The people of the world get 75 percent of their sustenance—either directly, or indirectly as meat—from four crops: maize (corn), wheat, rice and soybeans. The world's rising population—now predicted by the United Nations to reach 10.1 billion by century's end—has been fed thanks to rising yields of all four of these crops during the past century. Humanity's predilection for burning fossil fuels, however, is now contributing to the slowing of such rising yields, cutting harvests of wheat 5.5 percent and maize 3.8 percent from what they could have been since 1980, according to a new analysis of yields.

"On a global scale, we can see pretty clearly significant changes in the weather for most places where we grow crops," explains agricultural scientist David Lobell of Stanford University's Woods Institute for the Environment, who led the analysis published in the May 6 issue of Science. "Those changes are big enough to sum up to pretty big losses for wheat and corn."

Using U.N. Food and Agriculture Organization data going back to 1980 for crop yields in all major crop-growing regions of the world, and pairing that with temperature and precipitation data for their growing seasons, Lobell and his colleagues found that warming temperatures were reducing yields—although changes in precipitation did not appear to be having an effect, yet.

Those temperature changes are the result of increasing concentrations of atmospheric greenhouse gases, such as carbon dioxide (CO2), largely as a result of burning fossil fuels and agricultural practices. But CO2 also helps rice, soybeans and wheat grow. In fact, the researchers suggest the extra CO2 boosted yields for these crops by roughly 3 percent during the period studied. Unfortunately, in the case of wheat, that wasn't enough to overcome the loss in yields resulting from warming temperatures. "Temperature effects are already overriding CO2 effects," Lobell notes.

Of course, this loss of yield translates directly into food prices, which have been rocketing upward in recent months and years. The new analysis suggests that the climate-related yield loss has contributed as much as 18.9 percent to the average price of a given crop during the period of the study. Climate change "is not disastrous but it's a multibillion-dollar-per-year effect already," Lobell says.

More troubling, further climate change is already locked in: Current CO2 levels imply warming of at least another degree Celsius by 2100. That means areas that have not been affected to date, such as the "corn belt" of the U.S., may soon see the same or worse impacts. "No climate change [in the U.S. corn belt] meant productivity improvements all went to increased yields. By contrast, in Europe the improvements [in yield] went to counteracting the effects of higher temperature," says agricultural economist Gerald Nelson of the International Food Policy Research Institute (IFPRI), who was not involved in the study but found it convincing. "We need to think seriously about breeding crops for dealing with higher temperatures."

Already, agricultural companies such as Monsanto are investing in developing such strains for maize and soybeans, but that leaves less profitable wheat and rice at the mercy of shrinking public sector agricultural research budgets. "Working on a trait like drought resistance is more complex than introducing a trait like insect resistance," says plant breeder Robert Reiter, vice president of biotechnology at Monsanto. "We don't have a lot of genes [to work with] that help produce more grain under water stress."

Nevertheless, Monsanto plans to begin field trials of such a hybrid corn next year, and hopes to introduce it for sale as early as 2013. "Our first gene close to commercialization—what it seems to be doing is helping the plant basically maintain more normal metabolic levels as opposed to trying to shut those processes down under stress," Reiter explains. "We may be taking [genetic] leads from corn and putting them into wheat to help it be drought tolerant and high-yielding."

But IFPRI's Nelson also noted that extreme weather at "fragile points" in a crop's growing cycle, such as high temperatures during the few weeks of flowering in maize and rice plants, could have big impacts. "There are these narrow windows where a small spike in temperature can have a big effect…. Agronomists know these sensitivities but they haven't been looking at them in the context of future climate change."

Adapting agriculture to face a hotter—and potentially drier—future has become a necessity. "We're not saying the sky is falling and food is becoming scarcer and scarcer," Lobell adds. "But there's a real drag on food production from climate change already."