Sir Nicholas Stern estimates that the average per capita carbon footprint will need to be reduced to 2 tonnes per person (with an expected population of 9 billion people in 2050) for us to limit the adverse impacts of climate change.

CNN: Your ask the expert: Carbon trading, part 2

But some economists think Lord Stern's cost estimates are too low.

ECONOMIST: Is it worth it?

Research by Panos Ipeirotis, a computer expert at the Stern School of Business at New York University, estimates that Mechanical Turk alone engages 500, 000 active workers in more than 100 countries, with workers heavily concentrated in two countries: the United States (with 50 percent of the total) and India (with 40 percent).

FORBES: On the New York Times Stupidity Over Amazon's Mechanical Turk