Determining how atmospheric carbon dioxide concentrations have changed during the history of the Earth

The reconstruction of ancient atmospheric carbon dioxide concentrations is essential to understanding the history of the Earth and life. It is also an important guide to identifying the sensitivity of the Earth system to this greenhouse gas and, therefore, constraining its future impact on climate. However, determining the concentration of CO2 in ancient atmospheres is a challenging endeavour requiring the application of state-of-the-art analytical chemistry to geological materials, underpinned by an understanding of photosynthesis and biochemistry. It is truly an interdisciplinary challenge. Carbon dioxide and water vapour are the two largest contributors to the greenhouse effect in the atmosphere. Water vapour is so close to its condensation point in the atmosphere that it is difficult to increase its concentration without raising the temperature. In contrast, the concentration of CO2 in the atmosphere (pCO2) can be changed by altering the balance between its sources and sinks. Thus, although CO2 is only a minor component in the atmosphere compared with water vapour (390 parts per million (ppm) compared with up to 40 000 ppm), changes in its concentration due to natural and human activity are possible and important controls on greenhouse warming. Carbon exists on Earth in various reservoirs, most importantly the biosphere, sedimentary rocks and the ocean–atmosphere system. Movements of carbon between these reservoirs are governed by a number of processes. Firstly, growth of the biosphere and the marine ‘biological pump’ can transfer carbon from the ocean–atmosphere system to sediments, and can remove CO2 from the atmosphere. Secondly, vulcanism has multiple impacts but primarily transfers carbon from the buried sedimentary reservoir back into the ocean–atmosphere system. Thirdly, the chemical weathering of rocks can either remove carbon from the atmosphere system or add it, depending on the nature of the rocks being weathered. Finally, within the ocean–atmosphere system itself, CO2 can be moved either into or out of the atmosphere depending on the balance of ocean carbonate chemistry and circulation. Crucially, the balance of these sources and sinks changes over time, and any imbalance can cause either a ‘drawdown’ or an increase in pCO2. Movements of significant amounts of carbon between reservoirs happen on long, geological timescales, and natural variations in the global carbon cycle have caused CO2 concentrations to rise and fall in the atmosphere over the last 2 billion years. More recently, pCO2 has started to increase owing to the burning of fossil fuels by humans. This anthropogenic input has caused pCO2 to rise from a pre-industrial age value of 270–280 ppm to 391 ppm in 2011. Although it is highly likely that this rise in pCO2 has caused the small increase in observed temperature over the last century, there is legitimate debate about the effect of increasing pCO2 over the next century and beyond. One approach to examine what the Earth’s climate system would be like at higher pCO2 levels is to look at what the climate was like during periods of Earth’s history when it was higher than today. To do so requires an understanding of how atmospheric CO2 has evolved over the past few million years. For the past 800 000 years, pCO2 can be measured by analysing remnants of ancient atmosphere trapped