jennings: yes. the scientists kept working, taking advantage of the final days of the short arctic summer. the 66 core samples they collected during their month at scientists around the world for >> peter demenocal: this is the largest core depository in the world. paleo climatologist at columbia university. he says the cores collected in greenland are like a black box of the earth's inner workings. this one he collected just south of greenland. >> peter demenocal: so this is today's climate and we've had about 10,000 years of relatively warm climate. and then we go ten thousand years in the past - boom, there's the last ice age. this is when long island was formed and cape cod was formed. >> peter demoncal: and you can go on, and you can just find this color. it's filled with these rocks, what we call ice rafted detritus, until this period when - whoa, there's another warm phase. and then another cold phase, and then another warm phase. a short cold phase, a longer warm phase and then - boom, another ice age. cold, warm, cold, warm, today. >> alfonsi: how do we know that the warming we're seeing now, how do we not know it's part of this warm, cold, warm, cold? >> peter demenocal: that