The East Asian summer monsoon (EASM) is the most important climate system over East Asia, providing about 40 - 50% of the annual mean precipitation over southern China. Understanding the mechanisms responsible for the EASM’s decadal weakening is important. Natural factors (solar variability, volcanic eruptions, and internal variability), and anthropogenic factors (greenhouse gas and anthropogenic aerosols) may play a key role. A team of scientists, including a DOE researcher at Pacific Northwest National Laboratory, examined the responses of EASM to the natural and anthropogenic factors simulated in the 17 most recent Coupled Model Intercomparison Program phase 5 models in the Intergovernmental Panel on Climate Change’s Assessment Report 5. The team found that aerosol forcing plays a primary role in driving the weakened low-level monsoon circulation. The preferential cooling over continental East Asia caused by aerosols affects the monsoon circulation through reducing the land-sea thermal contrast and results in higher sea-level pressure over northern China. In the upper level, both natural forcing and aerosol forcing contribute to the observed southward shift of East Asian subtropical jet through changing the meridional temperature gradient. The weakening of EASM circulation is partly reproduced in simulations under all-forcing runs. A comparison of separate forcing runs shows that the aerosols play a primary role in the weakening of EASM in the all-forcing run. The contribution of natural forcing is nearly negligible. The greenhouse gas forcing influences a slightly enhanced rather than weakened monsoon circulation. The combined effects of greenhouse gasses and aerosols make East Asia cooler than other neighboring regions.