It may only be possible to achieve the target of limiting global warming to 2℃ above pre-industrial levels if carbon dioxide emissions begin to fall rapidly within the next two decades and eventually decrease to zero. This finding, published online in Nature Climate Change this week, highlights that the task of limiting warming to 2℃ is becoming increasingly daunting. Decisions about how soon, how quickly and by how much future carbon dioxide emissions are reduced will determine whether the internationally agreed goal of limiting global temperature rise to 2℃ can be met. Pierre Friedlingstein and colleagues used a climate model to explore which combinations of these three factors are consistent with meeting this goal over the coming millennium. They find that if the sensitivity of the climate system to carbon dioxide is approximately as expected, the target can only be attained if medium to high emission-reduction rates begin within the next 20 years and net emissions eventually fall to zero or become negative. If climate sensitivity is high, the authors show that the target is even less attainable.