Climate Change and the Oceans

我提出的研究涉及地球气候系统内的两个基本反馈过程/大气二氧化碳浓度和海洋环流强度。反馈过程放大了气候的任何外部强迫，例如，地球绕太阳轨道的变化。这两个过程在现代气候系统中都扮演着重要的角色，但它们在过去的作用，特别是它们在未来气候变化中的预测作用，是不确定的。这项提议的主要目的是通过重建这两个系统在过去关键时刻的行为来减少这种不确定性。拟议研究的第一部分涉及大气中二氧化碳的浓度。二氧化碳是一种重要的温室气体，因此大气中二氧化碳含量的变化被认为对地球气候有重要的控制作用。事实上，在过去的250万年里，地球的气候在极端寒冷的时期(称为冰期)和相当的温暖时期(像今天这样的间冰期)之间振荡。重要的是，气候的这些波动伴随着大气二氧化碳浓度(PCO2)的变化。冰盖消长的总驱动力是地球围绕太阳的轨道的微妙变化，这影响了在高纬度地区接收到的太阳辐射的数量和季节分布。二氧化碳浓度的变化可能使这种轨道强迫变得全球化并增强。改变二氧化碳的确切机制尚不清楚，但可能与海洋有关(海洋是地球上最大的碳储量，可以足够快地做出反应)。为了审查这一作用，我建议使用浮游和底栖有孔虫(在整个海洋中常见的钙质单细胞原生生物，保存在深海沉积物中)的硼同位素组成，生成海洋各种敏感区域过去海洋酸度(PH)的记录。由于海洋的酸度在很大程度上决定了二氧化碳，我将能够从这一记录中确定在冰盖消长期间二氧化碳是如何从深海储存和释放的，并有可能分离出它在导致从一种气候状态向另一种气候状态转变的过程中所起的作用。这项提案的第二部分涉及海洋的循环。赤道从太阳接收更多的热量，比两极更热，正是这种温度梯度驱动了海洋(和大气)环流。海洋环流的大西洋部分是循环系统中特别重要和敏感的部分。在这里，墨西哥湾流将温暖的咸水从低纬度带到北大西洋。然后这些水冷却，向大气失去热量，变得稠密(含盐的冷水比新鲜的温水更稠密)，结果是下沉。然后，它们向南深流，形成了对流单体的返回臂。这种循环方式释放的热量不仅对改善欧洲海洋气候很重要，而且可以通过确定形成海冰的北冰洋的水温来影响地球的整体气候。海冰的反射率很高，更多的海冰导致更多的太阳能量被反射回太空，因此可以影响全球气温。这种环流模式在未来气候情景中的作用是不确定的；减少这种不确定性的一种方法是研究过去地质时期的海洋环流。我提议使用一种新的分析技术，包括对铁锰结壳(从深层海水中缓慢沉淀的金属结壳)进行激光显微取样，以重建过去气候明显比今天(上新世中期)更冷和更暖时的环流强度和模式。

The research I propose concerns two fundamental feedback processes within the Earth's climate system / the concentration of atmospheric CO2 and the circulation strength of the oceans. Feedback processes amplify any external forcing of the climate caused by, for example, variations in the Earth's orbit around the sun. Both processes play important roles in the modern climate system, yet their roles in the past, and in particular, their predicted roles in future climate change are uncertain. It is the principal aim of this proposal to reduce this uncertainty by reconstructing the behaviour of these two systems at key times in the past. The first part of the proposed research concerns the concentration of carbon dioxide in the atmosphere. CO2 is an important greenhouse gas such that variations in the amount of atmospheric CO2 are thought to have an important control on the Earth's climate. Indeed, over the last 2.5 million years the climate of the Earth has oscillated between periods of extreme cold (called glacial periods) and comparable warmth (interglacial periods like today). Importantly, these swings in climate have been accompanied by changes in the concentration of atmospheric CO2 (pCO2). The overall driving force for the waxing and waning of ice sheets are subtle variations in the orbit of the Earth around the sun, which influence the amount and seasonal distribution of solar radiation received at high latitudes. Variations in the concentrations of CO2 probably globalise and enhance this orbital forcing. The exact mechanisms responsible for altering pCO2 are not known but probably involve the oceans (the largest store of carbon on the planet that can respond with sufficient rapidity). In order to examine this role, I propose to generate a record of past ocean acidity (pH) in a variety of sensitive areas of the ocean using the boron isotopic composition of planktic and benthic foraminifera (calcareous single celled protists that are common throughout the ocean and are preserved in deep sea sediments). Since the acidity of the ocean largely determines pCO2, I will be able, from this record, to identify how CO2 is stored and released from the deep sea during the waxing and waning of the ice sheets and potentially isolate its role in causing the transitions from one climate state to another. The second part of this proposal concerns the circulation of the oceans. The Equator, which receives more heat from the sun, is hotter than the poles, and it is this temperature gradient that drives ocean (and atmospheric) circulation. The Atlantic portion of ocean circulation is a particularly important and sensitive part of the circulation system. Here, the Gulf Stream carries warm, salty water from the low latitudes to the North Atlantic. These waters then cool, lose heat to the atmosphere and become dense (salty cold water is denser than fresh warm water) and, as a result, sink. They then flow southward at depth, forming the return arm of the convection cell. The release of heat by this mode of circulation is not only important in ameliorating the climate of maritime Europe, but it can influence the overall climate of the planet by determining the water temperature in the Artic seas, where sea-ice forms. Sea-ice is highly reflective and more sea-ice results in more of the Sun's energy being reflected back into space, and hence can influence global temperature. The role of this circulation pattern in future climate scenarios is uncertain; one way to reduce this uncertainty is to examine ocean circulation in the geological past. I propose to use a new analytical technique, involving the laser microsampling of ferromanganese crusts (metallic encrustations that precipitate very slowly from seawater at depth) to reconstruct the strengths and patterns of circulation in the past when the climate was significantly colder (during glacial periods) and warmer than today (the Mid-Pliocene).