With the Copenhagen climate summit just three months away, the global warming community is starting to crank out numbers. Last week it was adaptation, and this week it’s mitigation.

The United Nations’ latest estimates indicate that the developing world will need $500 to $600 billion annually to leapfrog industrialized nations and chart a sustainable energy future (AFP). That’s roughly equal to 1 percent of the global gross domestic product. Toss in the investments that will be needed in the industrialized world, and you get a figure of roughly $1 trillion annually. The UN report assumes a goal of keeping atmospheric carbon dioxide levels below 450 parts per million, thus increasing the likelihood of averting more than 2 degrees Celsius of warming.

For an eye-popping estimate of an entirely different magnitude, we will now turn to China. The Financial Times got its hands on an economic analysis that is apparently making the rounds as China considers the costs of cleaning up its act such that emissions peak in 2030. Their numbers suggest that costs could reach $438 billion annually by 2030 – for China alone.

—

China might well roll out such numbers as they press their case for more resources from industrialized nations, but they will surely encounter a fair bit of skepticism. There’s plenty of room for argument and uncertainty, to be sure, but the UN figures come in on the high end of the range of previous estimates.

And it’s important to keep these numbers in perspective. The UN report estimates global energy investments at roughly $500 billion annually today, so we’re really talking about redirecting and then expanding investments. In terms of government expenditures, the report suggests that some $250 billion in subsidies went to fossil fuels in 2005, which is to say that we are already in up to our knees; the question is one of direction.

Quantifying all of this is notoriously difficult, of course, but at some level it needs to be done. Politicians need fodder as they negotiate over numbers for how much money is needed from whom to do what. The take home message is that rich and poor nations alike need to move forward together in a very big way. We’ll see if that message gets through in Copenhagen.