We're Killing the World’s Oceans

The ocean can still look beautiful while life beneath it is dying. Blue water, sunshine and even a tidy beach tell us very little about the health of what lies below. Tasmania, for example, has lost around 95 per cent of its giant kelp forests. Warmer, nutrient-poor waters and pressure from sea urchins are stripping away underwater forests that provide food and shelter for marine life.

As a former Threatened Species Commissioner, I find these losses deeply troubling. Country is connected. Our responsibilities don’t stop where the land meets the sea.

The oceans have absorbed more than 90 per cent of the excess heat trapped by human-caused warming since the 1970s. Much of the heat generated by our continued burning of coal, oil and gas has been accumulating underwater, beyond our everyday view.

But heat is only part of the problem. Warmer water holds less oxygen, and a bigger difference between warm surface water and cooler depths restricts the mixing that carries oxygen downwards. Globally, the oceans have lost about two per cent of their oxygen since the 1960s, with much larger losses in some places. That changes where animals can survive, grow and reproduce.

The wider effects of warming reach through entire ecosystems, altering food supplies and the places animals depend on to survive. Eastern North Pacific grey whale numbers have more than halved over the last decade. Changes in their Arctic and Subarctic feeding grounds have contributed to malnutrition, fewer births and increased deaths.

Meanwhile, absorbed carbon dioxide changes seawater chemistry, making it more acidic and harder for corals, oysters and other animals to build and maintain their skeletons and shells. Heating, oxygen loss and acidification put marine life under several pressures at once.

Australian Antarctic Division scientists found that elevated carbon dioxide levels sharply reduce krill egg hatching success. Their modelling warns that unchecked emissions are a huge threat to krill, which are foundational to marine food chains. Krill declines are already affecting the whales, seals and penguins that depend on them.

Australia’s Great Barrier Reef shows how quickly climate damage can unfold. Following the enormous 2024 bleaching event, hard coral cover fell by roughly one-quarter in the northern region and more than 30 per cent in the south. Across the continent, Ningaloo has also suffered. In early 2025, government surveys confirmed bleaching along the length of the reef. By October, researchers found two-thirds of tracked corals had died at surveyed northern Ningaloo lagoon sites.

Bleaching itself isn’t necessarily death: corals can survive if the stress eases. But prolonged and repeated heat stress kills them, and shorter gaps between events leave less time for recovery. The IPCC projects declines in coral reefs of up to 90 per cent at 1.5°C of warming, rising to more than 99 per cent at 2°C. Their future depends heavily on how much warming we allow.

South Australia offers another confronting example. The harmful algal bloom that began in March 2025 damaged marine ecosystems and coastal livelihoods. Its causes were complex: nutrients from River Murray floods, an ocean upwelling event, and a marine heatwave with temperatures about 2.5°C above average. The algae damaged fish gills, while decomposition consumed oxygen. Calling algae “natural” tells us little about why a bloom became so destructive.

For me, the response starts with cutting fossil fuel emissions rapidly and ending new coal, oil and gas expansion. Australia should take responsibility for the climate consequences of the fuels we export as well as those we burn ourselves. We also need cleaner waterways, effective marine protection, sustainable fishing, long-term monitoring and properly funded First Nations leadership in caring for Sea Country.

There’s still so much alive, and so much worth protecting. Every reduction in warming gives marine life a better chance.

You don’t need to be a marine scientist to demand action. Find your local MP and ask what they are doing to cut emissions and protect our oceans. Ask for decisions, funding and measurable results. The ocean can’t vote. But we can.