Abstract. Large-scale reforestation, afforestation, and forest restoration schemes have gained global support as climate change mitigation strategies due to their significant carbon dioxide removal (CDR) potential. However, there has been limited research into the unintended consequences of forestation from a biophysical perspective. In the Community Earth System Model version 2 (CESM2), we apply a global forestation scenario, within a Paris Agreement-compatible warming scenario to investigate the land surface and hydroclimate response. Compared to a control scenario where land use is fixed to present-day levels, the forestation scenario is up to 2 °C cooler at low latitudes by 2100, driven by a 10 % increase in evaporative cooling in forested areas. However, afforested areas where grassland or shrubland are replaced lead to a doubling of plant water demand in some tropical regions, causing significant decreases in soil moisture (~5 % globally, 5–10 % regionally) and water availability (~10 % globally, 10–15 % regionally) in regions with increased forest cover. While there are some increases in low cloud and seasonal precipitation over the expanded tropical forests, with enhanced negative cloud radiative forcing, the impacts on large-scale precipitation and atmospheric circulation are limited. This contrasts with the precipitation response to simulated large-scale deforestation found in previous studies. The forestation scenario demonstrates local cooling benefits without major disruption to global hydrodynamics beyond those already projected to result from climate change, in addition to the cooling associated with CDR. However, the water demands of extensive forestation, especially afforestation, have implications for its viability given uncertainty in future precipitation changes.

中文翻译：

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全球森林扩张对全球和区域水文的影响

摘要。大规模重新造林、造林和森林恢复计划因其巨大的二氧化碳清除（CDR）潜力而作为气候变化减缓战略获得了全球支持。然而，从生物物理学的角度对造林的意外后果进行的研究还很有限。在社区地球系统模型版本 2 (CESM2) 中，我们在符合《巴黎协定》的变暖情景中应用了全球造林情景，以调查陆地表面和水文气候响应。与土地利用固定在当前水平的控制情景相比，造林情景到 2100 年低纬度地区的气温将下降 2°C，原因是森林地区蒸发冷却增加 10%。然而，草地或灌木丛被取代的绿化地区导致一些热带地区植物需水量增加一倍，导致土壤湿度显着下降（全球~5%，区域性5%~10%）和可用水量（全球~10%，区域性~10%）。 10-15%（区域性））森林覆盖率增加的地区。虽然扩大的热带森林上空低云和季节性降水有所增加，负云辐射强迫增强，但对大范围降水和大气环流的影响有限。这与之前研究中发现的模拟大规模森林砍伐的降水响应形成鲜明对比。除了与 CDR 相关的降温之外，造林情景还展示了局部降温效益，不会对全球流体动力学造成重大破坏，超出了气候变化已预测造成的影响。然而，鉴于未来降水变化的不确定性，广泛造林，特别是造林的需水量对其生存能力产生影响。