The most likely explanation of the recent acceleration of warming is the increasing
accumulation in the atmosphere of so-called greenhouse gases. To understand the
issues surrounding global warming, particularly as affected by these gases, it is
necessary to master the notion of the Earth’s radiation balance, a topic peripheral (but
critical) to biological oceanography. So, briefly, the sun shines on the atmosphere,
delivering energy to areas perpendicular to its rays at the rate of 1365.6 to 1367.0
watts m−2 (varying with the sunspot cycle) mostly in the visible part of the spectrum
(Fig. 16.3). That mostly passes through the atmosphere and heats the land and the sea.
Of course, radiance on any given area depends upon the solar zenith angle. Warmed,
the land and sea emit energy as light of longer wavelength, infrared light (IR), back
toward space. The Earth warms until its IR emissions to beyond the atmosphere equal,
on average, the incoming energy. That is the radiation balance, and the composite of
temperatures at which it is achieved depends strongly upon absorption of the IR
emissions by gases in the atmosphere. Typical in-bound light is almost entirely in a
wavelength band from 350 to 750 nm, while at surface temperatures averaging ∼15°C
the wavelength of outbound light peaks from 12 to 20 μm (the “15 micron” band, a
factor of ∼38 longer). Those wavelengths are absorbed by atmospheric gases:
transferred into vibrations of their molecular bonds and into increased molecule
velocity (higher temperature). That both warms the atmosphere and requires that the
surface be warmer to emit sufficient amounts of the “15 μm” IR, and somewhat
shorter wavelengths that are not so strongly absorbed, to attain balance. Balance is
attained at relatively short time scales (a few days).
Fig. 16.3 Increasing global average surface atmospheric temperature from 1979–2009
and solar irradiance (W m−2) incident at the outside of the atmosphere over the same
interval. (NOAA data.)