The presence of moisture deficits in Miocene New Zealand

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Earth's hydrological cycle is expected to intensify in response to global warming, with a "wet-gets-wetter, dry-gets-drier" response anticipated over the ocean. Subtropical regions (∼15°-30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterized by wetter conditions. Here we use an integrated data-modeling approach to reconstruct global and zonal-mean rainfall patterns during the early Eocene (∼56-48 million years ago). The Deep-Time Model Intercomparison Project (DeepMIP) model ensemble indicates that the mid-(30°-60°N/S) and high-latitudes (>60°N/S) are characterized by a thermodynamically dominated hydrological response to warming and overall wetter conditions. The tropical band (0°-15°N/S) is also characterized by wetter conditions, with several DeepMIP models simulating narrowing of the Inter-Tropical Convergence Zone. However, the latter is not evident from the proxy data. The subtropics are characterized by negative precipitation-evaporation anomalies (i.e., drier conditions) in the DeepMIP models, but there is surprisingly large inter-model variability in mean annual precipitation (MAP). Intriguingly, we find that models with weaker meridional temperature gradients (e.g., CESM, GFDL) are characterized by a reduction in subtropical moisture divergence, leading to an increase in MAP. These model simulations agree more closely with our new proxy-derived precipitation reconstructions and other key climate metrics and imply that the early Eocene was characterized by reduced subtropical moisture divergence. If the meridional temperature gradient was even weaker than suggested by those DeepMIP models, circulation-induced changes may have outcompeted thermodynamic changes, leading to wetter subtropics. This highlights the importance of accurately reconstructing zonal temperature gradients when reconstructing past rainfall patterns. Plain Language SummaryAs the world warms, the atmosphere is able to hold more moisturehowever, this moisture will not fall evenly across the globe. Some regions are expected to become wetter, whereas other regions will become drier. This is the basis of the familiar paradigm "wet-gets-wetter, dry-gets-drier" and is largely supported by future model projections. However, evidence from the geological record contradicts this hypothesis and suggests that a warmer world could be characterized by wetter (rather than drier) subtropics. Here, we use an integrated data-modeling approach to investigate the hydrological response to warming during an ancient warm interval (the early Eocene, 56-48 million years ago). We show that models with weaker latitudinal temperature gradients are characterized by a reduction in subtropical CRAMWINCKEL ET AL.

Section: Methodsmentioning

confidence: 99%

Global and Zonal‐Mean Hydrological Response to Early Eocene Warmth

Cramwinckel

Burls

Fahad

et al. 2023

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“…MAT, MDR, WQT, CQT, WMP, and DMP were derived from WORLDCLIM (Fick & Hijmans, 2017) and NPP were derived from MODIS (Running et al., 2015). The modern‐day calibration leaf physiognomy dataset used here (Supporting Information Table S1) is a Southern Hemisphere‐based dataset (Kennedy et al., 2014), supplemented with sites from Chile (Hinjosa et al., 2006) and southeastern Australia (Reichgelt et al., 2019).…”

Section: Methodsmentioning

confidence: 99%

Plant Proxy Evidence for High Rainfall and Productivity in the Eocene of Australia

Reichgelt

Greenwood

Steinig

et al. 2022

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During the early to middle Eocene, a mid‐to‐high latitudinal position and enhanced hydrological cycle in Australia would have contributed to a wetter and “greener” Australian continent where today arid to semi‐arid climates dominate. Here, we revisit 12 southern Australian plant megafossil sites from the early to middle Eocene to generate temperature, precipitation, and seasonality paleoclimate estimates, net primary productivity (NPP), and vegetation type, based on paleobotanical proxies and compare them to early Eocene global climate models. Temperature reconstructions are uniformly subtropical (mean annual, summer, and winter mean temperatures 19–21°C, 25–27°C, and 14–16°C, respectively), indicating that southern Australia was ∼5°C warmer than today, despite a >20° poleward shift from its modern geographic location. Precipitation was less homogeneous than temperature, with mean annual precipitation of ∼60 cm over inland sites and >100 cm over coastal sites. Precipitation may have been seasonal with the driest month receiving 2–7× less than the mean monthly precipitation. Proxy‐model comparison is favorable with a 1,680 ppm CO2 concentration. However, individual proxy reconstructions can disagree with models as well as with each other. In particular, seasonality reconstructions have systemic offsets. NPP estimates were higher than modern, implying a more homogenously “green” southern Australia in the early to middle Eocene when this part of Australia was at 48–64°S and larger carbon fluxes to and from the Australian biosphere. The most similar modern vegetation type is modern‐day eastern Australian subtropical forest, although the distance from coast and latitude may have led to vegetation heterogeneity.

“…These higher temperatures were accompanied by increased humidity in many parts of the world (e.g. Hui et al, 2018;Trayler et al, this volume), although reconstructions from New Zealand suggest the highest drought potential during the MCO (Reichgelt et al, 2019). Tropical forests expanded and were more extensive than today (Bonnefille, 2010;Jaramillo et al, this volume), also indicating a warmer and possibly wetter climate.…”

Section: Accepted Articlementioning

confidence: 97%

“…Estimates for the MCO propose global mean annual temperatures 5°C–6°C warmer than present‐day (Hui et al., 2018), with the northern high‐latitudes and northern temperate zones potentially 14°C and 9°C warmer than pre‐industrial (Pound et al., 2012). These higher temperatures were accompanied by increased humidity in many parts of the world (e.g., Hui et al., 2018; Trayler et al., 2020), although reconstructions from New Zealand suggest the highest drought potential during the MCO (Reichgelt et al., 2019). Tropical forests expanded and were more extensive than today (Bonnefille, 2010; Jaramillo et al., 2020), also indicating a warmer and possibly wetter climate.…”

Section: Terrestrial Climate Recordsmentioning

confidence: 99%

The Miocene: The Future of the Past

Steinthorsdottir

Coxall

Boer

et al. 2021

271

179

The Miocene epoch (23.03-5.33 Ma) was a time interval of global warmth, relative to today.Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhouse interval-the Miocene Climatic Optimum (MCO). Floras, faunas, ice sheets, precipitation, pCO 2 , and ocean and atmospheric circulation mostly (but not ubiquitously) covaried with these large changes in climate. With higher temperatures and moderately higher pCO 2 (∼400-600 ppm), the MCO has been suggested as a particularly appropriate analog for future climate scenarios, and for assessing the predictive accuracy of numerical climate models-the same models that are used to simulate future climate. Yet, Miocene conditions have proved difficult to reconcile with models. This implies either missing positive feedbacks in the models, a lack of knowledge of past climate forcings, or the need for re-interpretation of proxies, which might mitigate the model-data discrepancy. Our understanding of Miocene climatic, biogeochemical, and oceanic changes on broad spatial and temporal scales is still developing. New records documenting the physical, chemical, and biotic aspects of the Earth system are emerging, and together provide a more comprehensive understanding of this important time interval. Here, we review the state-of-the-art in Miocene climate, ocean circulation, biogeochemical cycling, ice sheet dynamics, and biotic adaptation research as inferred through proxy observations and modeling studies. Plain Language Summary During the Miocene time period (∼23-5 million years ago),Planet Earth looked similar to today, with some important differences: the climate was generally warmer and highly variable, while atmospheric CO 2 was not much higher. Continental-sized ice sheets were only present on Antarctica, but not in the northern hemisphere. The continents drifted to near their modernday positions, and plants and animals evolved into the many (near) modern species. Scientists study the Miocene because present-day and projected future CO 2 levels are in the same range as those reconstructed for the Miocene. Therefore, if we can understand climate changes and their biotic responses from the Miocene past, we are able to better predict current and future global changes. By comparing Miocene climate reconstructions from fossil and chemical data to climate simulations produced by computer models, scientists are able to test their understanding of the Earth system under higher CO 2 and warmer conditions than those of today. This helps in constraining future warming scenarios for the coming STEINTHORSDOTTIR ET AL.