Patterns, mechanisms, and legacies of abrupt climate change: examining the Younger Dryas in eastern North America

Abstract

Increasing global temperatures from anthropogenic greenhouse gas emissions are driving widespread climatological and ecological changes globally. Abrupt global changes that share rates of climate change similar to those experienced today (Overpeck et al. 2003; Williams and Burke 2019) are recorded throughout the geologic record and offer important insights that can help predict future anthropogenic change. The Deglacial period (19,000 to 11,000 years before present) after the Last Glacial Maximum has been a key interval for understanding ecological and climatological responses to increasing greenhouse gas concentrations and a warming climate (COHMAP Project Members 1988; Nolan et al. 2018; Mottl et al. 2021). Imposed on this gradual warming are abrupt climate oscillations that onset within decades to centuries, last for millennia, and are commonly attributed to changes in the Atlantic meridional overturning circulation forced by the input of freshwater into the North Atlantic Ocean. The most recent of these millennial-scale climate events is the Younger Dryas (ca. 12,900 to 11,700 years before present) and caused spatially complex climate changes globally. In this dissertation, we first aim to determine the spatial patterns of climate change and the atmospheric mechanisms responsible for driving abrupt climate change regionally in eastern North America through the use of organic temperature biomarkers (brGDGTs) and climate models. Second, we seek to disentangle the contributions of glacial and millennial-scale climate variability upon modern patterns of species richness in eastern North America. Chapter 2 seeks to determine the spatial fingerprint of Younger Dryas temperature changes in eastern North America. We develop a spatially dense multiproxy network of temperature reconstructions relying upon statistical transfer functions applied to fossil-pollen abundances and an independent proxy, based on organic biomarkers (brGDGTs). This analysis indicates that temperature changes during the Younger Dryas followed a dipole pattern in eastern North America. Temperatures lowered abruptly in maritime Canada and the northeastern United States nearly synchronously with temperature records from Greenland (Severinghaus et al. 1998). Cooling is also reconstructed in the Great Lakes region but delayed by ~400 years. Sites south of 35°N exhibited an antiphased response and lack YD cooling, with Florida sites indicating a thermal maximum. Warming in Florida during the Younger Dryas suggests that the ‘bipolar-seesaw’ conceptualization is an oversimplification of the spatial patterns of global climate changes. Focus must be placed on constraining regional climate changes to refine the mechanisms of abrupt climate change. Chapter 3 aims to better understand the atmospheric mechanisms for these antiphased temperature changes in eastern North America. We accomplish this by combining our multiproxy temperature network with a synthesis of hydroclimate reconstructions to compare against four climate models with meltwater hosing experiments that resemble the onset of the Younger Dryas. Precipitation changes followed a tripole pattern with wetting in the northeastern United States and Florida and drying from the Great Lakes region to the Carolinas, in contrast to the temperature dipole resolved in Chapter 2. Analysis of the climate models highlights the dual role of ice sheets and meltwater-induced weakening of the Atlantic meridional overturning circulation as the key drivers of the reconstructed warming and wetting in the southeastern United States. Reduced northward oceanic heat transport in the Atlantic Ocean increased the latitudinal temperature gradient and strengthened the jet stream, leading to upper-level divergence over eastern North America and the transport of warmer and moister air into the southeastern United States. For Chapter 4, we use our multiproxy temperature and precipitation reconstruction from prior chapters, alongside 11 climate simulations of millennial-scale climate events forced by meltwater pulses, to assess whether legacies of these climate changes can be detected in the contemporary diversity of amphibians, birds, mammals, reptiles, and trees in eastern North America. Generalized additive models that use both contemporary and paleoclimatic predictors suggest that past millennial scale climate oscillations have left an imprint on contemporary amphibian and arboreal biodiversity, though the exact role of past climate changes remains uncertain. Generalized additive models that use the multiproxy network of Younger Dryas climate reconstructions and a subset of the climate models analyzed suggest that greater millennial scale climate variability is predictive of greater contemporary biodiversity. However, generalized additive models that use four of the climate models suggest that millennial-scale climate stability is predictive of greater contemporary richness in eastern North America. Disagreement in the sign, magnitude, and spatial fingerprint of climate changes among the 11 climate simulations and the multiproxy climate reconstructions precludes further refining the role of millennial-scale climate oscillations at this time. This uncertainty highlights that caution should be used when attempting to model contemporary biodiversity based on individual paleoclimatic simulations. Higher resolution climate simulations forced with accurate boundary conditions are necessary to constrain the relationship between past millennial-scale climate changes and contemporary biodiversity.