Our oceans may hold enough carbon in 80 years’ time to trigger a sixth mass extinction, a scientist has warned based on his mathematical prediction.

Daniel Rothman, geophysics professor at MIT, said he made the calculation based on the significant changes in the carbon cycle over the last 540 million years and everything we know so far about the previous five mass extinctions that occurred during this time.

In his paper, Rothman proposes that mass extinction occurs when one of these two thresholds are crossed – one, where changes occur at rates faster than our ecosystems can adapt in a long-timescale carbon cycle and, two, where changes are of a significantly large magnitude for carbon perturbations that take place over shorter timescales.

Given the recent rise in carbon dioxide emissions due to human activity, he said we would reach this threshold by the year 2100 and once that happens, it could lead to an unstable environment, and ultimately, mass extinction.

Rothman calculated the critical amount of carbon to be about 310 gigatons, which would be enough to trigger the events that could lead to a mass wipe-out of all the major species on Earth.

But it doesn’t mean mass extinction will follow immediately. Rothman believes it will take another 10,000 years for the ecological disasters to play out but warns that by 2100, the world would be tipped into “unknown territory”.

“This is not saying that disaster occurs the next day,” Rothman said. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict.

“In the geologic past, this type of behaviour is associated with mass extinction.”

The Earth has endured five mass extinction events, each involving processes that wreaked havoc on the the carbon cycle.

Having previously done some work on the end-Permian extinction, Rothman identified 31 events in the last 542 million years where a significant change occurred in our planet’s carbon cycle after looking through hundreds of scientific papers.

Looking at the geochemical record, Rothman noted changes in the relative abundance of two carbon isotopes – carbon-12 and carbon-13 – for each mass extinction event.

He then created a mathematical formula based on the total mass of carbon that was added to the oceans during each event and the timescale of each event.

“It became evident that there was a characteristic rate of change that the system basically didn’t like to go past,” Rothman said.

“Then it became a question of figuring out what it meant.”

The research is published in Science Advances. </s> Over the past 540 million years Earth has suffered five mass extinction events, the worst of which wiped out more than 9 per cent of marine life on the planet. A new study has suggested that the next such catastrophe might not be too far away.

Massachusetts Institute of Technology (MIT) geophysicist and mathematician, Daniel Rothman has been busy studying previous mass extinctions. He reckons the next one might be a mere 83 years away.

The previous five catastrophic events each unfolded over millions of years and involved the natural cycle of carbon through the oceans and atmosphere being disturbed, resulting, in some cases, the death of almost all life on Earth.

The award-winning mathematician identified two “thresholds of catastrophe” that, if exceeded, would upset the natural order of the cycle, leading to an unstable environment and eventually a mass extinction.

The first relates to changes in the carbon cycle over a period of thousands or millions of years. A mass extinction will occur if the rate of change in the cycle occurs faster than global ecosystems can adapt.

The second pertains to the size or magnitude of the carbon flux over a shorter period, as has been the case over the last century.

Therein lies a problem, however, as Rothman says: “How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?”

“So I sat down one summer day and tried to think about how one might go about this systematically.”

He set about searching through hundreds of geochemistry papers trying to find an answer, classifying the events as either long or short in duration. Throughout his studies, Rothman identified 31 instances over the past 542 million years in which a major change occurred in the carbon cycle.

Following this, he devised a mathematical formula to determine the total mass of carbon added to the oceans during each event, after which “it became evident that there was a characteristic rate of change that the system basically didn't like to go past.”

Rothman estimates this to be 310 gigatons. He thinks that given the rise of carbon dioxide over the last century, a sixth mass extinction could be on the way as estimates suggest that humans will add roughly 310 gigatons to the cycle by 2100.

READ MORE: ‘What the frack! We’re destroying all life, we have a huge problem’

“This is not saying that disaster occurs the next day,” Rothman said. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”

Rothman's paper was published Wednesday in the journal Science Advances. </s> CBS Local — Will the human race live to see the next century on Earth? According to a mathematician at MIT, the next extinction-level event for the planet may begin as early as the year 2100.

Professor Daniel Rothman has published a formula in the journal Science Advances that looks at the planet’s carbon cycle. According to Rothman, the massive amounts of CO2 emissions produced around the world are in line with previous mass extinction events in the planet’s history.

“The modern critical size for the marine carbon cycle is roughly similar to the mass of carbon that human activities will likely have added to the oceans by the year 2100,” Rothman said. The MIT professor adds that in four of the last five extinction events the critical level of carbon in his model was surpassed.

The study doesn’t draw a direct conclusion as to how life will begin to die out in the 22nd century.

“This is not saying that disaster occurs the next day,” Rothman admits. The mathematician adds that his model only points to a dangerous trend in the history of the Earth over the last 540 million years. “If left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.” </s> Climate change will have sent Earth on the way to its sixth mass extinction in less than a lifetime, new research warns.

By the year 2100, about 310 gigatons of carbon will have been added to the oceans - a potential 'tipping point' for ecological disaster, according to the study.

When CO2 dissolves into the ocean - as in the case of the End-Permian mass extinction - life on Earth is at risk.

Professor Daniel Rothman from the Massachusetts Institute of Technology came up with a simple mathematical formula to predict when the next mass extinction would take place.

The formula predicts that by the end of the century oceans will hold enough carbon to launch a mass extermination of species in the future.

The model showed the critical extra amount required is about 310 gigatons, which is around the best case scenario projected by the Intergovernmental Panel on Climate Change (IPCC).

And it's well below the worst of more than 500 gigatons that would far exceed the line.

In all scenarios the study found by the end of the century the carbon cycle will either be close to, or well beyond, the threshold for catastrophe.

It also found although mass extinction won't soon follow at the turn of the century the world may have tipped into 'unknown territory.'

Professor Rothman says it would take some time, about 10,000 years, for such ecological disasters to play out.

He said: 'This is not saying disaster occurs the next day.

'It's saying - if left unchecked - the carbon cycle would move into a realm which would be no longer stable and would behave in a way that would be difficult to predict.

'In the geologic past this type of behaviour is associated with mass extinction.'

In the modern era CO2 emissions have risen steadily since the 19th century, but deciphering whether this could lead to mass extinction has been challenging.

That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions which have taken place over little more than a century.

Professor Rothman's analysis identified 'thresholds of catastrophe' in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

He had previously done work on the end-Permian extinction, or Great Dying.

Since then conversations with colleagues spurred him to consider the likelihood of a sixth, raising an essential question.

Professor Rothman said: 'How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?.

'So I sat down one summer day and tried to think about how one might go about this systematically.'

His unique equation factored in the critical rate and magnitude of change in the carbon cycle to the timescale that separates fast from slow change from 31 events in the last 542 million years in which a significant change occurred in the carbon cycle.

For each, including the five mass extinctions, he noted the change in carbon, expressed in the relative abundance of two telltale chemicals in the geologic records.

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.

'It became evident that there was a characteristic rate of change that the system basically didn't like to go past,' said Professor Rothman.

There was a common threshold that most of the 31 events appeared to stay under. They were relatively benign - not enough to destabilise the system toward catastrophe.

In contrast four of the five mass extinctions lay over the threshold - with the 'Great Dying' being the farthest over the line.

'Then it became a question of figuring out what it meant,' said Professor Rothman.

He added: 'There should be ways of pulling back emissions of carbon dioxide.

'But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present.'

Humans have emitted 1,540 billion tonnes of CO2 since the industrial revolution.

That's equivalent to burning enough coal to form a square tower 72 feet wide stretching 240,000 miles from Earth to the Moon.

Half of these have remained in the atmosphere causing a rise in levels at least 10 times faster than any known natural increase during Earth's long history.

Most of the other half has dissolved into the ocean, causing acidification.

The full results of the study were published today in the journal Science Advances. </s> New Delhi: Courtesy climate change as well as human behaviour towards nature and its beings, the world may need to prepare itself for its sixth mass extinction event, which researchers have said will begin by about 2100.

The reason? The amount of carbon being pumped into the atmosphere.

The threat that the ecosystem is facing due to this, has scientists worried and going by the rate at which the world is losing out on its biodiversity, they say that the possibility of a mass extinction in the future looms larger than ever.

Scientists came to this conclusion after a comprehensive mathematical study of the five previous events in the last 540 million years.

According to a report in the Independent UK, professor Daniel Rothman, co-director of the Massachusetts Institute of Technology’s Lorenz Centre, theorised that disturbances in the natural cycle of carbon through the atmosphere, oceans, plant and animal life played a role in mass die-offs of animals and plants.

After studying such changes based on when they took place 31 times, he found that four out of the five previous mass extinctions occurred when the disruption crossed a “threshold of catastrophic change”.

The 'Great Dying', known as the worst mass extinction of all – that took place 248 million years ago when 96 percent of species died out – breached one of these thresholds by the greatest margin.

Based on his analysis of these mass extinctions, Professor Rothman developed a mathematical formula to help predict how much extra carbon could be added to the oceans – which absorb vast amounts from the atmosphere – before triggering a sixth one.

The answer came as a shock.

For the figure of 310 gigatons is just 10 gigatons above the figure expected to be emitted by 2100 under the best-case scenario forecast by the Intergovernmental Panel on Climate Change. The worst-case scenario would result in more than 500 gigatons, the report said.

While many scientists argue that the sixth mass extinction has already begun, the total number of species that have disappeared from the planet comes nowhere near the most apocalyptic events of the past and the rate of species loss is comparable.

Professor Rothman emphasised that dramatic changes to the carbon cycle wasn't a necessary occurrence for a mass extinction to take place – as shown by the absence of this during the Late Devonian extinction more than 360 million years ago.

The study, published in the journal Science Advances, noted that events such as volcanic eruptions, climate change and other environmental factors could also play a role.

Does this mean that mass extinction will soon follow at the turn of the century? As per a report in Phys.org, Rothman says it would take some time—about 10,000 years—for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

The Independent UK further quoted him saying, “By analyzing 31 carbon isotopic events during the past 542 million years, I identify the critical rate with a limit imposed by mass conservation.

“The modern critical size for the marine carbon cycle is roughly similar to the mass of carbon that human activities will likely have added to the oceans by the year 2100.” </s> According to the latest findings by cosmic doomsayers, Planet X is due to eclipse the sun this Saturday, September 23, marking the start of The Rapture. Although sceptics - such as NASA - have denied the very existence of Nibiru, there are still many conspiracists who are convinced that the end is nigh. But they may in fact be right - in planetary terms at least. One US geophysicist and award-winning mathematician has been studying Earth's last five mass extinction events - and believes the next one could be just 83 years away.

Daniel Rothman, of the Massachusetts Institute of Technology, has identified two "thresholds of catastrophe" that upset the world's natural order, resulting in environmental instability and, eventually, mass extinction. The first regards changes in the carbon cycle over a period of thousands or millions of years. If changes occur faster than the ecosystem can adapt, then a mass extinction will ultimately occur.

The second threshold involves the magnitude of the carbon flux. In this scenario, it is not the speed of the changes, merely the size of them which will result in extinction. Mr Rothman has now predicted that, with humans increasing the rate of carbon dioxide emissions at a faster rate over a shorter period of time, the tipping point for the sixth mass extinction could occur as early as 2100. He calculated that by then, mankind will have added some 310 gigatons of carbon to the oceans, which he believes is the critical amount needed to trigger a future apocolyptic event. However, Mr Rothman said the likelihood of ecological catastrophe occurring as soon as this point is reached is unlikely.

In his paper, publicised in Science Advances, he said: "This is not saying that disaster occurs the next day. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. "In the geologic past, this type of behaviour is associated with mass extinction." Earth has suffered five mass extinctions in the past 540 million years, with the worst wiping out more than 9 per cent of the planet's marine life. </s> Sedimentary rocks at Meishan, China. These rocks contain signatures of a disturbance in the carbon cycle immediately preceding Earth's greatest mass extinction. Credit: Shuzhong Shen

In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world.

The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether this recent spike in carbon could lead to mass extinction has been challenging. That's mainly because it's difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today's disruptions, which have taken place over just a little more than a century.

Now Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences and co-director of MIT's Lorenz Center, has analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events. He has identified "thresholds of catastrophe" in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.

In a paper published in Science Advances, he proposes that mass extinction occurs if one of two thresholds are crossed: For changes in the carbon cycle that occur over long timescales, extinctions will follow if those changes occur at rates faster than global ecosystems can adapt. For carbon perturbations that take place over shorter timescales, the pace of carbon-cycle changes will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event.

Taking this reasoning forward in time, Rothman predicts that, given the recent rise in carbon dioxide emissions over a relatively short timescale, a sixth extinction will depend on whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates to be roughly equivalent to the amount of carbon that human activities will have added to the world's oceans by the year 2100.

Does this mean that mass extinction will soon follow at the turn of the century? Rothman says it would take some time—about 10,000 years—for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into "unknown territory."

"This is not saying that disaster occurs the next day," Rothman says. "It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction."

History follows theory

Rothman had previously done work on the end-Permian extinction, the most severe extinction in Earth's history, in which a massive pulse of carbon through the Earth's system was involved in wiping out more than 95 percent of marine species worldwide. Since then, conversations with colleagues spurred him to consider the likelihood of a sixth extinction, raising an essential question:

"How can you really compare these great events in the geologic past, which occur over such vast timescales, to what's going on today, which is centuries at the longest?" Rothman says. "So I sat down one summer day and tried to think about how one might go about this systematically."

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. By searching through hundreds of published geochemistry papers, he identified 31 events in the last 542 million years in which a significant change occurred in Earth's carbon cycle. 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-12 and carbon-13. 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.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," Rothman says.

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.

"Then it became a question of figuring out what it meant," Rothman says.

A hidden leak

Upon further analysis, Rothman found that the critical rate for catastrophe is related to a hidden process within the Earth's natural carbon cycle. The cycle is essentially a loop between photosynthesis and respiration. Normally, there is a "leak" in the cycle, in which a small amount of organic carbon sinks to the ocean bottom and, over time, is buried as sediment and sequestered from the rest of the carbon cycle.

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 10,000 years. For much shorter events, the critical threshold is no longer tied to the rate at which carbon is added to the oceans but instead to the carbon's total mass. Both scenarios would leave an excess of carbon circulating through the oceans and atmosphere, likely resulting in global warming and ocean acidification.

The century's the limit

From the critical rate and the equilibrium timescale, Rothman calculated the critical mass of carbon for the modern day to be about 310 gigatons.

He then compared his prediction to the total amount of carbon added to the Earth's oceans by the year 2100, as projected in the most recent report of the Intergovernmental Panel on Climate Change. 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 300 gigatons of carbon to the oceans by 2100, while more than 500 gigatons will be added under the worst-case scenario, far exceeding the critical threshold. In all scenarios, Rothman shows that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.

"There should be ways of pulling back [emissions of carbon dioxide]," Rothman says. "But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

More information: "Thresholds of catastrophe in the Earth system," Science Advances (2017). Journal information: Science Advances "Thresholds of catastrophe in the Earth system,"(2017). advances.sciencemag.org/content/3/9/e1700906 </s> It doesn't come along very often, but after some 540 million years, this world we live on has witnessed five mass extinctions – and the next curtain could fall before the century is up.

That's the grim assessment of a new mathematical analysis of Earth's revolving carbon cycle, with calculations predicting our horrendous output of CO2 emissions is inching us towards a 'threshold of catastrophe' the planet hasn't breached in millions of years.

MIT geophysicist Daniel Rothman investigated fluctuations in the carbon cycle – the pathway carbon traces through Earth's land, oceans, and atmosphere – that have occurred over the last 542 million years.

By analysing 31 established carbon isotopic events recognised by geochemists, Rothman identified the ebb and flow of carbon–12 and carbon–13 – two isotopes of carbon whose abundance has varied considerably in Earth's history.

From this, he constructed a database to assess how much carbon mass was pumped into the world's oceans in each historical event. In most of these episodes, the carbon volume stayed under a certain threshold.

But in some of them – including four of the past five mass extinction events that exterminated multitudes of life-forms on the planet – the threshold was breached.

Now, we all know that correlation doesn't equal causation, but in light of all the other evidence we have on how dangerous high levels of carbon are to life on our planet, a disturbing pattern is definitely emerging.

"It became evident that there was a characteristic rate of change that the system basically didn't like to go past," as Rothman puts it.

Now for the bad news.

Per Rothman's calculations, there are two ways carbon levels can exceed this threshold of catastrophe. One is where CO2 emissions slowly swell over thousands and millions of years, slowly triggering a global calamity.

The other case occurs on a much shorter timescale, where an immense shift in carbon volumes moving through the carbon cycle happens in the space of say, decades and years. Sound familiar?

In this context, Rothman predicts it would take about 310 gigatonnes of carbon added to the world's oceans for us to pass the threshold – which is roughly the minimum amount expected to be contributed by the year 2100 at the rate things are going, at which point the researcher says we'll enter "unknown territory".

"This is not saying that disaster occurs the next day," Rothman says.

"It's saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behaviour is associated with mass extinction."

In other words, unless humanity does something to drastically turn around our carbon situation – and there's cause for real optimism on that front, friends – we could lock in a dangerous extermination.

Not just for humanity necessarily either, but for many a critter that ever walked, chirped, buzzed, or purred.

It wouldn't happen overnight, mind you, but such an epic die-off could play out over something like 10,000 years or so, Rothman suggests, and the phenomenon could crystallise as soon as 2100 if things don't change.

Of course, it's only one perspective on how the world's carbon scenario could materialise, and Rothman doesn't pretend he has all the answers, but he does hope we take these numbers as another piece of evidence to galvanise our slow-moving selves into action.

"There should be ways of pulling back [emissions of carbon dioxide]," he says.

"But this work points out reasons why we need to be careful, and it gives more reasons for studying the past to inform the present."

The findings are reported in Science Advances. </s> Earth has supported life for 3.5 billion years, but its hospitality is hardly consistent. Natural disasters have triggered at least five mass extinctions in the past 500 million years, each of which wiped out between 50 and 90 percent of all species on the planet. The most recent occurred about 65 million years ago, when an asteroid ended the reign of dinosaurs and opened new doors for mammals.

Now it's happening again. A 2015 study reported the long-suspected sixth mass extinction of Earth's wildlife is "already underway." And a new study calls the loss of that wildlife a “biological annihilation” and a “frightening assault on the foundations of human civilization.” Researchers from the Universidad Nacional Autónoma de México found that the rate of population loss is extremely high — even among species that are not considered endangered. They also found that up to half of all individual animals have been lost in the last few decades.

A 2016 study published in the journal Science also suggests this sixth mass extinction is killing off large ocean dwellers (like sharks, whales, giant clams, sea turtles and tuna) in disproportionately greater numbers than smaller animals. That's a reversal from past extinctions, when there was a slight connection between smaller size and going extinct.

And while previous extinctions were often linked to asteroids or volcanoes, this one is an inside job. It's caused mainly by one species — a mammal, ironically. The current crisis is the handiwork of humans, and we have a "unique propensity to cull the largest members of a population," the authors of the 2016 study write.

Many scientists have been warning us for years, citing a pace of extinctions far beyond the historical "background" rate. Yet critics have argued that's based on inadequate data, preserving doubt about the scope of modern wildlife declines. To see if such doubt is justified, the 2015 study compared a conservatively low estimate of current extinctions with an estimated background rate twice as high as those used in previous studies. Despite the extra caution, it still found species are disappearing up to 114 times more quickly than they normally do in between mass extinctions.

Here are six important things to know about life in the sixth mass extinction:

Once abundant in eastern North America, passenger pigeons were driven extinct by a surge of hunting and habitat loss. (Photo: Seabamirum/Flickr)

Extinction is a natural part of evolution, having already claimed an estimated 99 percent of all species in Earth's history. But things can get ugly when too many species die out too quickly, creating a domino effect capable of bringing down ecosystems. In the new study, researchers used a background rate of two mammal extinctions per 10,000 species per 100 years (2 E/MSY), which is double the background rate used in many previous studies. When they compared that to a conservative estimate of modern-day extinctions, they found no way to avoid calling this a mass extinction.

"Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 114 times higher than the background rate," the study's authors write. "Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way."

Deforestation is one of the primary threats for endangered species. (Photo: Fedorov Oleksiy/Shutterstock)

The No. 1 cause of modern wildlife declines is habitat loss and fragmentation, representing the primary threat for 85 percent of all species on the IUCN Red List. That includes deforestation for farming, logging and settlement, but also the less obvious threat of fragmentation by roads and other infrastructure.

And even where habitats aren't being razed or divided, they're increasingly altered by other human activities. Invasive species now threaten a variety of native plants and animals around the world, either by killing them directly or by outcompeting them for food and nest sites. Pollution is pervasive in many places, from chemicals like mercury that accumulate in fish to the plastic debris that slowly kills sea turtles, sea birds and cetaceans. Entire ecosystems are now migrating due to climate change, leaving behind less mobile or adaptable species. And in some parts of the world, poachers are obliterating rare species to meet demand for wildlife parts like rhino horn and elephant ivory.

The lemur tree frog is critically endangered and listed on the IUCN Red List. (Photo: G.J. Verspui/Shutterstock)

The number of vertebrate species that have definitely gone extinct since 1500 is at least 338, according to the new study. (That doesn't include the less stringent categories of "extinct in the wild" (EW) and "possibly extinct" (PE), which push the total up to 617.) More than half of those extinctions have occurred since 1900 — 198 in the "extinct" (EX) category, plus another 279 in EW and PE.

Even under the most conservative estimates, the extinction rates for mammals, birds, amphibians and fish have all been at least 20 times their expected rates since 1900, the researchers note (the rate for reptiles ranges from 8 to 24 times above expected). Earth's entire vertebrate population has reportedly fallen 52 percent in the last 45 years alone, and the threat of extinction still looms for many — including an estimated 41 percent of all amphibian species and 26 percent of mammals.

"There are examples of species all over the world that are essentially the walking dead," Ehrlich says.

4. It's probably still worse than we think.

Insecticides can weaken native pollinators like bees, raising concerns about food supplies. (Photo: Bjorn Watland/Flickr)

The new study is intentionally conservative, so the actual rate of extinctions is almost certainly more extreme than it suggests. "We emphasize that our calculations very likely underestimate the severity of the extinction crisis," the researchers write, "because our aim was to place a realistic lower bound on humanity's impact on biodiversity."

The study also focuses on vertebrates, which are typically easier to count than smaller or subtler wildlife like mollusks, insects and plants. As another recent study pointed out, this leaves much of the crisis unexamined. "Mammals and birds provide the most robust data, because the status of almost all has been assessed," write the authors of that study, published June 8 in the Proceedings of the National Academy of Sciences. "Invertebrates constitute over 99% of species diversity, but the status of only a tiny fraction has been assessed, thereby dramatically underestimating overall levels of extinction."

By incorporating data on terrestrial invertebrates, they add, "this study estimates that we may already have lost 7% of [contemporary] species on Earth and that the biodiversity crisis is real.

About 1 billion people rely on fish as their main source of animal protein, according to the World Health Organization. (Photo: Albert Pego/Shutterstock)

Humans are hardly an endangered species, with a global population of about 7.2 billion and growing. But fortunes can change quickly, as we've demonstrated in recent decades with lots of other wildlife. And despite our best efforts to buffer ourselves against the whims of nature, civilization remains reliant on healthy ecosystems for food, water and other resources. Adjusting to mass extinctions would be a challenge under any circumstances, but it's especially daunting in the context of climate change.

"If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on," says Gerardo Ceballos of the Universidad Autónoma de México, lead author of the new study. "We are sawing off the limb that we are sitting on," Ehrlich adds.

6. Unlike an asteroid, we can be reasoned with.

An artist's rendering of the asteroid widely credited with wiping out the dinosaurs. (Photo: NASA)

Previous mass extinctions may have been inevitable, but it's not too late to stop this one. While the authors of the new study acknowledge the difficulty of curbing lucrative destruction like deforestation, not to mention climate change, they note it is still possible. It's even gaining momentum, thanks to growing public awareness as well as high-profile attention from governments, corporations and even the pope.

"Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species," the study's authors write, "and to alleviate pressures on their populations — notably habitat loss, over-exploitation for economic gain and climate change."

That won't be easy, but at least it's more of a chance than the dinosaurs got.

Editor's note: This story has been updated since it was originally published in June 2015.
MIT physics professor Daniel Rothman's mathematical formula, published in the peer-review journal Science Advances, predicts a sixth mass extinction may be set to occur within 10,000 years if by the year 2100 current carbon dioxide emissions are not reduced, and a critical amount of carbon (310 gigatons) is added to the oceans.