Twenty‐First Century Drought Projections in the CMIP6 Forcing Scenarios Earth's Future

There is strong evidence that climate change will increase drought risk and severity, but these conclusions depend on the regions, seasons, and drought metrics being considered. We analyze changes in drought across the hydrologic cycle (precipitation, soil moisture, and runoff) in projections from Phase Six of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble shows robust drying in the mean state across many regions and metrics by the end of the 21st century, even following the more aggressive mitigation pathways (SSP1-2.6 and SSP2-4.5). Regional hotspots with strong drying include western North America, Central America, Europe and the Mediterranean, the Amazon, southern Africa, China, Southeast Asia, and Australia. Compared to SSP3-7.0 and SSP5-8.5, however, the severity of drying in the lower warming scenarios is substantially reduced and further precipitation declines in many regions are avoided. Along with drying in the mean state, the risk of the historically most extreme drought events also increases with warming, by 200-300% in some regions. Soil moisture and runoff drying in CMIP6 is more robust, spatially extensive, and severe than precipitation, indicating an important role for other temperature-sensitive drought processes, including evapotranspiration and snow. Given the similarity in drought responses between CMIP5 and CMIP6, we speculate that both generations of models are subject to similar uncertainties, including vegetation processes, model representations of precipitation, and the degree to which model responses to warming are consistent with observations. These topics should be further explored to evaluate whether CMIP6 models offer reasons to have increased confidence in drought projections. Grant no. NA19OAR4310278

已有充分证据表明，气候变化会加剧干旱风险与干旱严重程度，但此类结论取决于所研究的区域、季节以及采用的干旱指标。本研究基于耦合模式比较计划第六阶段（CMIP6）的预估数据，分析了水文循环（hydrologic cycle）维度下的干旱变化特征，涵盖降水、土壤湿度与径流。多模式集合模拟结果显示，到21世纪末，即便在减排力度更强的共享社会经济路径（Shared Socioeconomic Pathways，SSP）1-2.6与SSP2-4.5情景下，全球多数区域在多数干旱指标下的平均干旱状况仍会呈现显著的干旱化趋势。干旱化显著的区域热点包括北美西部、中美洲、欧洲与地中海地区、亚马孙流域、非洲南部、中国、东南亚以及澳大利亚。但相较于SSP3-7.0与SSP5-8.5情景，低升温情景下的干旱化程度已大幅缓解，多数区域的降水进一步减少的情况也得以避免。除平均干旱状况加剧外，历史罕见极端干旱事件的发生风险也随升温而上升，部分区域的极端干旱风险增幅可达200%至300%。相较于降水变化，CMIP6模式中土壤湿度与径流的干旱化趋势更为显著、空间覆盖范围更广且程度更剧烈，这表明其他受温度调控的干旱过程，包括蒸散发（evapotranspiration）与积雪过程，发挥了重要作用。鉴于耦合模式比较计划第五阶段（CMIP5）与CMIP6模式在干旱响应特征上的相似性，本研究推测两代模式均面临类似的不确定性来源，包括植被过程、降水的模式表征能力，以及模式对升温的响应与观测结果的契合程度。需对上述议题展开进一步研究，以评估CMIP6模式是否能为提升干旱预估的可信度提供依据。项目编号：NA19OAR4310278