With all the focus on the potential future impacts of climate change and ocean acidification, it's possible to lose track of the fact that the CO 2 concentrations and temperatures have already risen significantly, and that we might already be seeing the impacts of climate change. In fact, plenty of evidence, from altered growing seasons and changed animal migrations, indicate that climate change is already having an effect on the biosphere.

Evidence is now beginning to build that things are hitting us where it hurts. Last year, data was published that indicated commercially important shellfish were already being hurt by ocean acidification. Now, a paper that will be released by Science later today indicates that rising temperatures are cutting into agricultural productivity.

The authors of the new paper have combined a variety of public databases that track crop locations, productivity, temperature trends, and changes in precipitation. They then focus on four specific crops that, in total, account for about 75 percent of the calories that humans consume: maize, wheat, rice, and soybeans.

Since 1980, there haven't been any statistically significant trends in precipitation. When it comes to temperature, however, the story was very different, with many countries experiencing temperature increases that were a standard deviation or more from the historic norm. That includes just over half of the soybean producing nations, 65 percent of rice and maize growers, and 75 percent of the countries that grow wheat. Notably absent from that list: the US, where the trends in temperature have been very minor.

Based on the crop yield models and data obtained by the authors, the effects of these temperature changes were somewhat mixed (the study focused on the 1980-2008 period). Compared to a projection in which the temperatures remained stable, rice productivity dropped in many areas where it's cultivated, but productivity at higher latitudes offset this, leaving the global impact minimal. The same was true for soybeans. Wheat and maize, however, were very different stories, with temperatures driving a 3.1 percent drop in productivity for maize, and a 4.9 percent drop for wheat.

For context, the authors note that this is one Mexico's worth of maize, and the equivalent to France's annual wheat crop. "Climate trends were large enough in some countries to offset a significant portion of the increases in average yields that arose from technology, CO 2 fertilization, and other factors," the authors conclude.

But carbon dioxide impacts plant growth through mechanisms other than climate. The authors reran their calculations using data that estimates the boost in productivity that's gained from the additional CO 2 , which plants require for photosynthesis. As long as water and fertilizers aren't constrained, rice and soybeans would have actually seen productivity gains during the study period, and some of the loss of wheat productivity would be offset. Maize is not affected by increased carbon dioxide.

Overall, if you exclude the carbon fertilization, temperature changes have translated into an average price increase of 19 percent on the global agricultural market. The offset of carbon fertilization drops that figure to 6.4 percent.

As the authors emphasize, these changes are taking place against a backdrop of technological innovation that has generally sent agricultural yields upwards. The work does suggest, however, that we may not be seeing the full benefit of these changes. It also does not account for extreme precipitation and temperature events, which can cut into yields dramatically, but won't be visible in the monthly averages used by the authors.

The key question that the study doesn't address is whether the agricultural community is beginning to respond to some of the challenges posed by climate change. This is best illustrated by the example of Russia, where the authors calculate that temperature changes have knocked 15 percent off the annual wheat yield. Rising temperatures have the potential to turn Russia into one of the agricultural winners in a warming climate, but will require that farmers and the infrastructure they need relocate to optimal growing areas. If they continue to farm in existing locations, then the sorts of losses seen in this study could mount.

Science, 2011. DOI: 10.1126/science.1204531 (About DOIs).