You would have to go back at least 15 million years to find carbon dioxide levels on Earth as high as they are today, a UCLA scientist and colleagues report in the online edition of the journal Science.

Synthetic fertilizers have dramatically increased food production worldwide. But the unintended costs to the environment and human health have been substantial. Nitrogen runoff from farms has contaminated surface and groundwater and helped create massive “dead zones” in coastal areas, such as the Gulf of Mexico. And ammonia from fertilized cropland has become a major source [...]