That's how long we have before it is no longer likely that we will be able to avoid dangerous levels of anthropogenic climate change. The estimate is from the new economics foundation's 100 months campaign, and is based on current greenhouse gas concentrations and emissions trends. Here's the short version from the Guardian:
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.(The technical note explaining their methodology is here)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.
We found that, given all of the above, 100 months from today we will reach a concentration of greenhouse gases at which it is no longer "likely" that we will stay below the 2C temperature rise threshold. "Likely" in this context refers to the definition of risk used by the IPCC. But, even just before that point, there is still a one third chance of crossing the line.
What happens when we cross that line? According to the IPCC's Fourth Assessment Report, it means hundreds of millions of people facing food and water shortages, 30% of all species going extinct, the death of most coral reefs, and an increasing chance of positive feedback leading to runaway emissions. It will make the world a much worse place for us and our children, so its something we definitely want to avoid.
But can we? Yes, if we shift our economy to be less reliant on fossil fuels. And it can be done - the US for example can shift to almost completely renewable electricity by 2018, reducing global emissions by 8% and removing a major source of emissions growth. Other countries have similar opportunities. The quicker we act, the better our chances. And we should act quickly - because 2 degrees is just not a risk we can afford to take.
