Let’s call this the "Null Hypothesis." Logically therefore, the burden of proof is on alarmists to demonstrate that the Null Hypothesis is not adequate to account for empirical climate data; alarmists must provide convincing observational evidence for Anthropogenic CC (ACC).
Even more fundamentally, THEY'RE the ones proposing we do something new. They can't very well take the position "We think there's this danger, and we're going to take steps to guard against it, unless YOU prove it's not true", especially when those steps are anti-freedom and anti-rights, and empower the government at the people's expense. YOU'RE the ones who think the status quo is wrong, YOU have the burden of proof.
They think the solution to their orchestrated crisis of climate change is for the government to control energy use and, therefore, the means of production.