The standard killing model

According to Professor Mike Benton from the University of Bristol, mass extinction is a lot more common than you might think. In fact there have been seventeen mass extinctions in the last 500 million years, and six of them involved global warming. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

This is the same kind of global warming that we are experiencing today.

The difference is that in the past it was massive volcanic eruptions lasting thousands of years that caused the global warming. These days it's burning fossil fuels.

With global warming the culprit in so many mass extinctions, scientists have been able to develop a theory on just what makes global warming so deadly. Professor Benton calls it the "standard killing model", and it goes like this.

In the mass extinctions of the past, wide-scale volcanic eruptions injected huge quantities of CO2 and sulphur dioxide (SO2) into the atmosphere. This caused global warming combined with acid rain, which killed off life on land.

In the oceans, the problem was getting enough oxygen. Dead matter and nutrients from the extinction on land were flushed into the sea, feeding massive algae blooms, which sucked up oxygen. As sea temperatures rose, oxygen levels fell even further, creating dead zones across the oceans.

CO2 wasn't the only greenhouse gas to raise temperatures. Methane hydrate is a substance that usually remains frozen and stored deep in the oceans. With rising temperatures, the methane thawed and bubbled up into the atmosphere. As a greenhouse gas, the methane meant more global warming, killing off even more life.

All this raises the question — with levels of CO2 again rising, are we about to follow in the footsteps of earlier extinctions? Are we now seeing a return of the killing model?

Global warming

The killing model starts with rising CO2 levels. Because it's a greenhouse gas the CO2 warms the Earth, raising temperatures to the point where nothing can survive.

For most animals somewhere between 35 °C to 40 °C (95 °F to 104 °F) is when temperatures become unbearable. Go much higher than that, and the heat becomes lethal. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

This is what what happened 252 million years ago, during the worst extinction in half a billion years — the Permian extinction. Volcanoes pumped 36,000 gigatons of CO2 into the atmosphere. 36,000 gigatons of carbon heralded history’s biggest mass extinction — Ars Technica Temperatures rose to a global average of 40 °C (104 °F). 96% of marine life was wiped out, along with 70 percent of life on land. After the extinction, it took 10 million years for life to recover. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

Jumping forward to the present day and temperatures are again on the rise. They don't have far to go to reach mass extinction levels.

One Chinese study says "a temperature increase of 5.2  °C (9.36 °F) above the pre-industrial level at present rates of increase would likely result in mass extinction". Thresholds of temperature change for mass extinctions — research article

Another study by EU/Australian scientists had similar results. They found that "5-6 degrees (9-10.8  °F) of average warming globally is enough to wipe out most life on the planet". Losing species risks domino extinction — press release

Japanese research on the same topic found that global mass extinction started at a much higher 7-9  °C (12.6-16.2 °F). The bigger the temperature change, the larger the extinction event, reveals researcher — press release

These temperatures are reachable. In a worst-case global warming scenario (RCP8.5 or SSP5-8.5), we could reach and exceed mass extinction-level temperatures in the next century, or even the end of this one. Multi-century dynamics of the climate and carbon cycle under both high and net negative emissions scenarios — research article

The most pessimistic global warming scenario (SSP5-8.5) leads to mass extinction level temperatures this century. Reversing CO2 emissions by 2040 (SSP5-3.4) avoids these temperatures. Charles D. Koven et al., CC BY 4.0

Or we could avoid mass extinction level temperatures by following a more moderate scenario. The SSP5-3.4 scenario, where we cap greenhouse gas emissions by 2040 and keep on reducing them afterwards, keeps global temperatures well away from mass extinction levels.

What the future holds, whether it's the worst case scenario RCP8.5 tracks cumulative CO2 emissions - research article or a moderate scenario, Emissions – the ‘business as usual’ story is misleading — science article depends on how much CO2 we emit in the next few decades.

Dead zones

According to the killing model, marine life is particularly vulnerable to mass extinction.

It's because of a physical property of water — as water gets hotter it can't hold as much oxygen.

Sometimes it can't hold any oxygen at all. During the Permian extinction oxygen levels in the oceans dropped by 80%, and half of the ocean seafloor became oxygen free. Worst mass extinction event in Earth’s history was caused by global warming analogous to current climate crisis — Mongabay

Combine this with lethal surface temperatures and fish were facing what scientists like to call a double whammy.

Marine life during the Permian extinction had two options — cooking to death near the ocean's surface, or suffocating in the oxygen-free water on the seabed. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

And dying caused more dying. Dead matter and nutrients left by the extinction fed massive algae blooms in the oceans, which used up oxygen, suffocating even more marine life and expanding dead zones further. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

The result — by the end of the Permian extinction, 96% of life in the oceans was wiped out. Only a tiny percentage of sea life survived the extinction. By comparison, on land, 30% of life survived. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

Back in the present we can see signs of the oceans warming again. Upper ocean temperatures could reach between 35 and 50% of temperatures during the Permian extinction by the year 2100, if greenhouse gas emissions aren't reduced. Worst mass extinction event in Earth’s history was caused by global warming analogous to current climate crisis — Mongabay

Falling oxygen levels together with a rise in algal blooms have led to a massive expansion of dead zones. In the last fifty years, dead zones have grown by 4.5 million square kilometers (1.7 million square miles). Added together, dead zones in the world's oceans equal the size of Europe. Global warming, pollution supersize the oceans’ oxygen-depleted dead zones — Mongabay

With figures like these, a reappearance of the killing model doesn't seem quite so unlikely.

Methane blow-outs

The killing model doesn't stop with CO2. There's also the methane.

Methane is a powerful greenhouse gas, much more than CO2. If some methane managed to escape from a gas field for example, in its first 20 years in the atmosphere it would warm the planet about 84-87 times more than the same amount of CO2 could. A hundred years later it would still be warming the planet, but not as much — only about 28-36 times more than CO2. Methane matters — NASA Earth Observatory

Undersea methane hydrate. USGS, Public domain

There are vast reserves of frozen methane, called methane hydrate, stored at the bottom of the world's oceans. If enough of it warms and thaws, it has the potential to push global warming to lethal levels.

According to Dr Uwe Brand from Brook University and colleagues, "global warming triggered by the massive release of carbon dioxide may be catastrophic, but the release of methane from hydrate may be apocalyptic." Methane Hydrate: Killer cause of Earth's greatest mass extinction — research article

When the methane hydrate thawed at the end of the Permian extinction, it released enough methane gas to send average global temperatures past 36 °C (97 °F). Methane Hydrate: Killer cause of Earth's greatest mass extinction — research article It's more than twenty degrees (36 °F) hotter than the average global temperature today. What is the average temperature on Earth? — science article

If it happened before, could it happen again?

Some scientists like Dr. David McKay of the Stockholm Resilience Centre are not so concerned. Dr McKay explains that any methane thaw will be "a gradual chronic leak" rather than a "sudden, catastrophic release". Fact-Check: is an Arctic “Methane Bomb” about to go off? — Climatetippingpoints.info

One reason is that the oceans are so huge that it could take "hundreds or thousands of years" for global warming to have an effect on temperatures at the bottom of the ocean, according to Dr Carolyn Ruppel from the US Geological Survey. The interaction of climate change and methane hydrates — research article

Not all scientists agree. Karin Andreassen and her team of scientists from Norway's CAGE Centre believe that "there is a large difference between slow, steady degassing" and abrupt "blow-outs" that could "eject massive fluxes of methane into the atmosphere". Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor — research article

Dr Andreassen warns that blow-outs have the potential to impact the climate "not only in the past but also the future". Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor — research article

The blow-outs that Dr Andreassen was referring to are plain to see. Since 2014, across the Yamal Peninsula in Siberia, 17 craters have mysteriously appeared, the result of methane blow-outs. Scientists estimate 7,185 more potential blow-out sites in the region. More than 400 sealed ‘craters’ are ticking time bombs from a total 7000+ Arctic permafrost mounds — news article

Blow-outs on land eject methane directly into the atmosphere. There's nothing holding back the methane. But their size and power mean that, even underwater, they can push large amounts of methane up through the ocean and into the air.

In one case an underwater blow-out off the Shetland Islands coast several million years ago sent somewhere between 0.02 and 0.03 of a gigaton of methane into the atmosphere. Scientists suspect the blast was due to an earthquake rather than global warming, but it gives an idea of how much methane there is in a blow-out. Evidence for massive emission of methane from a deep‐water gas field during the Pliocene — research article

The blow-outs don't need to be one-off events either. At the bottom of the Barents Sea, just north of Norway, lie 100 craters, remnants of methane blow-outs 12,000 years ago, at the end of the last ice age. The explosions were powerful enough to form craters a kilometre (0.6 miles) across in hard bedrock. Ancient Arctic ‘gas’ melt triggered enormous seafloor explosions — Science News Explores

Sonar image of kilometre-wide craters on the Arctic seafloor, formed by methane blow-outs at the end of the last ice age. Andreia Plaza Faverola/CAGE, Permitted use

At the time of the blow-outs, water temperatures on the seabed, which scientists call the "bottom temperature", had warmed to 6 °C (10.8 °F). Deglacial bottom water warming intensified Arctic methane seepage in the NW Barents Sea — research article

CAGE scientists found that under a worst case RCP8.5 scenario, bottom temperatures in the Barents Sea could rise to 5 °C (9 °F) by the end of the century. It's very close to temperatures during the last blow-out 12,000 years ago. Deglacial bottom water warming intensified Arctic methane seepage in the NW Barents Sea — research article

The sixth mass extinction

That brings us to the final stages of the killing model — mass extinction, on land and in the oceans. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

The predictions of the killing model are close to coming true. There is a long list of scientists who are now saying that we are at the start of the world's sixth global mass extinction. The Sixth Mass Extinction: fact, fiction or speculation? — research article

Lemurs — the world's most endangered mammals. James Kemp, CC BY-NC-ND 2.0

Nearly 1 million species are estimated to be at risk of extinction. Erosion of global functional diversity across the tree of life — research article

At current rates, scientists predict we will be in the midst of a global mass extinction in three centuries. Has the Earth’s sixth mass extinction already arrived? — research article

Could we out-kill our way to the number 1 spot on the list of the world's worst extinctions — the Permian extinction, that took the world 10 million years to recover from?

According to the killing model, during the Permian extinction the main threats were the lethal levels of heat, acid rain, and lack of oxygen in the seas. Hyperthermal-driven mass extinctions: killing models during the Permian–Triassic mass extinction — review article

These days the threats are so much more. There's also habitat loss, pollution, overfishing and overhunting, invasive species and pathogens, and the ever-expanding human population. Has the Earth’s sixth mass extinction already arrived? — research article

And then there's the threat to ourselves.

Dr Rodolfo Dirzo from Stanford University and colleagues warn that the destruction of nature is a "threat to the existence of civilization that could even threaten the persistence of humanity." Circling the drain: the extinction crisis and the future of humanity — research article

Just like any other species, we rely on other animals and plants to survive. Without bees to pollinate our crops for example, three out of four crops would disappear from the shelves of our supermarkets. Why bees matter - FAO

That's just one species. As the mass extinction progresses, millions of species will be wiped out.

Which one will be the nail in the coffin for humanity? The extinction of livestock or crops that we depend on for food? Or the the extinction of a species that we would never know about, like a rare tropical plant that could have provided a cure for the next pandemic?

By the time we find out, it will be too late. Humans, like millions of other species, will have gone extinct.