Climate change has brought about major changes in ocean stability faster than previously thought, according to a study published on Wednesday, which has raised alarms about its role as a global thermostat and the marine life it supports.
Research published in the journal The nature analyzed 50 years of data and tracked how surface water “decouples” from the deeper ocean.
Climate change has disrupted the mixing of the oceans, a process that helps store most of the world’s excess heat and a significant proportion of CO2.
Surface water is warmer – and therefore less dense – than the water below, a contrast that is intensified by climate change.
Global warming also causes massive amounts of fresh water to flow into the seas from melting ice sheets and glaciers, lowering the salinity of the upper layer and further reducing its density.
This increasing contrast between the density of the ocean layers makes mixing harder, so that oxygen, heat and carbon are less able to penetrate the deep sea.
“Similar to a layer of water on top of the oil, surface water in contact with the atmosphere mixes less efficiently with the underlying ocean,” said lead author Jean-Baptiste Sallee of the Sorbonne University and the CNRS National Center for Scientific Research in France.
He said, while scientists were aware that the process was ongoing, “here we show that this change has taken place at a much faster pace than previously thought: more than six times faster.”
The report used global observations of temperature and salinity obtained between 1970 and 2018 – including those from electronically tracked marine mammals – with a focus on the summer months, which have more data.
He said the barrier layer separating the ocean surface and the deep layers has strengthened globally – as measured by density contrast – at a much higher rate than previously thought.
Researchers have also found that, contrary to their expectations, winds strengthened by climate change have also acted to deepen the ocean’s surface layer by five to 10 meters per decade in the last half century.
A significant number of marine animals live in this surface layer, with a phytoplankton-dependent food web.
But as the wind grows, the phytoplankton is shaken deeper, away from the light that helps them grow, disrupting the potential trophic network.
These are not “small changes that only some experts care about,” Sallee told AFP.
“They represent a fundamental change in the underlying structure of our oceans. Much more pronounced than we thought until now.”
Deep and worrying
The oceans play a crucial role in mitigating the effects of climate change by absorbing about a quarter of man-made CO2 and absorbing more than 90% of the heat generated by greenhouse gases, according to the Intergovernmental Panel on Climate Change (IPCC).
“But through stabilization, the role of the ocean in dampening climate change is made more difficult because it is more difficult for the ocean to absorb these vast amounts of heat and carbon,” Sallee said.
Scientists are increasingly sounding the alarm about the potential implications of warming on our oceans.
In 2019, research published in the US The works of the National Academy of Sciences calculated that climate change would empty the ocean of nearly a fifth of all living creatures, measured en masse, by the end of the century.
Climate scientist Michael Mann warned in September that the results of a co-authored study The Nature of Climate Change – which suggested that the stratification of global oceans increased by 5.3 percent from 1960 to 2018 – had “profound and worrying” implications.
These included potentially more intense hurricanes caused by warming ocean surfaces.
And in February, research in Geosciences of nature found that the northern extension of the Gulf Stream – the large oceanic heat-carrying current that influences weather in Europe and sea level in the United States – has been the weakest in a thousand years, probably due to climate change.
They said increased rainfall and melting of the Greenland ice sheet increased freshwater in the upper ocean, disrupting the normal cycle that carries hot, salty surface water north from the equator and sending deep, low-salinity water back south.
© Agence France-Presse