IN THE PAST 150 years, the concentration of carbon dioxide in the atmosphere has risen from 280 parts per million (ppm) to 410 ppm. For farmers this is mixed news. Any change in familiar weather patterns caused by the atmospheric warming this rise is bringing is bound to be disruptive. But more carbon dioxide means more fuel for photosynthesis and therefore enhanced growth-sometimes by as much as 40%. And for those in temperate zones, rising temperatures may bring milder weather and a longer growing season. (In the tropics the effects are not so likely to be benign.) What is not clear, though, and not much investigated, is how rising CO_2 levels will affect the relation between crops and the diseases that affect them.
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