2 00:00:10,620 --> 00:00:15,252 There are some lessons from the past that make this particularly worrisome. 3 00:00:15,252 --> 00:00:20,040 one is a trace of sea level through time 4 00:00:20,040 --> 00:00:23,360 through the last deglaciation. 5 00:00:23,360 --> 00:00:27,300 Sea level was 120 meters lower during the last ice age. 6 00:00:27,300 --> 00:00:31,940 And when it rose to form the interglacial world that we live 7 00:00:31,940 --> 00:00:35,620 in today, it didn't just rise in a smooth way, but it seemed 8 00:00:35,620 --> 00:00:36,770 to go in pulses. 9 00:00:36,770 --> 00:00:41,510 And in particular, there was this one interval called Meltwater Pulse 1A. 10 00:00:41,510 --> 00:00:46,260 Which was a huge amount of water, a very large change in sea level 11 00:00:46,260 --> 00:00:48,800 in a very short time, and glaciologists 12 00:00:48,800 --> 00:00:51,130 aren't even sure where this water came from. 13 00:00:52,480 --> 00:00:57,310 Another class of events from the paleo record is called Heinrich events. 14 00:00:57,310 --> 00:01:02,540 What's left behind today are 16 00:01:02,540 --> 00:01:07,260 layers of pebbles in the sediments of the Atlantic Ocean. 17 00:01:07,260 --> 00:01:08,680 They're much too big to have 18 00:01:08,680 --> 00:01:11,710 been blown there by wind. 19 00:01:11,710 --> 00:01:14,280 They must have been carried in ice. 20 00:01:14,280 --> 00:01:21,210 They are called ice-rafted debris. At various specific times 21 00:01:21,210 --> 00:01:25,840 during the glacial time, the Laurentide ice 22 00:01:25,840 --> 00:01:31,380 sheet, in North America, seemed to just collapse and dump 23 00:01:31,380 --> 00:01:34,110 vast armadas of icebergs, carrying 24 00:01:34,110 --> 00:01:37,110 these little pebbles into the Atlantic ocean. 25 00:01:37,110 --> 00:01:42,910 They made layers of ice-rafted debris, that we find today. 26 00:01:42,910 --> 00:01:46,600 The trouble is that models of ice sheets 27 00:01:46,600 --> 00:01:52,720 Can't generate this much ice-rafted debris. 28 00:01:52,720 --> 00:01:55,310 It's clear that ice sheets know a trick 29 00:01:55,310 --> 00:01:59,200 or two about melting that we're only now figuring out. 30 00:01:59,200 --> 00:02:01,800 And if anything, the models that we have for the 31 00:02:01,800 --> 00:02:05,860 ice sheets and their response to climate are probably too sluggish.