A worrisome forecast for the world's crops

Studies on rising ozone pollution, shorter winters, and an expanding tropical belt do not bode well for agriculture.

By , Columnist

If you are concerned mainly with temperature when you think about climate change, your perspective is too narrow.

Think also about other atmospheric changes such as rising ozone pollution. A recent study indicates that its increasing harmful effect on plants could cut the global economic value of crop production by 10 to 12 percent by this century's end. The research projects that regions such as the United States, China, and Europe would become net food importers.

Think, too, of how plants respond to a warmer environment. New research shows that a longer growing season is not always beneficial. Or consider new evidence that the tropical zone already is expanding faster than computer-based climate simulations have forecast.

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These examples from the latest research make the point that ecologists trying to anticipate global change still have a lot to learn.

The new ozone projection was a shocker even for the Massachusetts Institute of Technology researchers who conducted it. The study projects that growing worldwide fossil fuel burning will boost global average ozone concentrations 50 percent by 2100 unless emissions are seriously restricted.

"Even assuming that best-practice technology for controlling ozone is adopted worldwide, we see rapidly rising ozone concentrations in the coming decades," study leader John Reilly explained when MIT announced the results in late October. "That result is both surprising and worrisome."

Effects will vary from region to region and over time. Taking everything into account – including breeding ozone-resistant crops and other adaptations – global crop production could still take a serious economic hit.

You might think that a longer growing season would be good for plants. Sometimes it is. But many plants need a winter chill to respond vigorously to an earlier spring. With too short a winter, those plants are slow to wake up. Xiaoyang Zhang with Earth Resources Technology in Camp Springs, Md., and colleagues see this phenomenon reflected in satellite data and climate records from 1982 to 2005.

In October, they reported in Geophysical Research Letters that, north of latitude 40 degrees north, shorter winters still give plants enough of a cold soak to have a vigorous spring green-up. South of 35 degrees north, however, plants don't hibernate long enough. They are slow to green-up when winter is over. The data show that the transition zone between the two regimes has been moving northward about 0.1 latitude degrees a year. The researchers expect this trend to continue as global warming proceeds.

Dian J. Seidel at the NOAA Air Resources Laboratory in Silver Springs, Md., and colleagues reported earlier this month on Nature Online that the tropical belt also has expanded faster than climate simulations have predicted. This poleward expansion could significantly alter such crucial elements as jet stream flow patterns and the course of storm tracks. The scientists note that "the edges of the tropical belt are the outer boundaries of the subtropical dry zones and their poleward shift could lead to fundamental shifts in ecosystems and human settlements."

Dr. Zhang and his team say their study "suggests that there is still much to be learned about this aspect of climate change."

That goes for every other aspect of the subject as well.

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