When is the next extinction

Starting million years ago, this extinction event eliminated about 75 percent of all species on Earth over a span of roughly 20 million years. In several pulses across the Devonian, ocean oxygen levels dropped precipitously, which dealt serious blows to conodonts and ancient shelled relatives of squid and octopuses called goniatites.

The worst of these pulses, called the Kellwasser event, came about million years ago. The eruption would have spewed greenhouse gases and sulfur dioxide, which can cause acid rain. Asteroids may also have contributed. Though it may sound surprising, land plants may have been accessories to the crime.

During the Devonian, plants hit on several winning adaptations, including the stem-strengthening compound lignin and a full-fledged vascular structure. These traits allowed plants to get bigger—and for their roots to get deeper—than ever before, which would have increased the rate of rock weathering.

The faster rocks weathered, the more excess nutrients flowed from land into the oceans. The influx would have triggered algae growth, and when these algae died, their decay removed oxygen from the oceans to form what are known as dead zones. In addition, the spread of trees would have sucked CO 2 out of the atmosphere, potentially ushering in global cooling. To add to the puzzle, not only did some creatures go extinct during the late Devonian, but species diversification slowed down during this time.

The slowdown may have been caused by the global spread of invasive species , as high sea levels let creatures from previously isolated marine habitats mix and mingle, which let ecosystems around the world homogenize. The cataclysm was the single worst event life on Earth has ever experienced. Over about 60, years, 96 percent of all marine species and about three of every four species on land died out. Of the five mass extinctions, the Permian-Triassic is the only one that wiped out large numbers of insect species.

Marine ecosystems took four to eight million years to recover. Find out more about the devastation of the Permian-Triassic mass extinction. A sail-backed edaphosaurus forages amid a Permian landscape in this artist's depiction. These primitive predators, along with their close relatives the dimetrodons, though dinosaur-like in appearance, are actually considered the forerunners of mammals. Scientists think their large back fins were used to regulate body temperature.

The eruption triggered the release of at least Adding insult to injury, magma from the Siberian Traps infiltrated coal basins on its way toward the surface, probably releasing even more greenhouse gases such as methane. The resulting global warming was downright hellish. In the million years after the event, seawater and soil temperatures rose between 25 to 34 degrees Fahrenheit.

By At the time, almost no fish lived near the Equator. As temperatures rose, rocks on land weathered more rapidly, hastened by acid rain that formed from volcanic sulfur. Just as in the late Devonian, increased weathering would have brought on anoxia that suffocated the oceans. Climate models suggest that at the time, the oceans lost an estimated 76 percent of their oxygen inventory. Life took a long time to recover from the Great Dying, but once it did, it diversified rapidly.

Different reef-building creatures began to take hold, and lush vegetation covered the land, setting the stage for a group of reptiles called the archosaurs: the forerunners of birds, crocodilians, pterosaurs, and the nonavian dinosaurs. But about million years ago, life endured another major blow: the sudden loss of up to 80 percent of all land and marine species. An artist's rendering shows hatchling nothosaurs heading for the safety of water as a hungry but terrestrial Ticinosuchus attacks near a lagoon in ancient Switzerland.

Nothosaurs lived during the mid- and late Triassic period and were among the earliest reptiles to take to the sea. Because nothosaurs may have had to come ashore to lay eggs, the eggs and hatchlings would have been vulnerable to Ticinosuchus. Yet once the hatchlings reached deeper waters, they were safe—for the moment. At the end of the Triassic, Earth warmed an average of between 5 and 11 degrees Fahrenheit, driven by a quadrupling of atmospheric CO 2 levels. Does this mean that mass extinction will soon follow at the turn of the century?

Rothman says it would take some time — about 10, years — for such ecological disasters to play out. In the geologic past, this type of behavior is associated with mass extinction. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction, raising an essential question:.

He eventually derived a simple mathematical formula based on basic physical principles that relates the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change.

He hypothesized that this formula should predict whether mass extinction, or some other sort of global catastrophe, should occur. Rothman then asked whether history followed his hypothesis. For each event, including the five mass extinctions, Rothman noted the change in carbon, expressed in the geochemical record as a change in the relative abundance of two isotopes, carbon and carbon He also noted the duration of time over which the changes occurred.

He then devised a mathematical transformation to convert these quantities into the total mass of carbon that was added to the oceans during each event. Finally, he plotted both the mass and timescale of each event. In other words, he observed a common threshold that most of the 31 events appeared to stay under.

While these events involved significant changes in carbon, they were relatively benign — not enough to destabilize the system toward catastrophe. In contrast, four of the five mass extinction events lay over the threshold, with the most severe end-Permian extinction being the farthest over the line.

The cycle is essentially a loop between photosynthesis and respiration. Rothman found that the critical rate was equivalent to the rate of excess production of carbon dioxide that would result from plugging the leak. Any additional carbon dioxide injected into the cycle could not be described by the loop itself.

One or more other processes would instead have taken the carbon cycle into unstable territory. He then determined that the critical rate applies only beyond the timescale at which the marine carbon cycle can re-establish its equilibrium after it is disturbed. Today, this timescale is about 10, years. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.

From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about gigatons. The IPCC projections consider four possible pathways for carbon dioxide emissions, ranging from one associated with stringent policies to limit carbon dioxide emissions, to another related to the high range of scenarios with no limitations.

The best-case scenario projects that humans will add gigatons of carbon to the oceans by , while more than gigatons will be added under the worst-case scenario, far exceeding the critical threshold. Mass extinctions are just as severe as their name suggests. The most recent, 66 million years ago, saw dinosaurs disappear. Read More. The past events were caused by catastrophic alterations of the environment, including massive volcanic eruptions or collision with an asteroid.

The sixth mass extinction -- the one happening now -- is different: Scientists say it's caused by humans. Extinction breeds extinctions. While life on Earth has bounced back after each of these events, it took millions of years to restore the number of species. Amur leopards are now critically endangered. When one species in the ecosystem disappears, it erodes the entire ecosystem and pushes other species toward annihilation. The researchers use amphibians as an example of this phenomena.