| Summary: | 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 ...
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