Climate extremes during the 20th and 21st centuries simulated by the CSIRO Mk3.6 climate model with anthropogenic and natural forcings

Abstract

Analyses of observed climate records around the world have shown a general trend over recent decades towards increases in the frequency of warm days and nights and decreases in the frequency of cold days and cold nights. In many regions, this has resulted in changes to the hydrological cycle. Studies have also shown that anthropogenic climate change will lead to more extreme events in many parts of the globe. However, it is unclear to what degree changes in radiative forcings, such as increased concentration of greenhouse gases, changed aerosol loading, solar variability and volcanic aerosols have contributed to the observed trends in climate extremes in various regions of the globe during the 20 th century. In this study, the CSIRO Mk3.6 coupled climate model was used to perform a number of attribution and experiments following the Coupled Model Intercomparison Project Phase 5 (CMIP5) experimental design. Results from these experiments were used to evaluate the relative impacts of various radiative forcings on climate extremes during the 20 th century. These experiments include an ensemble of simulations with all forcings (anthropogenic and natural), natural forcings only, and all forcings with anthropogenic aerosols held constant at pre-industrial levels. Using the simulated daily data from these experiments, climate extreme indices defined by the Australian Government Bureau of Meteorology were computed for the 1851 to 2005 period. These include a number of temperature based indices, such as warm spell duration, hot days and warm nights, as well as a number of precipitation based indices, such as daily rainfall intensity, heavy precipitation days and consecutive dry days. This analysis quantifies the relative contributions of various radiative forcing factors to the simulated changes in extreme indices for the 1971 to 2005 period. Trends in the climate extreme indices were computed to show the simulated changes across the globe and in the Australian region during this period. In addition, climate change projection experiments for the 21 st century have been performed using the CMIP5 experimental design and several Representative Concentration Pathways (RCPs) with the same model. Results from these experiments were used to compute the set of climate extreme indices for the 2006 to 2115 period. Changes in these climate extreme indices in the Australian region for 30-year periods centered at 2030, 2050 and 2070 compared to baseline period (1971 - 2000) have been analysed. Results from the analysis show that natural forcings cause only modest changes in the temperature and precipitation based extremes during the 20th century. Simulations with all forcings show statistically significant changes in the temperature based extremes over most land areas and these changes have greater magnitude than those due to natural forcings. Furthermore, the rates of increase of temperature based extreme indices are much greater in experiments accounting for anthropogenic forcings if only the greenhouse effect was considered without the cooling effect of anthropogenic aerosols. For example, the cooling impact of anthropogenic aerosols during the 1971 to 2005 period on the warming in global average mean screen temperature was 0.65°C. Less significant changes can been seen for precipitation based extremes compared to temperature based extremes for all simulations. However, the analyses show that enhanced greenhouse gases may have suppressed the hydrological cycle in the Australian region during later part of 20 th century, but this may have been counteracted by increased anthropogenic aerosols to varying degree. Projections of temperature based extremes show continuing warming in the 21 st century in the Australian region for all scenarios considered. These changes are consistent with the rate of increases in mean temperature. Projected changes in precipitation extremes show a strong tendency towards drying in the Australian region. However, extreme precipitation is projected to become more intense over tropical Australia.