The temperature of the Earth’s surface is determined by the balance of radiative
fluxes of energy at the surface. Downward radiation from the Sun and atmosphere
is balanced by upward radiation emitted by the Earth, as well as, energy absorbed
by the Earth, particularly by the oceans.
On average, each square meter of the upper regions of the atmosphere receives 342
watts of solar radiation (W/m²). The atmosphere absorbs on average 67W/m² and
reflects 77 W/m². About 198 W/m² reaches the Earth's surface, of which 168 W/m²is absorbed and 30 W/m² is reflected back to space (Nielsen, R. 2005, 'Solar Radiation').
Obviously, the solar irradiance at any one location can vary greatly from these averages.
In particular, locations closer to the Equator receive significantly more radiation on
average and regions near the poles receive significantly less, especially during winter time when solar irradiation can be zero.
The radiation of the Sun and the Earth is approximately determined by Planck's Law,
which describes the spectral density of electromagnetic radiation emitted by a black
body in thermal equilibrium at a given temperature T.
The figure below shows the radiation spectrum from the Sun reaching the atmosphere
(yellow) and the Earth’s surface (red). The atmosphere is mostly transparent to the
radiation from the sun which is peaked in the visible spectrum due to the relatively
hot 5,778 C average temperature of Sun’s surface.
Because the global average surface temperature is only 15 C, the radiation emitted
by the Earth is in the infrared (IR) spectrum. A majority of the Earth's IR radiation is absorbed and re-emitted in all directions by the Earth's atmosphere.
A straightforward calculation using well-established physics principles shows that
without greenhouse gases in the earth’s atmosphere the average temperature of the planet would be -18 C instead of the actual 15 C. The dominant greenhouse gas is water vapor, which has strong absorption across them IR spectrum emitted by the Earth, with the exception of several "infrared atmospheric windows", with the most significant window being between 8 and 14 microns emitted wavelength (see the figure below).
The main source of anthropogenic (due to activities of mankind) global warming
(AGW) and resulting climate change (CC) is greenhouse gases including CO2,
methane, and CFCs produced in large quantities since the beginning of the industrial age. CO2 receives the majority of attention because we emit it in large quantities from
activities such as burning fossil fuels. In pre-industrial times the level was 280 parts
per million (ppm) and is currently at 410 ppm. The figure below shows the increase in CO2 at the Hawaii Mauna Loa observatory since 1958.
There are no scientifically valid arguments to dispute that anthropogenic greenhouse
gases warm the planet; the only valid arguments regard the magnitude of the warming,
especially when “feedback” effects with other climate parameters, especially water vapor are included. The warming is not just a theoretical prediction. It has been observed in multiple experiments. Harries (Nature 2001) compares the IR spectrum from Earth as measured from satellites in 1970 and 1997, and the increase in absorption by greenhouse gases are clearly seen in the figure below.
Simarialy, Evans 2006 presented data showing the emission signatures of greenhouse gases in the downward IR radiation in the atmosphere as seen below.
