Global warming is ‘fundamentally’ changing the structure of our world’s oceans


Climate change has caused major changes in the stability of the oceans faster than previously thought, according to a study published Wednesday, raising alarms about its role as a global thermostat and the marine life it supports.

Research published in the journal Nature examined 50 years of data and tracked how surface water “decouples” from the deeper ocean.

Climate change has altered the mixing of the oceans, a process that helps store most of the world’s excess heat and a significant proportion of CO2.

The water on the surface is warmer, and therefore less dense, than the water below, a contrast that is intensified by climate change.

Global warming is also causing huge amounts of fresh water to flow into the seas from melting ice sheets and glaciers, reducing the salinity of the upper layer and further reducing its density.

This increasing contrast between the density of the oceanic layers makes mixing more difficult, so that oxygen, heat and carbon are less able to penetrate the depths of the sea.

“Similar to a layer of water on top of oil, surface waters in contact with the atmosphere mix less efficiently with the underlying ocean,” said lead author Jean-Baptiste Sallee of the Sorbonne University and the National Center for scientific research CNRS of France.

He said that although scientists knew this process was underway, “here we show that this change has occurred at a much faster rate than previously thought – more than six times faster.”

The report used global temperature and salinity observations obtained between 1970 and 2018, including those of electronically tracked marine mammals, with a focus on the summer months, which have the most data.

He said that the barrier layer that separates the ocean surface and the deep layers had been strengthened around the world, as measured by contrast in density, at a much higher rate than previously thought.

The researchers also found that, contrary to their expectations, winds strengthened by climate change had also acted to deepen the ocean’s surface layer by five to 10 meters per decade over the past half century.

A significant number of marine animals live in this surface layer, with a food web that depends on phytoplankton.

But as the winds pick up, the phytoplankton churn deeper, away from the light that helps them grow, which could disrupt the larger food web.

These “are not small changes that only concern some experts,” Sallee told AFP.

“They represent a fundamental change in the underlying structure of our oceans. Much more pronounced than we have previously thought.”

Deep and disturbing

The oceans play a crucial role in mitigating the effects of climate change by absorbing around a quarter of man-made CO2 and absorbing more than 90 percent of the heat generated by greenhouse gases, according to the Intergovernmental Panel for Climate Change (IPCC).

“But as it stabilizes, the ocean’s role in buffering climate change becomes more difficult, as it becomes more difficult for the ocean to absorb these large amounts of heat and carbon,” Sallee said.

Scientists are increasingly sounding the alarm about the possible implications of warming our oceans.

In 2019, research published in the US procedures of the National Academy of Sciences he calculated that climate change would empty the ocean of nearly a fifth of all living things, measured by mass, by the end of the century.

Climate scientist Michael Mann warned in September that findings from a study he co-authored Nature Climate Change – which suggested that the global stratification of the oceans had increased by 5.3 percent between 1960 and 2018 – had “profound and troubling” implications.

These included potentially more intense hurricanes driven by warming of the ocean surface.

And in February, research in Nature Geoscience found that the northern extent of the Gulf Stream, the vast heat-carrying ocean current that influences climate in Europe and sea level in the US, was the weakest in over a thousand years, probably due to the climate change.

They said increased rainfall and the melting of the Greenland ice sheet have increased fresh water in the upper ocean, disrupting the normal cycle that carries warm and salty surface water north from the equator and sends deep water. low salinity to the south.

© Agence France-Presse

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