PARIS—World food prices jumped 28 percent in 2021 to their highest level in a decade and hopes for a return to more stable market conditions this year are slim, the U.N.’s food agency said on Thursday. The Food and Agriculture Organization’s (FAO) food price index, which tracks the most globally traded food commodities, averaged 125.7 points in 2021, the highest since 131.9 in 2011. The monthly index eased slightly in December but had climbed for the previous four months in a row, reflecting harvest setbacks and strong demand over the past year. Higher food prices have contributed to a broader surge in inflation as economies recover from the coronavirus crisis and the FAO has warned that the higher costs are putting poorer populations at risk in countries reliant on imports. In its latest update, the food agency was cautious about whether price pressures might abate this year. “While normally high …