Ocean sensitivity to atmospheric change is well established. But just how sensitive the oceans are remains a surprise to science.
LONDON, 30 June, 2020 – As greenhouse gas emissions soar, ocean sensitivity has quietly helped humanity to slow global heating: the seas have responded by absorbing more and more carbon dioxide from the atmosphere.
But should humans come to grips with the challenge of looming climate catastrophe and start to reduce emissions, the oceans could respond again – by absorbing less and slightly slowing the fall of the mercury in the global thermometer.
And there is even an immediate chance to test this proposal: if so, then oceans that have been each year absorbing more and more carbon from the atmosphere as greenhouse gas ratios rise will go into brief reverse, because of the global economic shutdown and fall in emissions triggered by the global pandemic of Covid-19.
For the first time in decades, the oceans could take up less carbon dioxide in 2020, according to a new study by US scientists in the American Geophysical Union journal AGU Advances.
“We didn’t realise until we did this work that these external forcings, like changes in the growth of atmospheric carbon dioxide, dominate the variability in the global ocean on year-to-year timescales. That’s a real surprise,” said Galen McKinley, of Columbia University’s Lamont-Doherty Earth Observatory.
Feedback in action
“As we reduce our emissions and the growth rate of atmospheric carbon dioxide slows down, it’s important to realise that the ocean carbon sink will respond by slowing down.”
The research should not be interpreted as an invitation to go on burning fossil fuels. It is another lesson in the intricacy of the traffic between atmosphere, rocks, oceans, and living things in an evolving world. And it is more immediately an exquisite example of what engineers call feedback.
In cases of negative feedback, the agency of change also triggers a way of slowing that change. Since 1750 – the birth of the Industrial Revolution – human economies have added 440 billion tonnes of carbon to the planetary atmosphere.
For most of human history carbon dioxide ratios in the atmosphere had hovered around 285 parts per million. They have now gone beyond 400 ppm, and global average temperatures have already risen by more than 1°C.
They’d be even higher but for the oceans, which have responded by absorbing around 39% of all that extra carbon from coal, oil and gas combustion. So the oceans are sensitive to atmospheric change, and respond.
“There will be a time when the ocean will limit the effectiveness of mitigation actions, and this should be accounted for in policymaking”
The latest study is a lesson in how sensitive: Professor McKinley and her colleagues used computer models to try to understand better why the ocean uptake of carbon varies.
In the early 1990s, the ocean absorption of carbon dioxide varied: dramatically at first, because a devastating volcanic eruption of Mt Pinatubo in the Philippines in 1991 that darkened the stratosphere also accelerated ocean uptake.
And then the ocean uptake started to slow, as the skies cleared but also as the collapse of the Soviet Union and its satellite nations changed the global pattern of fuel use. It went on declining until 2001, when fossil fuel use started to accelerate. And then the ocean sink started once again to become more absorbent.
Such research is a reminder of how much scientists still don’t know about the machinery of the planet. That greenhouse gas from fossil fuel combustion drives global heating is not now in doubt. But the precise speed, and the drivers and brakes of positive and negative feedback, remain less certain.
Many feedbacks are positive: as the Arctic warms, carbon plant remains frozen in the permafrost will start to decay, release more methane and carbon dioxide, and accelerate warming.
As the sea ice retreats, and the ice reflects less sunlight, the exposed blue seas will absorb ever more radiation, to turn up the planetary temperatures. A warner world will be a wetter one, which may also mean a rise in the rate of warming.
But the ocean is not the only example of negative feedback. More carbon dioxide seems to mean more vigorous plant growth, and there is clear evidence that the world’s great forests are an important carbon sink: an example of negative feedback. That is why almost all governments recognise the importance of forest conservation.
Action however is uneven, forests are still being degraded, and there is alarming evidence that at some point, as temperatures get too high, the tropical forests could start surrendering the carbon they have for millennia absorbed, and become agents of positive feedback.
Professor McKinley warns that – as global emissions are cut – there will be a phase during which ocean uptake slows. If so, then planetary temperature rise will not slow as fast as hoped: extra carbon dioxide will linger, to contribute to warming.
“We need to discuss this coming feedback. We want people to understand that there will be a time when the ocean will limit the effectiveness of mitigation actions, and this should also be accounted for in policymaking.” – Climate News Network
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