I think talk of "the theory" is ambiguous, so let's break this down a bit.<p>(All the claims I'm going to list should really be made in a quantitative form. They all can be; I'm just too lazy right now to look up the numbers.)<p>Theory 1: <i>More CO2 in the atmosphere tends to produce warmer climate.</i> This is, on the face of it at least, a consequence of well-established physics. The burden of proof here is on anyone wishing to deny it, just as it would be for "if I drop a dachsund from the top of the Leaning Tower of Pisa, it will fall to the ground".<p>Theory 2: <i>Human activity over recent decades has increased the amount of CO2 in the atmosphere.</i> Yup, the burden of proof here would be on those who advocate this theory. As it happens, there's no shortage of evidence. See, e.g., <a href="http://www.gcrio.org/ipcc/qa/05.html" rel="nofollow">http://www.gcrio.org/ipcc/qa/05.html</a> .<p>Theory 3: <i>Human activity such as we've seen over recent decades is likely to lead to warmer and warmer climates, if CO2 emissions aren't reduced and other drastic steps aren't taken.</i> It seems to me that, <i>given Theory 1 and Theory 2</i>, the burden of proof is on anyone who wishes to reject Theory 3, because it's basically a consequence of those two.<p>(If you strengthen Theory 3 to include the claim that human activity over the last several decades <i>has in fact</i> led to warmer climates, then I agree that the burden of proof is on whoever's making that extra claim. It seems to me that the evidence for that claim is in fact very strong, and with that evidence in place the rest of Theory 3 becomes even more the default position in the absence of strong disconfirming evidence.)