Ecological and societal disruptions by modern climate change are critically determined by the time frame over whichclimates shift beyond historical analogues. Here we present a new index of the year when the projected mean climate ofa given location moves to a state continuously outside the bounds of historical variability under alternative greenhouse gasemissions scenarios. Using 1860 to 2005 as the historical period, this index has a global mean of 2069 (618 years s.d.) fornear-surface air temperature under an emissions stabilization scenario and 2047 (614 years s.d.) under a ‘business-as-usual’scenario. Unprecedented climates will occur earliest in the tropics and among low-income countries, highlighting thevulnerability of global biodiversity and the limited governmental capacity to respond to the impacts of climate change.Our findings shed light on the urgency of mitigating greenhouse gas emissions if climates potentially harmful to biodiversityand society are to be prevented.

RCP: a novel probe design bias correction method for Illumina Methylation BeadChip