of this is plants taking
CO2 out of the atmosphere and converting it (and water) into
carbohydrates via photosynthesis. In the industrial era, human activities have added greenhouse
gases to the atmosphere, primarily through the burning of fossil fuels and clearing of forests.
Adding more of a greenhouse gas, such as CO2, to the atmosphere
intensifies the greenhouse effect, thus warming Earth’s climate. The amount of warming depends
on various feedback mechanisms. For example, as the atmosphere warms due to rising levels of
greenhouse gases, its concentration of water vapour increases, further intensifying the greenhouse
effect. This in turn causes more warming, which
causes an additional increase in water vapour, in a self-reinforcing cycle. This water vapour
feedback may be strong enough to approximately double the increase in the greenhouse effect due
to the added CO2 alone.
Additional important feedback mechanisms involve clouds. Clouds are effective at absorbing
infrared radiation and therefore exert a large greenhouse effect, thus warming the Earth. Clouds
are also effective at reflecting away incoming solar radiation, thus cooling the Earth. A change
in almost any aspect of clouds, such as their type, location, water content, cloud altitude,
particle size and shape, or lifetimes, affects the degree to which clouds warm or cool the Earth.
Some changes amplify warming while others diminish it. Much research is in progress to better
understand how clouds change in response to climate warming, and how these changes affect climate
through various feedback mechanisms.