When Dr. Roy Spencer looked up summer temperature data for the U.S. Corn Belt, it showed no warming trend for over a century. But that was before temperatures were “adjusted” by National Oceanic and Atmospheric Administration climate scientists — now the same data shows a significant warming trend.

Spencer, a climate scientist at the University of Alabama in Huntsville, said that the National Climatic Data Center made large adjustments to past summer temperatures for the U.S. Corn Belt, lowering past temperatures to make them cooler. Adjusting past temperatures downward creates a significant warming trend in the data that didn’t exist before.