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Author Topic: The Holocene Extinction  (Read 24410 times)


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Re: The Holocene Extinction
« Reply #300 on: July 10, 2019, 12:15:51 PM »
Breaching a 'carbon threshold' could lead to mass extinction

Daniel Rothman, professor of geophysics and co-director of the Lorenz Center in MIT's Department of Earth, Atmospheric and Planetary Sciences, has found that when the rate at which carbon dioxide enters the oceans pushes past a certain threshold—whether as the result of a sudden burst or a slow, steady influx—the Earth may respond with a runaway cascade of chemical feedbacks, leading to extreme ocean acidification that dramatically amplifies the effects of the original trigger....

...What does this all have to do with our modern-day climate? Today's oceans are absorbing carbon about an order of magnitude faster than the worst case in the geologic record—the end-Permian extinction. But humans have only been pumping carbon dioxide into the atmosphere for hundreds of years, versus the tens of thousands of years or more that it took for volcanic eruptions or other disturbances to trigger the great environmental disruptions of the past. Might the modern increase of carbon be too brief to excite a major disruption?

According to Rothman, today we are "at the precipice of excitation," and if it occurs, the resulting spike—as evidenced through ocean acidification, species die-offs, and more—is likely to be similar to past global catastrophes.

"Once we're over the threshold, how we got there may not matter," says Rothman, who is publishing his results this week in the Proceedings of the National Academy of Sciences. "Once you get over it, you're dealing with how the Earth works, and it goes on its own ride.

The Paper itself

Characteristic disruptions of an excitable carbon cycle

The history of the carbon cycle is punctuated by enigmatic transient changes in the ocean’s store of carbon. Mass extinction is always accompanied by such a disruption, but most disruptions are relatively benign. The less calamitous group exhibits a characteristic rate of change whereas greater surges accompany mass extinctions. To better understand these observations, I formulate and analyze a mathematical model that suggests that disruptions are initiated by perturbation of a permanently stable steady state beyond a threshold. The ensuing excitation exhibits the characteristic surge of real disruptions. In this view, the magnitude and timescale of the disruption are properties of the carbon cycle itself rather than its perturbation. Surges associated with mass extinction, however, require additional inputs from external sources such as massive volcanism. Surges are excited when CO2 enters the oceans at a flux that exceeds a threshold. The threshold depends on the duration of the injection. For injections lasting a time ti≳10,000 y in the modern carbon cycle, the threshold flux is constant; for smaller ti, the threshold scales like ti−1. Consequently the unusually strong but geologically brief duration of modern anthropogenic oceanic CO2 uptake is roughly equivalent, in terms of its potential to excite a major disruption, to relatively weak but longer-lived perturbations associated with massive volcanism in the geologic past.


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Re: The Holocene Extinction
« Reply #302 on: July 17, 2019, 12:25:35 PM »
Joshua trees facing extinction


 In the best-case scenario, major efforts to reduce heat-trapping gasses in the atmosphere would save 19 percent of the tree habitat after the year 2070. In the worst case, with no reduction in carbon emissions, the park would retain a mere 0.02 percent of its Joshua tree habitat.

The team's findings were published recently in Ecosphere. Project lead Lynn Sweet, a UCR plant ecologist, said she hopes the study inspires people to take protective environmental action. "The fate of these unusual, amazing trees is in all of our hands," she said. "Their numbers will decline, but how much depends on us."


They found that Joshua trees have been migrating to higher elevation parts of the park with cooler weather and more moisture in the ground. In hotter, drier areas, the adult trees aren't producing as many younger plants, and the ones they do produce aren't surviving.

Joshua trees as a species have existed since the Pleistocene era, about 2.5 million years ago, and individual trees can live up to 300 years. One of the ways adult trees survive so long is by storing large reserves of water to weather droughts.

Younger trees and seedlings aren't capable of holding reserves in this way though, and the most recent, 376-week-long drought in California left the ground in some places without enough water to support new young plants. As the climate changes, long periods of drought are likely to occur with more frequency, leading to issues with the trees like those already observed.

An additional finding of this study is that in the cooler, wetter parts of the park the biggest threat other than climate change is fire. Fewer than 10 percent of Joshua trees survive wildfires, which have been exacerbated in recent years by smog from car and industrial exhaust. The smog deposits nitrogen on the ground, which in turn feeds non-native grasses that act as kindling for wildfires.

As a partner on this project, the U.S. Park Service is using this information to mitigate fire risk by removing the invasive plants.