Modeling the Regional and Global Climate Responses to Holocene Land Surface Change

Widespread land surface changes took place in the Northern Hemisphere throughout the Holocene epoch (11,700 years ago to the present). The most notable of these changes were large increases in vegetation cover that occurred throughout the African Sahara. Yet, the varied responses of regional and global climate to this land surface change are not well understood because few modeling studies have directly incorporated it in their experiments. This dissertation presents new Earth system modeling results that include land surface changes in the African Sahara and Northern Hemisphere mid- and high latitudes to directly identify the responses of regional and global climate to these changes. The chapters in this dissertation provide various resolutions to several challenges in reconciling Holocene model and proxy reconstructions of both hydroclimate and temperature. Chapters 2 and 3 investigate challenges related to the impact of African Saharan greening, known as the “Green Sahara”, on the West African monsoon during the mid-Holocene. Conclusions from these chapters better constrain the mid-Holocene response of African hydroclimate to a wide array of land surface processes associated with the Green Sahara. Chapter 2 explores the competing impacts of the direct radiative and indirect aerosol-cloud effects associated with the vegetation-induced reduction in dust aerosols, providing an improved understanding of the West African monsoon response to vegetation and dust forcing in the mid- Holocene. Chapter 3 examines how vegetation-induced changes in the isotopic composition of precipitation and soil water help to constrain estimates of the monsoon’s northernmost limit. By showcasing a previously unknown positive anomaly in the isotopic composition of precipitation, this chapter suggests a mid-Holocene northernmost limit of ~23–28°N for the West African monsoon. Chapters 4 and 5 expand the spatial and temporal scope of this dissertation and investigate external climate responses, both regional and global, to Holocene land surface change. Both of these chapters offer increased Northern Hemisphere vegetation as a resolution to challenges related to reconstructing Holocene climate with model simulations and geochemical proxies. Chapter 4 explores the opposing isotopic signature of the North American monsoon in comparison with other Northern Hemisphere land monsoons and shows that the Green Sahara is likely responsible for this opposing behavior by modulating the local Walker circulation. Seasonal exploration of hydroclimatic and stable water isotopic changes elucidates the factors contributing to the evolution of the North American monsoon. Chapter 5 investigates the controversial model-data disagreement in reconstruction of Holocene global temperatures known as the Holocene Temperature Conundrum. By inclusion of Holocene increases in vegetation in the African Sahara and Northern Hemisphere mid- and high latitudes, the results of this chapter suggest that vegetation change drives a mid-Holocene maximum in annual global mean temperatures and better aligns model simulations with proxy data. Taken together, findings from this dissertation highlight the importance of vegetation in driving past climate change and in reconciling results from model simulations with geochemical proxy data.