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Forests, it's often said, are the lungs of Earth.

Global warming is driving leafy tropical canopies close to temperatures where they can no longer transform sunlight and CO2 into energy, threatening total collapse if the thermometer keeps climbing, according to a study Thursday.

Global warming is driving leafy tropical canopies close to temperatures where they can no longer transform sunlight and CO2 into energy, threatening total collapse if the thermometer keeps climbing, according to a study Thursday.If tropical forest's average surface temperature warms 4C above current levels -- widely considered a worst-case scenario -- "we're predicting possible total leaf death," he said.

Global warming is driving leafy tropical canopies close to temperatures where they can no longer transform sunlight and CO2 into energy, threatening total collapse if the thermometer keeps climbing, according to a study Thursday.If tropical forest's average surface temperature warms 4C above current levels -- widely considered a worst-case scenario -- "we're predicting possible total leaf death," he said.

Only a tiny percentage of the leaves of the upper canopy have already crossed the threshold of 47 ° C. But this percentage could quickly increase because the leaves warm up faster than air. If air temperatures were to rise continuously by 0.03°C per year, this massive leaf mortality could occur in just over a century. The discovery of a small proportion of overheating canopy is "a warning sign" that must prompt action, says co-author Joshua Fisher of Chapman University in California. But there are already worrying signs in the Amazon, which is warmer than other forests.