How are oceans faring in a heating world?

Earth's oceans are home to more than 250,000 species, among them tiny plankton, colorful coral reefs and the gigantic blue whale. Over a billion people rely on food from the sea as a significant source of nutrition.  

The international community is now meeting in Nice, France, to hash out solutions to better protect the planet's vulnerable and plundered ocean waters. But what are the areas of concern at the UN Ocean Conference?

Warmer oceans mean fewer underwater inhabitants

As the planet heats up, huge swaths of underwater life are at stake.

With rising temperatures, corals lose their color as a stress response and these crucial habitats can then die. Coral bleaching affects about 84% of all reefs.

If ocean temperatures were to rise by 1.5 degrees Celsius (2.7 degrees Fahrenheit) compared with pre-industrial times, most reefs would die.

"With anything higher than 2 degrees Celsius, destruction would be inevitable," said Katja Matthes, director of the GEOMAR Helmholtz Center for Ocean Research Kiel in northern Germany.

Warm water also absorbs less oxygen and that puts many more underwater creatures at risk. New research shows that oceans are already warming to a depth of 2,000 meters (about 6,600 feet).

"That means plankton, fish and marine mammals run out of oxygen. We see death zones here in Germany's Baltic Sea where life is virtually unable to exist anymore," said Matthes.

Overfishing puts stress on marine ecosystems  

Excessive and unregulated fishing also puts a strain on marine ecosystems. Environmental organization WWF estimates the number of overfished stocks globally has tripled in the past 50 years. Fish populations have no chance to replenish if they're overly exploited. 

The problem is especially dire in the Mediterranean Sea, where over half of fish populations, such as herring, sardines and anchovies, are considered overexploited.

"That has an impact on the food chain of bigger marine mammals and in turn affects an entire ecosystem," said Matthes.

Fish are the most important source of protein for over a billion people. Over 600 million people depend on oceans for their livelihoods — especially in China, Indonesia and India.

More plastic than fish in oceans by 2050

By 2050 the weight of all fish combined could be exceeded by something else: plastic waste. Every year, about 8 to 10 million tons of plastic ends up in the water. That's according to estimates by the World Resources Institute, a nonprofit organization based in Washington D.C.

It can take hundreds of years for plastic to break down. The persistent waste and microplastic particles are causing increasing problems for marine life.

Rising sea temperatures affect weather patterns

Global ocean currents have a major influence on monsoon periods in South America and Asia and Europe's relatively mild climate.

The Gulf Stream, for example, as part of the Atlantic Meridional Overturning Circulation (AMOC), brings warm water from the tropics to the North Atlantic Ocean. This helps regulate Europe's mild air temperatures and, thus, is central to the continent's high agricultural yields.

According to researchers, rising temperatures could alter the AMOC. There are signs that the Gulf Stream is already slowing down. Without it, northern Europe would be 5 to 15 degrees Celsius colder, according to Germany's environment agency UBA.

Oceans as an 'ally in the fight against climate change'

Sea surface temperatures set new records in 2023 and 2024, according to the latest report from Copernicus, the European Union's Earth observation program. Water expands as it warms. This is one of the main reasons for sea level rise, alongside melting land ice.

The sea is getting warmer, because it absorbs CO2 and other greenhouse gas emissions. It's sucked up about a third of human-made emissions.

"Without this storage function, the temperature in the atmosphere would already be unbearable," said marine scientist Carlos Duarte, who's based at the King Abdullah University of Science and Technology in Saudi Arabia.

"The ocean is our ally in the fight against climate change," said Matthes, "but only as long as we maintain its function." As water temperatures rise, oceans absorb less CO2. 

And higher CO2 levels turn the sea acidic, killing off mussels and corals, Matthes added.

Adjusting to more acidic conditions is hard on many creatures, and means they can lack the energy needed to grow and reproduce.

How are oceans protected right now?

To counteract these threats, countries have established marine protected areas. The largest of these is located off the coast of Hawaii in the US. 

The kind of protection provided in such zones varies from country to country. Wind farm construction and fishing are often prohibited. Currently, there are protected areas in less than 9% of the world's oceans — but fishing is prohibited in just 3% of these.

Goal of cutting down on plastic in oceans

"We can't solve all the problems with marine protected areas. These zones don't matter to climate change or the plastic floating in the ocean," said Duarte.

The UN has pushed for an international treaty for years to cut down on plastic pollution. Recent negotiations stalled thanks to oil and gas producing nations such as Saudi Arabia and Russia. The next round of talks is scheduled for August in Switzerland.

Research into alternatives to conventional plastic is another avenue scientists are pursuing. Japanese researchers have developed a substance that is supposed to dissolve in ocean saltwater within hours.

However, such new alternatives offer no solution to the already huge amounts of plastic waste in oceans.

Who is allowed to exploit ocean resources?

About 40% of the oceans are governed by national law. These are the areas within a radius of approximately 370 kilometers (about 230 miles) around a state. Beyond this line are the high seas, which don't belong to any nation and are often referred to as the "common heritage of mankind."

For the longest time, this area wasn't regulated at all.

"As a result, many of the ocean's resources were plundered without anyone being held accountable," said Duarte.

So far, only 1% of the high seas is protected because the international community could not agree on any other region besides Antarctica.

The International Convention on the High Seas, signed by 134 nations in 2023 after 15 years of negotiations, aims to close this gap.

However, it only becomes binding when at least 60 countries have ratified the treaty. So far, 49 have done so, among them many smaller countries, as well as Bangladesh and France. Germany and the US have not yet ratified the treaty.

The international community has also agreed to protect biodiversity. The goal is to place 30% of the oceans under protection by 2030, so within the next five years.

That's ambitious, said Duarte, adding: "It will take time for our current actions to have a noticeable impact in the future."

Nevertheless, he said he's optimistic. "If we agree on this protection now, we will be able to leave our children and grandchildren an ocean in 2050 that looks roughly like the one our grandparents knew."

This article was originally published in German and was adapted by Sarah Steffen.

Edited by: Anke Rasper