2 00:00:11,410 --> 00:00:15,630 Another proxy we need for the past, another thing that we'd really like 3 00:00:15,630 --> 00:00:18,630 to know, is how bright the sun was. 4 00:00:18,630 --> 00:00:23,250 That way, when trying to piece together past changes in climate, we can 5 00:00:23,250 --> 00:00:28,100 say how much of that could be driven by changes in the intensity of the sun. 6 00:00:28,100 --> 00:00:32,690 There's a proxy for the solar 7 00:00:32,690 --> 00:00:36,430 intensity that you never would have believed, 8 00:00:36,430 --> 00:00:39,120 until you'd seen it for yourself, and seen 9 00:00:39,120 --> 00:00:42,360 how the proof of it is always 10 00:00:42,360 --> 00:00:46,950 to compare what the proxy is trying to measure 11 00:00:46,950 --> 00:00:50,420 with the real data for when you have measurements. 12 00:00:50,420 --> 00:00:53,470 And if it works reliably in that time span then you 13 00:00:53,470 --> 00:00:56,780 can project it into the past and make estimates of the past. 14 00:00:56,780 --> 00:01:01,840 This proxy for the solar intensity relies on the magnetic 15 00:01:01,840 --> 00:01:08,660 field from the sunc which shields the earth, to some extent, from cosmic rays. 16 00:01:08,660 --> 00:01:13,840 Cosmic rays are coming in from space all the time and when they come in and hit 17 00:01:13,840 --> 00:01:16,900 the atmosphere they can do some nuclear chemistry and 18 00:01:16,900 --> 00:01:23,120 create these radioactive elements like carbon 14 and beryllium 10. 19 00:01:23,120 --> 00:01:26,960 And those are then deposited in ice cores. 20 00:01:26,960 --> 00:01:32,090 We can measure the concentration, in particular beryllium 10, in ice cores 21 00:01:32,090 --> 00:01:36,310 in order to back out what the solar intensity was in the past. 22 00:01:36,310 --> 00:01:42,000 And so here you see a plot of that correlation going 23 00:01:42,000 --> 00:01:47,060 through the instrumental record and it seems to work remarkably well. 24 00:01:53,180 --> 00:01:56,980 Temperature reconstructions of the last thousand years or 25 00:01:56,980 --> 00:02:02,860 so are colloquially and affectionately called the hockey stick because 26 00:02:02,860 --> 00:02:05,640 the very first one of these showed very flat 27 00:02:05,640 --> 00:02:09,810 temperature and then a strong increase just at the end. 28 00:02:09,810 --> 00:02:14,060 This has been repeated now a dozen times, 29 00:02:14,060 --> 00:02:18,230 with different methods for illuminating the the noise in the tree 30 00:02:18,230 --> 00:02:20,270 ring records, for example. 31 00:02:20,270 --> 00:02:23,400 And they're all basically coming up with the same picture. 32 00:02:23,400 --> 00:02:29,040 You have period of general warmth during medieval times. 33 00:02:29,040 --> 00:02:33,680 This was the time when the Vikings were sailing the world and 34 00:02:33,680 --> 00:02:38,548 people were building cathedrals in gratitude to benevolent god in Europe. 35 00:02:38,548 --> 00:02:43,440 After that came a time from about 1300 to about 1750 called 36 00:02:43,440 --> 00:02:48,610 the Little Ice Age. This was a time with nasty 37 00:02:48,610 --> 00:02:53,770 weather in Europe. Not just cold, but periods of 38 00:02:53,770 --> 00:02:58,780 drought, periods of intense rain, periods of just strange kind of nasty weather. 39 00:03:00,020 --> 00:03:02,750 Right in the middle of this time is a period called 40 00:03:02,750 --> 00:03:06,710 the Maunder Minimum which was a time when there were no sun spots. 41 00:03:06,710 --> 00:03:10,340 Right after the telescope was invented, 42 00:03:10,340 --> 00:03:14,045 just a few decades after that, and sun spots were discovered, 43 00:03:14,045 --> 00:03:17,900 for some reason the sunspots went away on the sun. 44 00:03:17,900 --> 00:03:21,430 The intensity of the sun today is found 45 00:03:21,430 --> 00:03:24,170 to be correlated with the number of sun spots. 46 00:03:24,170 --> 00:03:28,340 We have this roughly 11 year sun spot cycle and when 47 00:03:28,340 --> 00:03:32,132 there's lots of sun spots the sun is brighter than when there are fewer. 48 00:03:32,132 --> 00:03:35,720 We know that this period is 49 00:03:35,720 --> 00:03:39,760 a time when when the sun was relatively cool, 50 00:03:39,760 --> 00:03:44,160 and that can explain why the Little Ice Age was colder. 51 00:03:44,160 --> 00:03:47,680 You can see that the reconstructed temperatures from 52 00:03:47,680 --> 00:03:50,950 the tree rings have a lot more variability in them. 53 00:03:50,950 --> 00:03:54,730 compared to that from the bore hole temperatures which are very smooth. 54 00:03:54,730 --> 00:03:57,020 They can only tell us about the larger changes 55 00:03:57,020 --> 00:04:00,250 in temperature rather than all the little wiggles. 56 00:04:00,250 --> 00:04:01,510 And you see also 57 00:04:01,510 --> 00:04:06,490 where the tree rings don't match the instrumental record in the last few 58 00:04:06,490 --> 00:04:12,110 decades of the hockey stick period that we mentioned before, 59 00:04:12,110 --> 00:04:14,800 the decline in sensitivity of the tree rings during that time. 60 00:04:16,690 --> 00:04:21,510 In general, the temperature changes we see in the last 1,000 years 61 00:04:21,510 --> 00:04:26,300 are roughly comparable to the temperature changes we've seen in the last 30 years. 62 00:04:26,300 --> 00:04:28,400 The Little Ice Age was maybe one degree C 64 00:04:28,400 --> 00:04:34,600 colder than 1950 temperature on Earth. 65 00:04:34,600 --> 00:04:41,080 The Medieval warm time was maybe 1/2 a degree warmer than the 1950 temperature. 68 00:04:41,080 --> 00:04:42,970 And where we are today is about 0.7 or 0.8 69 00:04:42,970 --> 00:04:45,780 degrees warmer than that temperature. 70 00:04:45,780 --> 00:04:50,640 The temperature changes we've seen so far are comparable to those 71 00:04:50,640 --> 00:04:55,960 that have affected history just in the last 1000 years or so. 72 00:04:55,960 --> 00:04:57,870 And these were significant changes. 74 00:04:59,850 --> 00:05:04,060 The Little Ice Age was significantly different than the Medieval time. 75 00:05:04,060 --> 00:05:06,000 But it wasn't apocalyptic. 76 00:05:07,380 --> 00:05:09,640 There are some false conclusions that can be drawn from 77 00:05:09,640 --> 00:05:12,670 this record too that I want to warn you of. 78 00:05:12,670 --> 00:05:17,120 One is since we see some natural climate change, 79 00:05:17,120 --> 00:05:19,590 does that mean that climate change is harmless? 80 00:05:19,590 --> 00:05:20,480 No. 81 00:05:20,480 --> 00:05:22,180 Not really. There's no reason to draw that. 82 00:05:22,180 --> 00:05:27,846 And it also doesn't mean that humans can't also change the climate. 83 00:05:27,846 --> 00:05:29,870 If the climate changed in the past it 84 00:05:29,870 --> 00:05:33,940 might have been because the sun changed its intensity 85 00:05:33,940 --> 00:05:35,550 or something like that. 86 00:05:35,550 --> 00:05:41,190 It doesn't mean that we are excluded from ever effecting earth's temperature. 87 00:05:41,190 --> 00:05:43,380 And any other conclusion that I've heard drawn 88 00:05:43,380 --> 00:05:46,940 is that, since the Medieval time was actually 89 00:05:46,940 --> 00:05:51,660 a good time to be in Europe, relative to the Little Ice Age, when it was worse, 90 00:05:51,660 --> 00:05:55,760 does that mean that any amount of warming is good? 91 00:05:55,760 --> 00:05:56,840 Not really. 92 00:05:56,840 --> 00:05:59,010 Because the forecast 93 00:05:59,010 --> 00:06:03,050 for the end of the century totally blows all of this off the scale. 94 00:06:03,050 --> 00:06:08,140 The forecast for the year 2100 is like 3 or 4 degrees centigrade. 95 00:06:08,140 --> 00:06:10,510 A little bit of warming might be good. 96 00:06:10,510 --> 00:06:12,860 A lot of warming is a different story altogether.