The mechanism by which carbon dioxide traps heat in the atmosphere is commonly referred to as the "greenhouse effect." Stated very simply, CO2 is nearly transparent to the solar radiation emitted from the sun, but largely opaque (in certain wavelengths) to the thermal radiation emitted by the earth. Hence, it allows most of the incoming solar radiation from the sun to pass unimpeded through the atmosphere and warm the earth’s surface; but when the earth’s surface reradiates energy back to space, a significant portion of this thermal radiation is absorbed and reradiated by the atmosphere’s CO2 molecules back to the earth’s surface, incrementally warming the planet. Without this effect of water vapor (which is responsible for the lion’s share of the warming), CO2, and other radiatively-active trace gases in the air, the planet’s average temperature would be about 34°C cooler than it is presently.
So has the greenhouse effect increased in recent years as the air’s carbon dioxide concentration has risen? In an attempt to answer this question, Harries et al. (2001) analyzed the difference between the spectra of outgoing longwave radiation obtained by two orbiting spacecraft that looked down upon the earth at periods of time separated by a span of 27 years. The data utilized were obtained over a specific area in the central Pacific (10°N-10°S, 130°W-180°W) and a "near-global" area of the planet (60°N-60°S). The data were further constrained by masking out land/island areas and areas believed to contain clouds.
The results of their analysis showed a number of differences in the land-masked and cloud-cleared data, which the authors attributed to changes in atmospheric concentrations of CH4, CO2, O3, CFC-11 and CFC-12 that occurred over the 27-year period separating the times of their two sets of measurements. Hence, they concluded their results provided "direct experimental evidence for a significant increase in the earth’s greenhouse effect" over the 27-year time interval. Such a conclusion, however, is somewhat misleading, for it does not provide direct experimental evidence for a significant increase in earth’s total greenhouse effect. It does so only for the cloud-free part of the atmosphere located over a portion of the planet’s oceans. Furthermore, research that has been conducted on the cloudy portion of the atmosphere over the oceans has revealed the presence of a highly negative feedback phenomenon that is capable of totally overpowering any temperature increase forced by the rise in greenhouse gases (Lindzen et al., 2001).
Unfortunately, the work of Harries et al. tells us nothing about earth’s climatic response to the inferred increase in radiative forcing, which is what the climate change debate is all about, i.e., trying to evaluate the competing effectiveness of various positive and negative feedbacks that come into play when there is a small change in the radiative properties of the cloudless atmosphere. In fact, the authors’ finding is so rudimentary as to be essentially meaningless. Assuming, for example, that their handling of their data is correct - and this is a huge assumption they spend over half their paper discussing - they have simply verified the definition of the greenhouse effect! Hence, the debate continues.
Harries, J.E., Brindley, H.E., Sagoo, P.J. and Bantges, R.J. 2001. Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997. Nature 410: 355-357.
Lindzen, R.S., Chou, M.-D. and Hou, A.Y. 2001. Does the earth have an adaptive infrared iris? Bulletin of the American Meteorological Society 82: 417-432.