a, Mean annual temperatures of an example grid cell (small square on map) exceed historical climate bounds (grey area) for three consecutive years starting in 2012 (blue arrow) and for 11 consecutive years after 2023 (green arrow); after 2036 (red arrow) all subsequent years remained outside the bounds (data from the Geophysical Fluid Dynamics Laboratory Earth System Model 2G). b, c, Effect of using different historical reference periods (b) and different numbers of consecutive years exceeding historical bounds (c) on the projected timing of climate departure from recent variability for global multi-model averages under RCP85. d, Comparison of the projected timing of climate departure from recent variability under RCP85, using the ‘historical’ and the ‘historicalNat’ experiments as reference to set the bounds of historical climate variability.
ABSTRACT: Ecological and societal disruptions by modern climate change are critically determined by the time frame over which climates shift beyond historical analogues. Here we present a new index of the year when the projected mean climate of a given location moves to a state continuously outside the bounds of historical variability under alternative greenhouse gas emissions scenarios. Using 1860 to 2005 as the historical period, this index has a global mean of 2069 (±18 years s.d.) for near-surface air temperature under an emissions stabilization scenario and 2047 (±14 years s.d.) under a ‘business-as-usual’ scenario. Unprecedented climates will occur earliest in the tropics and among low-income countries, highlighting the vulnerability 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 biodiversity and society are to be prevented.