SAN DIEGO — The mood among economic forecasters gathered for their annual meeting last weekend was dark. They warned one another about President Trump’s trade war, about government budget deficits and, repeatedly, about the inability of central banks to fully combat another recession should one sweep the globe anytime soon.

Among the thousands of economists gathered for the profession’s annual meeting, there was little celebration of Mr. Trump’s economic policies, even though unemployment is at a 50-year low, wages are rising and the economy is experiencing its longest expansion on record.

Underlying their sense of foreboding was a widespread sentiment that the current expansion is built on a potentially shaky combination of high deficits and low interest rates — and when it ends, as it is bound to do eventually, it could do so painfully.

Those concerns were echoed on Wednesday by economists at the World Bank, who called the worldwide expansion “fragile” in their latest “Global Economic Prospects” report. The report forecasts a slight uptick in growth in 2020 after a sluggish year bogged down by trade tensions and weak investment. But it said “downside risks predominate,” including the potential escalation of trade fights, sharp slowdowns in the United States and other wealthy countries and financial disruptions in emerging markets like China and India.