Ocean productivity at risk as climate warms

Runaway climate change will alter the pattern of ocean productivity and circulation and play perhaps irreversible havoc with fish catches.

Global ocean productivity – the annual bloom of algae and the cornucopia of molluscs, shrimp, krill, squid, fish and marine mammals that depend on this flowering of the blue planet – could be in serious decline by 2300,  thanks to climate change.

The harvest from the North Atlantic could fall by almost two thirds. The decline in the Western Pacific could drop by 50%. The overall productivity of the oceans from pole to pole will be at least 20% less.

Global warming that is already melting the ice caps and increasingly making the seas more acidic has been blamed for changes in fishery hauls and damage to reef ecosystems.

But the latest study looks not at the immediate consequences of profligate human combustion of fossil fuels, but at the very long-term consequences of turning up the planetary thermometer.

Scientists report in the journal Science that three centuries of continuous rise in carbon dioxide levels in the planet’s atmosphere, as a consequence of fossil fuel combustion, could raise global average temperatures by 9.6°C.

“Marine ecosystems everywhere to the north will be increasingly starved for nutrients, leading to less primary production by phytoplankton”

This is ten times the warming already observed. It will change wind patterns, melt almost all the sea ice and increase ocean surface temperatures.

And with this increase in temperature comes change in the growth of phytoplankton, on which ultimately all marine life depends. There will be shifts in ocean circulation that will take nutrients from the surface and deposit them in the deepest waters.

Antarctic waters could become richer in nutrients. But the world’s human population is centred in the northern hemisphere. “Marine ecosystems everywhere to the north will be increasingly starved for nutrients, leading to less primary production by phytoplankton, which form the base of ocean food chains,” said Keith Moore, an earth system scientist at the University of California, Irvine, who led the study.

“By looking at the decline in fish food over time, we can estimate how much our total potential fisheries could be reduced.”

Research of this kind is based on computer simulation of an entire planetary ocean system over the next 280 years. Leaders from almost all the world’s nations vowed in Paris in 2015 to contain global warming, and other studies have shown that world commercial fisheries would benefit from such action.

Delayed response

But time is running out: the oceans have yet to respond fully to the greenhouse gases that have already built up in the atmosphere in the last century or so.

“The climate is warming rapidly now, but in the ocean, most of that added heat is still right at the surface. It takes centuries for that heat to work its way into the deeper ocean, changing the circulation and removing the sea ice, which is a big part of this process,” Dr Moore said.

“This is what’s going to happen if we don’t put the brakes on global warming, and it’s pretty catastrophic for the oceans.

“There is still time to avoid most of this warming and get to a stable climate by the end of this century, but in order to do that, we have to aggressively reduce our fossil fuel use and emissions of greenhouse gas pollutants.”