Quantifying Asian summer monsoon responses to global warming target temperatures

Abstract

In this thesis, we quantify the climatic changes over Asian summer monsoon regions at global warming levels specified in the Paris Agreement, using available simulations from multiple global climate models. First, the changes in summer monsoon rainfall over South Asia and East Asia are assessed under the 1.5 °C and 2.0 °C global warming conditions using multiple atmospheric GCM simulations participating in the “Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI)” project. In particular, differences between 2.0 °C and 1.5 °C warmer worlds are examined to evaluate possible benefits of global warming mitigation. The summer mean and extreme precipitation is projected to increase over the Asian monsoon regions, which are considerably higher in the 2.0 °C simulations than in the 1.5 °C simulations. A moisture budget analysis reveals that an increase in atmospheric humidity and surface evaporation contributes majorly to the mean precipitation increase, under the additional warming conditions, with a good inter-model agreement. Changes in daily precipitation characteristics show that future extreme precipitation events are intensified and occur more frequently with stronger global warming levels. A significant relationship between surface air temperature and extreme intensity demonstrates that the enhanced moistening under larger warming conditions, through the Clausius–Clapeyron (C–C) relation, is a major driver of the intensification of precipitation extremes. When repeating the analysis using atmosphere-ocean coupled climate models (CMIP5 RCP8.5 scenario runs), results are largely consistent with those from HAPPI simulations, i.e., increase in mean and extreme precipitation and more frequent and intensified extreme events. Secondly, this thesis provides a first investigation of the difference in Asian monsoon changes at Paris Agreement target temperatures under equilibrium (EQ) versus transient (TR) warmer worlds using available CMIP5 (EQ: RCP2.6/4.5 scenario, TR: RCP8.5 scenario) simulations. Mean precipitation is projected to increase more under EQ than under TR conditions. A moisture budget analysis shows that evaporation is a major contributor to the precipitation difference (EQ – TR). In particular, anthropogenic aerosol reduction in EQ is found to induce additional local warming over Asia monsoon regions, resulting in larger evaporation than in TR. The intensity and frequency of extreme precipitation increase more in EQ world as well, more strongly over East Asia than over South Asia. Similarly to the HAPPI-based results, there is a strong temperature dependency of extreme precipitation intensity, in line with the C–C relation. An examination based on CO2 only forcing simulations indicates much smaller difference in Asian summer monsoon rainfall between EQ and TR warmer worlds (abrupt4xCO2 and 1pctCO2, respectively), supporting the dominant role of aerosol forcing differences. In addition, considering large inter-model difference in aerosol sensitivity, the sensitivity test is performed to the use of higher sensitivity models and also single model large ensemble simulations (CESM-LE). Results overall support our findings, reaffirming that both mean and extreme precipitation over the Asian monsoon regions would increase more in EQ warmer world than in TR warmer world. Thirdly, this thesis analyzes changes in the monsoon duration and the areal and population exposure to precipitation extremes at specific warming levels of the Paris Agreement using HAPPI, CMIP5, and CESM-LE simulations. In accordance with the increase in summer mean precipitation under global warming, the duration of monsoon seasons over the Asian monsoon region are projected to increase, more strongly over East Asia. It is found that the delay of monsoon retreat is a major contributor to the monsoon season expansion. With increased intensity and frequency of extreme events under global warming, areal and population exposure to the summer extreme precipitation events are expected to increase over the Asia monsoon region. Differences between 1.5 °C and 2 °C warmer worlds indicate that global warming mitigation will reduce the duration of summer monsoon and the risks of extreme precipitation events. Larger increases in monsoon duration and population exposure in EQ warmer world than in TR warmer world are observed over East Asia at 1.5 °C warming level. Results of this thesis demonstrate that future climate changes over the Asian summer monsoon regions are affected much by global warming levels, providing a quantitative assessment of avoided impacts by half a degree less warming. Also, it is shown that the future changes in Asian monsoon rainfall characteristics are highly dependent on global warming pathways, in particular, aerosol emission differences.