Enhanced Global Monsoon in Present Warm Period Due to Natural and Anthropogenic Forcings

In this study, we investigate global monsoon precipitation (GMP) changes between the Present Warm Period (PWP, 1900-2000) and the Little Ice Age (LIA, 1250-1850) by performing millennium sensitivity simulations using the Community Earth System Model version 1.0 (CESM1). Three millennium simulations are carried out under time-varying solar, volcanic and greenhouse gas (GHG) forcing, respectively, from 501 to 2000 AD. Compared to the global-mean surface temperature of the cold LIA, the global warming in the PWP caused by high GHG concentration is about 0.42 degrees C, by strong solar radiation is 0.14 degrees C, and by decreased volcanic activity is 0.07 degrees C. The GMP increases in these three types of global warming are comparable, being 0.12, 0.058, and 0.055 mm day(-1), respectively. For one degree of global warming, the GMP increase induced by strong GHG forcing is 2.2% degrees C-1, by strong solar radiation is 2.8% degrees C-1, and by decreased volcanic forcing is 5.5% degrees C-1, which means that volcanic forcing is most effective in terms of changing the GMP among these three external forcing factors. Under volcanic inactivity-related global warming, both monsoon moisture and circulation are enhanced, and the enhanced circulation mainly occurs in the Northern Hemisphere (NH). The circulation, however, is weakened in the other two cases, and the GMP intensification is mainly caused by increased moisture. Due to large NH volcanic aerosol concentration in the LIA, the inter-hemispheric thermal contrast of PWP global warming tends to enhance NH monsoon circulation. Compared to the GHG forcing, solar radiation tends to warm low-latitude regions and cause a greater monsoon moisture increase, resulting in a stronger GMP increase. The finding in this study is important for predicting the GMP in future anthropogenic global warming when a change in natural solar or volcanic activity occurs.