I devoted six years to carbon accounting, building models for the Australian government to estimate carbon emissions from land use change and forestry (Google on "FullCAM"). When I started that job in 1999 the evidence that carbon emissions caused global warming seemed pretty conclusive, but since then new evidence has weakened the case that carbon emissions are the main cause. I am now skeptical. As Lord Keynes famously said, "When the facts change, I change my mind. What do you do, sir?"