LONDON (Reuters) – A gradual rise by the Japanese yen in recent weeks culminated in a dramatic overnight surge — firing a warning shot for world markets and the global economy in 2019.

Historically, outsizes yen gains in short periods, such as the Russian default in 1998 and the global market meltdown in 2008, are a harbinger of stress for global markets. Market watchers say the yen’s latest ascent is a sign that the global economy is set for a rocky ride ahead.

Signs are growing that the global economy is headed for a slowdown. In an environment like that, the yen tends to thrive. Japan’s large current account surplus means global markets consider it a safe haven.

Global surveys this week showed activity in European and Chinese factories are slowing. And falling demand forced Apple (AAPL.O) to issue a rare cut in its sales forecast, sending tremors through global markets.

The yen’s roaring higher was a sign of just how widespread concern over the health of the global economy had become, said Ulrich Leuchtmann, head of FX research at Commerzbank.

Japanese investors tend to invest a large portion of their savings overseas, then bring the money home during extreme market stress, driving the yen higher.