Why most climate scientists believe that human influence now dominates natural variability in climate

The Boston Review has a nice article on climate change written for a general audience. The whole article is a bit long, but worth the read. The authors main conclusion is that almost all climate scientists today believe that man's influence on the climate dominates that of natural variability.
To quote from the article; this figure shows the results of two sets of computer simulations. In the first (blue) uses only natural forcings, such as variable solar output and dimming owing to aerosols produced by known volcanic eruptions. The second (in red) adds anthropomorphic influences such as sulfate aerosols and greenhouse gases. The solid curves show the averages of multiple simulations. The observed global average surface temperature is depicted by the black curve. One observes that the two sets of simulations begin to diverge during the 1970s and today have no overlap at all. This exercise has been repeated many times with the same qualitative result: one cannot simulate the evolution of the climate over last 30 years without including in the simulations mankind'’s influence on sulfate aerosols and greenhouse gases.

So what does this mean?

Projections based on climate models suggest that the globe will continue to warm another 3 to 7°F over the next century.

The warming of already hot regions is expected to be somewhat less, while the warming of cold regions like the arctic is projected to be more, with nighttime temperatures increasing more rapidly than daytime temperatures.

huge quantities of water were locked up in the great continental ice sheets. As polar regions warm, it is possible that portions of the Greenland and Antarctic ice sheets will melt, increasing sea level. Should Greenland ice cap to melt, sea level would increase by around 22 feet—flooding many coastal regions including much of southern Florida and lower Manhattan.

The twin perils of flood and drought both increase substantially in a warmer world. Expect it to rain harder in places that are already wet and at the same time expect the intensity, duration, or geographical extent of droughts to increase.

It is possible that negative-feedback mechanisms that we have not contemplated or have underestimated will kick in, sparing us from debilitating consequences. Equally likely there are positive feedbacks that could make matters worse than projected.