Do you want to know the real reason for the advances by Isis in Iraq and Syria? Changing lightbulbs in America. This is the explanation given by John McCain, Republican chair of the Senate armed services committee. At the weekend he blamed Barack Obama's inability to magic away Islamic State on the president's belief that climate change is "the biggest enemy we have". Never mind the role of the Iraq war — which Mr McCain supported — in destabilising the region, destroying the Iraqi army and creating the opportunities Isis has exploited. Never mind the propagation of Salafi doctrines by Saudi Arabia, which McCain bravely confronts by grovelling before its tyrants. It's the Better Buildings Challenge and the Solar Instructor Training Network that allowed Isis to capture Ramadi and Palmyra.
In fact there is a connection, but it strengthens Obama's contention that "climate change constitutes a serious threat to global security". One of the likely catalysts for the 2011 uprising in Syria was a massive drought — the worst in the region in the instrumental record — that lasted from 2006 to 2010. It caused the emigration of one and a half million rural workers into Syrian cities, and generated furious resentment when Bashar al-Assad's government failed to respond effectively. Climate models suggest that man-made global warming more than doubled the likelihood of a drought of this magnitude.
But this is nothing by comparison to the real threats to global security; in fact, to threats that make global security, as understood by McCain and Obama, look almost frivolous. As the evidence accumulates, it now seems that climate change was the commonest cause of mass extinction in the Earth's prehistory.
In the media, if not the scientific literature, global catastrophes have long been associated with asteroid strikes. But as the dating of rocks has improved, the links have vanished. Even the famous meteorite impact at Chicxulub in Mexico, widely blamed for the destruction of the dinosaurs, was out of synch by over 100,000 years.
The story that emerges repeatedly from the fossil record is mass extinction caused by three deadly impacts, occurring simultaneously: global warming, the acidification of the oceans and the loss of oxygen from seawater. All these effects are caused by large amounts of carbon dioxide entering the atmosphere. When seawater absorbs CO2, its acidity increases. As temperatures rise, circulation in the oceans stalls, preventing oxygen from reaching the depths.
The great outgassings of the past were caused by volcanic activity that was orders of magnitude greater than the eruptions we sometimes witness today. The dinosaurs appear to have been wiped out by the formation of the Deccan Traps in India: an outpouring of basalt on such a scale that one river of lava flowed for 1500km. But that event was dwarfed by a far greater one, 190 million years earlier, that wiped out 96% of marine life as well as most of the species on land. What was the cause? It now appears that it might have been the burning of fossil fuel.
Before I explain this extraordinary contention, it's worth taking a moment to consider what mass extinction means. This catastrophe, at the end of the Permian period 252 million years ago, wiped out not just species within the world's ecosystems, but the ecosystems themselves. Forests and coral reefs vanished from the fossil record for some 10 million years. When, eventually, they were reconstituted, it was with a different collection of species, that evolved to fill the ecological vacuum. Much of the world's surface was reduced to bare rubble. Were such an extinction to take place today, it would be likely to eliminate almost all the living systems that sustain us. When plants are stripped from the land, the soil soon follows.
The latest research into the catastrophe at the end of the Permian is summarised in two articles by the geologist John Mason on the Skeptical Science site. The strongest clues all seem to point to the same conclusion: that the extinctions were triggered by the eruption of an igneous belt even bigger than the Deccan plateau: the Siberian Traps.
As well as CO2, the volcanoes there produced sulphur dioxide, chlorides and fluorides, causing acid rain and the depletion of ozone. But because the residence time of carbon dioxide in the atmosphere is greater than that of these other gases, it's likely to have been the major cause of extinction. The change of state — including a rise in oceanic temperatures of between six and ten degrees — was too sudden and sustained to permit the majority of lifeforms to adapt. The onset of mass extinction coincides with a giant carbon spike "so distinctive that it serves as a marker-horizon all over the world".
So where did the carbon dioxide come from? Some of it would have bubbled out of the magma. But, enormous as the eruptions were, this alone seems insufficient to account for either the total volume of emissions or the ratio of isotopes (the different atomic forms) of the carbon entering the atmosphere. Fossil fuel seems to fill the gap. The volcanoes exploded through the Tunguska sedimentary basin, cooking much of the coal, petroleum and methane it contained. Particles of coal fly ash have been found in rocks as far away as the Canadian Arctic. Rising temperatures might also have destabilised methane hydrates — a frozen form of natural gas — causing the kind of runaway feedback that terrifies some climate scientists today. Yes: the geological record suggests that fossil fuel burning might have eliminated most life on Earth.
And today? According to a paper published in 2013, the current rate of ocean acidification, caused by the burning of fossil fuels, is faster than at any time in the past 300 million years. During the Permian mass extinction, the eruption of the Siberian Traps through the Tunguska basin seems to have produced between one and two gigatonnes of carbon dioxide a year. Today fossil fuel burning produces 30 gigatonnes a year.
Isis? Global security? If anyone were to survive a mass extinction on the scale of the Permian catastrophe, they would look back and shake their heads, amazed that we could have considered such issues more important.