Climate change mitigation via a reduction in the anthropogenic emissions of carbon dioxide (C02) is the principle requirement for reducing global warming, its impacts, and the degree of adaptation required. Here, the trade-offs between delaying mitigation action and the strength of mitigation action required to meet particular atmospheric CO2 concentrations are explored using a conceptual model of emission trajectories and a simple Earth system model. The results show that avoiding dangerous climate change is more likely if global mitigation action commences as soon as possible and that starting mitigation earlier is also more effective than acting more aggressively once mitigation has begun, given realistic limits of rates of decarbonisation. A detailed examination of the latest datasets on CO2 emissions from the combustion of fossil fuels and cement production show a significant shift in the dominant drivers of global CO2 emissions, with a substantial growth in emissions from coal since 2002, and coal surpassing oil as the main source of emissions from fossil fuels in 2006. When compared to the Intergovernmental Panel on Climate Change (IPCC) scenarios, recent emissions are shown to be higher than five of the six Special Report on Emission Scenarios (SRES) emission scenarios, and the growth rate in emissions for 2000 to 2007 are higher than the growth rates for the current decade, in four of the six scenarios. If the post-2002 emissions continue, driven by a growth in coal which is the most carbon intensive fossil fuel, then the task of mitigation becomes more challenging, the importance of building adaptive capacity more pressing and calls into question whether mitigation alone is sufficient to meet the aspiration of avoiding dangeriiious climate change. Given the significant and widening gap between the current trajectory of CO2 emissions and the trajectory that would provide the greatest probability of avoiding dangerous climate change, there has been a resurgence of interest in geoengineering in recent years. Climate geoengineering seeks to rectify the current radiative imbalance via either (1) reducing incoming solar (shortwave) radiation or (2) removing CO2 from the atmosphere and transferring it to long-lived reservoirs, thus increasing outgoing longwave radiation. A critical review of the geoengineering literature shows that shortwave geoengineering can rectify a global radiative imbalance but ocean acidification and residual regional climate changes would still occur and the intervention could bring about unforeseen Earth system responses that may in turn increase the radiative imbalance. Creation of CO2 sinks (longwave geoengineering) involves less risk than shortwave geoengineering, as it acts upon the primary cause of the radiative imbalance and has a more limited capacity for `failure'. Geoengineering does not provide a `solution' to anthropogenic climate change. In order to meet the ultimate objective of the United Nations Framework Convention on Climate Change (UNFCC), demonstrable and significant mitigation action must get under way soon, with the creation of CO2 sinks a potential complement. The necesscity of undertaking geoengineering will ultimately be dicated by the magnitude of climate interference judged to be dangerous and the strength of mitigation agreed and adhered to by the international community in Copenhagen in December 2009