5 biggest threats to our oceans - and what we can do about them
Oceans cover more than two-thirds of our planet and give us food, energy and other resources. But worldwide, this fantastic underwater world is in danger. Is there still time to prevent the worst?
Surfing, boating, long walks on the beach - yes, we love our oceans. And yet, we treat them horribly, even though we need them to survive.
DW takes a closer look at the five biggest man-made threats to these massive bodies of water - and why we should try desperately to save the oceans while we still have a chance.
1. Depleted fish stocks
Eating fish and seafood is good for our health and many people worldwide, particularly in low-income countries, rely on these important sources of protein. In the past, the number of fish and other sea creatures caught by humans could be replenished through natural reproduction. Today, however, we take out more than what nature can deliver.
According to the UN's Food and Agriculture Organization (FAO), humans extracted more than 81 million tons of fish and seafood from the oceans in 2015, an increase of 1.7 percent compared to 2014.
Around 30 percent of global fish stocks are overexploited or have already collapsed; 58 percent are at their limit.
The countries with the largest marine capture rates in 2015 were China, Indonesia and the US. In total, 23 countries are responsible for 80 percent of the world's fish and seafood production - most of them high-income countries.
A growing human population needs plenty of fish - too many?
Conventional aquaculture, often seen as a solution, actually makes matters even worse. This industrial mass fish farming, partly in cages in the oceans, uses up huge amounts of seafood as feeding material.
Aquaculture farms also pollute their surrounding areas with excrement and facilitate the spread of fish diseases.
More rigorous fishing quotas and better fishery management could help. Fish stocks can recover within decades or even years, but action needs to be taken soon before some species are critically endangered or even lost forever.
And yes: limiting the amount of fish and seafood we eat to a reasonable level and backing away from eating endangered species would also help.
2. Ocean acidification
Carbon dioxide emissions have increased significantly since the beginning of industrialization in the 19th century. But the concentration of CO2 in the atmosphere has only risen by around 40 percent.
That's because much of the CO2 ends up in the oceans, as the gas dissolves in water. Oceans, therefore, help slow global warming - but that help comes at a price.
When CO2 dissolves in water it increases ocean acidity, leading to a drop in pH, the scale used to measure acidity or basicity. In 1870, the average pH of seawater was 8.2; today, it's at 8.1. By 2100, that value is predicted to even become more acidic, dropping to 7.7.
Though it appears to be only a minor change, that drop of 0.1 corresponds to a 150 percent increase in acidity. Many sea creatures aren't able to cope with such an extreme change, and will stop reproducing and eventually die out.
In 2005, oyster farms along the Californian coast were forced to close because seawater there had become too acidic for oyster larvae. They died - and with them a whole industry.
Mollusks, such as mussels, are particularly sensitive to a drop in pH. But fish species can suffer as well. In 2015, a study published in the scientific journal Global Change Biology showed that acidification might even have a detrimental effect on plankton, tiny marine organisms that are a crucial food source for many fish and mammals.
Unfortunately, there is only one way to stop ocean acidification, and that's to reduce the amount of CO2 emissions - as soon as possible.
3. A warming world
The oceans not only store CO2 - they also store heat. In fact, they absorb more than 93 percent of the heat generated by man-made CO2 emissions, warming the water.
Between 1900 and 2008, seawater surface temperature rose by 0.62 Celsius (1.12 Fahrenheit) on average. In some areas in the Western Pacific Ocean, that increase was as much as 2.1 Celsius.
Warming is major problem for many underwater organisms, and in particular corals.
A coral, before and after bleaching
Corals are creatures that form a hard exoskeleton out of calcium carbonate. Inside, they harbor colorful photosynthetic algae. When the surrounding water gets too warm, corals expel the algae and finally starve to death, an event called coral bleaching.
One-third of the Great Barrier Reef in Australia, an expanse the size of Italy, has already been affected by coral bleaching.
Only a reduction in CO2 emissions can prevent seawater temperatures from any further increases.
Meanwhile, researchers are also working on breeding corals that are more resilient to warmer water temperatures, or learning how to boost their sexual reproduction. Hopefully, these efforts might help coral reefs survive in a warming world.
Read more: Sexual healing for dying coral reefs
4. Pollution everywhere
The oceans were once a big waste dump for sailors, cruise ships and coastal towns. For some, they still are. Even though our attitude toward the sea has changed, there is still an awful lot of trash building up in the oceans.
Five huge trash vortexes have formed in the world's oceans, areas where the currents trap trillions of pieces of plastic and other debris. These garbage patches are estimated to measure between 700,000 and 15 million square kilometers (up to 5.7 million square miles).
Plastic waste washes ashore around the world, including here in Ghana
But as much as 99 percent of plastic waste never reaches these vortexes. A lot ends up on shorelines, polluting coasts and putting seabirds, turtles and other wildlife in danger.
Most of the waste, though, decomposes or is broken up into tiny pieces - microplastics, distributing themselves on the ocean floor, in sea ice at the poles and even in fish who ingest them as food.
And there are other pollutants as well, like nitrate and phosphate from industrial farms that enter the oceans via rivers. These substances cause algae to bloom. When algae die, they are decomposed by bacteria, which reduces the water's oxygen content so that nothing else can grow there.
Industrial wastewater and emissions also add dangerous metals and chemicals to the oceans, including lead, mercury and persistent organic pollutants. The latter accumulate in the fat of whales, sharks and other animals.
Is there a solution to this mess?
The Ocean Cleanup, a Dutch foundation, has announced a plan to start extracting plastic from an area known as the Great Pacific garbage patch in 2018 using a floating system developed for this purpose.
Apart from that, efforts to reduce plastic waste and implement stricter rules concerning wastewater treatment are necessary around the globe.
5. Buried treasure
You might think that our oceans have already seen enough exploitation. But the big rush may be yet to come.
Deep in the oceans, valuable natural resources are waiting to be uncovered. One example is manganese nodules, rocks on the seafloor composed of iron and manganese hydroxide. Manganese is used to produce industrial metal alloys, in particular stainless steel.
Manganese nodules and other precious minerals line the ocean floor
Experts estimate there are more than 7 billion tons of manganese in the oceans, more than in reserves on land.
Many countries have already secured claims on the seafloor, areas where they plan to begin mining operations as soon as the process is allowed and becomes economically viable.
Other precious metals like cobalt, nickel, thallium and rare earth elements are also known to exist down below, ready for extraction. But research shows that such metallic nodule fields are hot spots of biodiversity.
Last year, researchers discovered a "ghostlike" octopus living close to these nodules which they called Casper. Mining operations could dramatically affect these delicate ecosystems.
Strict rules on deep sea mining are needed to prevent the worst.