[Brucelee]

An except:



What are the take-home messages:


The temperature effect of atmospheric carbon dioxide is logarithmic, not exponential.

The potential planetary warming from a doubling of atmospheric carbon dioxide from pre-Industrial Revolution levels of ~280ppmv to 560ppmv (possible some time later this century - perhaps) is generally estimated at less than 1 °C.

The guesses of significantly larger warming are dependent on "feedback" (supplementary) mechanisms programmed into climate models. The existence of these "feedback" mechanisms is uncertain and the cumulative sign of which is unknown (they may add to warming from increased atmospheric carbon dioxide or, equally likely, might suppress it).

The total warming since measurements have been attempted is thought to be about 0.6 degrees Centigrade. At least half of the estimated temperature increment occurred before 1950, prior to significant change in atmospheric carbon dioxide levels. Assuming the unlikely case that all the natural drivers of planetary temperature change ceased to operate at the time of measured atmospheric change then a 30% increment in atmospheric carbon dioxide caused about one-third of one degree temperature increment since and thus provides empirical support for less than one degree increment due to a doubling of atmospheric carbon dioxide.

There is no linear relationship between atmospheric carbon dioxide change and global mean temperature or global mean temperature trend -- global mean temperature has both risen and fallen during the period atmospheric carbon dioxide has been rising.
The natural world has tolerated greater than one-degree fluctuations in mean temperature during the relatively recent past and thus current changes are within the range of natural variation. (See, for example, ice core and sea surface temperature reconstructions.)

Other anthropogenic effects are vastly more important, at least on local and regional scales.
Fixation on atmospheric carbon dioxide is a distraction from these more important anthropogenic effects.
Despite attempts to label atmospheric carbon dioxide a "pollutant" it is, in fact, an essential trace gas, the increasing abundance of which is a bonus for the bulk of the biosphere.
There is no reason to believe that slightly lower temperatures are somehow preferable to slightly higher temperatures - there is no known "optimal" nor any known means of knowingly and predictably adjusting some sort of planetary thermostat.

Fluctuations in atmospheric carbon dioxide are of little relevance in the short to medium term (although should levels fall too low it could prove problematic in the longer-term).

Activists and zealots constantly shrilling over atmospheric carbon dioxide are misdirecting attention and effort from real and potentially addressable local, regional and planetary problems.

Remember: Water vapor and carbon dioxide are major greenhouse gases. Water vapor accounts for about 70% of the greenhouse effect, carbon dioxide somewhere between 4.2% and 8.4%. Much of the wavelength bands where carbon dioxide is active are either at or near saturation. Water vapor absorbs infrared over much the same range as carbon dioxide and more besides. Clouds are not composed of greenhouse gas -- they are mostly water droplets -- but absorb about one-fifth of the longwave radiation emitted by Earth. Clouds can briefly saturate the atmospheric radiation window (8-13µm) through which some Earth radiation passes directly to space (those hot and sticky overcast nights produce this effect - that is greenhouse but has nothing to do with carbon dioxide). Greenhouse gases can not obstruct this window although ozone absorbs in a narrow slice at 9.6µm. Adding more greenhouse gases which absorb in already saturated bandwidths has no net effect. Adding them in near-saturated bands has little additional effect