Higher temperatures brought on by global warming will decrease crop yields in the Midwest by about 4 percent, according to a recent Department of Agriculture study. But increased levels of carbon dioxide in the atmosphere will offset some of that decline, increasing crop yields.The Des Moines Register's Philip Brasher writes Sunday of the tradeoffs for agriculture in the gradual warming of the atmosphere. Brasher reports: "The USDA study estimates Midwest soybean yields could increase by nearly 10 percent over the next 30 years, mostly because of the increased carbon dioxide levels. Corn yields would fall 3 percent. The increased carbon dioxide only slightly offsets the impact of increased heat on corn."Other scientists believe the lab results showing increased yields from carbon dioxide will be lessened in the fields And, "There is a dark side to the increase in carbon dioxide, too: a potential increase in weeds that are resistant to herbicides. Think weeds on steroids."