The final countdown
Time is fast running out to stop irreversible climate change, a group of global warming experts warns today. We have only 100 months to avoid disaster. Andrew Simms explains why we must act now - and where to begin
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    •    Andrew Simms
    •    The Guardian, Friday August 1 2008

Planet earth viewed from space. Photograph: Corbis
If you shout "fire" in a crowded theatre, when there is none, you understand that you might be arrested for irresponsible behaviour and breach of the peace. But from today, I smell smoke, I see flames and I think it is time to shout. I don't want you to panic, but I do think it would be a good idea to form an orderly queue to leave the building.

Because in just 100 months' time, if we are lucky, and based on a quite conservative estimate, we could reach a tipping point for the beginnings of runaway climate change. That said, among people working on global warming, there are countless models, scenarios, and different iterations of all those models and scenarios. So, let us be clear from the outset about exactly what we mean.
The concentration of carbon dioxide (CO2) in the atmosphere today, the most prevalent greenhouse gas, is the highest it has been for the past 650,000 years. In the space of just 250 years, as a result of the coal-fired Industrial Revolution, and changes to land use such as the growth of cities and the felling of forests, we have released, cumulatively, more than 1,800bn tonnes of CO2 into the atmosphere. Currently, approximately 1,000 tonnes of CO2 are released into the Earth's atmosphere every second, due to human activity. Greenhouse gases trap incoming solar radiation, warming the atmosphere. When these gases accumulate beyond a certain level - often termed a "tipping point" - global warming will accelerate, potentially beyond control.

So, how exactly do we arrive at the ticking clock of 100 months? It's possible to estimate the length of time it will take to reach a tipping point. To do so you combine current greenhouse gas concentrations with the best estimates for the rates at which emissions are growing, the maximum concentration of greenhouse gases allowable to forestall potentially irreversible changes to the climate system, and the effect of those environmental feedbacks. We followed the latest data and trends for carbon dioxide, then made allowances for all human interferences that influence temperatures, both those with warming and cooling effects. We followed the judgments of the mainstream climate science community, represented by the Intergovernmental Panel on Climate Change (IPCC), on what it will take to retain a good chance of not crossing the critical threshold of the Earth's average surface temperature rising by 2C above pre-industrial levels. We were cautious in several ways, optimistic even, and perhaps too much so. A rise of 2C may mask big problems that begin at a lower level of warming. For example, collapse of the Greenland ice sheet is more than likely to be triggered by a local warming of 2.7C, which could correspond to a global mean temperature increase of 2C or less. The disintegration of the Greenland ice sheet could correspond to a sea-level rise of up to 7 metres.
In arriving at our timescale, we also used the lower end of threats in assessing the impact of vanishing ice cover and other carbon-cycle feedbacks (those wanting more can download a note on method from onehundredmonths.org). But the result is worrying enough.

Andrew Simms is policy director and head of the climate change programme at NEF (the new economics foundation). The material on climate models for this article was prepared by Dr Victoria Johnson, researcher at NEF on climate change. For regular suggestions for what individuals and groups can do to take action, and links to a wide range of organisations supporting the focus on the 100 months countdown, go to: onehundredmonths.org. The Green New Deal can be downloaded at neweconomics.org