Department of History, Stockholm University, SE-10691 Stockholm, Sweden.
Centre for Medieval Studies, Stockholm University, SE-10691 Stockholm, Sweden.
Nature. 2016 Apr 7;532(7597):94-8. doi: 10.1038/nature17418.
Accurate modelling and prediction of the local to continental-scale hydroclimate response to global warming is essential given the strong impact of hydroclimate on ecosystem functioning, crop yields, water resources, and economic security. However, uncertainty in hydroclimate projections remains large, in part due to the short length of instrumental measurements available with which to assess climate models. Here we present a spatial reconstruction of hydroclimate variability over the past twelve centuries across the Northern Hemisphere derived from a network of 196 at least millennium-long proxy records. We use this reconstruction to place recent hydrological changes and future precipitation scenarios in a long-term context of spatially resolved and temporally persistent hydroclimate patterns. We find a larger percentage of land area with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions are more widespread between the twelfth and nineteenth centuries. Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data across the Mediterranean, western USA, and China have operated consistently over the past twelve centuries. Using an updated compilation of 128 temperature proxy records, we assess the relationship between the reconstructed centennial-scale Northern Hemisphere hydroclimate and temperature variability. Even though dry and wet conditions occurred over extensive areas under both warm and cold climate regimes, a statistically significant co-variability of hydroclimate and temperature is evident for particular regions. We compare the reconstructed hydroclimate anomalies with coupled atmosphere-ocean general circulation model simulations and find reasonable agreement during pre-industrial times. However, the intensification of the twentieth-century-mean hydroclimate anomalies in the simulations, as compared to previous centuries, is not supported by our new multi-proxy reconstruction. This finding suggests that much work remains before we can model hydroclimate variability accurately, and highlights the importance of using palaeoclimate data to place recent and predicted hydroclimate changes in a millennium-long context.
准确地模拟和预测全球变暖背景下的局地到大陆尺度的水文气候响应至关重要，因为水文气候对生态系统功能、作物产量、水资源和经济安全有着重大影响。然而，水文气候预测仍然存在很大的不确定性，部分原因是可用的仪器测量数据长度较短，难以评估气候模型。在这里，我们利用至少 196 个跨越过去 12 个世纪的千年长代理记录网络，呈现了北半球过去 12 个世纪水文气候变化的空间重建。我们利用这一重建，将最近的水文变化和未来的降水情景置于长期的、空间分辨率高且时间持续性好的水文气候模式中。我们发现，在 9 到 11 世纪以及 20 世纪，有更大比例的陆地地区相对湿润，而在 12 到 19 世纪，干燥地区更为广泛。我们的重建结果显示，在过去的 12 个世纪里，在仪器数据中观察到的地中海、美国西部和中国之间明显的干湿气候交替模式始终存在。我们利用最新汇编的 128 个温度代理记录，评估了重建的过去 12 个世纪里，北大西洋地区的百年尺度水文气候与温度变化之间的关系。尽管在温暖和寒冷气候条件下，大面积的地区都出现了干燥和湿润的情况，但特定地区的水文气候和温度之间存在显著的共变关系。我们将重建的水文气候异常与耦合大气-海洋环流模型模拟结果进行了比较，发现工业化前时期两者吻合度较高。然而，与前几个世纪相比，模拟结果中 20 世纪的平均水文气候异常有所增强，而这一点并未得到我们的新多代理重建结果的支持。这一发现表明，在我们能够准确模拟水文气候变化之前，还有很多工作要做，并强调了利用古气候数据将近期和预测的水文气候变化置于千年时间尺度上的重要性。
