Warm nights could flood the air with C[O.sub.2].

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The warming effects of climate change usually conjure up ideas of parched and barren landscapes broiling under a blazing sun, its heat amplified by greenhouse gases, but a study led by researchers at Princeton (N.J.) University suggests that hotter nights actually may wield much greater influence over the planet's atmosphere as global temperatures rise--and eventually could lead to more carbon flooding the atmosphere.

Since measurements began in 1959, nighttime temperatures in the tropics have had a strong influence over year-to-year shifts in the land's carbon-storage capacity, or "sink," the researchers report in Proceedings of the National Academy of Sciences. Earth's ecosystems absorb about 25% of the excess carbon from the atmosphere, and tropical forests account for about one-third of land-based plant productivity.

During the past 50 years, the land-based carbon sink's "interannual variability" has grown by 50%100%. The researchers used climate- and satellite-imaging data to determine which of various climate factors--including rainfall, drought, and daytime temperatures--had the most effect on the carbon sink's swings. They found the strongest association with variations in tropical nighttime temperatures, which have risen by about 0.6[degrees]C since 1959.

First author William Anderegg, associate research scholar at Princeton's Environmental Institute, explains that he and his colleagues determined that warm nighttime temperatures lead plants to put more carbon into the atmosphere through a process known as respiration. Just as people are more active on warm nights, so, too, are plants. Although plants take up carbon dioxide from the atmosphere, they also internally consume sugars to stay alive. That process, known as respiration, produces carbon dioxide...

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