Past climates inform our future
Past climates inform our future
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DOI：
10.1126/science.aay3701
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发表时间：
2020-11
期刊：
影响因子：
56.9
通讯作者：
J. Tierney;C. Poulsen;I. Montañez;Tripti Bhattacharya;R. Feng;H. Ford;B. Hönisch;G. Inglis;S. P
中科院分区：
文献类型：
--
作者：
J. Tierney;C. Poulsen;I. Montañez;Tripti Bhattacharya;R. Feng;H. Ford;B. Hönisch;G. Inglis;S. P
过去的未来 气候预测不确定性的一个主要原因是我们对大气中二氧化碳含量一定增加会导致多少变暖的认识不准确。古气候记录有可能帮助我们加深这种理解，因为它们记录了如此广泛的环境条件。蒂尔尼等人。回顾数据收集、统计和建模方面的最新进展，这些进展可能有助于我们更好地了解大气二氧化碳水平上升将如何影响未来的气候。科学，本期第 14 页。 eaay3701 背景 人为排放正在迅速改变地球气候，将其推向前所未有的变暖状态。尽管这种破坏不存在完美的模拟，但地球的历史包括过去的气候状态——“古气候”——它们为我们变暖的世界的未来提供了教训。地球过去的这些时期跨越了巨大的温度范围、降水模式、冰冻圈范围和生物圈适应，并且对于提高我们对气候系统关键要素如何受到温室气体水平影响的理解越来越重要。新的地球化学和统计方法的兴起，以及古气候模型的改进，使得可以根据古气候数据对气候模型进行正式评估。特别是，鉴于一些最新一代的气候模型对大气二氧化碳浓度加倍具有很高的敏感性，古气候在限制平衡气候敏感性（ECS）及其对气候背景状态的依赖性方面发挥了新的作用。进展在过去十年中，越来越多的研究利用古气候温度和二氧化碳估计值来推断遥远过去的温暖和寒冷气候状态下的 ECS。最近的研究支持这样一种范式，即 ECS 强烈依赖于状态，随着 CO2 浓度的增加而增加。对始新世等过去温暖气候的模拟进一步强调了云反馈在二氧化碳水平增加的情况下对高 ECS 的贡献所发挥的作用。古气候为评估未来冰盖稳定性和随之而来的海平面上升提供了关键限制，包括海洋冰崖不稳定等阈值过程的可行性。除了全球范围的变化之外，对过去水循环变化的分析加深了我们对水文气候动态驱动因素的理解，这与区域气候预测和社会影响高度相关。新的和不断扩展的技术，例如对有孔虫单壳的分析，正在产生次季节气候信息，可用于研究外部气候强迫如何影响年内和年际变化模式。对古气候与背景状态（例如古新世-始新世最高热量）的异常、短暂偏离的研究为当前的人为畸变、其对地球系统的影响以及恢复的时间尺度提供了关键背景。许多进展消除了气候模型和代理数据之间的“语言障碍”，促进更直接地使用古气候信息来限制模型性能。将地球化学示踪剂（例如水同位素）直接纳入模型模拟中越来越普遍，这种做法极大地改善了模型与代理的比较。植根于贝叶斯推理的新统计方法的发展导致了对古气候数据不确定性的更彻底的量化。此外，数据同化等技术允许将代理数据和模型数据正式组合成混合产品。这种综合提供了对过去气候的全面了解，并且可以对我们没有直接代理的气候变量（例如云量或风速）施加限制。展望使用古气候信息作为模型目标的一个普遍担忧是，非二氧化碳强迫，例如气溶胶和微量温室气体，人们并不了解，尤其是在遥远的过去。尽管迄今为止的证据表明此类强迫是二氧化碳的次要因素，但地球化学代理和建模的未来改进有望解决这一问题。新的、快速发展的地球化学技术有可能改善对陆地生物圈、气溶胶和微量气体的限制；同样，生物地球化学循环现在可以纳入古气候模型模拟中。除了约束力之外，将代理信息转化为定量估计以解释代理系统中的不确定性也至关重要。已经开发出统计工具来实现这一目标，这应该可以更轻松地为模型评估创建稳健的目标。随着古气候信息量化的增加，我们建议建模中心在其模型性能的评估和陈述中纳入对过去气候的模拟。这种做法可能会缩小气候敏感性、冰盖和水循环的不确定性，从而改善未来的气候预测。过去的气候为未来的气候情景提供了背景。过去（上）和未来（下）的气候都根据相对于工业化前条件的全球年平均地表温度的估计变化来着色，范围从蓝色（较冷）到红色（较暖）。 “可持续性”、“中间道路”和“高排放”分别代表共享社会经济路径 (SSP) SSP1-2.6、SSP2-4.5 和 SSP5-8.5 估计的 2300 年全球温度异常。在过去和未来的情况下，气候变暖都与二氧化碳的增加有关（如箭头所示）。妈，几百万年前。随着世界变暖，迫切需要改进对气候变化的预测。尽管最新的地球系统模型提供了前所未有的数量特征，但基本的不确定性继续笼罩着我们对未来的看法。过去的气候提供了观察地球系统如何应对高二氧化碳的唯一机会，强调了古气候学在限制未来气候变化方面的基本作用。在这里，我们回顾了古气候信息与气候预测的相关性，并讨论了新兴方法的前景，以进一步从过去的气候中获得见解。代理方法和解释的进步为利用过去的气候进行模型评估铺平了道路——我们认为这种做法应该被广泛采用。
The future in the past A major cause of uncertainties in climate projections is our imprecise knowledge of how much warming should occur as a result of a given increase in the amount of carbon dioxide in the atmosphere. Paleoclimate records have the potential to help us sharpen that understanding because they record such a wide variety of environmental conditions. Tierney et al. review the recent advances in data collection, statistics, and modeling that might help us better understand how rising levels of atmospheric carbon dioxide will affect future climate. Science, this issue p. eaay3701 BACKGROUND Anthropogenic emissions are rapidly altering Earth’s climate, pushing it toward a warmer state for which there is no historical precedent. Although no perfect analog exists for such a disruption, Earth’s history includes past climate states—“paleoclimates”—that hold lessons for the future of our warming world. These periods in Earth’s past span a tremendous range of temperatures, precipitation patterns, cryospheric extent, and biospheric adaptations and are increasingly relevant for improving our understanding of how key elements of the climate system are affected by greenhouse gas levels. The rise of new geochemical and statistical methods, as well as improvements in paleoclimate modeling, allow for formal evaluation of climate models based on paleoclimate data. In particular, given that some of the newest generation of climate models have a high sensitivity to a doubling of atmospheric CO2, there is a renewed role for paleoclimates in constraining equilibrium climate sensitivity (ECS) and its dependence on climate background state. ADVANCES In the past decade, an increasing number of studies have used paleoclimate temperature and CO2 estimates to infer ECS in the deep past, in both warm and cold climate states. Recent studies support the paradigm that ECS is strongly state-dependent, rising with increased CO2 concentrations. Simulations of past warm climates such as the Eocene further highlight the role that cloud feedbacks play in contributing to high ECS under increased CO2 levels. Paleoclimates have provided critical constraints on the assessment of future ice sheet stability and concomitant sea level rise, including the viability of threshold processes like marine ice cliff instability. Beyond global-scale changes, analyses of past changes in the water cycle have advanced our understanding of dynamical drivers of hydroclimate, which is highly relevant for regional climate projections and societal impacts. New and expanding techniques, such as analyses of single shells of foraminifera, are yielding subseasonal climate information that can be used to study how intra- and interannual modes of variability are affected by external climate forcing. Studies of extraordinary, transient departures in paleoclimate from the background state such as the Paleocene-Eocene Thermal Maximum provide critical context for the current anthropogenic aberration, its impact on the Earth system, and the time scale of recovery. A number of advances have eroded the “language barrier” between climate model and proxy data, facilitating more direct use of paleoclimate information to constrain model performance. It is increasingly common to incorporate geochemical tracers, such as water isotopes, directly into model simulations, and this practice has vastly improved model-proxy comparisons. The development of new statistical approaches rooted in Bayesian inference has led to a more thorough quantification of paleoclimate data uncertainties. In addition, techniques like data assimilation allow for a formal combination of proxy and model data into hybrid products. Such syntheses provide a full-field view of past climates and can put constraints on climate variables that we have no direct proxies for, such as cloud cover or wind speed. OUTLOOK A common concern with using paleoclimate information as model targets is that non-CO2 forcings, such as aerosols and trace greenhouse gases, are not well known, especially in the distant past. Although evidence thus far suggests that such forcings are secondary to CO2, future improvements in both geochemical proxies and modeling are on track to tackle this issue. New and rapidly evolving geochemical techniques have the potential to provide improved constraints on the terrestrial biosphere, aerosols, and trace gases; likewise, biogeochemical cycles can now be incorporated into paleoclimate model simulations. Beyond constraining forcings, it is critical that proxy information is transformed into quantitative estimates that account for uncertainties in the proxy system. Statistical tools have already been developed to achieve this, which should make it easier to create robust targets for model evaluation. With this increase in quantification of paleoclimate information, we suggest that modeling centers include simulation of past climates in their evaluation and statement of their model performance. This practice is likely to narrow uncertainties surrounding climate sensitivity, ice sheets, and the water cycle and thus improve future climate projections. Past climates provide context for future climate scenarios. Both past (top) and future (bottom) climates are colored by their estimated change in global mean annual surface temperature relative to preindustrial conditions, ranging from blue (colder) to red (warmer). “Sustainability,” “Middle road,” and “High emissions” represent the estimated global temperature anomalies at year 2300 from the Shared Socioeconomic Pathways (SSPs) SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. In both the past and future cases, warmer climates are associated with increases in CO2 (indicated by the arrow). Ma, millions of years ago. As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.