Chapter 11 - Photosynthesis in action: The global view

Abstract

Life and the earth's environment have evolved closely together over earth's history. The appearance of water-splitting photosynthesis has oxygenated the atmosphere enabling life on earth as we know it. The oxygenation of the atmosphere is a consequence of organic matter burial in deep sea sediments. With burial, organic matter escaped oxidation and conversion back to carbon dioxide. Buried organic material has accumulated and been transformed over long-timescales via geological processes into fossil fuels. Fossil fuel compounds store energy gained by photosynthesis in form of energy held in atomic bonds. In the 19th century, humans have discovered how to tap effectively into the vast energy amounts which these fossil fuels hold. This seemingly near-infinite resource has enabled very rapid growth of the human population, spurring an ever increasing demand and thereby creating one of the most burning problems that humans face today. This process consumes atmospheric oxygen (O2) and releases carbon dioxide (CO2). Because carbon dioxide is a greenhouse gas, and because of the large amounts of fossil fuels burned so-far, atmospheric CO2 levels have by now increased by ~45% compared to preindustrial time (AD 1750). In a global context, photosynthesis is thus at the root of life on earth, and at the same time, reversal of its billions of years legacy now threatens life as we know it. The purpose of this chapter is to tell the story of photosynthesis in a global context, including efforts to use Earth-System computer models to predict the future consequences of fossil fuel burning will be for climate and life on earth. The chapter is structured into three components. We will first sketch how the earth system co-evolved with life and specifically oxygenic photosynthesis over geological time-scales. We will then show evidence for changes of atmospheric composition as a consequence of fossil fuel burning. We finally explain how photosynthesis and the carbon cycle are included in Earth-System models in order to predict future climate and possible feedbacks of the land vegetation with climate, what these models predict with regards to the land carbon cycle and how this compares with observations of land vegetation responses to a changing climate up to today.