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Increased leaf abundance is a double-edged sword

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A new global assessment reveals that increases in leaf abundance are causing boreal areas to warm and arid regions to cool. The results suggest that recent changes in global vegetation have had impacts on local climates that should be considered in the design of local mitigation and adaptation plans. A substantial portion of the planet is greening in response to increasing atmospheric carbon dioxide, nitrogen deposition, global warming and land use change. The increase in leafy green coverage, or leaf area index (LAI), will hold important implications for climate change feedback loops, yet quantifying these impacts on a global scale can be challenging. Here, Giovanni Forzieri and colleagues analyzed satellite data of global LAI coverage between 1982 and 2011. Their results varied depending on the local biome, where increased LAI in boreal regions caused a reduction in surface albedo (reflection of sunlight), and thus resulted in a warming effect; in contrast, increased LAI in arid regions caused an increase in transpiration, and thus drove a cooling effect. What's more, the authors found that these relationships between LAI and surface biophysics were amplified up to five times under extreme warm-dry and cold-wet years. They estimate that, across about 60% of the global vegetated area, greening has buffered warming by about 14%; for the remaining areas, which mostly include boreal zones, LAI trends have amplified the raise in air temperatures, leading to an additional warming of about 10%.

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http://dx.doi.org/10.1126/science.aal1727

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