Think tank: Climate affects security (AP)

New York Mayor Michael Bloomberg, right, tours the Seattle Art Museum sculpture park with director Mimi Gates, Friday, Nov. 2, 2007, in Seattle.  Bloomberg, a potential independent presidential candidate, proposed a national 'pollution pricing' plan Friday that envisions taxing companies directly for the greenhouse gases they release. (AP Photo/Elaine Thompson)AP - Climate change could be one of the greatest national security challenges ever faced by U.S. policy makers, according to a new joint study by two U.S. think tanks.