Dynamics of the Earth System and the Ice-Core Record (DESIRE)

Atmospheric composition and climate are closely linked because compounds such as carbon dioxide and methane are greenhouse gases: increases in their concentration are expected to warm the atmosphere. Such increases have occurred in the last two centuries, and are expected to accelerate in the next few decades. However, exactly how these concentrations and climate will evolve together depends on processes that link them within the so-called Earth System. Our understanding of these processes is expressed in models that represent and connect parts of the system such as the growth of vegetation, ocean circulation, atmospheric circulation and chemistry, etc. However, the best way we have of validating whether these models are correctly representing the Earth is by looking at the past. Various palaeoclimate records provide us with a view of how climate has behaved in the past. The ice core record is particularly valuable because it shows how both climate and atmospheric composition have evolved over the last 800,000 years. During this time, the Earth has passed into and out of glacial states many times, and it turns out that the principal greenhouse gases and climate have varied together during this period. Carbon dioxide and methane have high concentrations during warm interglacials and low concentrations in cold glacials. They thus offer numerous examples of how climate reacts to changes in atmospheric composition, and strong clues about how the sources and sinks of carbon dioxide and methane react to climate change. Our current understanding is that methane increases when climate warms because of a combination of expanded wetland sources and diminished atmospheric sinks. However, we lack many details about these sources and sinks, and have no clear evidence to differentiate their respective roles. For carbon dioxide, the changes are believed to stem mainly from processes in the Southern Ocean, but within this view there are a number of competing hypotheses. This proposal will combine the strongest elements of the relevant observational and modelling communities in the UK and France. Firstly, we will examine both the ice core and other datasets to provide as many constraints as possible on the causes of change in concentration of carbon dioxide and methane. This will involve particularly new measurements of isotopes of carbon that are diagnostic of sources, and new measurements of marine sediments in the Southern Ocean that can constrain mechanisms for changes in the carbon cycle. Particular aspects of the emission and processing of methane and carbon dioxide will be considered in order to make necessary improvements in models. We will then use a variety of models of different levels of complexity to explore the major changes seen in the ice record: between cold glacials and warm interglacials, between different interglacials, and at other particular times in the last 800,000 years that may allow us to differentiate the operation of certain mechanisms. Detailed models, including the new QUEST Earth System Model, will be used to assess the production and loss of methane at particular times in the record. Models of lower complexity will be run over longer time periods to determine the expected signal in different palaeoclimate archives of various mechanisms for changes in carbon dioxide, with a view to narrowing the uncertainties on the importance of each mechanism. Models will also be used to test whether we can understand the different climates seen in past interglacials knowing the energy input from the Sun and the concentrations of greenhouse gases seen in ice cores. The end result of this project will be an improved ability to simulate the past, a better understanding of the processes that control atmospheric composition, climate and the carbon cycle and, as an end result, an improved representation of all relevant processes in models used to predict the future evolution of the Earth System.