Efforts to anticipate how climate change will affect future food availability can benefit from understanding the impacts of changes to date. Here we show that in the cropping regions and growing seasons of most countries, with the important exception of the United States, temperature trends for 1980-2008 exceeded one standard deviation of historic year-to-year variability. Models that link yields of the four largest commodity crops to weather indicate that global maize and wheat production declined by 3.8% and 5.5%, respectively, compared to a counter-factual without climate trends. For soybeans and rice, winners and losers largely balanced out. Climate trends were large enough in some countries to offset a significant portion of the increases in average yields 16 that arose from technology, CO2 fertilization, and other factors.