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Title: Earth System impact of historical and future reforestation
Author: Koch, Alexander
ISNI:       0000 0004 7661 2329
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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Both aggressive mitigation strategies and negative emission technologies are required to limit global warming to 2 ◦ C. Tropical forest restoration is widely seen as a potential way to achieve negative emissions. Yet its maximum potential and the carbon cycle response to it have not been quantified. This PhD starts by looking at the only comparable large scale reforestation event in human history - the cessation of agriculture caused by the epidemics-driven deaths of 90 % of the indigenous population of the Americas after 1492 CE. I quantify the magnitude of vegetation regrowth and carbon uptake after European arrival in the Americas (55.8 million ha and 7.4 Pg C respectively) based on published population, land use, and biomass estimates. This is compared to the observed changes in climate and the carbon cycle at the time. Given the uncertainty in the observations, a series of idealised experiments are performed with an Earth System Model (CESM). These show a high sensitivity of the carbon cycle impact to the land use reconstruction used and that factorial forcing experiments would be required to conclusively determine the impact of the 1492 CE land use change. To better understand the Earth system response to large scale tropical forest restoration such an experiment is performed with a state-of-the-art Earth System Model. An experiment where all land use in the tropics (1,815 million ha) is rapidly stopped under an aggressive mitigation scenario (RCP 2.6) is performed with HadGEM2-ES. The resulting secondary succession results in a small CO2 (∼9.5 ppm) reduction by 2100 CE, but does not translate to a detectable decline in global temperature. Furthermore, negative carbon cycle feedbacks offset 65% of the benefits from terrestrial carbon uptake. I conclude that reforestation alone is insufficient to provide the substantial negative emissions.
Supervisor: Brierley, C. ; Lewis, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available