Donald Trump's victory in the US election, and the rise in populist sentiment in Europe, does not explain the slowdown in global trade. The trade to GDP ratio had been rising steadily for many years but it has fallen in recent years. The decline in the trade to GDP ratio has little to do with rising populism. It is more easily explained by slower GDP growth.
One reason for slower GDP growth is a decline in investment spending which is caused by uncertainty and other factors. Investment spending is trade intensive because a relatively small number of nations produce high technology capital goods. When demand for capital goods decline global trade will decline.
The China growth miracle is over. That can only happen once. China's double digit GDP growth in over. China's raw material imports have normalized and so have its exports. GDP growth in China has become more dependent on internal consumption.
Global supply chain growth was fueled by transport and logistics efficiency. We are reaching the limit in transport and logistics efficiency. Production efficiency will continue to continue to increase. More goods will be produced domestically.
Donald Trump plans to stimulate the US economy by investing in infrastructure. That will promote GDP growth and stimulate trade but its not clear the politics in the US will allow the government to find productive infrastructure investments with our current level of polarization.