Deep ocean temperatures through time Valdes, Paul J; Scotese, Christopher R; Lunt, Daniel J
Climate of the past,
07/2021, Letnik:
17, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Benthic oxygen isotope records are commonly used as a proxy for global mean
surface temperatures during the Late Cretaceous and Cenozoic, and the
resulting estimates have been extensively used in ...characterizing major
trends and transitions in the climate system and for analysing past climate
sensitivity. However, some fundamental assumptions governing this proxy have
rarely been tested. Two key assumptions are (a) benthic foraminiferal
temperatures are geographically well mixed and are linked to surface
high-latitude temperatures, and (b) surface high-latitude temperatures are well
correlated with global mean temperatures. To investigate the robustness of
these assumptions through geological time, we performed a series of 109
climate model simulations using a unique set of paleogeographical
reconstructions covering the entire Phanerozoic at the stage level. The
simulations have been run for at least 5000 model years to ensure that the
deep ocean is in dynamic equilibrium. We find that the correlation between
deep ocean temperatures and global mean surface temperatures is good for the
Cenozoic, and thus the proxy data are reliable indicators for this time
period, albeit with a standard error of 2 K. This uncertainty has not
normally been assessed and needs to be combined with other sources of
uncertainty when, for instance, estimating climate sensitivity based on
using δ18O measurements from benthic foraminifera. The
correlation between deep and global mean surface temperature becomes weaker
for pre-Cenozoic time periods (when the paleogeography is significantly
different from the present day). The reasons for the weaker correlation
include variability in the source region of the deep water (varying
hemispheres but also varying latitudes of sinking), the depth of ocean
overturning (some extreme warm climates have relatively shallow and sluggish
circulations weakening the link between the surface and deep ocean), and the
extent of polar amplification (e.g. ice albedo feedbacks). Deep ocean
sediments prior to the Cretaceous are rare, so extending the benthic foraminifera
proxy further into deeper time is problematic, but the model results
presented here would suggest that the deep ocean temperatures from such time
periods would probably be an unreliable indicator of global mean surface conditions.
Past greenhouse periods with elevated atmospheric CO₂ were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree ...than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO₂, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.
A reduction in dust over North Africa during the mid‐Holocene “Green Sahara” period could have amplified precipitation, helping reconcile climate model simulations with paleo‐precipitation ...reconstructions. Here we test this using general circulation model simulations including interactive dust. We calculate a dust‐precipitation amplification factor using three different dust configurations to evaluate the sensitivity to dust optical properties and particle size range. The resultant amplification ranges from −20% to 50%. With more absorbing dust properties, there is a large negative net radiative effect and hence a larger impact on the hydrological cycle. With the inclusion of particles greater than 1 μm in radius, the precipitation amplification is reversed. Based on the simulations which best match observed Saharan dust properties, we conclude that there was a limited enhancement of precipitation due to reduced dust during the mid‐Holocene, meaning other aspects of the climate system should be the focus of future research.
Plain Language Summary
From around 11,000–4,000 years ago, the area that is the modern day Sahara desert was covered in vegetation—the “Green” Sahara. This was brought about by long‐term variations in the orbit of the Earth and the resultant changes in the distribution of incoming sunlight. However, when this period is simulated with climate models, the simulated rainfall increase is not adequate to maintain vegetation. We also know that this time period showed much reduced atmospheric dust from the desert. Here we study the effect of this dust reduction using a coupled climate‐dust model. We find that the reduction in dust had little impact on rainfall change, because dust has opposing effects on heat and radiation in the atmosphere that approximately balance over North Africa. To understand this response, we configured different versions of our climate‐dust model and show that the result is highly dependent on the physical characteristics of the dust particles that are given to the model. With the more realistic and up to date measurements from Saharan particles, the rainfall feedback is small. This means that future research needs to focus on rainfall processes in the atmosphere to better understand this “Greening” of the Sahara.
Key Points
Dust was previously shown to reconcile GCM and proxy rainfall anomalies for mid‐Holocene North Africa
We use GCM simulations to show dependence on dust properties, a more realistic configuration shows no rainfall amplification
This motivates a focus on the land surface and atmospheric convection response in this region
Abstract
The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the ...intensity of the African monsoon system. However, most climate models cannot reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we utilise a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800 kyr which have good agreement with NAHPs identified in proxy data. Our results show that precession determines NAHP pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacial periods, enhanced ice-albedo driven cooling suppresses NAHP amplitude at precession minima, when humid conditions would otherwise be expected. This highlights the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary.
Constraining the greenhouse gas forcing, climatic warming and estimates of climate sensitivity across ancient large transient warming events is a major challenge to the palaeoclimate research ...community. Here we provide a new compilation and synthesis of the available marine proxy temperature data across the largest of these hyperthermals, the Paleocene–Eocene Thermal Maximum (PETM). This includes the application of consistent temperature calibrations to all data, including the most recent set of calibrations for archaeal lipid-derived palaeothermometry. This compilation provides the basis for an informed discussion of the likely range of PETM warming, the biases present in the existing record and an initial assessment of the geographical pattern of PETM ocean warming. To aid interpretation of the geographic variability of the proxy-derived estimates of PETM warming, we present a comparison of this data with the patterns of warming produced by high pCO2 simulations of Eocene climates using the Hadley Centre atmosphere-ocean general circulation model (AOGCM) HadCM3L. On the basis of this comparison and taking into account the patterns of intermediate-water warming we estimate that the global mean surface temperature anomaly for the PETM is within the range of 4 to 5°C.
While paleoclimate records show that the Earth System is characterized by several different tipping points, their representation within Earth System models (ESMs) remains poorly constrained. This is ...because historical observations do not encompass variations large enough to provoke such regime changes, and paleoclimate conditions are rarely used to help develop and tune ESMs, which potentially ignores a rich source of information on abrupt climate change. A critical example is the early to mid-Holocene "greening" and subsequent rapid desertification of the Sahara, which most ESMs fail to reproduce, casting doubt on the representation of land-atmosphere coupling and monsoon dynamics. Here, we show that this greening and abrupt termination can be successfully simulated with one ESM after optimizing uncertain model components using both present-day observations and crucially mid-Holocene (6,000 y before present) reconstructions. The optimized model displays abrupt threshold behavior, which shows excellent agreement with long paleoclimate records that were not used in the original optimization. These results suggest that in order to realistically capture climate-system thresholds, ESMs first need to be conditioned with appropriate paleoclimate information.
The Cretaceous/Paleogene mass extinction, 66 Ma, included the demise of non-avian dinosaurs. Intense debate has focused on the relative roles of Deccan volcanism and the Chicxulub asteroid impact as ...kill mechanisms for this event. Here, we combine fossiloccurrence data with paleoclimate and habitat suitability models to evaluate dinosaur habitability in the wake of various asteroid impact and Deccan volcanism scenarios. Asteroid impact models generate a prolonged cold winter that suppresses potential global dinosaur habitats. Conversely, long-term forcing from Deccan volcanism (carbon dioxide CO₂-induced warming) leads to increased habitat suitability. Short-term (aerosol cooling) volcanism still allows equatorial habitability. These results support the asteroid impact as the main driver of the non-avian dinosaur extinction. By contrast, induced warming from volcanism mitigated the most extreme effects of asteroid impact, potentially reducing the extinction severity.
Elucidating the source(s) of Meltwater Pulse 1a, the largest rapid sea level rise caused by ice melt (14–18 m in less than 340 years, 14,600 years ago), is important for understanding mechanisms of ...rapid ice melt and the links with abrupt climate change. Here we quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a, by driving an ice sheet model with two transient climate simulations of the last 21,000 years. Ice sheet perturbed physics ensembles were run to account for model uncertainties, constraining ice extent and volume with reconstructions of 21,000 years ago to present. We determine that the North American ice sheet produced 3–4 m global mean sea level rise in 340 years due to the abrupt Bølling warming, but this response is amplified to 5–6 m when it triggers the ice sheet saddle collapse.
Key Points
The cause of the largest rapid sea level rise ever recorded, Meltwater Pulse 1a (14.6 ka), is unresolved
We quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a
The Bolling warming likely triggered the saddle collapse mechanism contributing 5‐6 m or more to MWP1a from the North American ice sheet
During abrupt climate changes of the last glacial period paleorecords show large amplitude changes in the dust cycle. We use Earth System model simulations to evaluate processes operating across ...these events. Idealized Heinrich stadial‐like simulations show a southwards migration of tropical rainfall that dries the Sahel and reduces wet deposition causing a widespread enhancement of tropical dust loading. However, several discrepancies with marine core dust deposition reconstructions are evident. Simulations with a more limited freshwater forcing (0.4 Sv instead of 1.0 Sv) and weaker cooling over the North Atlantic (less than 3°C) show a switch in sign of the stadial dust deposition anomaly in several regions, improving agreement with paleorecords. The simulated dust cycle therefore displays in places a non‐linear response to abrupt change. The global‐mean stadial dust radiative forcing in the more realistic simulations is around −0.2 to −0.6 W m−2 and so could represent an amplifying feedback during these events.
Plain Language Summary
Mineral dust in the atmosphere is mostly sourced from arid regions like the Sahara desert. The amount and geographical spread of this dust in the atmosphere is sensitive to environmental conditions such as drought. Paleoclimate records show that rapid cooling events centered on the North Atlantic during the last ice‐age also saw large increases in the rate of dust deposited over the ocean. Earth System models are a computational tool that can be used to understand the links between climate and the dust cycle. Using a series of such simulations we found that abrupt cooling events in the North Atlantic can lead to a massive increase in dust over the equatorial (tropical) regions. However, the model's response is in many places non‐linear, meaning that the change in dust is very sensitive to the magnitude of the climate changes occurring. This non‐linearity is mostly due to interactions between changes in the water cycle and the dust. We show that a less severe cooling produces a more realistic dust response when evaluated against paleo‐dust records. Since dust scatters incoming sunlight, increased atmospheric dust during these events may have amplified and even extended the cold phases.
Key Points
Freshwater hosing forcing increases tropical dust loading in Earth System model simulations of the last glacial maximum
Simulated response is non‐linear with forcing in some regions, particularly North Africa
Weaker events improve agreement with data and emitted dust produces a self‐amplifying feedback
Palaeoclimate model simulations are an important tool to improve our understanding of the mechanisms of climate change. These simulations also provide tests of the ability of models to simulate ...climates very different to today. Here we present the results from two brand-new simulations using the latest version of the UK's physical climate model, HadGEM3-GC3.1; they are the mid-Holocene (â¼6 ka) and Last Interglacial (â¼127 ka) simulations, both conducted under the auspices of CMIP6/PMIP4. This is the first time this version of the UK model has been used to conduct palaeoclimate simulations. These periods are of particular interest to PMIP4 because they represent the two most recent warm periods in Earth history, where atmospheric concentration of greenhouse gases and continental configuration are similar to the pre-industrial period but where there were significant changes to the Earth's orbital configuration, resulting in a very different seasonal cycle of radiative forcing.
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