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Evidence suggests that the end-Permian extinction, 251 million years ago, may have been triggered by a rise of temperatures of 6 °C: it almost wiped out life on Earth. Forests, swamps and savannahs were washed away, land turned to desert; the warming oceans, which lose oxygen as temperatures rise, would have become stagnant and toxic. Some scientists believe the warming of the ocean would have been enough to trigger the release of huge clouds of methane from the sea bed, poisoning the atmosphere. Life on Earth did survive 6 °C of warming, but those changes took place over 10,000 years. Human releases of carbon dioxide are almost certainly happening faster than any natural releases since the beginning of life on Earth.

Writer Mark Lynas has studied historical records to examine the potential effects of temperature rises of up to 6 °C. Here is a summary of his six-degree scenario:

+1 °C: Deserts spread across parts of the United States, turning farmland to dust from Canada in the north to Texas in the south. The Gulf Stream could switch off – plunging Europe into an icy winter. Coral reefs around the world are wiped out.

+ 2 °C: Oceans turn increasingly acidic, killing off plankton and affecting sea life. European summers are plagued by heat waves as strong as the killer of 2003. Wildfires spread around the Mediterranean. Greenland tips into irreversible melt, accelerating sea-level rise and threatening coastal cities. The polar bear and walrus become extinct.

+ 3 °C: Deserts spread across parts of Africa, driving millions of refugees to surrounding countries. A permanent El Niño grips the Pacific, causing weather chaos worldwide. Drought and wild fires rage across the Amazon, destroying swathes of forest, releasing yet more carbon. World food running short. Water shortages threaten parts of India, Pakistan, Australia and Peru.

+4 °C: Tens of millions become refugees as rising waters threaten the Nile Delta and low-lying Bangladesh. The West Antarctic ice sheet collapses, pumping 5 metres of water into global sea levels. Southern Europe becomes like the Sahara, with deserts spreading in Spain and Portugal.

+5 °C: The Earth is hotter than at any time for 55 million years. Desert belts expand across Europe, America and Asia. Some populations try to move towards the poles. Most of the world is uninhabitable.

+6 °C: Huge firestorms sweep the planet as methane hydrate fireballs ignite. Seas release poisonous hydrogen sulphide. Most of life on Earth has been extinguished. Humanity’s survival is in question.