Terrestrial Ecosystem Interactions with Global Climate and Socio-Economics

The global terrestrial carbon cycle plays a pivotal role in regulating the atmospheric composition of greenhouse gases. Currently it is responsible for removing 25-30% of our emissions every year, an important ecosystem service that may not persist in the long term. Global warming tends to decrease the CO2 sink capacity of the world’s oceans and ecosystems leading to a larger fraction of our emissions remaining in the atmosphere contributing to further warming, which in turn further decreases the sink capacity. Here past, present and future variations and drivers of terrestrial ecosystem fluxes are analysed and projected. Uncertainties and robustness as well as their origin are assessed. It is shown that short term variations such as droughts significantly influence global fluxes even though occurring in limited regions. Additionally, seasonal changes may alter the present balance between uptake and release of carbon by stimulating the process of release of carbon through respiration dominating in the winter more than the photosynthesis and net uptake of carbon in the growing season. These variations may be more important for the carbon cycle than long term trends. Ecosystem models represents powerful tools to further our understanding of the functioning of terrestrial ecosystems as well as allowing for projections of the potential future evolution of the carbon cycle. It is demonstrated that ecosystem models generally capture the main global spatial and temporal distributions of carbon fluxes. However, future projections are accompanied by large uncertainties, a result of a combination of uncertainties in climate predictions by climate models and uncertainties in the response of ecosystems to changes in the drivers. It is shown that uncertainties arising from differences in climate projections in terms of seasonal changes and short term variability dominate uncertainties arising from different scenarios of future atmospheric CO2 concentrations. Additionally, representation of sea surface temperature variations and especially variations associated with El Niño-Southern Oscillation in climate models stands out as particularly important for accurate predictions of future carbon fluxes. Human activity contributes with a significant perturbation of the carbon-cycle and the climate system. Climate-economy models have the potential to supply direct policy-relevant estimates for climate mitigation. Here it is demonstrated that correct representation of the climate system and the carbon cycle in climate-economy models is important for making accurate estimates of future economy