Roger Jones (Australia) 1; Jim Ricketts (Australia) 1
1 - Victoria University Melbourne
This presentation is the result of 25 years’ work on reconciling abrupt changes in past climates with projected gradual changes in future climate. The major question was ñwhy do we see so many abrupt changes in the past, but future warming is always presented as a gradual process?î
The main tool used was the bivariate test, used to check inhomogeneities in climate data for quality control. Abrupt changes in rainfall due in Australia caused by climate regime changes were detected. Analysing post-1997 shifts in SE Australian climate, the test showed that warming in both observations and regional model simulations was also step-like.
Expanded to global scale, this work showed that historical warming is dominated by shifts, and that in climate models, future warming follows suit. This suggests rival hypotheses: (1) change and variability are independent of each other and warming follows gradually increasing radiative forcing, or (2) that they interact and climate change is actually enhanced climate variability. These hypotheses were subject to severe testing, which is successful if one hypothesis passes with high probability, and its rivals can be ruled out with high probability. Trend-like change failed these tests; in 2017, the results were published in Earth System Dynamics.
Instead of atmosphere warming occurring in situ, all available heat is stored in the ocean and emitted as part of regime changes. These occur when climate becomes unstable and shifts to a new, higher energy regime. On decadal time scales, climate is a series of steady-state regimes, separated by shifts, over the longer term forming a complex trend. The theory and supporting evidence as to why will be presented.
For Europe’s mean annual temperature (GHCNv4), from 1910 there was little detectable warming until 1988, when warming increased by 0.5°C (p<0.01), followed by a further shift of 0.5°C in 1999 (p<0.01), and another of similar magnitude in 2014 (p<0.05). From the 1910_2000 anomaly, the 1910_1987 mean anomaly was -0.09°C, 1988_1999 0.45°C, 1999_2013 0.95°C and 2014_2017 1.5°C. The 1988 shift mainly occurred in May_October and the 1998 shift in November_April. The 2014 shift appears to be mainly in summer and fall.
Since 1910, the European continent has warmed by over 1.5°C and over 90% of that warming has occurred in less than 3% of the elapsed time. The implications for managing current and future risk in a two-speed climate will be discussed.