TRAcing the fate of Glacial-Interglacial Carbon ('TRAGIC')

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，极大地放大人类已经造成的气候变暖时，我们是否会到达一个“临界点”？由于地球气候系统的复杂性，有时仅仅是因为我们缺乏对地球被戳的所有不同方式做出反应的想象力，即使使用我们最好的气候模型，也很难全面回答这些问题。如果我们能更好地了解地质历史中记录的涉及大气温室气体和气候变化的事件，这将对我们测试和改进我们的计算机模型和对未来气候变化的预测有很大帮助。我们之所以关注这里，是因为30多年来对古冰中的微小气泡所含的二氧化碳(CO2)进行了艰苦的测量。这些测量表明，大气中的二氧化碳浓度在过去100万年中经历了巨大的波动，随着巨大冰盖和寒冷冰期的增长而大致同步下降，随着条件变暖而上升。由于二氧化碳是一种温室气体，较低的浓度显然有助于解释为什么冰川时期的气候比现在冷得多。那么，是什么导致二氧化碳在冰川时期降低(而在像今天这样的间冰期上升)呢？令人惊讶的是，尽管科学家们已经寻找了30多年的答案，但仍然没有人确切地知道。人们提出了许多不同的假说；一些涉及海洋环流的变化，另一些涉及海洋表面生命的营养物质(如铁)的供应。一些建议的改变将被证明是不重要的，其他可能是拼图的关键部分。我们怎样才能从这么多的可能性中选择正确的答案呢？我们认为，布里斯托尔大学的一名博士生产生的新信息掌握着关键。他测量了生活在海底的微小生物的外壳的组成。各种元素和化合物在形成时从海水中并入贝壳中，元素硼的确切数量及其同位素组成取决于海洋的酸度。因此，有可能重建自上一次冰川以来深海酸度是如何变化的。这一点很重要，因为在冰河时代，大气中的碳有很大一部分可能是以溶解的形式储存在深海中的，而且由于向水中加入二氧化碳会使水变得更酸性，所以有可能重建那里到底有多少碳。在这个项目中，我们将测试是什么导致了大气中二氧化碳的冰期-间冰期变化。由于我们没有多余的地球副本来试验和检验想法，也没有时光机可以回到过去直接测量深海中的碳储量，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效应，以及海洋中碳和营养物质的循环以及与底层深海沉积物的交换。我们将使用这个模型来预测，如果为观测提出的各种假设中的任何一个是正确的，那么地质记录将是什么样子。符合我们假设的最接近的那个也最接近真相(尽管我们永远不能绝对知道发生了什么)。我们工作的结果将是更好地理解全球碳循环如何以及为什么会随着冰川-间冰期循环而波动，从而增强我们对能够预测未来(化石燃料)碳在不同水库之间可能如何交换，以及气候和海洋环流的变化可能如何调节这种交换的信心。

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 better understand events recorded in the geological past involving changes in atmospheric greenhouse gases and climate. Our focus here is prompted by painstaking measurements that have been made for more than 30 years of the amount of carbon dioxide (CO2) contained in minute bubbles trapped in ancient ice. These measurements reveal that the concentration of CO2 in the atmosphere has undergone large fluctuations over the course of the past million years, falling in approximate step with the growth of massive ice sheets and cold glacial periods and rising as conditions get warmer. As CO2 is a greenhouse gas, lower concentrations clearly help explain why climate was much colder during glacial intervals than today. So what then drives CO2 lower during glacial times (and up during inter-glacial periods such as today)? Amazingly, although scientists have been searching for the answer for over 30 years, still no-one knows for sure. Many different hypotheses have been forwarded; some involving changes in ocean circulation, others the supply of nutrients (such as iron) to life at the ocean surface. Some suggested changes will turn out to be unimportant, others may be key parts of the puzzle. How can we choose the correct answer from such a variety of possibilities? We believe that new information generated by a PhD student at the University of Bristol holds the key. He has measured the composition of the shells of minute organisms living at the bottom of the ocean. Various elements and compounds are incorporated from seawater into the shells as they form, and the exact amount of the element boron and its isotopic composition depends on ocean acidity. Hence, it is possible to reconstruct how the acidity of the deep ocean has changed since the last glacial. This is important because much of the 'missing' atmospheric carbon during glacial times may have been stored in dissolved form in the deep ocean, and since adding CO2 to water makes it more acidic, it is possible to reconstruct how much carbon there really was down there. In this project we will test ideas for what caused the glacial-interglacial changes in atmospheric CO2. 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 make direct measurements of carbon storage in the deep ocean, 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 glacial-interglacial cycles, increasing our confidence in being able to predict how (fossil fuel) carbon may be exchanged between different reservoirs in the future, and how changes in climate and ocean circulation may modulate this.