WEBVTT Kind: captions Language: en 00:00:08.269 --> 00:00:15.330 We live a big life on a small planet. The human population has grown from five to 00:00:15.330 --> 00:00:21.630 over seven billion in one generation, and consumption has escalated too. Building 00:00:21.630 --> 00:00:27.240 homes, factories, and roads often leads to a better quality of life. But it's also 00:00:27.240 --> 00:00:34.140 increased pollution dumped into the air and water. Greenhouse gases like carbon 00:00:34.140 --> 00:00:39.120 dioxide have raised the global temperature. Our collective impact is so 00:00:39.120 --> 00:00:46.620 large it is even dramatically changing the chemistry of the ocean. Activities 00:00:46.620 --> 00:00:53.160 such as burning fossil fuels release carbon dioxide, or CO2, into the air, but a 00:00:53.160 --> 00:00:58.649 large part is absorbed by the ocean. CO2 in the water causes the ocean to become 00:00:58.649 --> 00:01:03.300 more acidic, and a more acidic ocean means it's harder for creatures - like 00:01:03.300 --> 00:01:08.640 reef building corals and shellfish, to create and maintain their calcium 00:01:08.640 --> 00:01:13.800 carbonate skeletons and shells and that weakens them. 00:01:13.800 --> 00:01:22.110 Locally trash, boats anchoring on the reef, destructive fishing practices, and the wrong 00:01:22.110 --> 00:01:27.600 sunscreen worn by swimmers, can also sicken corals. These local issues 00:01:27.600 --> 00:01:35.250 compound the global threat of a warming and acidifying ocean. The world's largest 00:01:35.250 --> 00:01:42.180 reef, the Great Barrier Reef of Australia, can be seen from space. It is home to 00:01:42.180 --> 00:01:49.020 more than 1,500 species of fish and estimated to be 500,000 years old. 00:01:49.020 --> 00:01:56.329 One of the wonders of the natural world. But in 2016 most of the Great Barrier 00:01:56.329 --> 00:02:03.200 Reef's corals bleached and almost 25% died from an astonishingly long period 00:02:03.200 --> 00:02:14.019 of intensely high water temperatures driven by global warming. 00:02:14.019 --> 00:02:18.470 None of the central and northern sections of the reef were spared from 00:02:18.470 --> 00:02:22.730 the heat stress that causes bleaching. The northern third was hit the hardest 00:02:22.730 --> 00:02:29.900 and two-thirds of its corals died. And tragically mass die-offs are again 00:02:29.900 --> 00:02:34.120 happening on the reef for an unprecedented second year in a row in 2017. 00:02:34.120 --> 00:02:39.230 Even worse, it was not an isolated incident. 00:02:39.230 --> 00:02:44.019 All of the world's corals are in danger. 00:02:44.140 --> 00:02:50.860 We have lost already approximately 50% of the world's reefs. This has occurred 00:02:50.860 --> 00:02:57.700 really in the last three decades. So fast when we're talking about an organism 00:02:57.700 --> 00:03:02.620 that has been on the planet and been in symbiosis with its microalgae for over 00:03:02.620 --> 00:03:10.810 50 million years. And the incidence of bleaching is increasing in frequency and 00:03:10.810 --> 00:03:18.100 intensity as the planet warms. It's not every year that water temperatures stay 00:03:18.100 --> 00:03:23.739 high enough, long enough, for corals to bleach. In fact extreme high temperatures 00:03:23.739 --> 00:03:28.989 used to come in cycles, often driven by climate events like El Nino. 00:03:28.989 --> 00:03:33.040 Due to climate change the higher temperatures are becoming more frequent, more intense, 00:03:33.040 --> 00:03:39.670 and more widespread. The underwater heat waves that have devastated the corals on 00:03:39.670 --> 00:03:44.739 the Great Barrier Reef are part of the third global coral bleaching event. 00:03:44.739 --> 00:03:51.010 This one has now lasted three years by far the longest, most widespread, and most 00:03:51.010 --> 00:03:57.280 damaging yet and it continues even now. It's feared that corals could now face 00:03:57.280 --> 00:04:07.030 bleaching so often that whole reefs and even species will be lost forever. NOAA's 00:04:07.030 --> 00:04:12.280 coral reef watch program, and others, use satellites to monitor the heat stress 00:04:12.280 --> 00:04:16.690 that can cause coral bleaching and use climate models to predict where 00:04:16.690 --> 00:04:21.160 bleaching may occur on the world's tropical coral reefs in the next few 00:04:21.160 --> 00:04:29.020 months. But what can we do to reduce the risks? Can corals make a comeback? 00:04:29.020 --> 00:04:33.780 Watch our next video in this series to find out.