The burden of proof is to prove a negative?
Absolutely. This isn't a matter of publishing a scientific paper, it's a matter of public policy. We consider this normal when it comes to other matters -- for instance, suppose a pharmaceutical company wants to market a new drug. The burden of proof is on them to demonstrate to the FDA that the drug is safe. The burden of proof is not on the FDA to prove that the drug is dangerous or harmful.
Furthermore, consider what we know about carbon dioxide in the atmosphere:
- there's a lot more of it than there was before the industrial revolution
- the increase roughly corresponds to the amount of increase we would expect from the burning of fossil fuels as a result of human activity
- carbon dioxide is known to be a greenhouse gas, which prevents heat from escaping back out into space
None of the above statements should be controversial. What is controversial, is how the increased carbon dioxide is going to interact with the rest of the environment, and for that we use computer models. These models may be flawed, but the worst models will always be better than sticking your head in the sand and hoping for the best. The best computer models are saying that we can expect temperatures to rise.
Unlike the pharmaceutical example above, we already know from the non-controversial facts of the matter that any result other than an increase of temperatures would be surprising. If, say, a pharmaceutical company wished to market a drug containing cyanide, the FDA would and should certainly be interested whether there were some mechanism in place that is expected to render the cyanide harmless.
If you need an additional reason why we should not demand positive proof, consider the form which positive proof would take: simply, to let things be as they are, and to wait and see if an environmental catastrophe ensues. If we were to take that course of action, and a catastrophe did ensure (as we have every reason to believe it would), we would have lost much time in implementing a solution, and the harm would be far greater.