Eugene S. Takle
© 2005
The indicator usually discussed for pointing to the impact of greenhouse gases is the
global mean surface temperature (actually "surface" here means temperature measured at
about 2 meters above the earth's surface as is done at all weather stations). However,
Wild and Ohmura (2005) suggest that the influence of greenhouse gases can be more
directly assessed by measuring the "downward longwave radiation." Since greenhouse
gases absorb longwave radiation (also called "infrared radiation") escaping from the
earth and re-radiate it back (down) to the earth, measurements over a long period of
time of the downward longwave radiation should indicate whether or not higher levels of
greenhouse gases are potentially influencing our climate.
Downward longwave radiation together with solar radiation describes the rate of heating
at the Earth's surface. Air temperature, on the other hand is an imperfect measure of
heating. It describes additional heat put into air near the surface, but some of the
heat provided by downward longwave radiation and solar radiation goes into heat of the
soil, oceans, or to melt ice. What is left over heats the atmosphere. The report of
Wild and Ohmura shows that downward longwave radiation has been increasing steadily
since about 1970 (as has the surface temperature) and is a more clear indicator of
global warming.
Reference
Wild, M., A. Ohmura, 2005: RSRN longwave downward radiation
measurements combined with GCMs show promise for greenhouse detection studies. GEWEX
Newsletter, 14 (4), 9-10.