Let's return one last time to the Summary For Policymakers written in 2001 by the Intergovernmental Panel on Climate Change (IPCC), the source many global warming alarmists cite as the "consensus" view that proves we must act to prevent further global warming.
The complete chart from that report showing influences on warming in the industrial era (since 1750), is reproduced here, and this time we want to pay particular attention to the very bottom of the chart. You can click on the chart at any time for a larger view.
The bottom of the chart represents the IPCC's estimate of our level of scientific understanding of each of the factors contributing to the warming of planet earth since 1750. They claim a "high" or "medium" scientific understanding of carbon dioxide (C02), nitrous oxide (N2O), methane (CH4), halocarbons, and tropospheric ozone (O3) as warming factors. They also claim a medium level of scientific understanding of the cooling factor stratospheric ozone.
Now let's note what they admitted (in 2001) that science does NOT understand very well. Warming of the planet caused by solar radiation? They classify scientific understanding of solar-induced warming (over which man obviously has no control) as "very low." Similarly, they confess a "very low" level of understanding of the cooling effects caused by changes in land use, and the indirect effects of aerosols -- how tiny particles we release into the atmosphere affect cloud formation and precipitation, which cool the earth.
What else do we understand poorly? According to the IPCC, our understanding of global cooling from sulphates, organic carbon from burning of fossil fuels released as aerosols, and biomass burning. Also, the effects of aviation contrails, mineral dust, and carbon black from fossel fuel burning are poorly understood.
To summarize, of the major factors influencing the warming of the planet (according to the IPCC, the scientific "consensus"), we have a high level of understanding of greenhouse gases, a medium level of understanding of the effects of ozone, and a low or very low level of understanding of eight other factors: sulphate, carbon black, organic carbon, biomass burning, mineral dust, indirect effects of aerosols, aviation contrails, and the effects of the sun. I think this last item cannot be repeated enough: the IPCC admits to a "very low" level of scientific understanding of the role changes in the sun play in global warming in the industrial age.
Two global warming factors we understand well, eight factors we do not. How can accurate climate models even exist, when eight out of a dozen or so factors are poorly understood? This is a no-brainer. It is impossible to even create a reliable computer model to predict future climate and temperature when most of the major factors in such a model are poorly understood. Such was the admission of the 2001 report by the IPCC, clear to anyone who would examine their summary closely.
The "consensus" of climate scientists reports that we are heading for disastrous global warming. It does so on the basis of computer climate models. The models must take into account many factors for which our level of scientific understanding is admittedly "very low." Why should the public pay any heed to predictions that are based on such huge uncertainties?
Next time, we will update this information by examining the most recent IPCC report from 2007. The basic conclusions do not change. Their estimates are, by their own admission, subject to a very wide range of possible outcomes. The uncertainty of any such estimates is built-in, via computer models, which must necessarily take into account many things we understand poorly.