Predictable hydrological and ecological responses to Holocene North Atlantic variability Bryan N. Shumana,1, Jeremiah Marsicekb, W. Wyatt Oswaldc, and David R. Fosterd aDepartment of Geology and Geophysics, University of Wyoming, Laramie, WY 82072; bDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706; cInstitute for Liberal Arts and Interdisciplinary Studies, Emerson College, Boston, MA 02116; and dHarvard Forest, Harvard University, Petersham, MA 01366 Edited by David J. Sauchyn, University of Regina, Regina, SK, Canada, and accepted by Editorial Board Member David W. Schindler February 5, 2019 (received for review August 20, 2018) Climate variations in the North Atlantic region can substan- AMOC is strong, heat is transferred northeast into the subpolar tially impact surrounding continents. Notably, the Younger Dryas region, while the NAM cools (1, 6). In winter, the NAO expresses chronozone was named for the ecosystem effects of abrupt similar atmospheric temperature anomalies (Fig. 1D). Multi- changes in the region at circa (ca.) 12.9–11.7 ka (millennia before century variability in the NAM is recorded by alkenones from 1950 AD). Holocene variations since then, however, have been cores such as OCE326-GGC30 off Nova Scotia (7) (Fig. 1A,dark hard to diagnose, and the responsiveness of terrestrial ecosystems blue line) and by correlated changes in pollen from coastal lakes continues to be debated. Here, we show that Holocene climate (8). Consistent with AMOC or NAO dynamics, the centennial variations had spatial patterns consistent with changes in Atlantic variability shows an antiphased relationship with temperatures in overturning and repeatedly steepened the temperature gradient Greenland, which is sensitive to the state of the subpolar North > between Nova Scotia and Greenland since 8 ka. The multicentury Atlantic [as represented by the Greenland Ice Sheet Project 2 changes correlated with hydrologic and vegetation changes in the (GISP2) ice core (9); light blue line in Fig. 1A]. northeast United States, including when an enhanced temperature Three patterns of temperature variation in the Greenland– gradient coincided with subregional droughts indicated by water- NAM gradient exist at 102–104 y (Fig. 1 A and B). First, long- level changes at multiple coastal lakes at 4.9–4.6, 4.2–3.9, 2.8–2.1, term progressive cooling of the NAM, apparent in the original SCIENCES and 1.3–1.2 ka. We assessed the variability and its effects by rep- alkenone time series from OCE326-GGC30 (7), would have ENVIRONMENTAL licating signals across sites, using converging evidence from mul- > tiple methods, and applying forward models of the systems decreased the temperature gradient by 5 °C over the Holocene involved. We evaluated forest responses in the northeast United (Fig. 1B, orange line). However, the cooling does not appear in States and found that they tracked the regional climate shifts in- adjacent continental records, which indicate lower (rather than cluding the smallest magnitude (∼5% or 50 mm) changes in effective precipitation. Although a long-term increase in effective precipita- Significance > > tion of 45% ( 400 mm) could have prevented ecological commu- ECOLOGY nities from equilibrating to the continuously changing conditions, The importance of climate change arises from effects on nat- our comparisons confirm stable vegetation–climate relationships ural resources like water and ecosystems. Diagnosing the pre- and support the use of fossil pollen records for quantitative dictability of these effects in the past can help to anticipate paleoclimate reconstruction. Overall, the network of records indi- future changes, while also clarifying the paleoclimate record. cates that centennial climate variability has repeatedly affected We examined >8,000 y of climate variations and their effects in the North Atlantic region with predictable consequences. the North Atlantic region, where ocean and atmosphere pro- cesses have often produced global consequences and altered drought | vegetation | North Atlantic | Holocene | climate climates from one century to the next. We found that repeated temperature variations across the region followed patterns his paper examines the expression of Holocene climate var- consistent with climate dynamics that affected water and for- Tiability in the North Atlantic region and then tests hypotheses ests in North America. Forest plant communities changed rap- about the attendant ecological responses on adjacent continents. idly and predictably, which underscores both the risk of future Both aspects of the paper relate to the overarching hypothesis that change and the potential to reconstruct paleoclimates using multicentury variability in Atlantic Meridional Overturning Cir- fossil evidence. culation (AMOC) (1, 2) or in the atmospheric dynamics underlying Author contributions: B.N.S., W.W.O., and D.R.F. designed research; B.N.S. and J.M. per- the North Atlantic Oscillation (NAO) (3) had substantial influ- formed research; B.N.S. and J.M. analyzed data; B.N.S. wrote the paper; and J.M., W.W.O., ences on synoptic climates and thus the ecosystems of the North and D.R.F. contributed to drafting the paper. Atlantic region during the Holocene. If so, quantitative paleo- The authors declare no conflict of interest. environmental records across the region should contain replicable This article is a PNAS Direct Submission. D.J.S. is a guest editor invited by the and synoptically articulated signals, although the importance of Editorial Board. these climate signals to ecological dynamics has been uncertain. Published under the PNAS license. Marine records of Holocene AMOC vary (2, 4, 5), but the Data deposition: All data analyzed for this study are publicly available through the Na- climate dynamics affecting the hydrology and ecology of adjacent tional Oceanic and Atmospheric Administration Paleoclimate database, including the temperature reconstructions from GISP2 (https://www.ncdc.noaa.gov/paleo-search/study/ landmasses must have also included large-scale atmospheric 2475)andGGC30(https://www.ncdc.noaa.gov/paleo-search/study/6409). The lake-level changes even if multicentury climate variability in the region reconstructions are archived at https://www.ncdc.noaa.gov/paleo-search/study/16094 originated in the ocean. Therefore, we first describe changes in the (for Davis Pond), https://www.ncdc.noaa.gov/paleo-search/study/16095 (for Deep Pond), and https://www.ncdc.noaa.gov/paleo-search/study/23074 (for New Long Pond). The marine and atmospheric temperature gradient across the north- pollen-inferred precipitation reconstructions are archived at https://www.ncdc.noaa. west Atlantic, which may relate to the spatial temperature “fin- gov/paleo-search/study/23072. gerprint” of AMOC variability (6) (Fig. 1 C and D). AMOC 1To whom correspondence should be addressed. Email: [email protected]. variability produces contrasting temperature changes in the North This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Atlantic Ocean along the North American margin (NAM) and in 1073/pnas.1814307116/-/DCSupplemental. the larger subpolar region surrounding much of Greenland. When www.pnas.org/cgi/doi/10.1073/pnas.1814307116 PNAS Latest Articles | 1of6 Downloaded by guest on September 25, 2021 Fig. 1. Holocene temperature histories (A) from Greenland, GISP ice core (9), and the NAM of the North Atlantic, core GGC30 (7), include negatively cor- related multicentury departures from the shared long-term trends (black dashed line in A). The NAM temperatures in A have been linearly detrended to account for a >5 °C cooling trend (8, 10) to visually highlight the short-term variability, which is consistent with the temperature contrast produced today (C and D) by AMOC changes in the Gulf Stream and North Atlantic Current regions (1, 6). The difference in the nondetrended Holocene temperatures from the two regions (orange line in B) correlates with the effective precipitation (Eff. Precip.) changes indicated by lake-level changes in Massachusetts, United States (14) (blue line in B). The scatter plot in B shows the correlation (Pearson’s product moment correlation: r = 0.90; P < 0.0001; n = 220) between the two series over the last 8,000 y after the regional influence of the Laurentide ice sheet rapidly diminished (8). Maps show winter (December to February) sea-surface temperature (SST) (C) and air temperature (D) differences between AD 2008–2017 and AD 1958–1967: a decade of weak AMOC (6) and positive NAO (33) minus a decade of strong AMOC (6) and negative NAO (33). Shades of blue indicate lower than normal temperatures during the weak AMOC period, and red indicates higher than normal temperatures during the weak AMOC period based on the Hadley Centre Sea Ice and Sea Surface Temperature data set (34) in C and the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis Project (35) in D. The blue line in B is mean of the recon- structions from Deep and New Long ponds (8, 14); shading indicates the 95% uncertainty in the mean. Insets in C and D show the location of the lake-level records and the inset map in Fig. 2. A generalized additive model represents the mean multimillennial trends as the dashed line in A.Calyr,calendaryear. higher) than present temperatures during the Younger Dryas shoreline sediments in small lakes, which shifted through time and and early Holocene (10). Removing the linear cooling trend, produced direct stratigraphic