At the AEA meetings in Boston this January Robert Barro talked about global warming. He has a draft paper out, "Environmental Protection, Rare Disasters, and Discount Rates," where he argues that the biggest concern in global warming ought to be insuring against low-probability disasters, rather than remedying the more likely levels of warming which cause relatively little damage. William Nordhaus estimated the damage from unmitigated global warming of 3 degrees Centigrade at 2.5% of world GDP. (http://yosemite.epa.gov/ee/epa/eerm.nsf/vwAN/EE-0564-14.pdf/$file/EE-0564-14.pdf). That's a lot of GDP, but an ordinary recession has the same effect— say, a 1% decline in GDP and 1.5% lost potential growth. Most recessions are followed by a bounceback to the trend line for potential GDP, but that doesn't always happen. Laurence Ball found that by 2013 U.S. potential GDP was still 4.7 percent below the trend before the Great Recession of 2009. (http://www.newrepublic.com/article/117976/us-potential-gdp-vs-gdp-shows-great-recessions-long-term-damage) Thus, we in the US have already experienced double the likely loss from global warming (assuming global warming resumes again after its 14 year halt).
What Barro points out (as Martin Weitzmann has been doing for some time) is that it's the improbably but disastrous downside that we should be looking at. This is even more true if you think the most likely scenario has changed to be a permanent stop to global warming rather than just a 14-year hiatus. Whatever the most likely scenario, we still should worry about the possibility that current science is wrong, and global warming will be much worse than anyone expects.