Modelling Middle Pliocene warm climates of the USA

Southwestern North America, like many subtropical regions, is predicted to become drier in response to anthropogenic warming. However, during the Pliocene, when carbon dioxide was above pre-industrial levels, multiple lines of evidence suggest that southwestern North America was much wetter. While existing explanations for a wet Pliocene invoke increases in winter rain, recent modeling studies hypothesize that summer rain may have also played an important role. Here, we present the first direct evidence for an intensified Pliocene monsoon in southwestern North America using leaf wax hydrogen isotopes. These new records provide evidence that the Pliocene featured an intensified and expanded North American Monsoon. Using proxies and isotope-enabled model simulations, we show that monsoon intensification is linked to amplified warming on the southern California margin relative to the tropical Pacific. This mechanism has clear relevance for understanding present-day monsoon variations, ...

Journal of Geophysical Research, 2006

Earth and Planetary Science Letters, 2019

The nature and controls of orbital-scale climate variability in North America (NA) are subjects of ongoing debates. On the basis of previous cave records from Southwestern United States, two mutually incompatible hypotheses have been proposed. One links NA orbital-scale climate variability to Northern Hemisphere (NH) summer insolation forcing in a manner analogous to low-latitude monsoon systems, while the other suggests that it is not causally tied to either changes in global ice-volumes or NH summer insolation. Here we report new cave oxygen isotope (δ 18 O) records from Buckeye Creek Cave (BCC), West Virginia, east central North America, covering most of the past three glacial-interglacial periods (∼335 to 45 kyr ago). The BCC δ 18 O record exhibits a strong precession-band cycle, which is in-phase with changes in global ice-volumes (i.e., sea level), sea surface temperatures in the NE Gulf of Mexico and is consistent with the results from published cave records from Nevada and Devils Hole. As with global ice-volume, the BCC records lag summer insolation at 65 • N by ∼5000 yr, which stands in contrast with records of low-latitude monsoon variability in South America and Asia, which are in phase and out-of-phase with changes in summer insolation and sea level, respectively. Provided the degree of lag to summer insolation provides a measure of competing forcing from global ice-volume and summer insolation, our data suggest that NA orbital-scale climate variability is dominantly driven by ice-volume forcing. In addition, the sea surface temperatures in the NE Gulf of Mexico and changes in northern high-latitude cryosphere may be also important in explaining the unusually low δ 18 O values at times of the intermediate ice-volume periods in BCC and other NA cave records.

In eastern North America, annual precipitation increased by >40% over the Holocene, largely in response to melting of the Laurentide Ice Sheet. The change substantially raised lake levels and transformed conifer-dominated ecosystems into mesic deciduous forests. δ2H values of terrestrially derived leaf-wax n-alkanes can facilitate diagnoses of the climate dynamics involved by reconstructing δ2H values of mean annual precipitation (δ2HMAP). However, competing influences on δ2HMAP values in the mid-latitudes, such as changes in moisture sources and in the seasonal distribution of precipitation, can generate confounding effects. To test δ2HMAP sensitivity to potential changes associated with the final Holocene phases of deglaciation in eastern North America, we used 14 fossil-pollen records to reconstruct monthly precipitation changes and to model δ2HMAP values during the Holocene. The pollen-inferred precipitation increased by 100-200 mm during both cold and warm seasons, but model...

Quaternary Science Reviews, 2010

Earlier studies indicated that the general pattern of the Holocene climate in the northeastern United States changed from cool and dry (11.6e8.2 ka; 1 ka ¼ 1000 cal yr BP) to warm and wet (8.2e5.4 ka) to warm and dry (5.4e3 ka) to cool and wet (after 3 ka). A new w35-year resolution stable isotope record of endogenic calcite from a sediment core for Lake Grinnell in northern New Jersey provided a chance to examine the Holocene climate variations of the region in a finer detail. After the Younger Dryas cold climate reversal, the d 18 O fluctuated around a constant value of À7.4& until 5.8 ka, thereafter shifted to a steadily decreasing trend to the most recent value of À8.2&. Responding to this shift, the widely observed hemlock decline in the northeastern USA occurred about w350e500 (AE143.5) years later. Detrended d 18 O and d 13 C records show a clear covariance at 910-year periodicity. The amplitudes of centennial-scale d 18 O variations became much smaller after 4.7 ka. At the same time, the dominant frequency of these variations changed from 330 to 500 years. We suggest that a non-linear response of atmospheric circulation to the gradual decrease in insolation is responsible for the shift in the climate trend at 5.8 ka as indicated by the deceasing d 18 O values. A dominant frequency shift in solar forcing and the decreased seasonal contrast of insolation might have caused the change in climate variability at 4.7 ka through modulating ocean and atmosphere circulations.

Global and Planetary Change

Marine microfaunal data and terrestrial pollen records indicate that the middle Pliocene (ca. 3 Ma) climate is the most recent period in geologic history with global temperatures nearly as warm as those predicted for the coming century. We used the GISS GCM to examine the Pliocene climate by specifying sea surface temperatures and vegetation distributions derived from U.S.G.S. data sets. The simulation resulted in 1.4°C warming, annually averaged over the Northern Hemisphere. Warming was greatest at high latitudes; consequently, the equator to pole temperature gradient decreased by 11.5°C. Surface air temperature increases were greatest in winter, as decreased snow and sea ice triggered a positive albedo feedback effect. At low latitudes, temperatures were mostly unchanged except for an anomalous 3°C cooling over eastern Africa. This anomaly is supported by palynological data and, in the simulation, was a response to the weakening of the Hadley circulation, which caused subtropical clouds and evapotranspiration rates to increase. Evaporation and precipitation rates decreased over the oceans and the appearance of negative P-E anomalies might have implications for the Pliocene thermohaline circulation. The hydrological cycle intensified over the continents, where annual evaporation, rainfall, and soil moisture all increased. However, simulated summer drought conditions are not corroborated by terrestrial records, pointing to deficiencies in either the model, the boundary conditions, or the terrestrial data interpretations. The Pliocene SST pattern implicates increased ocean heat flux as a component force behind the middle Pliocene warmth, since levels of CO2, large enough to cause the extreme high latitude temperatures, would generate more tropical warming than is indicated by floral and faunal records. Surface energy fluxes, calculated by the GCM, indicate that an increased meridional ocean heat flux of 32% could reproduce the data-derived SST distribution, despite weakened atmospheric transports. The decreased wind stress values suggest that any increase of ocean heat transports would probably have resulted from a strengthening of the thermohaline circulation.

2011

A sediment core spanning similar to 7000 cal yr BP recovered from Stella Lake, it small sub-alpine lake located in Great Basin National Park, Nevada. was analyzed for subfossil chironomids non-biting midges), diatoms, and organic content (estimated by loss-on-ignition (LOI)). Subfossil chironomid analysis indicates that Stella Lake was characterized by a warm, middle Holocene, followed by a cool "Neoglacial" period, with the last two millennia characterized by a return to warmer conditions Throughout the majority of the core the Stella Lake diatom-community Composition is dominated by small. periphytic taxa which are suggestive of shallow, cool, alkaline, oligotrophic waters with extensive seasonal ice cover A reconstruction of mean July air temperature (MJAT) was developed by applying a midge-based inference model for MJAT (two-component WA-PLS) consisting of 79 lakes and 54 midge taxa (r(jack)(2) = 0.55. RMSEP = 0.9 degrees C) Comparison of the chironomid-inferred temperature record to existing regional paleoclimate reconstructions Suggests that the midge-inferred temperatures correspond well to regional patterns This multi-proxy record provides valuable insight into regional Holocene climate and environmental conditions by providing a quantitative. reconstruction of peak Holocene warmth and aquatic ecosystem response to these changes In the Great Basin. a region projected to experience increased aridity and higher temperatures (C) 2009 University of Washington. Published by Elsevier Inc. All rights reserved.

Quaternary Science Reviews, 1998

Late Quaternary pollen data from eastern North America and pollen-climate response surfaces provide tests of the climate simulations from Version 1 of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM1) for 21, 16, 14, 11, and 6 ka. The model results are also compared to those from Version 0 of the NCAR model (CCM0). In contrast to CCM0, CCM1 used a slab ocean model to compute sea surface temperatures, included a seasonal cycle, and computed soil moisture. It also used an improved set of boundary conditions from 21 to 6 ka. In eastern North America, CCM1 simulated lower temperatures at the last glacial maximum (LGM) 21,000 years ago than did CCM0 and therefore was in better agreement with the pollen data than the CCM0 simulations were. The simulations by CCM1 for mean July temperatures from 16 to 11 ka, however, were much higher than those by CCM0 and were higher than present for much of the area south of the Laurentide ice sheet. These simulated conditions differ markedly from those reconstructed from the pollen data, which indicate July temperatures significantly lower than present from 16 to 11 ka. At 6 ka, CCM1 simulated climate conditions similar to today and therefore not too different from those simulated by CCM0 or those inferred from the pollen data. For 6 ka in the upper Midwest, however, CCM1 simulated moisture conditions similar to present, which is an improvement over CCM0. However, the new simulation still does not match the drier-thanpresent conditions inferred from the pollen data. Detailed analysis of the circulation variables and the surface-energy-budget terms simulated by CCM1 show that both dynamic and thermodynamic factors contributed to the major discrepancy in July temperatures from 16 to 11 ka. The simulated glacial anticyclone created less cloudiness and rainfall south of the ice sheet, and these conditions allowed the increased summertime insolation from 16 to 11 ka to overheat the surface. The impact of the ice sheet on model dynamics therefore led to overestimation of summer temperatures south of the ice for this time period.

Geological Society of America Bulletin, 2002

As global climate changes because of anthropogenic influences, it has become critical to better understand past climate and its various forcing mechanisms as a baseline for future comparison. To this end, we present a continental isotopic record from an 11.2-m-long wetland piston core sampled at 10-50 yr resolution; the core was taken in the heavily populated, economically vibrant northeastern United States (adjacent to Fayetteville Green Lake) and spans 14,600-3200 cal. yr B.P. We use a historically based correlation between ␦ 18 O calcite obtained from individual varves in a box core from Fayetteville Green Lake and winter atmospheric circulation over the northeast United States to examine the way in which changes in winter circulation have influenced ␦ 18 O in precipitation from 14,600 to 3200 cal. yr B.P. Our correlation analysis suggests that in periods during which the circumpolar westerlies are expanded, storms track more frequently from the Gulf of Mexico region, delivering precipitation with relatively high ␦ 18 O values to the study site. By contrast, contracted westerlies result in more frequent low-␦ 18 O precipitation cross-continental storms. By using this relationship we model winter-vortex latitudes over the northeast United States for the prehistoric oxygen isotope record, focusing on millennial-scale