How hard is this problem?

Read Paul Krugman's "Errors & Emissions: Could Fighting Global Warming be Cheap & Free?" here:

http://www.nytimes.com/2014/09/19/opinion/paul-krugman-could-fighting-global-warming-be-cheap-and-free.html?hp&action=click&pgtype=Homepage&module=c-column-top-span-region&region=c-column-top-span-region&WT.nav=c-column-top-span-region&_r=2