First, they looked at historical trends in the relationship of temperature and precipitation on agriculture profits.
They then used a climate model to predict temperature and precipitation in the U.S. Plugging those weather predictions into their estimated profit equations gave the predicted increase in profits.
Their results couldn’t preclude the possibility that there will be no effect of global warming on agricultural profits. It seems the debate is between those that think there will be a positive impact and those that believe there will be no impact. There’s no reason to believe there might be a negative impact of global warming on American agriculture.