Terrestrial ecosystem carbon dynamics and climate feedbacks p 289 “Recent evidence suggests that, on a global scale, terrestrial ecosystems will provide a positive feedback in a warming world, albeit of uncertain magnitude.” “It is evident that large uncertainties remain in our ability to assess terrestrial carbon-cycle-climate feedbacks over the coming decades. Current experiments give ambiguous results and do not provide definite conclusions on the importance of the mechanisms discussed above. Overall, it is likely that, at least on a global scale, terrestrial ecosystems will provide a positive, amplifying feedback in a warming world, albeit of uncertain magnitude. An important improvement in our understanding might be obtained by the combination of long-term multifactorial experiments with non-destructive ecosystem-level observations, such as whole-ecosystem flux measurements, and the integration of the results with ecosystem modelling in a multiple-constraint framework. As long as there is no fundamental understanding of the processes involved, simulations of coupled carbon-cycle-climate models can only illustrate the importance of, but do not show, a conclusive picture of the multitude of possible carbon-cycle-climate system feedbacks. Moreover, strong interactions between the natural processes described here and anthropogenic changes in land use, cover and management have to be expected.”
An Earth-system perspective of the global nitrogen cycle p 293 “Can management of the global nitrogen cycle help to mitigate climate change? Although various options have been proposed in the past, such as fertilization of forests and marine ecosystems, the scientific consensus is that their effectiveness is generally low, and that unintended negative consequences could be serious20. Therefore, the best strategy for reducing the potential threat from human activity in the ‘Anthropocene’ – this modern age in which humans have a significant impact on the Earth system – is to reduce the burning of fossil fuels.”
A steep road to climate stabilization p 297 ” … simulations carried out with coupled climate and carbon-cycle models indicate that changes in climate will result in even greater reductions in the ability of land and the ocean to absorb anthropogenic CO2 by the end of the twenty-first century5. These simulations suggest that the combination of warming and drying will limit photosynthesis by plants and stimulate the decomposition of organic matter in soil, reducing the capacity of land-based ecosystems to store carbon (see page 289). In addition, it is widely thought that global warming will result in slower ocean circulation, leading to a decrease in the amount of carbon that is exported from the surface to the deep ocean and thereby reducing the flux of carbon from the air to the ocean. So it seems that future warming will reduce carbon sinks, leaving more CO2 in the atmosphere and leading, in turn, to greater warming. This positive-feedback loop has implications for the pathway to stabilizing the concentrations of atmospheric greenhouse gases. If land-based and ocean ecosystems store less carbon than is expected in the future, then a greater effort will be needed, in terms of reducing anthropogenic emissions, to achieve a given concentration of atmospheric CO2. The potential importance of this effect is illustrated by simulations carried out for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). These simulations indicate that to stabilize atmospheric CO2 concentrations at 450 parts per million (generally accepted as ‘safe’) by 2100, cumulative emissions in the twenty-first century need to be reduced by a further 30% when this feedback is taken into account.