In a simple but important analysis, Mora and colleagues analyzed climate projections to identify when future warming will exceed the climate envelope of the past 150 years. They used multiple models (39) and seven climate variables (such as near-surface temperature and precipitation), to quantify global historical climate variability. They then used the future climate projections to identify when the future climate is expected to ‘depart’ from the historical envelope. In other words, in which future year should we expect a ‘cold’ year to be ‘hotter’ than any we’ve experienced over the past 1.5 centuries? The answer was surprisingly soon, 2047 (±14 years s.d.), for a ‘business-as-usual’ emissions scenario (RCP8.5). Furthermore, the ‘climate departure’ date is expected to occur soonest in the tropics and among low-income countries (due to low historical inter-annual variability in those regions). In addition to the ‘business-as-usual’ scenario, they did the same analysis for an emissions scenario that leads to stabilization of atmospheric carbon dioxide concentrations at 4.5 W/m2 by 2100 (RCP 4.5). The more optimistic scenario delays the date of climate departure by about 20 years (2069±18).
Mora, Camilo, Abby G. Frazier, Ryan J. Longman, Rachel S. Dacks, Maya M. Walton, Eric J. Tong, Joseph J. Sanchez, et al. 2013. “The Projected Timing of Climate Departure from Recent Variability.” Nature 502 (7470) (October 10): 183–187. doi:10.1038/nature12540.