is the Greenhouse Effect?
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.
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.