The year that is ending witnessed a remarkable economic performance. It began with robust growth in all the world's major developed and developing economies. Politicians rejoiced as just-released figures for the third quarter of 2017 showed that Japan's economy grew an annualized 2.5 percent, the U.S. expanded 3.3 percent, China grew 6.9 percent and India's economy expanded 6.3 percent, to name but four top performers. It is concluding, however, with plunges in markets worldwide and widespread expectations that 2019 could be the worst year since the Great Recession of 2009. Policymakers must begin preparing for the anticipated downturn, ensuring that they have the tools to respond. In some cases, that means ignoring the fulminations of politicians whose interests are more short-term and whose preferences could prove dangerous.

The end of the year is traditionally a time for markets to gain ground, but Wall Street appeared to be on track to have its worst December since 1931 — until Wednesday's big rally. Many sectors have fallen 20 percent, entering what is officially a "bear market." In Tokyo, the Nikkei 225 average remains well below where it ended the previous year. Markets elsewhere in the region have been similarly battered: Shanghai has lost 25 percent of the value of its composite index, while that in Hong Kong is 10 percent off.

More than half the fund managers in one global survey said they expect the global economy to weaken in the year ahead, the worst outlook since 2008. The International Monetary Fund has already cut its global growth forecasts — and there are warnings it will do so again in January — and economists who anticipated a recession in 2020 now fear it may arrive next year instead.