Ever-higher costs to service mounting debt loads pose a major risk to governments around the world, economists have warned. Global debt rose by $10 trillion in the first half of the year to top $365 trillion, according to research published by the Institute of International Finance on Wednesday. State debts are rising as yields on medium- and long-term government bonds issued by a slew of the world’s biggest economies hit their highest levels in more than a decade — including in the U.S., Japan, France and the U.K. Rising yields reflect growing investor discomfort at rising interest rates, persistent energy...