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Headed for a sixth mass extinction? MIT geophysicist warns oceans are on the brink

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07.12.2021

Earth’s skin already bears the scars of the climate crisis. Fires ravage forests, hurricanes swamp coastlines, floods drown city blocks and entire species disappear. But beneath the surface layer, in the planet’s rocks, lies evidence of past catastrophes much more severe.

Daniel Rothman, a professor of geophysics at Massachusetts Institute of Technology, believes we may be creeping toward a calamity for Earth’s life system as a whole — the planet’s sixth mass extinction event.

The staggering amount of carbon humans are pumping into the atmosphere and the oceans may soon cross a threshold that will disorder the planet’s carbon cycle and cause a cascade of disruptions we cannot fully envision, Rothman said.

What we do know is that such disruptions in the past have coincided with a series of mass extinctions in the 540 million years since life became abundant on the planet. And while the climate crisis is usually framed in terms of years, decades, or the next century, mass extinctions play out over thousands of years.

“Every time there has been a major event in the history of life, there has also been a major perturbation of the environment. These things tend to come together,” he said.

These disruptions are associated with the carbon cycle, or the exchange of carbon in the environment, “and we know that by traces that are left in the carbon chemistry of old rocks.”

“The average person doesn’t really think about such long time scales, of course they don’t, but that’s also geology’s gift to the world,” Rothman said.

There have been many disruptions to the carbon cycle in the past that were significant for the climate but did not result in mass extinctions, however. Rothman, from MIT’s Department of Earth, Atmospheric and Planetary Sciences, investigated what made the difference, or put another way, “Why were mass extinctions special?”

Carbon is a crucial element in biological compounds, and a major component in many minerals. It enters the atmosphere as carbon dioxide when we burn fuel.

The carbon cycle is a complex, nonlinear system that centers on a loop between photosynthesis and respiration. As part of the cycle, the element is exchanged between the atmosphere and the upper levels of the ocean. Too much carbon can outpace the ocean’s “damping mechanisms” and overwhelm the cycle, leading to a cascade of positive feedback that disrupts the entire system. The perturbation can take thousands of years to even out.

Too much carbon in the ocean renders it more acidic, and inhospitable to many types of life. Four of the five past mass extinction events appear to be associated with an increased rate of carbon cycle change, and “it looks as if there’s a special rate of change which is acting as a threshold.”

Rothman concluded that, on a scale of around 10,000 years, what matters is the rate at which carbon is added to the........

© The Times of Israel


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