Modelling the effects of land use on the climate
De Hertog, S. (2023). Modelling the effects of land use on the climate. PhD Thesis. Vrije Universiteit Brussel, Department of Hydrology and Hydraulic Engineering: Brussels. ISBN 9789464443714. xxvi, 162 pp. |  |
| Available in | Authors | | Document type: Dissertation
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| Authors | | Top | - De Hertog, S., more
- Nossent, J., revisor, more
- Huybrechts, P., revisor, more
| - De Graeve, I., revisor, more
- Pongratz, J., revisor
- Davin, E.L., revisor
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| Abstract | Land cover and land management changes can alter local, regional, and global climate. This can be by altering the biogeochemical fluxes due to a change in carbon sequestration potential which can alter global greenhouse gas concentrations and therefore global climate. However, it can also be through changes in the physical properties of the surface which in turn alter the energy and water balance and thereby affect regional to local climate. These effects are called the biogeophysical effects and remain poorly constrained, with high uncertainties about both their sign and magnitude. Land cover and land management changes are likely needed towards the future to achieve low-warming future scenarios through land-based mitigation. Therefore, it is of importance that land cover and land management induced climate effects are better understood in order to fully grasp their potential within the context of future mitigation and adaptation. This thesis aims to better constrain the effects of land cover and land management on climate and to illustrate their importance for future mitigation and adaptation potential. To this end, we use Earth System Models that fully asses the climate feedbacks arising from land cover and land management changes.
In the first study, we perform highly idealised simulations of four different types of land cover and land management conversions, (i) cropland expansion, (ii) afforestation, (iii) irrigation expansion and (iv) wood harvesting. These simulations are performed with three different Earth System Models to grasp the inter model uncertainty. The land cover and management changes are performed in a checkerboard pattern (alternating patches of change with patches of no change) to disentangle local from non-local biogeophysical effects. The effects of land cover changes on surface temperature are consistent with observational studies except over certain regions for some Earth System Models. Cropland expansion generally causes a local warming in the tropics and a cooling in boreal latitudes which are consistent across the earth System Models. This pattern was similar but of opposite sign for the local effects of afforestation. The non-local effects, in contrast, are not consistent across the models for cropland expansion (ranging from global cooling to regional warming) while these are consistently a non-local warming for afforestation. Irrigation generally causes a cooling, although the Earth System Models disagree whether this cooling is local or non-local. Wood harvesting was not found to have any clear effects on grid-scale surface temperature. The local surface temperature responses were mostly dominated by changes in turbulent heat fluxes. This analysis overall highlights the importance of separating local and non-local biogeophysical effects to better understand inter-model differences.
In the second study, we further analyse the idealised Earth System Model simulations for effects on evaporation, precipitation, and vertically integrated moisture flux convergence. We apply a moisture tracking algorithm to assess the effects of land cover and land management on both continental and local moisture recycling. The effects of land cover and land management changes over land are generally consistent across the different Earth System Models. Cropland expansion reduces evaporation, precipitation, and local moisture recycling, while afforestation and irrigation expansion cause the opposite effect. However, the signal does vary strongly in time and space and different patterns emerge within different Earth System Models, which relates to mechanisms dominating the overall change (from global circulation changes to more localised effects). Our results underline the importance of land cover and land management induced effects on moisture fluxes and moisture recycling and highlight some differences across Earth System Models which need to be considered before they can be applied to inform land-based adaptation strategies.
In the third study, we perform future simulations under a 1.5°C-compatible future scenario with one Earth System Model. These simulations are performed under different land cover and land management scenarios which represent two strongly different worlds. The sustainability scenario represents a world which converges socioeconomically and where greenhouse gas pricing and environmental protection is implemented globally. In the inequality scenario, in contrast, such degree of sustainability is only achieved in the wealthier countries while the rest of the world continues under current trends. The results were analysed for global mean temperature, heat stress and downstream socioeconomic impacts such as changes in labour capacity and temperature-related mortality. Achieving a sustainable land cover compared to the inequality scenario is shown to cause a global cooling of ca. 0.3 °C and a clear reduction in heat stress over land. Adverse impacts on humans generally decrease in a world with sustainable land cover change instead of inequality, with notably higher labour capacity and lower heat-related mortality over the tropics. Cold-related mortality, in contrast, rises in some locations such as Northwestern Europe. The adopted land cover and management scenario is crucial for assessing future climate change and should not be neglected within future mitigation and adaptation strategies.
Overall, this thesis advances the current understanding of the effects of land cover and land management changes on climate. We provide an improved understanding of biogeophysical effects by consistently separating local and non-local effects across different Earth System Models and land cover and management conversions specifically for surface temperature. Our results also highlight some remaining uncertainties related to specific Earth System Models, but also to the signal separation approach used which warrant more research. Finally, a second group of simulations illustrates the importance for including land cover and land management change induced climate effects within future land cover scenarios for temperature but also impacts on humans. Land cover and land management induced climate effects should therefore be carefully considered within the design of future land-based mitigation and adaptation pathways. |
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