As human development progresses rapidly, changes in land use become increasingly frequent, with deforestation emerging as a particularly rapid phenomenon since the industrial revolution. Consequently, understanding the ramifications of deforestation on climate change becomes paramount. While previous studies have predominantly examined the effects of deforestation on precipitation and temperature, the impacts of deforestation on drought, i.e. the interactions among precipitation, temperature, and other meteorological factors, remain understudied. This thesis aims to systematically analyze the influence of deforestation on drought across various temporal scales and geographical regions, drawing from a diverse range of data sources including observations and model outputs. The objective is to make significant contributions to the formulation of climate-focused land-use policies. Initially, historical forest and climate datasets spanning from 1992 to 2018 are employed alongside a series of linear models and analysis of variance methods to explore the impact of forest cover change, precipitation, and temperature on drought across diverse time scales and climate zones. Among these factors, precipitation emerges as the predominant driver of drought in equatorial, temperate, and snow regions, while temperature exerts control over drought occurrence in arid areas. Notably, precipitation moderates the influence of forest cover on long-term drought in arid regions, whereas temperature moderates the effects of forest cover changes on both short- and long-term drought in arid regions, as well as solely on long-term drought in temperate regions. Moreover, high forest cover fosters a combined effect of precipitation and temperature on long-term drought in arid and snow regions, whereas precipitation alone dominates under low forest cover conditions. Conversely, low forest cover triggers a robust combined effect of precipitation and temperature on drought in temperate regions. Subsequently, leveraging idealized deforestation experiment (deforest-global) and the pre-industrial control experiment (piControl) from the Land Use Model Intercomparison (LUMIP) project, deforestation accentuates dryness globally, particularly in equatorial regions, but enhancing moisture in dry zones. The impact on drought indices intensifies with longer time scales. Seasonally, deforestation amplifies short- term droughts in autumn and winter globally, notably impacting equatorial and northern polar regions. Snow zones witness significant seasonal shifts, becoming drier in winter and wetter in summer following global deforestation, while the northern dry regions experience heightened moisture, especially during autumn. Across Europe, forest alterations due to extensive land management policies are observed. Using idealized deforestation simulations (GRASS) and idealized forestation simulations (FOREST) from the Land Use and Climate Across Scales (LUCAS) project, an increase in drought severity is noted post-deforestation, particularly prominent in northern European regions and during prolonged drought periods. Additionally, significant monthly variability in the impact of deforestation on drought is evident, particularly in northern Europe (Scandinavia), where drought exacerbates during winter but tends to alleviate during summer months. Furthermore, an analysis of influencing factors in drought index calculation, namely precipitation and potential evapotranspiration, is conducted. In terms of climatic changes, alterations in precipitation closely correspond with changes in drought indices, whereas monthly fluctuations in potential evapotranspiration align with drought indices variations. These findings deepen our understanding of the intricate interplay between shifts in vegetation patterns and fluctuations in climate dynamics, thus serving as a cornerstone in advancing our ability to manage natural resources more effectively. Furthermore, they contribute significantly to our capacity to proactively mitigate the looming threats posed by climate change, paving the way for more sustainable and resilient ecosystems in the future.
