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 ABC FOR SUSTAINABLE CITIES
UNEP / UN HABITAT / FIDIC / GI-REC
Based on
Graphic: IPCC
