The Arctic. Home to a giant ocean predator（掠夺者）. The Orca, or killer whale. Off the coast of Norway we join an expedition under extreme conditions. The water is ice cold, the air temperature below freezing. Marine（海的） biologist（生物学家） Lucas Muller is getting ready to observe（遵守） a pod of orcas up close.

  Their world is under threat（威胁） from climate change. Lucas Muller isn't the only expert concerned（有关的） about dramatic（戏剧的） consequences for the oceans and their inhabitants（居民）. They're heading（率领） out to sea. Lucas Muller wants to know how climate change is affecting the Arctic Ocean, so he's looking for a school of herring（鲱）. As temperatures rise herring are expected to keep moving further north. Cold water is where they feed and it just so happens that herring attract orcas as their easy prey（捕食）.

  The fjords here are deep and wide so finding（找到） the orcas takes a bit of luck. Every day they could be somewhere else, but wherever（无论在哪里） there are herring there are orcas. It's Lucas Muller's first time here off the Norwegian（挪威的） island of Ševoja, inside the Arctic Circle. He has no illusions（幻觉） about how serious the situation is here. My greatest concern（关心） is that my greatest concern（关心） is that we're inflicting（造成） irreversible damage on the ocean as an ecosystem. The time to rescue（营救） nature will never be as good as it is today.

  If we destroy natural habitats（(动植物的)生活环境） today, it will take hundreds or thousands of years to recover（恢复） them. And in most cases we don't have the motivation（动机）, the ability or the knowledge to bring those habitats back. It's day one of the expedition. He and his team have been out since early morning, but so far they've not seen any herring. Time is of the essence. It's late November and there are only three to four hours of light a day.

  Let's commit to this, check this area out. Sure, good for me. The team also includes underwater cameraman Leonhard Rosenfeld. You have the black and white contrast of the fjord, the waves coming up, the wind whips up the waves and sometimes it looks like there are whales coming up and breathing, but it's just wishful thinking. This is closing in. It's early afternoon and they're freezing as they return to the harbor.

  We covered about 100 kilometers today. That's quite a distance. We didn't find any orcas. Tomorrow we'll try another fjord and hope that Mother Nature blesses us with a sighting so we can get them on camera. It's not just here in the Arctic that the ocean is changing. Outer temperatures are rising worldwide because the oceans absorb a lot of the warmth created by greenhouse gases in the atmosphere.

  Surveys down to a depth of 2,000 meters show that very clearly. Since the 1980s, ocean surface temperatures around the world have been rising steadily. In 2019, the temperature was 0.77 degrees Celsius above the average for the previous century. Many small sea creatures are unable to survive in the warmer water. Other fish species have begun migrating towards the poles in search of cooler conditions. Those who can't cover such huge distances will presumably die out and that will have consequences for humans too.

  For more than 750 million people worldwide, fish is an essential source of protein. Without that fish, many people will no longer be getting enough protein（蛋白质） to survive. And there's another problem. The oceans take in huge amounts of CO2. That's helped slow down climate change. But the marine ecosystem can't continue like this indefinitely.

  As Christopher Simmermann from Germany's Institute of Baltic Sea Fisheries knows all too well. The impact of climate change would already be far more severe and far more visible if the oceans hadn't acted as a buffer. But at some stage, that buffer（[计] 缓冲区） capacity will be exhausted and they will then need centuries to recover. He says profound changes are already visible along Germany's coasts. This year, we've proven for the first time that the productivity for one of our key herring stocks has dropped significantly, which will be reflected in the harvest in the coming years. Productivity for this stock is half what it was 30 years ago, because of climate change.

  And that will directly impact our coastal fisheries. For Lucas Muller, it's day two in the Arctic. The biologist is once again off to look for herring. But first, he applies a naked flame to the protective coating of his new goggles to prevent them from fogging up. Next, he prepares a belt with lead weights. He wants an extra 13 kilos to help him dive to a depth of 20 meters.

  Soon they get their first indication that herring are around, birds circling above the water. And just a few seconds later, the orcas arrive. The sense of anticipation is huge, even for these two experienced divers. Neither have ever been in the water with these giant predators. Lucas Muller dives without an oxygen cylinder. That allows him to move more quickly and get closer to the orcas without disturbing them.

  I took my last breath and dived. It's like another world. You're waiting in the depths for these huge shadows to approach. Then the entire pod swims past. Orcas use natural sonar, known as echolocation. Orcas use natural sonar（声纳）, known as echolocation（回波定位）, to communicate with each other and navigate their surroundings.

  The group functions as a unit, with the bulls swimming on the outside to protect the pod. Once I'd been down with the orcas two or three times and the bulls understood that I posed no threat, an inquisitive mother came right up close to me with her calf. Orcas are perfectly adapted to the frigid water, but Lucas Muller isn't even able to do that. On a day like today, he burns around 5,000 calories, and then comes back to his age. Orcas are perfectly adapted to the frigid water, but Lucas Muller is pushed to his limits and beyond. On a day like today, he burns around 5,000 calories.

  After four diving sessions, it's time to call it a day. We're going to leave them in peace. The weather's deteriorating, visibility is starting to get poor. It can be dangerous if you push your luck too far. A snowstorm is building up. It's high time they left.

  The marine biologist is not only interested in the Arctic, he goes diving all over the world. He spends six months of the year at a marine national park off the coast of Mozambique. Here, he's doing research into endangered shark species for his doctoral thesis, focusing especially on bull sharks. He takes water samples and then analyzes them with special filters for traces of genetic material left by the sharks.