Holocene climate change, ecosystem response and human impacts

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This research program studies the evolution of the earth ecosystem during the transition from the glacial to interglacial period. It has as goal understanding how the climate system has changed in the past, in response to various forcing factors including human impacts. We reconstruct past climates and vegetation using the fossil record, and develop and use archives of past climate and environmental changes such as lake sediment, ocean and ice cores. The main focus of this research program is the mapping and statistical analysis of databases of paleoclimates, paleo-vegetation and past human populations to understand the ecosystem evolution of North America, and, in the longer term, globally. The time scale is the past 20,000 years; the transition from a glacial period to an interglacial and into the Anthropocene. We will use a global database of past climates, derived from many different proxies (microfossils, geochemical parameters, isotopes, biomarkers, etc,) collected from lake sediments, ice cores, ocean sediments, tree-rings and other sources to understand how the climate has changed over the past 20,000 years. Using mapping and statistical analysis, we will study if the climate system changes slowly or abruptly, when were the maximum and minimum temperatures by region, how precipitation changed in time and space and when were major transitions in global and regional climates. We will explore new data-analytic methods, such as machine learning, in addition to classical methods of multivariate, time series and spatial statistics to summarize and map these data. Then we will use pollen databases, which are available from many parts of the world, to reconstruct past vegetation dynamics. Vegetation changes will be associated with the climate changes determined independently to see if terrestrial ecosystems respond rapidly to climate changes, or if there are thresholds or lags in vegetation change to climate variability of several timescales. An innovative part of the proposal is to quantify human impacts on the vegetation and climate over the Holocene. We use a global archaeological radiocarbon database to map and analyze human population density though time. These paleodemography results are interpreted as proportional to human impact on the environment, and are statistically associated with vegetation and climate changes to study (a) did past climate changes affect human population numbers, and/or (b) are vegetation composition and climate associated with changes in human numbers (early Anthropocene hypothesis). We also study human impact on natural fire regimes using a charcoal database. These studies are done at regional to continental scale, as appropriate, and the overall goal is a global analysis of how past climate changes drove vegetation responses, which in turn helped determine human population changes.