利用大气CO2浓度倍增试验探讨暖干与暖湿

自工业革命以来，大气CO2浓度持续升高；现代观测和未来预估都显示中国气候有变干的趋势。然而，地质重建记录显示，温暖时期往往伴随着湿润的气候条件，与现代观测和未来预估存在明显差别。地质记录反映的是平衡态下的气候，而现代观测或未来预估反映非平衡态下的气候变率。这一古气候与现代气候研究的差别，可能和气候态有关，但目前仍缺少深入研究和合理解释。文章利用挪威地球系统模式NorESM1-F，开展大气CO2浓度倍增的瞬变试验和平衡态试验，并用Sc-PDSI（Self calibrating-Palmer Drought Severity Index）指示非平衡态条件下和平衡态条件下中国的干湿变化。模拟结果表明：随着大气CO2浓度的升高，无论在非平衡态还是平衡态条件下，中国的年均降水量增加；但温度升高的同时潜在蒸散发加剧，使得我国气候整体变干。上述古气候"暖湿"与现代气候"暖干"之间的矛盾，源于二者对干湿定义的差别。古气候记录显示的暖湿，主要反映环境湿度（降水量、土壤湿度、植被量）在温暖的条件下增加；而现代观测显示的暖干，揭示的是中国的气象干旱→降水减去蒸发→在温暖的条件下加剧。

Investigation of warmer-drier and warmer-wetter mismatch in double atmospheric CO2 concentration experiments

Modern observations and future projections demonstrate a drying trend in China with increased atmospheric CO2 concentrations since the Industrial Revolution. However, many geological reconstructions show that climate is often wetter in warmer conditions, differing from the modern observations and future projections. The geological reconstructions reflect an equilibrated situation, whereas modern climate studies consider climate variabilities in a transient condition. This mismatch is likely relevant to the unequilibrated or equilibrated climate backgrounds but still needs further investigations. Here, we use the Norwegian Earth System Model(NorESM1-F) to conduct numerical experimental ensembles, in which atmospheric CO2 concentrations increase by one percent per year and remain constant after doubling. We further analyze the hydroclimate changes using the Self calibrating-Palmer Drought Severity Index(Sc-PDSI). Our simulations show that doubling atmospheric CO2 concentration leads to increased meteorological drought in China under both unequilibrated or equilibrated climate backgrounds. Although annual precipitation in the China region increases with the rise of atmospheric CO2 concentration at a rate of 0.413 mm/a, the increased precipitation does not necessarily indicate that the climate becomes wetter. With increased rainfall, surface air temperature also rises, leading to enhanced annual potential evapotranspiration at a rate of 1.234 mm/a. Since the rate of potential evapotranspiration increase is greater than the rate of precipitation change, meteorological drought is intensified in China. The above differences between paleoclimate and modern climate studies stem from their definitions of dryness and wetness, reflecting different aspects of the same change. Modern climate studies employ the definition of meteorological drought that indicates the residual between precipitation and evaporation. In contrast, paleoclimate studies emphasize rainfall, humidity, or vegetation, which generally increases in a warm climate.