Verdes calculated the amount of non-natural influence required to match the increases in temperature observed in the last 150 years. He plotted the influence over time. Then, he compared it to the evolution of greenhouse gasses, taking into account the cooling due to aerosols. With allowances for error, he found that influences attributable to greenhouse gasses mirror the graph of non-natural influence needed to explain the observed temperature increase of recent decades.
As the "debate" about the existence of global warming, and the causes of it, there are mounting evidence for human activities being a significant cause of it.
I'll edit this entry to include the exact reference once this paper has been published.
Edit: This is the exact citation to this paper:
Pablo F. Verdes, Phys. Rev. Lett., v.99, p.048501 (2007).
There is also a review of this work on PhysicsWeb.