Just $400 billion - 0.8% of global GDP. That's how much money one of the world's leading economists says governments around the world need to put towards "green" policies that will ensure global finances and the global environment prosper in the 21st century.
There's a catch, and it's a big one: the money needs to be set aside by this summer and spent by mid-2010.
The former chief economist of the world bank, Nicholas Stern, and three leading economists have compiled their report under the auspices of the Grantham Research Institute on Climate Change. Their aim was to determine whether it made sense to solve "the two great crises of our time" - one financial, one environmental - in one fell swoop.
The four were co-authors of the 2006 Stern Review, which estimated the economic cost of tackling climate change.