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Title: Investigating increases in terrestrial carbon uptake
Author: O'Sullivan, Michael Dominic
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2018
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Anthropogenic CO2 is the most important long-lived greenhouse gas, and with atmospheric concentrations over 40% above pre-industrial levels, it is the main cause of climate change. The terrestrial carbon sink has increased in proportion with anthropogenic CO2 emissions over the last century and dampened climate change. Yet, the mechanisms behind the increase remains a puzzle and this limits our predictive abilities to estimate climate-carbon feedbacks in projections of future climate changes. Since the turn of this century, the terrestrial sink has substantially increased during a time of rapid increase in fossil fuel CO2 emissions. In this thesis, the influence of increased diffuse radiation (due to anthropogenic aerosol) and nitrogen deposition (and carbon-nitrogen deposition synergies) associated with increased fossil fuel emissions on terrestrial carbon uptake, since the turn of this century is quantified. I also assessed interannual variations (IAV) and trends in climate-driven gross primary productivity (GPP) over the period 1982-2016. The distribution of atmospheric aerosols due to anthropogenic fossil fuel emissions was simulated with chemical transport-aerosol model TOMCAT-GLOMAP over the period 1998- 2010. The influence of aerosols on incoming solar radiation was calculated with the radiative transfer model Edwards-Slingo. Subsequently, the impact of variations in direct and diffuse radiation on net primary productivity (NPP) are quantified with the land-surface model JULES. Secondly, the Community Land Model (CLM4.5-BGC) was used to estimate the influence of changes in atmospheric CO2, nitrogen deposition, climate, and their interactions to changes in net primary production (NPP), and net biome production (NBP) over the historical (1901-2016) and modern (1990-2016) periods. Finally, the influence of climate on vegetation productivity is evaluated using upscaled flux tower observations (FLUXCOM), a satellite-based light use efficiency model (LUE), and a set of process based terrestrial biosphere models (TRENDYv6). I estimate that at global scale, changes in light regimes from fossil fuel aerosol emissions had only a small negative effect (-0.08 PgC/yr) on the increase in terrestrial NPP over the period 1998-2010 (overall increase of 1.7 PgC/yr). Hereby, the substantial increases in fossil fuel aerosol emissions and subsequent plant carbon uptake over East Asia (33% of the 0.53 PgC/yr increase) were effectively cancelled by opposing trends across Europe and North America. Over 1901-2016, nitrogen deposition and carbon-nitrogen synergistic effects contributed ~30% to increase in NBP (overall increase of 2.31 PgC/yr). However, since the turn of this century, nitrogen-related mechanisms had no contribution to the increasing sink. Opposing regional NBP trends due to nitrogen deposition (50 TgC/yr increase in East Asia, and 14 TgC/yr and 6 TgC/yr decreases in North America and East Europe, respectively) cancel at the global scale. Nonetheless, I find that increased nitrogen deposition in East Asia since the early 1990s contributed 50% to the overall increase in NBP over this regions, highlighting the importance of carbon-nitrogen interactions. Therefore, potential large-scale changes in nitrogen deposition could have a significant impact on terrestrial carbon cycling and future climate. Regarding the impact of climate variations on vegetation productivity, I find large differences in global and regional trends between the observational and modelling products. FLUXCOM has a small positive global GPP trend (0.02±0.01 PgC/yr2) and has a smaller IAV than both other products. The TRENDYv6 models simulate a positive global GPP trend of 0.09±0.06 PgC/yr2, whereas the LUE product has a smaller increase of 0.06±0.02 PgC/yr2. For TRENDYv6 and LUE, the global pattern is predominately driven by a large northern latitude increase (driven by a warming trend). The products agreed on the direction of trend across 60% of the vegetated land surface. Large areas of Eurasia and North America exhibit positive summer trends due to warming. In contrast, warming in central South America has decreased GPP. Negative precipitation trends in South US and Mongolia/China have reduced GPP, whereas a wetting trend in South Africa has enhanced productivity. There are clear differences in the sensitivity of each product to climate forcing, highlighting uncertainty in the processes that influence GPP. These differences in variability and trends in GPP, as well as underlying climatic controls in the three most commonly used products in carbon cycle studies, highlight the need for longterm observations of GPP, particularly in underrepresented regions (e.g. tropical forests), and the need to better constrain and improve GPP estimates from models.
Supervisor: Buermann, Wolfgang Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available