Implementing a climate action program in ten years is almost as expensive as starting a more ambitious effort to stop climate change today, according to a new study. The results come from the first large-scale comprehensive effort to simulate the future of climate that also takes into account uncertainty. Climate change itself, as well as the political, economic and technological measures that could be taken to combat it, are highly uncertain. We do not know exactly when some green technologies will be available, how much will they cost , how sensitive the global climate will be to a given change in greenhouse gas emissions, or the backdrop of global population growth and economic development against which all this will play out. (Sarah DeWeerdt)