Department of Atmospheric, Oceanic, & Earth Sciences, Center for Ocean-Land-Atmosphere Studies, George Mason University, Fairfax, VA 22030;
Department of Geology and Geophysics, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 2017 Dec 5;114(49):12888-12893. doi: 10.1073/pnas.1703421114. Epub 2017 Nov 20.
During the warm Miocene and Pliocene Epochs, vast subtropical regions had enough precipitation to support rich vegetation and fauna. Only with global cooling and the onset of glacial cycles some 3 Mya, toward the end of the Pliocene, did the broad patterns of arid and semiarid subtropical regions become fully developed. However, current projections of future global warming caused by CO rise generally suggest the intensification of dry conditions over these subtropical regions, rather than the return to a wetter state. What makes future projections different from these past warm climates? Here, we investigate this question by comparing a typical quadrupling-of-CO experiment with a simulation driven by sea-surface temperatures closely resembling available reconstructions for the early Pliocene. Based on these two experiments and a suite of other perturbed climate simulations, we argue that this puzzle is explained by weaker atmospheric circulation in response to the different ocean surface temperature patterns of the Pliocene, specifically reduced meridional and zonal temperature gradients. Thus, our results highlight that accurately predicting the response of the hydrological cycle to global warming requires predicting not only how global mean temperature responds to elevated CO forcing (climate sensitivity) but also accurately quantifying how meridional sea-surface temperature patterns will change (structural climate sensitivity).
在温暖的中新世和上新世时期，广阔的亚热带地区有足够的降水来支持丰富的植被和动物群。直到大约 300 万年前的上新世末期，全球冷却和冰期循环开始，干旱和半干旱亚热带地区的广泛模式才完全形成。然而，由于 CO2 上升导致的未来全球变暖的当前预测通常表明这些亚热带地区的干燥条件加剧，而不是恢复到更湿润的状态。未来的预测与这些过去的温暖气候有何不同？在这里，我们通过将典型的 CO2 倍增实验与由海面温度驱动的模拟进行比较来研究这个问题，该模拟与早期上新世的可用重建非常相似。基于这两个实验和一系列其他受干扰的气候模拟，我们认为，这个难题可以通过对不同的海洋表面温度模式的响应来解释，特别是减少了经向和纬向温度梯度。因此，我们的结果强调，准确预测水文循环对全球变暖的响应不仅需要预测全球平均温度对升高的 CO2 强迫的响应（气候敏感性），还需要准确量化经向海表面温度模式将如何变化（结构气候敏感性）。
