WEBVTT FILE 1 00:00:00.120 --> 00:00:04.250 Arctic sea ice reached its annual minimum 2 00:00:04.250 --> 00:00:08.450 extent September 19, and again on September 23, 2018. 3 00:00:08.450 --> 00:00:12.640 NASA works with the National Snow and Ice Data Center to track sea ice in the Arctic. 4 00:00:12.640 --> 00:00:16.840 Each year, it grows to a maximum extent through the winter 5 00:00:16.840 --> 00:00:21.030 and shrinks to its minimum extent at the end of summer. 6 00:00:21.030 --> 00:00:25.350 This year’s minimum sea ice extent reached 1.77 million square miles. 7 00:00:25.350 --> 00:00:29.400 It’s tied with 2008 and 2010 as the sixth lowest 8 00:00:29.400 --> 00:00:33.590 sea ice minimum since consistent satellite records began. 9 00:00:33.590 --> 00:00:37.790 We need every single data point to string together into a really nice time series and that 10 00:00:37.790 --> 00:00:42.000 helps us understand interannual variability and also the long-term trend. 11 00:00:42.000 --> 00:00:46.080 NASA has been observing changes in the polar sea ice covers for over 40 years. 12 00:00:46.080 --> 00:00:50.270 NASA studies the Arctic and Antarctic 13 00:00:50.270 --> 00:00:54.500 sea ice covers in several ways. So NASA’s Operation IceBridge, 14 00:00:54.500 --> 00:00:58.590 it’s an airborne mission – they fly every spring over the sea ice cover to measure the snow and the sea ice 15 00:00:58.590 --> 00:01:02.830 And another way that we measure sea ice is using passive microwave. 16 00:01:02.830 --> 00:01:06.900 So this is an instrument that can see through clouds essentially and tells us where the ice is. 17 00:01:06.900 --> 00:01:10.980 In addition to the 40-year passive microwave record, 18 00:01:10.980 --> 00:01:15.090 a new NASA satellite called ICESat-2 will provide a new 19 00:01:15.090 --> 00:01:19.280 and important collection of sea ice observations. 20 00:01:19.280 --> 00:01:23.460 ICESat-2 just launched and what it’s measuring is really, really exciting. 21 00:01:23.460 --> 00:01:27.680 So I was talking before about passive microwave tells us where the sea ice is. 22 00:01:27.680 --> 00:01:31.890 What ICESat-2 is going to do is to tell us how thick the ice cover is. 23 00:01:31.890 --> 00:01:35.920 It’s measuring the freeboard of the ice cover; this is the amount of the ice that floats 24 00:01:35.920 --> 00:01:40.090 above the sea level line, just like an ice cube in a glass of water, and we can use 25 00:01:40.090 --> 00:01:44.210 that to calculate just how thick the underlying ice is. 26 00:01:44.210 --> 00:01:48.290 Thickness is an important measure of sea ice health, and studying it 27 00:01:48.290 --> 00:01:52.510 helps scientists understand how the Arctic is changing. 28 00:01:52.510 --> 00:01:56.700 We’re seeing a decline in sea ice thickness, in sea ice age, meaning that the ice 29 00:01:56.700 --> 00:02:00.880 is no longer perennial, but it’s transitioning more to seasonal type ice, 30 00:02:00.880 --> 00:02:04.990 and also in its extentThere are two types of ice in the Arctic, 31 00:02:04.990 --> 00:02:09.140 there’s old ice and young ice. Perennial ice being the stuff 32 00:02:09.140 --> 00:02:13.330 that lasts years, and then seasonal ice, the stuff that melts back 33 00:02:13.330 --> 00:02:17.510 every summer. So there are some pretty big differences between those two ice types 34 00:02:17.510 --> 00:02:21.690 Starting with the seasonal ice. This is the ice that forms when the 35 00:02:21.690 --> 00:02:25.760 ocean freezes, so it actually has salt in it, it’s very saline, because it’ 36 00:02:25.760 --> 00:02:29.860 forming from sea water. This stuff is usually thinner than the older ice 37 00:02:29.860 --> 00:02:34.050 and because it has more salt, it’s usually weaker in its structure, 38 00:02:34.050 --> 00:02:38.090 so it’s easier to break up. For the older ice, this stuff’s 39 00:02:38.090 --> 00:02:42.280 usually a lot thicker, a lot fresher and stronger, so it has more resilience 40 00:02:42.280 --> 00:02:46.410 during the summer melt season than thinner ice. 41 00:02:46.410 --> 00:02:50.590 With the successful launch of ICESat-2, NASA scientists will link the records of sea ice 42 00:02:50.590 --> 00:02:54.810 extent, age and thickness to better understand 43 00:02:54.810 --> 00:03:05.068 how Earth’s polar regions are changing.