Global Monsoon in a Changing Climate

Monsoons, the most energetic tropical climate system, exert a great social and economic impact upon billions of people around the world. This chapter reviews recent progress in our understanding of the global monsoon (GM) system and its associated precipitation changes in the present and future warming climates. The GM can be viewed as an integrated system of all regional monsoons over the globe that are driven by solar forcing and bounded by the planetary-scale overturning circulation. The GM precipitation (GMP), defined as the total summer monsoon precipitation amount within the GM area (GMA), experienced multi-decadal variability in the twentieth century. The observed GMP over land shows a slightly increasing trend from 1900 throughout the 1940s, and then a downward trend from the 1950s until the end of the 1970s; there was no clear trend after 1980. The GMP over the ocean has had more uncertainty over the past three decades, and trends are inconsistent among different global rainfall datasets. In the twenty-first century, the GMP is expected to increase robustly, based on the projections by the state-of-the-art coupled models that participated in Phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). The change in GMP under global warming is primarily due to changes to the hydrological cycle induced by warmer temperatures. The increase in water vapor contributes positively to moisture convergence and surface evaporation over the GMA, but is partly offset by the weakening of the monsoon circulation.