NOAA/WDS Paleoclimatology - Global 3Ma Transient CESM1.2 Climate Simulation

Driven primarily by variations in earth’s axis wobble, tilt, and orbit eccentricity, our planet experienced massive glacial/interglacial reorganizations of climate and atmospheric CO2 concentrations during the Pleistocene (2.58 Ma–11.7 ka). Even after decades of research, the underlying climate response mechanisms to these astronomical forcings have not been fully understood. To further quantify the sensitivity of the earth system to orbital-scale forcings we conducted an unprecedented quasi-continuous coupled general climate model simulation with the Community Earth System Model version 1.2 (CESM1.2, ~ 3.75° horizontal resolution), which covers the climatic history of the past 3 million years ago (3 Ma). In addition to the astronomical insolation changes, CESM1.2 is forced by estimates of CO2 and ice-sheet topography which were obtained from a simulation previously conducted with the CLIMBER-2 earth system model of intermediate complexity. Our 3 Ma simulation consists of 42 transient interglacial/glacial simulation chunks, which were partly run in parallel to save computing time. The chunks were subsequently merged, accounting for spin-up and overlap effects to yield a quasi-continuous trajectory. The computer model data were compared against a plethora of paleo-proxy data and large-scale climate reconstructions. For the period from the Mid-Pleistocene Transition (MPT, ~1 Ma) to the late Pleistocene we find good agreement between simulated and reconstructed temperatures in terms of phase and amplitude (-5.7 °C temperature difference between Last Glacial Maximum and Holocene). For the earlier part (3 Ma–1 Ma), differences in orbital-scale variability occur between model simulation and the reconstructions, indicating potential biases in the applied CO2 forcing. Our model-proxy data comparison also extends to the westerlies, which show unexpectedly large variance on precessional timescales, and hydroclimate variables in major monsoon regions. Eccentricity-modulated precessional variability is also responsible for the simulated changes in the amplitude and flavours of the El Niño-Southern Oscillation. We further identify two major modes of planetary energy transport, which played a crucial role in Pleistocene climate variability: the first obliquity and CO2-driven mode is linked to changes in the equator-to-pole temperature gradient; the second mode regulates the inter-hemispheric heat imbalance in unison with the eccentricity-modulated precession cycle. During the MPT, a pronounced qualitative shift occurs in the second mode of planetary energy transport: the post-MPT eccentricity-paced variability synchronizes with the CO2 forced signal. This synchronized feature is coherent with changes in global atmospheric and ocean circulations, which might contribute to an intensification of glacial cycle feedbacks and amplitudes. Comparison of this paleo-simulation with greenhouse warming simulations reveals that for a RCP8.5 greenhouse gas emission scenario, the projected global mean surface temperature changes over the next 7 decades would be comparable to the late Pleistocene glacial-interglacial range; but the anthropogenic warming rate will exceed any previous ones by a factor of ~100.地球的气候与大气二氧化碳浓度在更新世（Pleistocene，2.58 Ma~11.7 ka）期间发生过大规模的冰期/间冰期重组，这一过程主要受地轴摆动、倾斜度以及轨道偏心率变化驱动。即便经过数十年研究，学界对这些天文强迫下的气候响应机制仍未完全阐明。为进一步量化地球系统对轨道尺度强迫的敏感性，我们借助社区地球系统模型版本1.2（Community Earth System Model version 1.2, CESM1.2，水平分辨率约3.75°）开展了一项前所未有的准连续耦合通用气候模型模拟，模拟覆盖了过去3百万年（3 Ma）的气候历史。除天文日照变化外，CESM1.2还受二氧化碳与冰盖地形的估算值驱动，这些数据源自此前使用中等复杂度地球系统模型CLIMBER-2完成的模拟结果。本次3 Ma时长的模拟包含42个瞬态冰期/间冰期模拟块，为节省计算时间，部分模拟块采用并行运行方式。随后我们对这些模拟块进行拼接，并校正自旋启动与重叠效应，最终得到一条准连续的气候演化轨迹。我们将该气候模型数据与大量古代理数据及大型气候重建结果进行了对比。针对中更新世转型期（Mid-Pleistocene Transition, MPT，约1 Ma）至晚更新世时段，模拟与重建的温度在相位与振幅上均吻合良好——末次冰盛期与全新世的温度差为-5.7 ℃。而在更早的时段（3 Ma~1 Ma），模型模拟与重建结果间存在轨道尺度变率的差异，这表明所采用的二氧化碳强迫可能存在潜在偏差。我们的模型-代理数据对比还延伸至西风带与主要季风区的水文气候变量：西风带在岁差时间尺度上展现出超出预期的变率；受偏心率调制的岁差变率同样主导了厄尔尼诺-南方涛动（El Niño-Southern Oscillation）振幅与类型的模拟变化。我们进一步识别出两类对更新世气候变率至关重要的行星能量输送模态：第一类由地轴倾斜角与二氧化碳驱动，与赤道-极地温度梯度的变化相关联；第二类则与受偏心率调制的岁差周期协同，调控半球间热不平衡。在中更新世转型期，第二类行星能量输送模态出现显著的定性转变：转型后受偏心率调控的变率与二氧化碳强迫信号实现同步。这一同步特征与全球大气与海洋环流的变化相一致，或有助于强化冰期周期的反馈机制与振幅。将本次古气候模拟与温室变暖模拟对比后可见，在典型浓度路径8.5（RCP8.5）温室气体排放情景下，未来70年的全球平均地表温度变化幅度可与晚更新世冰期-间冰期温度范围相当，但人类活动导致的变暖速率将远超此前任何时期，达到约100倍。