Patterns and drivers of Holocene moisture variability in mid-latitude eastern North America

Proxy data for North American hydroclimate (e.g., pollen and other microfossils, sedimentological data, and stable isotopes) indicate major variations in moisture balance including persistent multimillennial droughts during the Holocene. When synthesised with paleoclimate model simulations, the proxies also allow the testing of hypotheses about the drivers and feedbacks involved in the past moisture variations. Recently, advances in (A) the availability of high-resolution proxy records, (B) the numerical methods used to extract the hydroclimate signal from the proxies, and (C) the transient Earth system model simulations of the Holocene, have opened exciting new avenues in unraveling the spatiotemporal progress, magnitudes, and causes of Holocene North American droughts.

Here, we apply new machine-learning based (boosted regression tree) pollen–climate calibrations to reconstruct annual water balance and July temperature from 66 fossil pollen sequences from the eastern North American mid-latitudes. Based on these data, we prepare synthesis paleoclimate reconstructions for three regions, spanning from the prairie–forest ecotone to the eastern seaboard, designated as Midwest (MW), Great Lakes (GL), and Northeast (NE). The proxy reconstructions are complemented by state-of-the-art model simulations, including the EC-Earth and MPI-ESM transient runs for 8–0 ka and CESM1 equilibrium runs for 12, 11, and 9 ka.

Our water balance reconstructions confirm the major regional offset in drought timing suggested by earlier proxy data, with NE reaching peak drought by 11 ka but with a progressively later timing seen westward in GL (~10 ka) and MW (~7 ka). This spatiotemporal pattern is also reproduced in the model simulations. In the early Holocene simulations, the dipole of wet conditions in MW vs. dry in NE can be clearly linked to dynamical changes in atmospheric circulation, linked to anticyclonic blocking over the residual Laurentide Ice Sheet. This confluence between our water balance reconstructions and the new-generation model simulations gives strong support to the hypothesis that the broadscale drought progress can be explained by the combination of decreasing summer insolation and the waning of the Laurentide glacial anticyclone, which diverted the northward moisture advection from the mid-continent towards the eastern seaboard. Beyond these multimillennial patterns, our proxy reconstructions show coherent centennial events in moisture and temperature. Wavelet analyses of the reconstructions and the transient simulations reveal significant periodicities in both water balance and July temperature, most commonly in the 0.2–0.6 ka wavelength range in the reconstructions and at 0.1–0.2 ka in the simulations.

In the MPI-ESM and EC-Earth simulations, the mid-Holocene drought is driven by a combination of lower-than-preindustrial precipitation together with increased potential evapotranspiration due to warmer summer temperatures. The relative drought through most of the Holocene, seen in both the reconstructions and the simulations, suggests that the recent and modern climate is unusually wet while drier conditions seem to be the norm during the Holocene. Looking towards the future, it is hence plausible that reverting the natural neoglacial cooling of the recent millennia with anthropogenic global warming might cause a return of drier conditions due to a higher evaporative demand that is not compensated by higher precipitation.