CalTech's Dr. Tapio Schneider gives a fairly standard talk on what we do and don't know about global climate change:

Carbon dioxide has been increasing dramatically; this increase is due to human activity, though some anthropogenic emissions are taken up by carbon sinks; carbon dioxide magnifies the greenhouse effect because it absorbs solar radiation; increasing carbon dioxide increases the radiative forcing in the lower atmosphere; global mean temperatures have generally risen with the increase in carbon dioxide concentrations; temperatures on land have increased than temperatures over water due to the thermal inertia of the oceans; various feedbacks within the climate system, both positive and negative, modify the extent to which increases in radiative forcing increases surface temperatures; the extent of these feedbacks represent the greatest uncertainty in projected climate change; despite the uncertainties, climate models do a fairly good job of replicating the temperature changes of the past 150 years; model projections for the future vary substantially in their projections – between 1.4 to 5.8 degrees Celsius by 2100) – largely due to difference assumptions regarding cloud feedbacks and future patterns of energy use; future warming will occur primarily on land, increasing the risk of summer droughts and increasing sea level.

Schneider adds that there is large thermal and dynamic inertia in the climate system, and therefore much of the climate forcing already underway will continue irrespective of whether greenhouse gas concentrations are stabilized in the near term (which is an exceedingly unrealistic). Thus, climate change is inevitable. The issue today is how much climate change we are willing to tolerate (or risk), not whether we will address climate change.

In response to questions, Schneider says the risks are serious, but there is no risk of a runaway greenhouse effect turning planet Earth into another Venus. He also stresses that the scientific findings do not determine what policies we should adopt.

Another questioner wonders about the effects of sulfates. It is assumed that sulfate aerosols led to global cooling between the 1940s and 1970s, and this can be seen regionally over the United States. The questioner wonders why there is not a similar regional cooling in parts of the world that have industrialized since, such as India and China, where sulfate pollution has not decreased as it has in the West. Schneider suggests this cooling effect has been swamped by the overall warming, but seems tentative in his response.