China's economy is slowing and facing increasingly powerful headwinds. Economists have been watching evidence pile up for some time, but Apple CEO Tim Cook's warning that sales of its iPhone have been slowing in China sent Apple shares, along with those of other technology companies, into a slide. Chinese authorities are trying to cushion the impact of a slowdown, but the causes are diverse, and sustained growth is increasingly difficult. Policy makers must prepare for troubles in China and protect their own economies from the fallout.

Cook was forced to revise downward Apple's growth forecasts because of slowing iPhone sales in China. In 2015, Apple boasted of multiple quarters in which revenue growth in greater China topped 100 percent per year; in November 2018, that number plummeted to 16 percent and it is expected to fall still further. Those grim numbers sparked a 10 percent drop in Apple's share price and worrying falls in the Dow Jones industrial average, Standard & Poor's 500-stock index and the Nasdaq composite index.

These problems are not Apple's alone. The stock price of Taiwanese companies that supply Apple quickly fell, as did Japanese manufacturers, such as Japan Display, which makes the liquid crystal displays for the iPhone XR. According to the Japan Machine Tool Builders' Association, orders from China of machine tools for electrical and precision machinery plummeted over 90.6 percent from the previous year in November. Consistent with that, manufacturing indexes across Southeast Asia have all fallen in the wake of Apple's announcement. This week, Samsung Electronics announced that it, too, would reduce its anticipated fourth-quarter profits. It blamed "mounting macro uncertainties" and "lackluster" demand for memory chips, which constitute a majority of its profits and more than a third of its sales.