Detecting and attributing nonlinear anthropogenic regional warming in southeastern Australia

Abstract

Nonlinear anthropogenic warming is detected and attributed as a series of step changes in observed and simulated climate for southeastern Australia (SEA). A stationary period of 1910–1967 and non‐stationary period of 1968–2010 was established using statistically significant step‐changes (pH0 < 0.01) in the relationship between observed minimum (Tmin) and maximum (Tmax) temperature (0.6°C in 1968) and Tmax and rainfall (P; 0.7°C in 1997). Regressions between these pairings during stationary conditions were used to determine how Tmin and Tmax would have evolved under non‐stationary conditions. Assuming these relationships remain constant, the resulting residuals were attributed to anthropogenic regional warming. This warming was initiated as step changes in 1968 forTmin (0.7°C) and 1973 for Tmax (0.5°C), coinciding with step changes in zonal (24–44°S) and southern hemisphere mean air temperatures (Tav). A step change in 1997 in Tmax (0.8°C) coincided with a statistically significant step change in global mean air temperature of 0.3°C. This analysis was repeated using regionally averaged output from eleven climate model simulations. Regional warming in all models commenced with step changes in Tmin ranging from 0.4 to 0.7°C between 1964 and 2003. Tmax underwent step changes ranging from 0.7 to 1.1°C simultaneously or within several decades. Further step changes, combined with rising trends, were simulated under increasing radiative forcing to 2100. This highlights limitations in the current use of the signal‐to‐noise model that considers anthropogenic climate change as a monotonic curve. The identification of multiple step changes in a changing climate provides important information for planning adaptation.