MODELING THE THERMAL AND CARBON DYNAMICS OF GLOBAL FRESHWATER ECOSYSTEMS

Freshwater ecosystems, including lakes, reservoirs, and ponds (lakes hereafter if not specifically noted), are a critical component of the earth system. They exert strong influences on the global water cycle, energy balance, and carbon budget. Lakes modulate local atmospheric boundary layer conditions by affecting fluxes of heat, moisture, and momentum. Lakes and wetlands are the single largest natural methane source. Covering only 3.7% of the Earth’s surface area, lakes are estimated to be responsible for 6-16% of natural methane emissions. Under global warming, a more severe emission scenario is expected. Due to enhanced microbial activities, old carbon exposed from thawed permafrost, and thermokarst lake expansion, the increase in methane emissions could be dramatic. The recent estimates still bear the largest uncertainties among all natural sources.

Understanding lake thermal regimes and carbon dynamics and the driving force under climate change is of great importance. The primary aim of this study was to investigate the mechanism of lake thermal regime and methane emission changes and their uncertainties through modeling. To achieve this, the algorithms of a 1-D lake biogeochemical model were improved. Simulations of methane emissions during the current period and the last decade of the century were run for the Finnish boreal region and the whole globe. Through model sensitivity tests and algorithm intercomparison, strategies for 1-D model improvement, calibration and running at large scales were developed to reduce human efforts and computational resources, which is often a pain point in applying lake models to regional and global issues. The spatiotemporal trends of total methane emissions were investigated, resulting in a lower value than previously assumed, which reduces the large gaps between satellite-inversed and statistically-upscaled numbers. The major factors controlling lake thermal changes in surface temperature, stratification and mixing, snow phenology, and methane emissions were examined. It was found that warming-induced direct factors, including longer ice-free days and enhanced microbial activities, play the dominant role in lake methane emission increase in the future compared to permafrost thaw and thermokarst lake surface area changes.

This thesis is organized in the form of a collection of published (or under-review) papers. The first two papers introduce my work on developing and improving the thermal module in 1-D lake models especially from a large-scale perspective by analyzing the relative importance and algorithm performances of different thermal processes. Both projects were part of the Inter-sectoral Impact Model Intercomparison Project (ISIMIP). In Chapter 2, I conducted simulations and sensitivity tests based on 53 lakes globally to validate the ALBM and examine the impacts of different thermal processes on the simulation accuracy. As a result, four processes including convective heat transfer, wind-induced turbulent mixing, light distinction, and snow density were identified as the key thermal processes. The parameter values in the corresponding algorithms together accounted for more than 95% of the variation in the modeled water temperature, stability, and ice phenology. Based on the conclusion in Chapter 2, an algorithm intercomparison study was carried out in Chapter 3 using the same global lake observation dataset. A model improvement and large-scale calibration strategy was proposed with the best-performing algorithm (or algorithm combination) and the parameter value selected for each process. The third paper presented in Chapter 4 is an application of the model to Finnish boreal lakes. Simulations of diffusive fluxes from all lakes in Finland were run for the 1990s and the first and last decade of the century. Furthermore, the mechanism of the increasing emissions was examined, and the increasing ice-free season was identified as a major impact factor compared to the warming water temperature. Finally, in Chapter 5, the global lake methane emissions were estimated and predicted, and the climate warming impacts were investigated. Through an improved simulation approach, I showed that the previous bottom-up calculation using the statistical upscaling method could overestimate the total emissions and our result could better fit into the top-down estimated global methane budget. Modeling tests were run to evaluate and compare the importance of various impact factors, including increasing air and water temperature, increasing ice-free season, carbon load from permafrost thaw, and thermokarst lake areal changes, on total lake methane emissions. It was found that although warming and permafrost carbon thaw would lead to higher methane production rates, methane oxidation in water columns could be an efficient sink to moderate the impacts.