Abstract
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 °C, by strong solar radiation is 0.14 °C, and by decreased volcanic activity is 0.07 °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% °C−1, by strong solar radiation is 2.8% °C−1, and by decreased volcanic forcing is 5.5% °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.
Highlights
Prediction of global monsoon precipitation (GMP) change is important for our daily life and for the whole society due to its effect on about two-thirds of the world’s population
We investigate GMP changes in response to individual greenhouse gas (GHG), solar and volcanic forcing in this study, by performing millennium experiments using Community Earth System Model version 1.0 (CESM1)
The cold Little Ice Age (LIA) and warm present warm period (PWP) can be simulated in each experiment forced by individual external forcing (Figure 1), and the warm PWP’s major contribution comes from the GHG forcing
Summary
Prediction of global monsoon precipitation (GMP) change is important for our daily life and for the whole society due to its effect on about two-thirds of the world’s population. Under the global warming induced by strong GHG concentration, the GMP was found to be enhanced from 2006 to 2100 under the Representative Concentration Pathway (RCP) 4.5 scenario in many GCMs [17] This intensification is mainly caused by the increasing land–ocean and inter-hemispheric thermal contrast due to the GHG forcing [6,17,18]. By analysing the observation and CMIP5 model results, Polson et al mentioned that the observed changes in NH summer monsoon precipitation over the second half the 20th century was mainly influenced by anthropogenic aerosols, not GHG or natural forcing [25]. We examine GMP differences between the present warm period (PWP) and LIA caused by external solar, volcanic and GHG forcing, respectively, by performing millennium sensitivity experiments.
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