Carbon cycle research as a challenge of the anthropocene

Abstract

The carbon cycle has a very distinctive role in the Earth system due to the fact that organic carbon components represent the basic form of energy storage by biological processes and that they form the building blocks of all life on Earth. Carbon is also involved in many physicochemical processes on all time scales. Biological processes have caused the accumulation of ancient organic carbon deposits in the form of oil, coal, peat, methane gas or gas hydrates over the millennia as well as massive deposits of inorganic carbon (e.g. limestones). The recent reactivation of these fossil fuel deposits for energy production by man over the very short time span of less than a century has led to the present sharp rise in the concentration of carbon dioxide (CO 2) in the atmosphere as shown in Figure 3.2.1.1. The concentration of CO2 in the atmosphere is one component, amongst several, influencing the heat balance of Earth over long time scales. Changes in its concentration are generally associated with changes in Earth's climate. Therefore, much attention in recent years has been focussed on understanding variations in CO2 concentration in the atmosphere. It has to be borne in mind, however, that other "greenhouse" gases are concomitantly released into the atmosphere enhancing the effect of CO2 (Prather et al. 2001). The most important ones are water vapour, methane (CH4), nitrous oxide (N2O) and tropospheric ozone (O3). They either originate directly from human activities, such as burning of plant biomass or release of methane from rice paddies and cattle, or their production is caused by changing environmental conditions, such as changes in the water cycle or emission of methane and dinitrogenoxide from suboxic areas on land and in the sea. The effect of these gases on the heat balance of Earth is amplified by changes in water vapour content in the atmosphere. Furthermore, aerosols may add or diminish the effects of "greenhouse" gases (Penner et al. 2001). Depending on the type of aerosol and its distribution, it can either reflect solar radiation back into space providing a cooling effect or it can absorb solar radiation and thus heat up certain strata of the atmosphere. Backscattering of solar radiation by clouds (cloud albedo) is a major feedback in the atmosphere to regulate the surface temperature of the Earth. At present, the heating effect of the "greenhouse" gases is thought to be compensated by the backscattering of solar radiation by clouds and aerosol particles. Recent estimates indicate that the heating impact of "greenhouse" gases may be reduced by 65%-80% due to albedo effects (Crutzen and Ramanathan, Dahlem Conference May 2003, in press). These authors argue that it is not likely that aerosols will increase at the same rate as CO2 concentrations in the atmosphere and, hence, the heating effect may rise more rapidly in future. High CO2 concentrations in the atmosphere, however, have also quite different effects on the environment. They increase photosynthesis in land plants, reduce the pH of water and can alter calcification processes. Experiments have shown that carbonate formation by marine organisms, including plankton, algae and corals, is inhibited at concentrations of ca. 800 ppm expected for the year 2100 (Kleypas et al. 1999, Riebesell et al. 2000). In fact, models predict a drastic reduction in carbonate formation (Figure 3.2.1.2) with unknown consequences for the marine ecosystem. It would endanger coral reefs, decrease their high biodiversity and may deteriorate the stabilisation by the reefs for coasts and tropical islands. It would also change the composition of plankton organisms in the open ocean with possible impacts on the production of fish. In addition to the above mentioned direct influences of rising CO2, indirect effects are suspected from climatic changes triggered by the increasing atmospheric concentrations. At present, rising global temperatures are being observed that are the result of the combined effects of "greenhouse" gases, water vapour, aerosols and clouds. These components are linked by various feedback processes, which enhance or compensate each other. Therefore, the understanding and modelling of such complex interacting systems is a major challenge for future research which requires to consider the linkages within the Earth system. Global average temperature increase predicted by different models range from 1.7 to 4.2°C with a mean of 2.8°C over the next 100 years. Sea level rises due to thermal expansion of sea water and melting of glaciers are predicted to amount to 0.48m (range 0.09-0.88m) over the same time span as a result of global warming. Such climatic changes will alter hydrological cycles, ocean stratification and current systems. The present knowledge of carbon flows and their effects on our climate is compiled most comprehensively in the IPCC (Intergovernmental Panel on Climate Change) reports (IPPC 2001). After a decade of basic research into the pivotal role of the element carbon in the Earth system, the international scientific community has now launched into a new effort to link the present understanding of the natural carbon cycle with the behaviour of human societies. This "Global Carbon Project" (GCP, see http://www. globalcarbonproject.org/) has defined the following central issues for the next decade of research: 1. Patterns and variability: What are the current geographical and temporal distributions of the major stores and fluxes in the global carbon cycle? 2. Processes and interactions: What are the control and feedback mechanisms . both anthropogenic and non-anthropogenic . that determine the dynamics of the carbon cycle? 3. Management of the carbon cycle: What are the likely dynamics of the carbonclimate system into the future and what points of intervention and windows of opportunity exist for human societies to manage this system? The GCP is a joint project of the four global change programmes IGBP, WCRP, IHDP and DIVERSITAS. Its main function is to address the above questions by integration of knowledge and data obtained by other research programmes. It has the task to link natural and human sciences with the aim to better understand the carbon cycle and to assess mitigation and adaptation options for society. This article will firstly give a brief overview over the present understanding of the variability and of critical processes in respect to atmospheric carbon concentrations. In the following some of the research issues will be highlighted that were identified in several workshops as being of particular relevance to link natural and socio-economic sciences in respect to carbon research. © Springer-Verlag Berlin Heidelberg 2006.