Perhaps most challenging of all will be managing the carbon cycle such that it continues to keep the planet in a stable climatic state. From a natural-science perspective, this will involve developing non-fossil-fuel-based energy sources such as biofuels, as well as finding ways of sequestering carbon such as by afforestation or air capture and storage.
Technologies that might have multiple benefits, such as using biomass taken from the farming cycle, are especially worthy of investigation. Biomass can potentially be used either as a low-carbon fuel or as a means of storing carbon. The latter case has the added benefit of generating products that could be traded for credits on the carbon market. Biomass that is transformed into long-lasting carbon materials can effectively remove atmospheric CO2, at least for the lifetime of the products, and as such is termed ‘carbon negative’8.
The global carbon cycle and its management cannot be studied from only a natural-science perspective. The disruption of the global carbon cycle is tightly linked to human development, and to the need for energy and food resources on land and in the seas. Scientific assessment of any management options thus clearly needs to accommodate the multitude of socioeconomic drivers in the modern world. Addressing this in a rational, scientific way poses a huge challenge that must be met in order to steer the Earth systems within acceptable bounds over the next 100 years and beyond.
To assess which anthropogenic emissions of carbon dioxide are compatible with the goal of limiting global warming to 2 °C, our climate model must include changes in the carbon cycle. Earth-system simulations by the Max Planck Institute for Meteorology demonstrate substantially reduced ‘permissible’ carbon dioxide emissions during the twenty-first century when a coupled carbon cycle is included (Roeckner, E. et al. Clim. Change doi:10.1007/s10584-010-9886-6, 2010).