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1. Expanded lacustrine sedimentation in the Qaidam Basin on the northern Tibetan Plateau: Manifestation of climatic wetting during the Oligocene icehouse

   https://doi.org/https://doi.org/10.1016/j.epsl.2021.116935  

   Wu, M.; Zhuang, G. S.; Hou, M.; and Liu, Z. (January 2021, Earth and planetary science letters)

   Bendick, R. (Ed.)

   The Qaidam Basin in the core area of arid Inner Asia has been considered undergoing continuous aridification over the Cenozoic. However, the Qaidam Basin is marked with expanded lacustrine sedimentation during the Oligocene, which contrasts with the fluvial or deltaic facies stratigraphically below (Eocene) and above (Miocene-present). The Oligocene lacustrine expansion challenges the idea of persistent aridification. To solve the conundrum, we reconstruct a long-term compound-specific hydrogen isotope (δ2H) record from sedimentary leaf wax n-alkanes to evaluate the paleoclimatic context before, during, and after the Oligocene lacustrine expansion. The δ2H results reveal three shifts at ca. 40 Ma, 34 Ma, and 24 Ma. The leaf wax δ2H values range from −176.8to −166.7from 51 to 40 Ma, followed by an abrupt increase of 23.9at 40 Ma. We interpret this rapid increase as enhanced aridification due to the coeval retreat of the Paratethys Sea from the region. At 34 Ma, the δ2H plunges across the Eocene-Oligocene transition (EOT). Post-EOT δ2H values are the lowest, vary with high amplitude from −187.1to −153.2, and are associated with the lacustrine facies expansion, indicating a wetter climate. By compiling the regional isotopic proxy studies, we observe the contrasting patterns in paleohydrology conditions since the EOT: the relaxation of aridity in the westerlies region versus the enhanced aridification in the East Asian summer monsoon region. We interpret that the west-east contrasting patterns represent the different climatic responses to global cooling: wetting in the west as a result of the enhanced moisture transport via westerlies replacing the subtropical high, and drying in the east due to the reduction in moisture content associated with weakening East Asian summer monsoon. Wetting in Inner Asia is synchronous with cooling in the ocean (North Atlantic) and on land (Xining Basin). Since 24 Ma, δ2H increases in response to warming during the latest Oligocene to the early Miocene when the subtropical high re-occupied Inner Asia, causing the aridity. This study reveals a dynamic climate in Inner Asia with different mechanisms responding to global change. 

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2. Hydroclimatic trends during 1950–2018 over global land

   https://doi.org/10.1007/s00382-021-05684-1  

   Dai, Aiguo (February 2021, Climate Dynamics)

   null (Ed.)

   Global hydroclimatic changes from 1950 to 2018 are analyzed using updated data of land precipitation, streamfow, and an improved form of the Palmer Drought Severity Index. The historical changes are then compared with climate model-simulated response to external forcing to determine how much of the recent change is forced response. It is found that precipitation has increased from 1950 to 2018 over mid-high latitude Eurasia, most North America, Southeast South America, and Northwest Australia, while it has decreased over most Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Pacifc coasts of Canada. Streamfow records largely confrm these precipitation changes. The wetting trend over Northwest Australia and Southeast South America is most pronounced in austral summer while the drying over Africa and wetting trend over mid-high latitude Eurasia are seen in all seasons. Coupled with the drying caused by rising surface temperatures, these precipitation changes have greatly increased the risk of drought over Africa, southern Europe, East Asia, eastern Australia, Northwest Canada, and southern Brazil. Global land precipitation and continental freshwater discharge show large interannual and inter-decadal variations, with negative anomalies during El Niño and following major volcanic eruptions in 1963, 1982, and 1991; whereas their decadal variations are correlated with the Interdecadal Pacifc Oscillation (IPO) with IPO’s warm phase associated with low land precipitation and continental discharge. The IPO and Atlantic multidecadal variability also dominate multidecadal variations in land aridity, accounting for 90% of the multidecadal variance. CMIP5 multi-model ensemble mean shows decreased precipitation and runoff and increased risk of drought during 1950–2018 over Southwest North America, Central America, northern and central South America (including the Amazon), southern and West Africa, the Mediterranean region, and Southeast Asia; while the northern mid-high latitudes, Southeast South America, and Northwest Australia see increased precipitation and runoff. The consistent spatial patterns between the observed changes and the model-simulated response suggest that many of the observed drying and wetting trends since 1950 may have resulted at least partly from historical external forcing. However, the drying over Southeast Asia and wetting over Northwest Australia are absent in the 21st century projections. 

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3. Emergence of the Southeast Asian islands as a driver for Neogene cooling

   https://doi.org/10.1073/pnas.2011033117  

   Park, Yuem; Maffre, Pierre; Goddéris, Yves; Macdonald, Francis A.; Anttila, Eliel S. C.; Swanson-Hysell, Nicholas L. (September 2020, Proceedings of the National Academy of Sciences)

   Steep topography, a tropical climate, and mafic lithologies contribute to efficient chemical weathering and carbon sequestration in the Southeast Asian islands. Ongoing arc–continent collision between the Sunda-Banda arc system and Australia has increased the area of subaerially exposed land in the region since the mid-Miocene. Concurrently, Earth’s climate has cooled since the Miocene Climatic Optimum, leading to growth of the Antarctic ice sheet and the onset of Northern Hemisphere glaciation. We seek to evaluate the hypothesis that the emergence of the Southeast Asian islands played a significant role in driving this cooling trend through increasing global weatherability. To do so, we have compiled paleoshoreline data and incorporated them into GEOCLIM, which couples a global climate model to a silicate weathering model with spatially resolved lithology. We find that without the increase in area of the Southeast Asian islands over the Neogene, atmosphericpCO₂would have been significantly higher than preindustrial values, remaining above the levels necessary for initiating Northern Hemisphere ice sheets. 

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4. Wildfire and Environmental Feedbacks in the Western Sahel: Responses to Cenozoic Boundary Shifts

   Blattmann, Franziska; Lupien, Rachel; Uno, Kevin T; deMenocal, Peter B (December 2024, AGU Abstracts)

   Wildfires are essential to terrestrial ecosystems, playing a crucial role in nutrient and carbon cycles, particularly in highly seasonal environments like the Western Sahel. Their occurrence is linked to complex feedback mechanisms between climate, landscape structure, vegetation and the carbon cycle. It is therefore central to understand wildfire dynamics in the context of paleoclimatic and environmental change. Here we present a record of 3 to 7 ringed polyaromatic hydrocarbons (PAHs), from five targeted study windows throughout the last 25 million years from ODP Site 959 in the Gulf of Guinea. The time windows target the effects of orbital forcings of the West African Monsoon on wildfire and vegetation responses in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene, including shifts in global temperatures, greenhouse gas concentrations, and regional land surface. Orbitally resolved PAH biomarkers can provide insight into fire activity and be coupled with changing precipitation patterns and biomes. We discuss PAH sources and how wildfire frequency is linked to the observed drying trend, climate variability, and vegetation expansion throughout the Cenozoic in the Western Sahel. These findings are central for understanding future wildfire dynamics in the vulnerable Western Sahel region in the light of global warming. 

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5. Climate Conundrum: A Wet or Dry European and Northern African Climate During the Middle Miocene

   https://doi.org/10.1029/2024GL109499  

   Acosta, R_P; Burls, N_J; Pound, M_J; Bradshaw, C_D; McCoy, J.; Gibson, M.; O’Keefe, J_M_K; Feakins, S_J (November 2024, Geophysical Research Letters)

   Abstract End of 21st‐century hydroclimate projections suggest an expansion of subtropical dry zones, with Mediterranean and Sahel regions becoming much drier. However, paleobotanical assemblage evidence from the middle Miocene (17‐12 Ma), suggests both regions were instead humid environments. Here we show that by modifying regional sea surface temperatures (SST) in an Earth System Model (CESM1.2) simulation of the middle Miocene, the increased ocean evaporation and integrated water vapor flux overrides any drying effects associated with warming‐induced land‐surface evaporation driven by atmospheric CO₂concentrations. These modifications markedly reduce the bias in the model‐data comparison for this period. A vegetation model (BIOME4) forced with simulated climatologies predicts both regions were dominated by mixed forest, which is largely consistent with the paleobotanical record. This study unveils the potential for wetter subtropical Mediterranean climates associated with warming, presenting an alternative scenario from future drying projections with localized SST warming governing regional climate change. 

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