Unchecked climate change could mean that the weather conditions hurting farmers this year will become increasingly common and result in higher costs for the federal government, according to a U.S. Department of Agriculture report.

The report, issued by the USDA’s Economic Research Service, found that if greenhouse gases are allowed to continue to increase, U.S. production of corn and soybeans—which are more susceptible to extreme heat during growing season—could decline as much as 80% in the next 60 years.

As...