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The Sahel is highly sensitive to flooding, droughts, and wildfires, risking food and other resources on which nearly 100 million people depend. Understanding how natural variations of precipitation and vegetation fluctuate during high-amplitude glacial- interglacial cycles can help constrain the regional sensitivity to a wide range of external forcings. Further, the interactions between climate and ecosystem changes remain uncertain for sub-Saharan Africa due to the lack of long, highly-resolved, quantitative, terrestrial records. Here we present precipitation and vegetation records from ~215 ka to present, derived from long leaf wax hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively. These geochemical records are derived from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced terrestrial leaf waxes. We find that, unlike many African records that are precessionally- driven, obliquity plays an important role in West African late Pleistocene hydroclimate, suggesting that a cross-equatorial insolation gradient may be more important in this area and certainly that drivers of orbital-scale precipitation change are regionally-specific. Further, vegetation changes appear to have a complex relationship with hydroclimate over this mid-late Pleistocene interval. A potential shift in this climate-environment coupling at MIS6 ~130 ka, which is a time when there is also a shift in forcing mechanisms in East Africa, suggests that the global boundary condition changes associated with large glacial- interglacial cycling may affect equatorial climate.more » « less
In the early Pleistocene, global temperature cycles predominantly varied with ~41‐kyr (obliquity‐scale) periodicity. Atmospheric greenhouse gas concentrations likely played a role in these climate cycles; marine sediments provide an indirect geochemical means to estimate early Pleistocene CO2. Here we present a boron isotope‐based record of continuous high‐resolution surface ocean pH and inferred atmospheric CO2changes. Our results show that, within a window of time in the early Pleistocene (1.38–1.54 Ma), pCO2varied with obliquity, confirming that, analogous to late Pleistocene conditions, the carbon cycle and climate covaried at ~1.5 Ma. Pairing the reconstructed early Pleistocene pCO2amplitude (92 ± 13 μatm) with a comparably smaller global surface temperature glacial/interglacial amplitude (3.0 ± 0.5 K) yields a surface temperature change to CO2radiative forcing ratio of
S[CO2]~0.75 (±0.5) °C−1·W−1·m−2, as compared to the late Pleistocene S[CO2]value of ~1.75 (±0.6) °C−1·W−1·m−2. This direct comparison of pCO2and temperature implicitly incorporates the large ice sheet forcing as an internal feedback and is not directly applicable to future warming. We evaluate this result with a simple climate model and show that the presumably thinner, though extensive, northern hemisphere ice sheets would increase surface temperature sensitivity to radiative forcing. Thus, the mechanism to dampen actual temperature variability in the early Pleistocene more likely lies with Southern Ocean circulation dynamics or antiphase hemispheric forcing. We also compile this new carbon dioxide record with published Plio‐Pleistocene δ11B records using consistent boundary conditions and explore potential reasons for the discrepancy between Pliocene pCO2based on different planktic foraminifera.
Ocean‐atmosphere dynamics in the north Pacific play an important role in the global climate system and influence hydroclimate in western North America. However, changes to this region's mean climate under increased atmospheric greenhouse gas concentrations are not well understood. Here we present new alkenone‐based records of sea surface temperature (SST) from the northeast Pacific from the mid‐Piacenzian warm period (approximately 3.3–3.0 Ma), an interval considered to be an analog for near‐future climate under middle‐of‐the‐road anthropogenic emissions. We compare these and other alkenone‐based SST records from the north Pacific to fully‐coupled climate model simulations to examine the impact of mid‐Pliocene CO2and other boundary conditions on regional climate dynamics and to explore factors governing model disagreement about regional temperature patterns. Model performance varies regionally, with Community Earth System Model 1.2 (CESM 1.2) and CESM2 performing best in regions with greater warming like the California Margin, though these models underestimate the warming evidenced in our new proxy record and others from the region. Single forcing simulations reveal a strong influence for prescribed land surface changes and higher CO2levels on coastal warming patterns along the California Margin in CESM2. Furthermore, differences in shortwave and longwave radiation and circulation between the models, likely related to changes in the atmospheric component of the model, may play a key role in the ability of models to capture regionally‐varying patterns of Pliocene warmth. Regional patterns of temperature change inferred from geochemical records could therefore help to understand the impacts of different model parameterization schemes on regional climate patterns.
Interrupting a long‐term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17–15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO2levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long‐term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Δ47) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7–21.5 Ma) to Barstovian (16.9–14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Δ47‐temperatures ranging from 17°C to 24°C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 ± 2°C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Δ47‐temperatures (8°C–10°C) concomitantly with elevated Δ47‐temperatures (ca. 22°C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Δ47‐temperature record, combined with carbon isotope (δ13C) and reconstructed soil water oxygen isotope (δ18Osw) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available records from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.
The Western Equatorial Pacific (WEP) warm pool, with surface temperatures >28
°C and a deep thermocline, is an important source of latent and sensible heat for the global climate system. Because the tropics are not sensitive to ice‐albedo feedbacks, the WEP's response to radiative forcing can be used to constrain a minimum estimate of Earth system sensitivity. Climate modeling of pCO2‐radiative warming projections shows little change in WEP variability; here we use temperature distributions of individual surface and subsurface dwelling fossil foraminifera to evaluate past variability and possible radiative and dynamic climate forcing over the Plio‐Pleistocene. We investigate WEP warm pool variability within paired glacial‐interglacial (G‐IG) intervals for four times: the Holocene‐Last Glacial Maximum, ~2, ~3, and ~4 Ma. Our results show that these surface and subsurface temperature distributions are similar for all G‐IG pairs, indicating no change in variability, even as pCO2‐radiative forcing and other boundary conditions changed on G‐IG timescales. Plio‐Pleistocene sea surface temperature (SST) distributions are similar to those from the Holocene, indicating WEP SSTs respond to pCO2‐radiative forcing and associated feedbacks. In contrast, Plio‐Pleistocene subsurface temperature distributions suggest subsurface temperatures respond to changes in thermocline temperature and depth. We estimate tropical temperature sensitivity for the mid‐Pliocene (~3 Ma) using our individual foraminifera SST data set and a previously published high‐resolution boron isotope‐based pCO2reconstruction. We find tropical temperature sensitivity was equal to, or less than, that of the Late Pleistocene.