Forget the K-Pg extinction that led to the demise of the dinosaurs 66 million-years-ago - the most devastating mass extinction in Earth’s history occurred 251 million-years-ago at the end of the Permian.

This event - appropriately nicknamed the Great Dying - is the closest life on our planet has ever come to being entirely extinguished.

The geological evidence suggests that the main cause of the extinction was climatic change.

Unmatched scale

Estimating the exact moment that the extinction took place has proven difficult. Unlike the clear iridium-rich clay layer that delineates the K-Pg event, most of the rock record for the end Permian has been eroded away by fluctuations in sea level.

Even the famous Meishan Section in China, although an invaluable resource to scientists, is too restrictive to offer a clear picture of what was happening at a global scale.

When compared with data collected from other sites around the world it is clear that the Great Dying was a series of extinction events that occurred during the Late Permian into the Early Triassic.

The scale of the extinction is unmatched by anything else in the geological record. Although there is little evidence to suggest that life itself was ever at risk of disappearing entirely, the statists are staggering.

It is estimated that up to 90 percent of all species became extinct within a few million years. The event led to the wholescale collapse of entire ecosystems as plankton and other producers disappeared.

Sea temperatures

Isotopic data shows intense warming at the end of the Permo-Triassic boundary, around 250 million-years-ago.

A 2012 study published in the journal Science reported a rise in temperature from 21°C to 36°C (the ocean temperature at the time of writing this article is about 17°C) in the Tethys, an ancient ocean that was situated at the equator, over an 800ky period.

A second rise in temperature in the Early Triassic lead to temperatures exceeding 40°C, a critical temperature for marine organisms.

Such extreme estimates have been criticized for methological flaws in the data. Despite this, the rise in ocean temperatures at the end Permian is well documented - it’s the extent of the warming that is subject of ongoing study.

Marine organisms with high oxygen demands - such as cephalopods - cannot survive long in waters exceeding 35°C. The extreme temperatures at the equator forced organisms to higher latitudes where conditions remained somewhat tolerable.

Global anoxia

The same phenomena is observed in terrestrial organisms. During the Permian and Triassic the continents were joined together to form a single landmass called Pangea.

The rise in ocean temperatures was likely the dominate factor driving the extinction, but it is not enough to account for the sheer magnitude of the event. Many researchers instead think that the extinction was driven by a complex range of factors.

Unoxidised carbon-rich minerals, notably pyrite, are characteristic of the end Permian occurring in marine deposits worldwide. Ocean waters at the time were structured with anaerobic bottom waters capped by oxygenated surface waters.

The warming of the oceans reduced the solubility of oxygen in the seawater, causing the concentration of the oxygen to decrease. This coupled with increased weathering of the continents - caused in part by higher sea levels - led to an influx of phosphates in to the oceans.

The increase in phosphates was good for life, in the short-term, because the extra nutrients supported more primary producers, notably plankton, in the oceans. But the increase in productivity was a double edged sword: as the increase in the amount of plankton meant that they sank to the bottom of the ocean where they further decreased the oxygen concentration.

Methane hydrates

The accumulation of organic matter on the seabed led to release of CO2 as the organics decayed. Such ocean anoxic events allowed the formation of organic rich shales, the source rocks for petroleum and natural gas.

This makes studying palaeoclimatology important not just from an environmental but also an economic perspective.

During cool periods methane gas becomes trapped at the bottom of the sea forming methane hydrate deposits. An increase in temperatures causes the trapped methane in be released, furthering global warming.

When released, methane rapidly degrades to CO2. This increases ocean acidity, anoxia, and contributes to global warming by releasing greenhouse gases in to the atmosphere. All three of these factors amplified the extinction rates.

The release of methane hydrates would also explain the negative carbon excursion seen at the end of the Permian. There is real concern that contemporary climate change may cause methane hydrates to melt, with a severe impact on the earth's climate.

Triggering factor

What triggered the global warming at the end of the Permian has been debated by researchers.

One theory suggests that the warming was triggered by a meteorite impact citing shocked quartz discovered in Antarctica. However, the shocked quartz appeared to have been plastic deformation structures more consistent with tectonic activity rather than a bolide impact. The meteor hypothesis is largely rejected by most scientists.

The most widely accepted explanation is volcanic activity. The Siberian Traps in Russia are a large expanse of basaltic lava with a volume roughly 4 million km3which formed during 300,000 years of continuous eruptions over a mantle plume.

The phenomenal amounts of greenhouse gases released would have caused major climatic changes. The amount of CO2 released by these volcanic eruptions would also account for the isotopic data.

Heeding a warning

The end Permian extinction should serve as a warning as to the dangers about extreme climatic change.

The release of greenhouse gases by volcanic activity led to warming of the oceans and the results release of trapped methane caused the climate to spiral out of control. Just replace volcanic with human in the previous sentence and you have our current situation.

As global temperatures continue to rise we may see a repeat of the end Permian, though nowhere near as extreme.

This Author

Jack Wilkin is a graduate research student at the Camborne School of Mines in the United Kingdom. His research focuses on the isotopic geochemistry of fossils from the Jurassic of Germany for paleoclimate studies.