The dry continental interior of Asia has remained arid throughout most of its geological history, yet the future of this unique ecosystem remains unclear. Here we use palynological and isotopic records to track vegetation and moisture throughout the warm early Eocene (57 to 44 million years ago) as an analogue for extreme atmospheric CO₂ scenarios. We show that rainfall temporarily doubled and replaced the regional steppe by forested ecosystems. By reconstructing the season of pedogenic carbonate growth, we constrain the soil hydrologic regime and show that most of this rainfall occurred during the summer season. This humid event is therefore attributed to an inland expansion of monsoonal moisture following the massive greenhouse gas release of the Palaeocene–Eocene Thermal Maximum as identified by a negative carbon isotope excursion. The resulting abrupt greening of the Central Asian steppe-desert would have enabled mammal dispersal and could have played a role in carbon cycle feedbacks by enhancing soil organic carbon burial and silicate weathering. These extreme Eocene proto-monsoons, albeit different from the topography-driven Asian monsoon today, highlight the potential for abrupt shifts in Central Asian rainfall and ecosystems under future global warming.

亚洲干燥的大陆内部在其大部分地质历史中一直保持干旱，但这一独特生态系统的未来仍不明朗。在这里，我们使用孢粉学和同位素记录来追踪整个温暖的始新世早期（57 至 4400 万年前）的植被和湿度，作为极端大气 CO ₂情景的类比。我们发现降雨量暂时增加了一倍，区域草原被森林生态系统取代。通过重建土壤碳酸盐生长的季节，我们限制了土壤水文状况，并表明大部分降雨发生在夏季。因此，这一潮湿事件归因于在古新世-始新世最热期（通过负碳同位素偏移确定）大量温室气体释放后季风水分的内陆扩张。由此产生的中亚草原沙漠突然绿化将使哺乳动物得以扩散，并可能通过增强土壤有机碳埋藏和硅酸盐风化而在碳循环反馈中发挥作用。这些极端的始新世原季风虽然与今天地形驱动的亚洲季风不同，但凸显了未来全球变暖下中亚降雨量和生态系统可能发生突变的可能性。