BERLIN/LONDON—European automakers reeling from a global chip shortage have in recent days hastened to assure shareholders that shortages of magnesium are not, for now, a risk to their production plans. However, car part suppliers and industry associations warn that if production in China, which accounts for 85 percent of global magnesium supply, does not pick up pace soon, automakers could be facing significant shortfalls of the material. Car makers do not ordinarily purchase large quantities of magnesium themselves. But part manufacturers supplying automakers with key components are heavily reliant on magnesium, which can be used by itself or combined with aluminum to reduce the weight of cars. Making vehicles lighter is particularly useful for extending the driving range of electric vehicles, crucial for Europe’s energy transition plans. Chip shortages have already extended delivery times for new cars, sometimes up to a year. Auto firms Stellantis and BMW say they do …