More Like this

1. North American Hydroclimate During Past Warms States: A Proxy Compilation‐Model Comparison for the Last Interglacial and the Mid‐Holocene

   https://doi.org/10.1029/2022PA004528  

   de Wet, C. B. ; Ibarra, D. E. ; Belanger, B. K. ; Oster, J. L. ( June 2023 , Paleoceanography and Paleoclimatology)

   During the mid‐Holocene (MH: ∼6,000 years Before Present) and Last Interglacial LIG (LIG: ∼129,000–116,000 years Before Present) differences in the seasonal and latitudinal distribution of insolation drove Northern Hemisphere high‐latitude warming comparable to that projected for the end of the 21st century in low emissions scenarios. Paleoclimate proxy records point to distinct but regionally variable hydroclimatic changes during these past warm intervals. However, model simulations have generally disagreed on North American regional moisture patterns during the MH and LIG. To investigate how closely the latest generation of models associated with the Paleoclimate Model Intercomparison Project (PMIP4) reproduces proxy‐inferred moisture patterns during recent warm periods, we compare hydroclimate output from 17 PMIP4 models with newly updated compilations of moisture‐sensitive North American proxy records during the MH and LIG. Agreement is lower for the MH, with models producing wet anomalies across the western United States (US) where most proxies indicate increased aridity relative to the preindustrial period. The models that agree most closely with the LIG proxy compilation display relative wetness in the eastern US and Alaska, and dryness in the northwest and central US. An assessment of atmospheric dynamics using an ensemble of the three LIG simulations that best agree with the proxies suggests that weaker winter North Pacific pressure gradients and steeper summer North Pacific and Atlantic gradients drive LIG precipitation patterns. Our updated compilations and proxy‐model comparisons offer a tool for benchmarking climate models and their performance in simulating climate states that are warmer than present.

    

   more » « less
2. Mid‐Holocene, Coral‐Based Sea Surface Temperatures in the Western Tropical Atlantic

   https://doi.org/10.1029/2019PA003571  

   Rodriguez, Luis G. ; Cohen, Anne L. ; Ramirez, Wilson ; Oppo, Delia W. ; Pourmand, Ali ; Edwards, R. Lawrence ; Alpert, Alice E. ; Mollica, Nathaniel ( July 2019 , Paleoceanography and Paleoclimatology)

   The Holocene is considered a period of relative climatic stability, but significant proxy data‐model discrepancies exist that preclude consensus regarding the postglacial global temperature trajectory. In particular, a mid‐Holocene Climatic Optimum, ~9,000 to ~5,000 years BP, is evident in Northern Hemisphere marine sediment records, but its absence from model simulations raises key questions about the ability of the models to accurately simulate climate and seasonal biases that may be present in the proxy records. Here we present new mid‐Holocene sea surface temperature (SST) data from the western tropical Atlantic, where twentieth‐century temperature variability and amplitude of warming track the twentieth‐century global ocean. Using a new coral thermometer Sr‐U, we first developed a temporal Sr‐U SST calibration from three modern Atlantic corals and validated the calibration against Sr‐U time series from a fourth modern coral. Two fossil corals from the Enriquillo Valley, Dominican Republic, were screened for diagenesis, U‐series dated to 5,199 ± 26 and 6,427 ± 81 years BP, respectively, and analyzed for Sr/Ca and U/Ca, generating two annually resolved Sr‐U SST records, 27 and 17 years long, respectively. Average SSTs from both corals were significantly cooler than in early instrumental (1870–1920) and late instrumental (1965–2016) periods at this site, by ~0.5 and ~0.75 °C, respectively, a result inconsistent with the extended mid‐Holocene warm period inferred from sediment records. A more complete sampling of Atlantic Holocene corals can resolve this issue with confidence and address questions related to multidecadal and longer‐term variability in Holocene Atlantic climate.

    

   more » « less
3. Greenland temperature and precipitation over the last 20 000 years using data assimilation

   https://doi.org/10.5194/cp-16-1325-2020  

   Badgeley, Jessica A. ; Steig, Eric J. ; Hakim, Gregory J. ; Fudge, Tyler J. ( January 2020 , Climate of the Past)

   Abstract. Reconstructions of past temperature and precipitation are fundamental to modeling the Greenland Ice Sheet and assessing its sensitivity to climate. Paleoclimate information is sourced from proxy records and climate-model simulations; however, the former are spatially incomplete while the latter are sensitive to model dynamics and boundary conditions. Efforts to combine these sources of information to reconstruct spatial patterns of Greenland climate over glacial–interglacial cycles have been limited by assumptions of fixed spatial patterns and a restricted use of proxy data. We avoid these limitations by using paleoclimate data assimilation to create independent reconstructions of mean-annual temperature and precipitation for the last 20 000 years. Our method uses oxygen isotope ratios of ice and accumulation rates from long ice-core records and extends this information to all locations across Greenland using spatial relationships derived from a transient climate-model simulation. Standard evaluation metrics for this method show that our results capture climate at locations without ice-core records. Our results differ from previous work in the reconstructed spatial pattern of temperature change during abrupt climate transitions; this indicates a need for additional proxy data and additional transient climate-model simulations. We investigate the relationship between precipitation and temperature, finding that it is frequency dependent and spatially variable, suggesting that thermodynamic scaling methods commonly used in ice-sheet modeling are overly simplistic. Our results demonstrate that paleoclimate data assimilation is a useful tool for reconstructing the spatial and temporal patterns of past climate on timescales relevant to ice sheets. 

   more » « less
4. Evidence for Decreased Precipitation Variability in the Yucatán Peninsula During the Mid‐Holocene

   https://doi.org/10.1029/2021PA004219  

   Serrato Marks, Gabriela ; Medina‐Elizalde, Martín ; Burns, Stephen ; Weldeab, Syee ; Lases‐Hernandez, Fernanda ; Cazares, Gabriela ; McGee, David ( May 2021 , Paleoceanography and Paleoclimatology)

   The Yucatán Peninsula (YP) has a complex hydroclimate with many proposed drivers of interannual and longer‐term variability, ranging from coupled ocean–atmosphere processes to frequency of tropical cyclones. The mid‐Holocene, a time of higher Northern Hemisphere summer insolation, provides an opportunity to test the relationship between YP precipitation and ocean temperature. Here, we present a new, ∼annually resolved speleothem record of stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg/Ca and Sr/Ca) ratios for a section of the mid‐Holocene (5.2–5.7 kyr BP), before extensive agriculture began in the region. A meter‐long stalagmite from Río Secreto, a cave system in Playa del Carmen, Mexico, was dated using U–Th geochronology and layer counting, yielding multidecadal age uncertainty (median 2SD of ±70 years). New proxy data were compared to an existing late Holocene stalagmite record from the same cave system, allowing us to examine changes in hydrology over time and to paleoclimate records from the southern YP. The δ¹⁸O, δ¹³C, and Mg/Ca data consistently indicate higher mean precipitation and lower precipitation variability during the mid‐Holocene compared to the late Holocene. Despite this reduced variability, multidecadal precipitation variations were persistent in regional hydroclimate during the mid‐Holocene. We therefore conclude that higher summer insolation led to increased mean precipitation and decreased precipitation variability in the northern YP but that the region is susceptible to dry periods across climate mean states. Given projected decreases in wet season precipitation in the YP’s near future, we suggest that climate mitigation strategies emphasize drought preparation.

    

   more » « less
5. Hot Air, Hot Lakes, or Both? Exploring Mid‐Holocene African Temperatures Using Proxy System Modeling

   https://doi.org/10.1029/2020JD033269  

   Dee, Sylvia G. ; Morrill, Carrie ; Kim, Seung Hye ; Russell, James M. ( May 2021 , Journal of Geophysical Research: Atmospheres)

   Climate models predict Africa will warm by up to 5°C in the coming century, stressing African societies. To provide independent constraints on model predictions, this study compares two notable reconstructions of East African temperatures to those predicted by Paleoclimate Model Intercomparison Project (PMIP3) and transient TraCE (Transient Climate Evolution) simulations, focusing on the Mid‐Holocene (MH, 5–8 kyr B.P.). Reconstructions of tropical African temperature derived from lake sedimentary archives indicate 1–2.5°C of warming during the MH relative to the 20th century, but most climate models do not replicate the warming observed in these paleoclimate data. We investigate this discrepancy using a new lake proxy system model, with attention to the (potentially non‐stationary) relationship between lake temperature and air temperature. We find amplified lake surface temperature changes compared to air temperature during the MH due to heightened seasonality and precessional forcing. Lacustrine processes account for some of the warming, and highlight how the lake heat budget leads to a rectification of the seasonal cycle; however, the simulated lake heating bias is insufficient to reconcile the full discrepancy between the models and the proxy‐derived MH warming. We find further evidence of changes in mixing depth over time, potentially driven by changes in cloud cover and shortwave radiative fluxes penetrating the lake surface. This may confound interpretation for glycerol dialkyl glycerol tetraethers (GDGT) compounds which exist in the mixed layer, and suggests a need for independent constraints on mixed layer depth. This work provides a new interpretive framework for invaluable paleoclimate records of temperature changes over the African continent.

    

   more » « less