Timing, Causes and Consequences of the Decline in Pliocene pCO2

人类活动导致大气中的二氧化碳浓度从工业化前的百万分之275 - 285增加到今天的百万分之380。这种温室气体对人类引起的地球变暖的贡献比任何其他气体都要大。因此，了解二氧化碳与气候之间的关系是地球科学家预测未来气候变化的主要研究重点。尽管通用循环模型是我们预测未来变化的主要工具，但它们仍然是需要验证的不完善的工具。因此，古代气候的研究现在是向决策者通报气候变化问题的一个组成部分。如果气候模型成功地再现了过去发生的大规模气候变化，这将使我们对它们对未来的预测更有信心。最有意义的类比将出现在最近的地质历史中，那里的地理结构、洋流和生态系统与今天相似。上新世中期（大约3.3到3.0 Ma）是地球历史上最近的一个时期，当时全球平均气温比现在高得多，海平面也比现在高得多。在这段相对温暖的时期之后，地球急剧冷却，冰盖增加：格陵兰岛的现代冰盖证明了我们目前的“冰窖”气候。人们普遍认为，二氧化碳浓度的降低是这一关键气候转变的原因，然而，这种理解取决于对上新世大气二氧化碳浓度的确切了解，它们是如何变化的，以及这些变化是如何影响气候的。然而，这目前是基于一些极低分辨率和有限的研究。因此，我们建议在确定上新世CO2浓度时，特别强调确定其相对于北半球冰川作用的下降时间。这将涉及对二氧化碳浓度的订正估计和对海洋表面温度的全球广泛估计，而海洋表面温度是过去无法直接确定的环境参数。我们将研究海洋微生物，有孔虫和球石虫的化石遗骸。这些生物在海底的泥浆中非常丰富，为过去的海洋气候数据提供了宝贵的档案，通过观察它们外壳的化学成分或它们生物合成的有机化合物，我们可以确定海洋的温暖程度或酸性程度。从这些参数中，我们还可以推断出他们的环境中有多少二氧化碳。我们将使用几种互补的方法来确定CO2浓度，包括有孔虫的硼同位素组成和有机化合物的碳同位素组成。直到最近，方法学的进步才使这种多学科方法成为可能。有了对过去300万年大气中二氧化碳演变的更好理解，我们将确定是什么自然过程导致了其浓度的变化。我们首先将二氧化碳变化的时间与构造事件（如印度尼西亚海道的关闭和全球冰量的变化）进行比较，以检验a)什么可能引发了二氧化碳的减少，b)这是否真的是当时观测到的全球变冷和北半球大冰盖扩张的主要驱动因素。最后，我们将使用气候模型来验证这些想法。首先，我们将使用一个耦合的海洋-大气模型来测试二氧化碳变化的幅度和时间是否与相关的冷却一致。其次，我们将把该模型与碳循环模型结合起来，以测试全球变冷期间的正反馈是否在从大气中去除二氧化碳方面发挥了重要作用。

Human activity has led to an increase in atmospheric carbon dioxide levels from 275 to 285 parts per million (ppm) in preindustrial times to >380 ppm today. This greenhouse gas contributes more to the human-induced warming of the planet than any other gas. Therefore, understanding the relationship between carbon dioxide and climate is a major research focus of Earth Scientists predicting future climate change. Although General Circulation Models are the primary tool by which we anticipate future change, they remain imperfect tools that require validations. Therefore, the study of ancient climate is now an integral part in informing policy makers on climate change issues. If climate models successfully reproduce large scale climate changes that occurred in the past, this will give us more confidence in their prediction for the future. The most informative analogues will be in the recent geological past where geographical configurations, ocean currents and ecosystems are similar to today. The Mid-Pliocene (about 3.3 to 3.0 Ma) is the most recent time in Earth's history when mean global temperatures were substantially warmer and sea levels much higher than they are today. Immediately following this period of comparative warmth, the Earth cooled dramatically and ice sheets grew: the modern ice sheet on Greenland is testament to our current 'icehouse' climate. A reduction in the concentration of CO2 is widely believed to be the cause of this critical climate transition, yet this understanding depends on knowing exactly what atmospheric CO2 concentrations were in the Pliocene, how they changed and how those changes impacted the climate. However, this is currently based on a few extremely low resolution and limited studies. Therefore, we propose to determine Pliocene CO2 concentrations with particular emphasis on determining the timing of its decrease with respect to the glaciation of the northern hemisphere. This will involve revised estimates of CO2 concentrations and globally widespread estimates of sea surface temperature, environmental parameters that cannot be directly determined for the past. We will study the fossil remains of sea-dwelling microscopic organisms, the foraminifers and coccolithophorids. These organisms are very abundant in the mud on the floor of the oceans, providing an invaluable archive of past ocean climate data, and by looking at the chemical composition of their shells or the organic compounds they biosynthesise we can determine how warm or how acidic the ocean was. And from such parameters, we can also deduce how much CO2 was in their environment. We will use several complementary approaches to determine CO2 concentrations, including the boron isotopic composition of foraminifera and the carbon isotopic composition of organic compounds. Methodological advances have only recently made such a multi disciplinary approach possible. Armed with this improved understanding of the evolution of CO2 in the atmosphere over the last ~3 million years, we will determine what natural processes resulted in changing its concentration. We will initially compare the timings of CO2 change to tectonic events such as the closure of the Indonesian Seaway and changes in global ice volume to examine a) what might have triggered the decrease in carbon dioxide, and b) if that truly was the main driver of global cooling and the expansion of the great northern hemisphere ice sheets observed at that time. Finally, we will test those ideas using climate models. First, we will use a coupled ocean-atmosphere model to test whether the magnitude and timing of carbon dioxide change is consistent with associated cooling. Second, we will couple that model to a model of the carbon cycle to test if positive feedbacks during global cooling played an important role in carbon dioxide removal from the atmosphere.