Abstract. Climate in the Amazon region is particularly sensitive to surface processes and properties such as heat fluxes and vegetation coverage. Rainfall is a key expression of land surface-atmosphere interactions in the region due to its strong dependence on forest transpiration. While a large number of past studies have shown the impacts of large-scale deforestation on annual rainfall, studies on the isolated effects of elevated atmospheric CO₂ concentration (eCO₂) on canopy transpiration and rainfall are scarcer. Here for the first time we make a systematic comparison of the plant physiological effects of eCO₂ and deforestation on Amazon rainfall. We use the CPTEC-Brazilian Atmospheric Model (BAM) with dynamic vegetation under a 1.5xCO₂ and a 100 % substitution of the forest by pasture grassland, with all other conditions held similar between the two scenarios. We find that both scenarios result in equivalent average annual rainfall reductions (Physiology: −252 mm, −12 %; Deforestation: −292 mm, −13 %) that are well above observed Amazon rainfall interannual variability of 5.1 %. Rainfall decrease in the two scenarios are caused by a reduction of approximately 20 % of canopy transpiration, but for different reasons: eCO₂-driven reduction of stomatal conductance in Physiology; decreased leaf area index of pasture (−66 %) and its dry-season lower surface vegetation coverage in Deforestation. Walker circulation is strengthened in the two scenarios (with enhanced convection over the Andes and a weak subsidence branch over east Amazon) but, again, through different mechanisms: enhanced west winds from the Pacific and reduced easterlies entering the basin in Physiology, and strongly increased easterlies in the Deforestation. Although our results for the Deforestation scenario are in agreement with previous observational and modelling studies, the lack of direct field-based ecosystem-level experimental evidence on the effect of eCO₂ in moisture fluxes of tropical forests confers a considerable level of uncertainty to any projections on the physiological effect of eCO₂ on Amazon rainfall. Furthermore, our results highlight the responsibilities of both Amazonian and non-Amazonian countries to mitigate potential future climatic change and its impacts in the region driven either by local deforestation or global CO₂ emissions.

中文翻译：

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与亚马逊地区大规模森林砍伐一样，CO ₂施肥效应可能导致降雨减少

摘要。亚马逊地区的气候对地表过程和属性（例如热通量和植被覆盖度）特别敏感。由于降雨强烈依赖于森林蒸腾作用，因此它是该地区地表-大气相互作用的关键表达。尽管过去的大量研究表明大规模森林砍伐对年降雨量的影响，但对于大气CO ₂浓度升高（eCO ₂）对冠层蒸腾和降雨的孤立影响的研究却很少。在这里，我们首次对eCO ₂和森林砍伐对亚马逊降雨的植物生理影响进行了系统的比较。我们在1.5xCO下使用具有动态植被的CPTEC巴西大气模型（BAM）₂，牧场用100％的森林替代，两种情况之间的所有其他条件都相似。我们发现，这两种情况均导致等效的年均降雨量减少量（生理：-252毫米，-12％；森林砍伐：-292毫米，-13％），远高于观测到的亚马逊降雨年际变化5.1％。两种情况下的降雨量减少是由于冠层蒸腾量减少了约20％引起的，但由于不同的原因：eCO ₂导致气孔导度在生理上降低；减少森林砍伐的草叶面积指数（-66％）及其干旱季节下层地面植被的覆盖率。在两种情况下，沃克环流得到加强（安第斯山脉上的对流增强和东亚马逊河上的弱沉降分支增强），但是又通过不同的机制：来自太平洋的西风增强，生理学上进入盆地的东风减少，并且强烈增加森林砍伐中的东风。尽管我们针对毁林情景的结果与先前的观察和建模研究相符，但是缺乏基于现场直接生态系统水平的实验证据来证明eCO ₂的影响热带森林湿气通量的变化，给任何关于eCO ₂对亚马逊雨量生理影响的预测都带来了相当大的不确定性。此外，我们的研究结果强调了亚马逊地区和非亚马逊地区国家在缓解潜在的未来气候变化及其因当地森林砍伐或全球CO ₂排放而对区域造成的影响方面的责任。