The global economy is gathering momentum, the International Monetary Fund has declared. That's probably correct and undeniably encouraging, but there's an ominous discord between this economic expansion and what's euphemistically called "political uncertainty" — that is, the stresses caused by surging anti-trade, anti-market, anti-immigrant populism.

This "uncertainty" could be the prelude to some seriously bad policies, enough to derail one or more leading economies and stall the global expansion. And there's another danger, less obvious but no less important: the prospect of chronic underperformance. Even if the new politics doesn't bring the ceiling down, it threatens to block the longer-term policies that would promote growth.

The current expansion is steady but unspectacular. The IMF's new forecasts say global output will rise by between 3 percent and 4 percent this year and next — slightly better than in 2016. So far, investors haven't taken fright over President Trump's anti-market rhetoric, or the implications of Brexit for the U.K. and the European Union, or any of the other sources of global instability. That could change in an instant, of course.

Yet even if all goes well, there's a problem. Advanced and emerging economies are settling into a pattern of growth that's disappointing by historical standards. This shortfall means persistent poverty, diminished opportunities and stagnant incomes for hundreds of millions of people. It helps explain the current state of politics.

This longer-term slowdown is starting to look permanent. Growth in productivity had already weakened in many advanced economies — under pressure from demographic trends, fewer breakthrough innovations and other forces. Then came the crash. Roughly a decade later, economies still haven't shaken it off. Lenders and investors are more cautious; governments are more heavily indebted; and central banks are still wrestling with unconventional monetary policy.

Smart economic policy can still help increase long-term growth — but it requires deliberate, intelligent action of the kind that the new politics makes far more difficult.

There are no quick fixes. Promote innovation. Add to human capital by improving schools and widening access to higher education. Create programs that help workers relocate across regions and industries. Build well-chosen infrastructure. Encourage competition. All these things take years to yield benefits. Patience is the watchword — and populism is, above all, impatient.

One more thing. Persistent tepid growth is especially difficult for central banks, because it implies correspondingly low interest rates. In the U.S., the normal interest rate — the one expected to prevail at full employment — used to be roughly 4 percent. Today it might be 2 percent or less. This means the Federal Reserve will have less room to cut interest rates when the next recession hits.

FROM AN EDITORIAL AT BLOOMBERG VIEW