Humanity has just about run out of time to address climate change. Scientists have pointed out that a rise in mean surface temperature of 2º Celsius above pre-industrial levels will put the Earth in dangerous, uncharted territory. Yet we currently are on a path toward an increase of 4º or more this century. The last chance for action has arrived. That chance lies in Paris in December 2015. Either governments will agree to decisive action, as they have promised, or we will look back at 2015 as the year when climate sanity slipped through our fingers.