Orbital Modulation of Eocene Carbon Cycle and Climate (OMECCC)

我们的气候系统对二氧化碳和甲烷等温室气体的排放有多敏感？当自然过程开始释放越来越多的温室气体，大大放大人类已经造成的变暖时，我们是否会达到一个“临界点”？由于地球气候系统的复杂性，有时也仅仅是因为我们对地球应对气候变化的各种不同方式缺乏想象力，因此，即使使用我们最好的气候模型，也很难对这些问题给出一个全面的答案。如果我们能够识别和理解地质历史上记录的涉及向大气中大量释放温室气体的事件，这将对我们测试和改进计算机模型以及对未来气候变化的预测有很大的帮助。迄今为止，我们发现的最有希望的事件之一被称为古新世-始新世极热期（或“PETM”）。它发生在大约5500万年前，也就是最后一只恐龙出现在地球上大约1000万年之后。它与地球陆地表面和所有海洋的突然大幅变暖有关。在地质记录中，生活在深海海底的生物的灭绝，生活在深海表面的微小浮游生物的外壳中富含红色粘土的沉积物异常稀少，以及这些外壳中不同元素和同位素比例的突然变化（地球化学家可以根据这些元素和同位素重建古代海洋的温度和各种有用的东西），这些都清楚地表明了这一点。全球碳循环突然中断的指标与地表温度上升之间的对应关系是温室效应变暖的标志，这也是科学家们如此兴奋的原因。但是，仔细观察从海底钻出的几米长的圆柱形泥浆，你可能会看到一些更微妙的事情——颜色的微小波动。如果你有一个昂贵的质谱仪，你也可以发现碳同位素的微小波动。全球碳循环似乎周期性地“摇摆不定”。为什么?这与PETM有什么关系——这次大事件是由一个小的碳循环摆动引发的吗？了解这些摆动的原因具有特殊的重要性，因为它们似乎与地球轨道上的一些摆动发生的频率相同，而古海洋学家（研究百万年前臭泥的科学家的一个奇特术语）利用沉积物中记录的相对规律的变化作为测量沉积物积累的时间流逝的手段。在这个项目中，我们将测试是什么导致了全球碳循环中的古代摆动。由于我们没有备用的地球副本来实验和测试想法，也没有时间机器来回到过去观察原因，我们的研究工具是地球系统的计算机表示。该模型考虑了海洋环流和温室变暖，以及海洋内碳和营养物质的循环以及与海底沉积物的交换。我们将使用这个模型来预测地质记录会是什么样子，如果为观测提出的各种假设中的任何一个是正确的。最符合我们假设的，也是最接近真相的（尽管我们永远无法绝对确定到底发生了什么）。我们的工作结果将提高对全球碳循环如何以及为什么在古代响应地球轨道波动的理解，增加我们使用观测到的摆动来判断时间的信心，并有望找到触发PETM事件的线索。

How sensitive is our climate system to the emission of greenhouse gases such as carbon dioxide and methane? Might we reach a 'tipping point', when natural processes start to release more and more greenhouse gases, greatly amplifying the warming that humans are already causing? Because of the complexity of the Earth's climate system, and sometimes simply because of our lack of imagination about all the different ways in which the Earth can respond to being poked, a comprehensive answer to these questions is extremely difficult to achieve, even with our best climate models. It would be a great help to us in testing and improving our computer models and predictions of future climate change if we could identify and understand events recorded in the geological past involving a massive release of greenhouse gases to the atmosphere. One of the most promising events we have discovered so-far is called the Palaeocene-Eocene Thermal Maximum (or 'PETM'). It occurred about 55 million years ago, some 10 million years after the last dinosaur walked the face of the Earth, and is associated with a sudden and substantial warming of both the Earth's land surface and all the oceans. It stands out clearly in the geological record by extinctions amongst organisms dwelling on the deep ocean floor, by red, clay-rich sediments unusually poor in the shells of microscopic plankton who lived in the surface far above, and by sudden changes in the ratios between different elements and isotopes imprinted into these shells (from which geochemists can reconstruct the temperature of the ancient ocean and all sorts of useful things). The correspondence between indicators of a sudden disruption of the global carbon cycle and increasing surface temperatures is the hallmark of greenhouse warming, and this is what has gotten scientists so excited. But look closely along the meters and meters of cylinders of mud drilled from the bottom of the ocean and you might see something more subtle going on -- minute fluctuations in color. And if you had an expensive mass spectrometer, you could also find tiny fluctuations in carbon isotopes as well. It seems that the global carbon cycle periodically 'wobbled'. Why? And what is the relationship with the PETM -- could this massive event have been set off by a small carbon cycle wobble? Understanding the reasons for these wobbles has a special importance as they seem to occur at the same frequency as some of the wobbles in the Earth's orbital, and paleoceanographers (a fancy term for scientists who study million year old stinking mud) use the relatively regular changes recorded in the sediments as a means of measuring the passage of time as the sediments build up. In this project we will test ideas for what caused the ancient wobbles in the global carbon cycle. As we do not have spare copies of our planet on which to experiment and test ideas, nor a time machine to go back and observed the cause, our research tool is a computer representation of the Earth system. This model accounts for ocean circulation and greenhouse warming, as well as the cycling of carbon and nutrients within the ocean and exchanges with the underlying deep-sea sediments. We will use this model to predict what the geological record would look like if any of the various hypotheses proposed for the observations were correct. The one that fits the closest we will assume is also closest to the truth (although we could never know what happened absolutely for sure). The result of our work will be an improved understanding of how and why the global carbon cycle fluctuated in response to the Earth's orbit in the ancient past, increase our confidence in using the observed wobbles to tell time, and hopefully may find clues to the trigger of the PETM event.