(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: Climate is complicated and there are lots of competing theories and unsolved mysteries. Until this is all worked out, one can't claim there is consensus on global warming theory. Until there is, we should not take any action.
This is similar to the "global warming is a hoax" article, but at least here we can narrow down just what the consensus is about.
Answer: Sure there are plenty of unsolved problems and active debates in climate science. But if you look at the research papers coming out these days, the debates are about things like why model predictions of outgoing longwave radiation at the top of the atmosphere in tropical latitudes differ from satellite readings, or how the size of ice crystals in cirrus clouds affect the amount of incoming shortwave reflected back into space, or precisely how much stratospheric cooling can be attributed to ozone depletion rather than an enhanced greenhouse effect.
No one in the climate science community is debating whether or not changes in atmospheric CO2 concentrations alter the greenhouse effect, or if the current warming trend is outside of the range of natural variability, or if sea levels have risen over the last century.
This is where there is a consensus.