How are our oceans being affected by climate change?

Like the bushires, [tragedies like the loss of kelp, lobster and fish populations] are only part of a far larger catastrophe. All around us, natural systems are collapsing and colliding in unsettling and unpredictable new ways. On land this process is marked out in the spasms of fire and flood and brutal heat that are devastating environments from the Arctic to Antarctica. But the world beneath the waves is also being transformed.

Over the past half a century the oceans have absorbed more than 90 per cent of the extra heat the greenhouse gases released by human activity have added to the planet’s systems. In 2022 alone that amounted to the equivalent of five bombs the size of the one that devastated Hiroshima every second of every minute of every day. As the amount of heat the oceans absorb continues to accelerate, ocean temperatures are soaring: in 2023 sea surface temperatures were the highest on record, and the rate at which ocean temperatures are rising has doubled since the 1960s. 

The heating of the oceans is driving extreme weather events such as cyclones and flooding on land. But as the spread of Noctiluca and sea urchins shows, it is also having dramatic effects on ocean ecosystems. Over the past fifty years bleaching events have helped wipe out around half the world’s coral cover, and ocean heatwaves are growing more common and more intense.

In 2022 ocean heatwaves hit the South China Sea, the Mediterranean and New Zealand, triggering mass deaths of marine life and hastening the influx of invasive species: in New Zealand, where water temperatures rose more than four degrees above normal, schools of fish beached themselves and anglers and scientists reported die-offs of sponges and kelp and hundreds of penguins. Warming water is even altering the ocean currents, slowing the vast systems that move water from one part of the ocean to the other, a process that has the potential to further accelerate shifts in the climate. 

The causes of this hastening disaster are both brutally simple and dizzyingly complex. Over the past half a millennium humans have transformed the planet, clearing forests, diverting rivers and draining wetlands and water tables, simplifying ecosystems by moving some species between continents and driving others to extinction, and finally—and most disastrously—by liberating trillions of tonnes of greenhouse gases by burning the reserves of stored carbon buried beneath the Earth’s surface. 

The vast bulk of this change has taken place in just under eighty years since World War II. As economic growth and technological change have shot upwards, consumption of energy, water and other resources has surged so precipitously that some have dubbed this process "the Great Acceleration".

This transformation has been so rapid and uncontrolled that it has already pushed the planet well beyond its safe operating zone: one 2023 study suggests that of nine planetary boundaries ranging from climate and water to the area and functional integrity of natural systems, we have already crashed through the safe and just limit of seven. The effects of this are being felt all around us, and will only grow worse in years to come.

Writing about recent research that suggests previous climate models have seriously underestimated the sensitivity of the Earth’s systems, meaning that even under lower emission scenarios temperatures could rise much faster and much higher than previously thought, the climate scientist Dr Joëlle Gergis describes feeling as if "a metaphorical tsunami" is approaching.

Gergis observes that this research suggests that on our current emissions trajectory we may reach two degrees of warming above pre-industrial levels by 2040. "The implications of this are unimaginable—we may witness planetary collapse far sooner than we once thought," writes Gergis. "How could we not understand that life as we know it is unravelling before our eyes? That we have unleashed intergenerational warming that will be with us for millennia?"

Deep Water: The world in the ocean by James Bradley is published by Scribe (£22)