A global rise in temperature of just 2°C would be enough to threaten life as we know it. But leading climate scientists think even this universally agreed target will be missed. Could dramatic action help?

It all seemed so simple in 2008. All we had was financial collapse, a cripplingly high oil price and global crop failures due to extreme weather events. In addition, my climate scientist colleague Dr Viki Johnson and I worked out that we had about 100 months before it would no longer be ‘likely’ that global average surface temperatures could be held below a 2°C rise, compared with pre-industrial times.

What's so special about 2°C? The simple answer is that it is a target that could be politically agreed on the international stage. It was first suggested in 1975 by the environmental economist William Nordhaus as an upper threshold beyond which we would arrive at a climate unrecognisable to humans. In 1990, the Stockholm Environment Institute recommended 2°C as the maximum that should be tolerated, but noted: ‘Temperature increases beyond 1°C may elicit rapid, unpredictable and non-linear responses that could lead to extensive ecosystem damage.’

To date [2007], temperatures have risen by almost 1°C since 1880. The effects of this warming are already being observed in melting ice, ocean levels rising, worse heat waves and other extreme weather events. There are negative impacts on farming, the disruption of plant and animal species on land and in the sea, extinctions, the disturbance of water supplies and food production and increased vulnerability, especially among people in poverty in low-income countries. But effects are global. So 2°C was never seen as necessarily safe, just a guardrail between dangerous and very dangerous change.

To get a sense of what a 2°C shift can do, just look in Earth's rear-view mirror. When the planet was 2°C colder than during the industrial revolution, we were in the grip of an ice age and a mile-thick North American ice sheet reached as far south as New York. The same warming again will intensify and accelerate human-driven changes already under way and has been described by James Hansen, one of the first scientists to call global attention to climate change, as a ‘prescription for long-term disaster’, including an ice-free Arctic.