We calculate that 100 months from 1 August 2008, atmospheric concentrations of greenhouse gases will begin to exceed a point whereby it is no longer likely we will be able to avert potentially irreversible climate change. 'Likely' in this context refers to the definition of risk used by the Intergovernmental Panel on Climate Change (IPCC) to mean that, at that particular level of greenhouse gas concentration, there is only a 66 - 90 per cent chance of global average surface temperatures stabilising at 2o Celsius above pre-industrial levels.1 Once this concentration is exceeded, it becomes more and more likely that we will overshoot a 2o C level of warming. This is the maximum acceptable level of temperature rise agreed by the European Union and others as necessary to retain reasonable confidence of preventing uncontrollable and ultimately catastrophic warming. We also believe this calculation to be conservative. The reasons why and the assumptions behind our conclusion are detailed below. "