The network of waterways in the Amazon River Basin emits three times as much carbon dioxide into the atmosphere as previously thought, according to a study appearing in the current edition of Nature. The finding suggests that tropical forest regions are not carbon “sinks” that help cleanse the world of excess CO2 emissions. Rather, the Amazon region produces about 2 trillion pounds of carbon dioxide every year — or about one-fifth as much as is generated annually by deforestation, burning fossil fuels, and other human activities. About 20 percent of the Amazon emissions is released from aquatic processes in the waterways, while the rest comes from soil and other organic matter swept into the river by rains and floods. The study is the second in recent months to challenge the received wisdom about carbon absorption in forested areas.