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We all rely on the ocean, whether we know it or not. Earth's vast expanses of water are key to the success of all life on Earth. We eat fish from the ocean, we breathe the oxygen it gives off, we feel the warmth of its huge currents. Without a healthy ocean, humans cannot thrive.
But the ocean has taken a pummelling and we have not been treating it well. It is facing down three huge threats: overfishing, pollution and climate change. Most of these are caused by human mismanagement. Nature is stretching to breaking point. If we don't stop, the ocean could be drastically changed within our lifetimes.
We asked the experts: unless we change our ways, what will the ocean look like in 50 years?
Rivers of pollution flow into the ocean every day, with little sign of slowing down. Marine animals and birds now regularly eat plastic, and so do humans. It is estimated that by 2050 there could be more plastic in the sea than fish.
As the plastic piles up, fish disappear. Since industrialised fishing began in the mid-twentieth century, the oceans have been transformed. Today's seas contain only 10% of the marlin, tuna, sharks and other large predators that were found in the 1950s. To stop this slaughter, conservationists are calling for a third of the world's oceans to be protected from fishing.
Museum curator Miranda Lowe explains, 'When we kill large predators, like marlin and sharks, it disrupts the oceans, with consequences that can affect us all.
'Overfishing puts the whole ocean ecosystem out of balance. But some animals, like penguins and turtles, can adapt and snack on jellyfish when other food runs low.'
As our appetite for seafood grows and fish populations crash worldwide, scientists think that jellyfish might be the big winners. With fewer fish around, jellyfish have less competition for the plankton they feed on. They also thrive in warmer waters and seas polluted by fertilisers from farming. In the oceans of the future, jellyfish could be one of the few things left on the menu.
If we fished more sustainably, the picture would be much brighter. If we all ate seafood less regularly and we allowed stocks time to recover, fish populations would be able to keep up with human appetites.
Even if we stopped all fishing and tackled plastic pollution, life underwater would still be struggling to deal with climate change.
Earth's oceans have been around for billions of years, and they've gone through periods of warming and cooling before. Museum scientists study these past periods of climate change so they can predict what will happen to ocean animals in the future.
Research leader Prof Richard Twitchett says, 'We have a really good idea of what oceans look like when the climate warms. It has happened to Earth many times before, and here in the Museum we have collections of fossil animals and plants that date back millions of years, so we can see how they responded. The rocks and fossils show us that as temperature increased in the past, oxygen levels fell and huge areas of the seafloor became uninhabitable.
'Warm water holds less oxygen, which is a big problem for marine animals. Anyone who owns a fish tank knows you need air bubbling through it because the fish inside will use up the oxygen.'
When the oxygen levels go down, dead zones will expand. These are areas of the ocean where the water quality is too poor to sustain life. Pollution will only add to the devastation unless we stop it.
Some animals will get smaller as their watery home gets hotter and oxygen disappears. Any animal with gills will suffer, including fish, oysters, octopus and crabs.
Other animals breathe air, like whales and turtles, so they get their oxygen from the atmosphere. They won't be affected in the same way, but everything else will. Smaller fish will have knock-on effects up the food chain, and result in smaller predators too, like sharks.
Richard explains, ‘We have measured the sizes of fossils to test this prediction and in almost all cases marine animals became smaller as temperatures rose in the past. So, it's looking likely that our fish portions will shrink in the future. Even aside from climate change, we've been overfishing for a long time now, and we have already eaten a lot of the bigger animals in the water. It's a double whammy.'
Warm water decreases an ecosystem's productivity too. Certain organisms like burrowing sea urchins and mud shrimp that live within the seafloor are really important for nutrient recycling. So if those animals get smaller, there would be less productivity within the oceans systems.
Climate change is causing ice sheets to melt at a rapid rate, which means sea levels everywhere will rise. That could be a tragedy for humans as coastal cities face flooding, but for marine animals it probably won't matter too much.
Richard explains, 'A metre or so of sea level rise isn't a lot at all compared to past ice ages when Earth saw sea levels changes of tens or hundreds of metres. The UK, for instance, has had much higher and much lower sea levels before, and marine ecosystems continued to thrive. Sea level alone is not a problem for marine animals.
'The impact on us, on our coastal cities or low-lying islands, is the real issue - not the impact on animals.'
The ocean is a major carbon sink. Any animal with a hard and calcareous skeleton, like plankton, stores carbon inside its tiny body. These miniscule organisms are powerhouses in the fight against climate change.
But ocean acidification can dissolve their precious skeletons, so it's vital that we work to stop that.
Richard says, 'Thriving calcareous plankton are the natural way of cutting carbon from the atmosphere. If you end up destroying those then you are in trouble.'
Many animals, including whales, also rely on krill and plankton as a food source. Ocean acidification would affect them too. Blue whales, the largest animal on Earth, have grown so because they are able to take advantage of the wealth of krill and plankton in our oceans. If they lost their food source, it's likely we'd lose them too.
Seaweed could become a staple in our diet. Many seaweeds are grown worldwide without artificial fertilisers or pesticides and they are a nutritious food, rich in protein. Seaweeds can also be used to feed livestock and as a biological alternative to plastic packaging.
Today, global production is limited by pests and diseases, but Museum scientists are working to help farmers put more seaweed on our plates.
Plastic isn't the only source of pollution. Chemicals, light and noise all affect marine life too.
For example, throughout their long lives, whales build up large plugs of wax inside their ear canals. Museum scientist Richard Sabin has analysed this earwax and identified traces of toxins which were once used in pesticides, paints and plastics. Whales eat these toxins and pass them onto their young. The chemicals build up over time and can damage the animals' immune systems and ability to reproduce.
Richard says, 'Many of these pollutants were banned decades ago but they are very persistent and are still circulating through marine ecosystems. In addition, our studies indicate the effects of other sub-lethal stressors such as ocean noise pollution, which hampers the ability of large whale species to communicate with each other. This is particularly important for solitary species like the blue whale who call to each other across large distances to find a mate, and social species like killer whales who communicate when they hunt.
'As apex predators in the ocean, cetaceans play an important role in maintaining the delicate balance of marine ecosystems. Imagine a world where we lose our ocean giants, our resident coastal cetacean communities, and the ability to show our children the beauty and extreme diversity of these incredible animals.'
Dead zones in the ocean remain a threat. Protecting this precious resource isn't an easy matter. The goal is making the whole ecosystem more resilient, more able to cope with change.
Lots of these problems are far bigger than any individual choice. Only collective action can solve them, which requires commitments from governments and business.
However, we can all make a difference by recycling our plastic waste, buying sustainable seafood, and reducing our own carbon emissions.