A model-based investigation of the long-term climate and carbon cycle consequences of fossil fuel emissions, land use changes and negative emissions

Abstract

In this thesis, we investigate the effects of anthropogenic perturbations such as fossil fuel emissions, deforestation, afforestation and negative emissions on the long-term climate and carbon cycle using simulations from an Earth system model. First, we analyze the climate and carbon cycle consequences of the differences in fossil fuel (an external source of carbon) and deforestation (an internal source) emissions on millennial timescales. While fossil fuel emissions result in substantial climate change that remains on millennial timescales, regrowth of vegetation following deforestation restores the climate system close to preindustrial conditions. This shows that the source of carbon emissions (external vs internal) matters for the climate and carbon cycle impacts.

Next, we compare the effectiveness of afforestation with an equivalent reduction in fossil fuel emissions for climate change mitigation. Our simulations show that for the same amount of reduction in atmospheric carbon, reducing fossil fuel emissions is more effective because the biogeochemical cooling effect (due to the removal of carbon from the atmosphere) of afforestation is partly offset by its biophysical warming effects (due to changes in land surface properties such as surface albedo, evapotranspiration, and surface roughness). However, our study also finds added benefits of afforestation such as a reduction in ocean acidification, despite its reduced effectiveness for cooling the climate.

Finally, we study the path dependence of the response of the climate system in scenarios where the carbon emitted into the atmosphere is completely removed by negative emission technologies such as direct air capture. The sensitivity of the climate and carbon cycle to the amount and duration of the emission and removal pulses are analyzed. We find that the irreversibility of changes in global mean surface temperature on timescales relevant to humanity increases with an increase in the amount and duration of the emission and removal pulses. Further, we show that the hysteresis in the climate system is larger for larger amounts and shorter duration of the emission and removal pulses. Our findings in the present thesis highlight the need for an early reduction in fossil fuel emissions in scenarios with or without the use of negative emissions for climate change mitigation.