The colour of the world’s oceans is changing – a visible sign of climate disruption that scientists say could have wide-ranging consequences for marine life. The shift comes as world leaders prepare to meet in Nice next month for a major United Nations summit on ocean conservation.
In the Arctic and Antarctic, the water is becoming visibly bluer. Melting ice is allowing more sunlight to reach below its surface, changing the types of microscopic life that flourish there.
These tiny organisms – particularly plankton – help define the ocean’s colour and play a key role in absorbing carbon dioxide.
“What gives the ocean its colour are primarily plankton communities,” explained Vincent Doumeizel, who advises the UN on ocean sustainability, in an interview with RFI.
Plankton might be tiny, but they are everywhere. They make up around 90 percent of all life in the oceans.
“When we think a drop of water is empty, it's actually filled with about 1 million different organisms – and a litre of seawater has around a billion,” Doumeizel said.
Most of these organisms are plankton: microscopic plants and animals that float with the currents and support nearly all ocean life.
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Balance under threat
This microscopic life form captures between 30 and 40 percent of global CO2 emissions. But warming seas and retreating glaciers are putting that balance at risk.
A study published in Nature found that more than half of the world’s oceans have changed colour over the past 20 years, with tropical and polar waters becoming increasingly blue or green. The changes were linked to climate-driven shifts in plankton and other microscopic organisms.
“The light will change, the temperature will change – even the salinity will shift as glaciers release cold, fresh water,” said Doumeizel. “Together, these changes will alter surface plankton – the communities that shape the ocean’s colour.”
One striking example already observed is the appearance of so-called “blood falls” – red-tinted streams flowing from some glaciers, caused by a pigmented microalga called Sanguina.
“This glacier blood will ultimately accelerate melting,” said Doumeizel, "because the glaciers, no longer being white, reflect less of the sun’s heat.”
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Green and red oceans
Doumeizel’s new book, Plancton, to be released this month, builds on the message of The Plankton Manifesto, a document co-written with international scientists calling for plankton to be recognised as vital to climate balance and marine life.
He describes them as the planet’s “invisible climate engine”.
Despite absorbing vast amounts of carbon and sustaining most ocean life, plankton remain largely overlooked in global environmental policy.
The Earth’s oceans haven’t always been blue. Around 3 billion years ago they were green, and later red, depending on the microscopic life that was dominant.
Doumeizel explains that chlorophyll-rich organisms once turned the seas green. Later, the oceans became red when iron in the water oxidised in the presence of oxygen – newly released by those same organisms.
In the future, oceans could shift again – turning red during toxic algae blooms or even purple if volcanic activity releases sulphur into the water.
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Science-led summit
Scientists say these changes are already visible and could accelerate without stronger protection of marine ecosystems. That pressure is now building ahead of next month’s UN summit in Nice, where world leaders are expected to respond with new commitments.
Ahead of the Nice summit, the European Commission is set to present its “Ocean Pact”. But a leaked draft has drawn criticism from several environmental groups, including Surfrider, WWF, ClientEarth and Oceana.
In a joint statement, the groups said: “If the document mentions advances in enforcing existing laws, it contains no concrete actions to address the most urgent threats to marine life and biodiversity.”
Campaigners also delivered a petition with 250,000 signatures calling for a ban on bottom trawling – which involves dragging weighted nets across the seabed – in marine protected areas.
French President Emmanuel Macron has pledged to make the summit science-led. “No action regarding the ocean should be taken without being guided by science,” he said in March, warning of attacks on research by “major powers”.
The summit, running from 9 to 13 June, will bring together 2,000 scientists and dozens of heads of state. Macron also wants more countries to ratify the High Seas Treaty, which needs 60 signatories to enter into force –but has only 21 to date.
The OECD issued a report in March warning that ocean economy growth could stall without stronger protections. It cited climate change, illegal fishing and territorial disputes as growing threats.
This article was partly adapted from an interview in French by RFI's Florent Guignard
