The 2010 Russian heat wave that killed thousands and cut into that country’s grain harvest was primarily due to natural variability, not human-spurred climate change, U.S. scientists said on Wednesday.

There was plenty of circumstantial evidence pointing to human-caused greenhouse gas emissions and the resulting rise in carbon dioxide in the atmosphere, but close investigation showed this was not a major factor, the scientists said in research published online in Geophysical Research Letters.

“It was an off-the-charts intensity event,” Randall Dole of the U.S. National Oceanic and Atmospheric Administration said at a telephone news briefing. “It certainly was the most extreme event we had seen, dating back to at least 1880,” when modern weather record-keeping began.