Department of Meteorology and Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 2013 Aug 27;110(35):14162-7. doi: 10.1073/pnas.1303365110. Epub 2013 Aug 5.
Projections of future climate depend critically on refined estimates of climate sensitivity. Recent progress in temperature proxies dramatically increases the magnitude of warming reconstructed from early Paleogene greenhouse climates and demands a close examination of the forcing and feedback mechanisms that maintained this warmth and the broad dynamic range that these paleoclimate records attest to. Here, we show that several complementary resolutions to these questions are possible in the context of model simulations using modern and early Paleogene configurations. We find that (i) changes in boundary conditions representative of slow "Earth system" feedbacks play an important role in maintaining elevated early Paleogene temperatures, (ii) radiative forcing by carbon dioxide deviates significantly from pure logarithmic behavior at concentrations relevant for simulation of the early Paleogene, and (iii) fast or "Charney" climate sensitivity in this model increases sharply as the climate warms. Thus, increased forcing and increased slow and fast sensitivity can all play a substantial role in maintaining early Paleogene warmth. This poses an equifinality problem: The same climate can be maintained by a different mix of these ingredients; however, at present, the mix cannot be constrained directly from climate proxy data. The implications of strongly state-dependent fast sensitivity reach far beyond the early Paleogene. The study of past warm climates may not narrow uncertainty in future climate projections in coming centuries because fast climate sensitivity may itself be state-dependent, but proxies and models are both consistent with significant increases in fast sensitivity with increasing temperature.
未来气候的预测取决于对气候敏感性的精确估计。近期温度代用指标的研究进展极大地提高了从古新世温室气候重建的变暖幅度，这要求我们仔细研究维持这种温暖以及这些古气候记录所证明的广泛动态范围的强迫和反馈机制。在这里，我们表明，在使用现代和古新世早期配置的模型模拟的背景下，这些问题有几种互补的解决方案。我们发现：（i）代表缓慢的“地球系统”反馈的边界条件变化在维持古新世早期高温方面起着重要作用；（ii）二氧化碳的辐射强迫在与古新世早期模拟相关的浓度下显著偏离纯对数行为；（iii）在该模型中，随着气候变暖，快速或“Charney”气候敏感性急剧增加。因此，增加强迫以及增加缓慢和快速敏感性都可以在维持古新世温暖方面发挥重要作用。这提出了一个均变问题：相同的气候可以由这些因素的不同组合来维持；然而，目前，这种组合不能直接从气候代理数据中约束。强烈的状态依赖快速敏感性的影响远远超出了古新世。过去温暖气候的研究可能不会缩小未来几个世纪的气候预测的不确定性，因为快速气候敏感性本身可能是状态依赖的，但代理和模型都与随着温度的升高快速敏感性显著增加相一致。
