The Natural Greenhouse Effect
The earth receives an enormous amount of solar radiation. Just above the
atmosphere, the solar power flux density averages about 1366 watts per square
meter, or 1.740×1017 W over the entire Earth. This figure greatly exceeds the
power generated by human activities. The difference between the natural
greenhouse effect and global warming is that- global warming is anthropogenic
whereas greenhouse effect is not.
The solar power hitting Earth is balanced over time by an equal amount of
power radiating from the Earth (as the amount of energy from the Sun that is
stored is small). Almost all radiation leaving the Earth takes two forms:
reflected solar radiation and thermal black body radiation.
Reflected solar radiation accounts for 30% of the Earth's total radiation: on
average, 6% of the incoming solar radiation is reflected by the atmosphere, 20%
is reflected by clouds, and 4% is reflected by the surface.
The remaining 70% of the incoming solar radiation is absorbed: 16% by the
atmosphere (including the almost complete absorption of shortwave ultraviolet
over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the
land and oceans.
This absorbed energy heats the atmosphere, oceans, and land and powers life
on the planet. It should be noted that the surface of the Earth is in constant
flux with daily, yearly and age long cycles and trends in temperature and other
variables for a variety of causes; thus these percentages apply on average only.
Like the Sun, the Earth is a thermal radiator. Because the Earth's surface is
much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates
that Earth radiates its thermal energy at longer wavelengths than the Sun. While
the Sun's radiation peaks at a visible wavelength of 500 nanometers, Earth's
radiation peak is in the longwave (far) infrared at about 10 micrometres.
The Earth's atmosphere is largely transparent at visible and near-infrared
wavelengths, but not at 10 micrometres (this is, probably, not entirely
coincidental: the transparency to "visible" wavelengths makes eyes adapted to
seeing these wavelengths useful; and eye that could see in a strongly-absorbed
wavelength would not be so useful).
Only about 6% of the Earth's total radiation to space is direct thermal
radiation from the surface. The atmosphere absorbs 71% of the surface thermal
radiation before it can escape. The atmosphere itself behaves as a radiator in
the far infrared, so it re-radiates this energy.
The Earth's atmosphere and clouds therefore account for 91.4% of its longwave
infrared radiation and 64% of Earth's total emissions at all wavelengths. The
atmosphere and clouds get this energy from the solar energy they directly
absorb; thermal radiation from the surface; and from heat brought up by
convection and the condensation of water vapor.
Because the atmosphere is such a good absorber of longwave infrared, it
effectively forms a one-way blanket over Earth's surface. Visible and
near-visible radiation from the Sun easily gets through, but thermal radiation
from the surface can't easily get back out. In response, Earth's surface warms
The power of the surface radiation increases by the Stefan-Boltzmann law
until it (over time) compensates for the atmospheric absorption. Another,
simpler, but essentially equivalent way of looking at this is that the surface
is heated by two sources: direct solar radiation, and thermal radiation from the
atmosphere; it is thus warmer than if heated by solar radiation alone.
The result of the greenhouse effect is that average surface temperatures are
considerably higher than they would otherwise be if the Earth's surface
temperature were determined solely by the albedo and blackbody properties of the