Global Warming

The global warming described above tends to be attributed to human influences, particularly the release of greenhouse gases into the atmosphere by human activities. The largest culprit is carbon dioxide. Atmospheric CO₂ has increased about 35% between 1850 and 2006. CO₂ is the second greatest contributor (after water vapor) to the natural greenhouse effect that maintains a habitable climate on Earth.
There is a problem with attributing the 20th century warming to increasing CO₂: most of the temperature increase occurred from 1910 to 1940, but only a third of the modern increase in CO₂ had occurred by then. Clearly, the 20th century temperature increase in general cannot be attributed to human activities. The emerging hypothesis is that post-1940 temperature change was suppressed by the cooling effect of aerosols (particulate pollution) in the atmosphere, which diminished since the 1970s.
The CO₂-temperature causal relation has been mentioned in connection with ice-core data dating to several hundred thousand years ago, correlating CO₂ and temperature, but it is increasingly clear that in the geologic past the temperature changes preceded the CO₂ changes (Schackleton, 2000) Rather than atmospheric CO₂ driving temperature changes, it is temperature changes which may have driven past changes in the global carbon cycle (including biomass and oceanic CO₂) resulting in atmospheric CO₂ changes (Indermuhle et al., 1999).
This hypothesis rests on general circulation models (GCMs), computer models which simulate the global atmosphere including influences from the Sun, greenhouse gases, aerosols, clouds, ocean temperatures, etc. Published models have come to produce somewhat consistent results. This consistency by itself does not affirm the models: many of the influences cannot be adequately included--in fact, some effects are not yet understood from a physical standpoint to permit their modeling from first principles. The solution, to incorporate such effects as empirical factors, is not inherently wrong, but problems arise when too much confidence is placed in the results. After all, when multiple non-greenhouse gas forcings are included in ad-hoc ways, the fact that the models can be adjusted to reproduce past climate is no longer a test of the models. This misplaced faith is illustrated by the intensive scrutiny of the MSU satellite temperature record, prompted in part because these observations did not match the models. The scrutiny was fruitful, but the models remain the subject of too much faith. In addition, the similar empirical adjustments used in different GCMs contributes to similar outcomes, giving a misleading apperance of "consensus".