The Maldives archipelago acts for over 25 myrs as a giant natural sediment trap in the eastern Arabian Sea. Drifts and periplatform deposits bear the record of environmental changes such as sea-level fluctuations but also of monsoon-driven changes of the surface and intermediate water mass current regime, and of wind-driven dust influx. Carbonate drifts in the Inner Sea indicate the establishment of a strong wind-driven current regime in the Maldives at 12.9 – 13 Ma. Ten unconformities, dissecting the Miocene to Recent drift sequences, attest to changes in current strength or direction. A major shift in the drift packages is dated at 3.8 Ma that coincides with the end of stepwise platform drowning and a reduction of the OMZ in the Inner Sea. The lithogenic fraction of the Maldives carbonate drifts provides a unique record of atmospheric dust transport during the past 4 myrs as grain size provides proxies for dust flux as well as wind transport capacity. Entrainment and long-range transport of dust in the medium to coarse silt size range is linked to the strength of the Arabian Shamal winds and the occurrence of convective storms which prolong dust transport. Dust flux and the size of dust particles increased between 4.0 and 3.3 Ma, corresponding to the closure of the Indonesian seaway and the intensification of the South Asian Monsoon. Between 1.6 Ma and the Recent, dust flux again increased and shows higher variability, especially during the last 500 kyr. Transport capacity increased between 1.2 and 0.5 Ma but slightly decreased since then. Dust transport varies on orbital timescales, with eccentricity control being the most prominent (400 kyr throughout the record, 100 kyr between 2.0 and 1.3 Ma, and since 1.0 Ma). Higher frequency cycles (obliquity and precession) are most pronounced in wind transport capacity. The published and ongoing studies of IODP Expedition 359 cores show that deposits surrounding carbonate platforms, i.e. carbonate drifts, bear a previously underestimated potential to add substantial knowledge for the understanding of the monsoon evolution on million-year, but also on shorter time scales. Potential targets for further research and drilling are for example the Laccadives, the Mascarene Plateau or the South China Sea platforms.
The Taurus Mountains form the southern margin of the Central Anatolian Plateau of Turkey and form an orographic barrier separating the cold, semi-arid interior to the north from the mild Mediterranean coast to the south. When and how they formed, and the extent which they have influenced the regional climate remains poorly constrained. The Attepe iron deposits sit on the northern part of the Eastern Taurus mountains at altitude of 1.5-2 km and consequently are ideally located to record interactions between climate and tectonics. (U-Th)/He ages of iron-oxide-oxyhydroxides from four mines within the Attepe iron deposits record ages of 1-5 Ma consistent with the persistence of hot humid climate conditions throughout the Pliocene and Pleistocene. In mines where samples are measured from different depths the age data are consistent with water table lowering rate of between 12.3 to 6.4 m/Myr. Translating these to rock uplift rates they are close to uplift/incision recorded within the Central Anatolian Plateau over the past 2 Ma, suggesting that the region was already at or close to its current elevation by the late Miocene. The latest goethite precipitation constrains the cessation of hot-humid climate to sometime in the last million years and implies that regional climate cooling, rather than surface uplift, was the main driver of aridification.
Our increasingly robust history of ancient climates indicates that high latitude glaciation is the ultimate product of an episodic cooling trend that began about 100-million years ago rather than a result of a yet-to-be identified modal change. Antarctic geography (continent surrounded by ocean) allowed ice to develop prior to significant glaciation in the Northern Hemisphere (ocean surrounded by land), but global ice volume generally increased as Earth cooled. The question of what caused the Ice Ages should be reframed as to “What caused the Cenozoic Cooling?” Records tell us that changes in temperature and CO2 levels rise and fall together, however it is not clear when CO2 acts as a driver versus when it is primarily an indicator of temperature change. The episodic nature of the cooling trend suggests other more dynamic phenomena are involved. It is proposed that oceanic meridional overturning circulation (MOC) plays a significant role in regulating Earth’s surface temperature. Robust MOC has a cooling effect which results from its sequestration of cold waters (together with their increased heat-absorbing potential) below the surface. Unable to better absorb equatorial insolation for great lengths of time, oceanic deep waters are not able to fully compensate for the heat lost by warm-water transport to Polar Regions. A lag-time between cooling and subsequent warming yields lower operating temperatures commensurate with the strength of global MOC. The long-term decline in global temperatures is largely explained by the tectonic reshaping of ocean basins and the connections between them such that MOC has generally, but not uniformly, increased. Geophysically Influenced MOC (GIMOC) has caused a significant proportion of the lowering of global temperatures in the Cenozoic Era. Short-term disruptions in MOC (and subsequent impacts on global temperatures) were likely involved in Late Pleistocene glacial termination events and may already be compounding present anthropogenic CO2 induced warming. The immediate impacts of AMOC strength on North Atlantic and European temperatures coupled with the delayed and opposing effects on global temperatures offer explanations for phenomena including: Younger Dryas, Little Ice Age and the bipolar seesaw. (Please see supplemental file: Oceanic Geophysics & History of Climate(ver #02).pdf listed below.)
A transient period of climate change, characterized by a global warming of ~2.5–5°C followed by a cooling to pre-excursion conditions, occurred during the last 300 kyr of the Maastrichtian (~66.34–66.02 Ma). This instability may have played a role in destabilizing marine and terrestrial ecosystems, priming the system for abrupt extinction at the K-Pg boundary, likely triggered by a large bolide impact. This pre-K-Pg warming event has often been linked to the main phase of Deccan Trap volcanism, however large uncertainties associated with radio-isotopic dating methods of basalts, along with low sedimentation rates and hiatuses in many studied sedimentary sequences, have long hampered a definitive correlation. To complement recent advances in dating of the traps, we have generated the first complete and highest resolution (2.5–4 kyr) benthic stable δ13C and δ18O record for the final million years of the Maastrichtian using the epifaunal foraminifera species Nuttallides truempyi from ODP Site 1262, Walvis Ridge, South Atlantic, calibrated to an updated orbitally-tuned age model. We then compare our data to other previously published geochemical data from other sites in the high, middle, and low latitudes. Our data confirms that the onset of the warming event coincides with the onset of the main phase of Deccan volcanism, strongly suggesting a causal link. Furthermore, spectral analysis of our extended late Maastrichtian-Early Eocene record suggests that the onset of the warming event corresponds to a 405-kyr eccentricity minima, in contrast to many transient warming events (hyperthermals) of the Paleogene, suggesting a control by orbital forcing alone is unlikely. A peculiar feature of the event, compared to other hyperthermals, is a muted carbon cycle response during warming, which may be related to the comparatively heavier δ13C signature of volcanogenic CO2 (–7‰), compared to other sources of light carbon invoked to explain Paleogene hyperthermals. The warming event coincided with minor extinctions of thermocline-dwelling foraminifera, along with dwarfing and blooms of the opportunistic disaster genera Guembelitria, suggesting that Deccan-induced climatic instability may have played a role in priming high-stress ecosystems which were tipped over a threshold into mass extinction during bolide impact.
Year-to-year variability of precipitation and temperature has significant consequences for water management decision making. “Whiplash” is a term which describes this variability at its most severe, referring to events at various timescales in which the hydroclimate switches between extremes. Tree-rings in semi-arid environments like the Truckee-Carson River Basin (California/Nevada watersheds with headwaters in the Sierra Nevada) can provide proxy records of hydroclimate as their annual growth is tied directly to limitations in water-year rainfall and temperature, but traditional metrics of reporting explained variance do not distinguish a reconstruction’s sensitivity to whiplash events. In this study, a pool of total ring width indices from five snow-adapted conifer species (Abies magnifica, Juniperus occidentalis, Pinus ponderosa, Pinus jeffreyi, Tsuga mertensiana) were used to develop a series of standardized reconstructions of water-year PRISM precipitation (P12) using stepwise linear regression. A nonparametric analysis approach was then used to determine positive and negative whiplash events in reconstructed and instrumental precipitation records. Hypergeometric distribution of the resulting timeseries datasets illustrates relationships between reconstructions and recorded whiplash events and allows for determination of patterns in tree-ring growth response. The results of this study suggest that ring-width indices from the assessed conifer species in the snow-belt of the Sierra Nevada are often able to record consecutive years of opposing extreme precipitation and report such events through derived models. Negative WL events are tracked more consistently across species in site-specific reconstructions of P12 than positive ones. It appears that residual effects of a preceding year’s drought or pluvial do not necessarily suppress records of WL, though sensitivity to precursor conditions in tracking of WL events may differ across species, and the absolute WL events captured in a reconstruction vary.
This paper addresses several issues concerning Milankovitch Theory and its relationship to paleoclimate data over the last 800,000 years. A model is presented that deconvolutes the precession index (precession modulated by the eccentricity) and the obliquity contributions to the percentage change between successive mean-daily-insolation minima and maxima. The sum of these contributions is in close agreement with the corresponding benchmark calculation of J. Laskar et al. The model predictions indicate that the precession index contribution dominates such insolation changes, and its time-dependent behavior correlates with the occurrence of interglacial and glacial periods and temperature trends during these periods. Best fit curves to the separate contributions appear as quasiperiodic waves that correlate with interglacial initiations and terminations through their constructive and destructive interference. However, a comparison of model predictions with the EPICA Dome C (EDC) data indicates delayed inceptions for Marine Isotope Stages 18d and 13c, which have also been noted by Parrenin et al. through a comparison of LR04 benthic 18O and EDC ice core datasets. Finally, the model enables the classification of interglacial periods into two distinct types that approximately account for their durations. This classification also enables a low-resolution estimation of the Holocene termination based solely on celestial mechanical forcing.
Volcanic flood basalt eruptions have been linked to or are contemporaneous with major climate disruptions, ocean anoxic events, and mass extinctions throughout at least the last 400M years of Earth’s history. Previous studies and recent history have shown that volcanically-driven climate cooling can occur through reflection of sunlight by H2SO4 aerosols, while longer-term climate warming can occur via CO2 emissions. We use the Goddard Earth Observing System Chemistry-Climate Model to simulate a four-year duration volcanic SO2 emission of the scale of the Wapshilla Ridge member of the Columbia River Basalt eruption. Brief cooling from H2SO4 aerosols is outweighed by dynamically and radiatively driven warming of the climate through a three orders of magnitude increase in stratospheric H2O vapor.
In a warmer world, the hydrological cycle will change in intensity and in its geographic behaviour. This, in turn, will change patterns of river flood and the risk associated with them. The Last Interglacial (LIG; 125,000 years ago) is the most recent instance of climate warmer than today - especially in the high northern latitudes-, sea level was higher, ice sheets were smaller and monsoons were stronger. We use daily output from multi-century LIG simulations of an ensemble of paleoclimate models, and study how global precipitation patterns and extremes deviate from the preindustrial climate. We validate these results by comparing them with the first compilation, to our knowledge, of global LIG precipitation patterns. Successively, we use the daily temperature and precipitation from the paleoclimate models to drive two global hydrological models (PCR-GLOBWB and CWATM), and simulate river discharges at 5-30’ resolution. With this, we force a hydrodynamic model, CaMa-Flood, and produce floods maps for different return periods. At the end of this model cascade, we look into what would happen if a climate similar to the LIG were to materialize in the coming decades: we combine the flood maps with maps of exposure through vulnerability relationships, and to calculate the risk that floods may pose to future people and assets.
Coastal risks are increasing due to the warming of the climate, resulting in rising mean sea levels and changes in storminess. Projections of future coastal flooding rely on global climate models based on greenhouse gas scenarios with inherent large uncertainties. The past warm climate of the Last Interglacial (LIG, ~127,000 years ago) is considered a partial analogue of a future warmer world. Therefore, understanding how coastal systems were affected by changes in atmospheric and relative sea levels during the LIG can inform us about possible future changes. In this contribution we will analyze extreme sea levels and coastal flooding during the LIG. The analysis is based on the hydrodynamic Global Tide and Surge Model (GTSM; Muis et al., 2016, doi: 10.1038/ncomms11969). To simulate storm surges during the LIG GTSM will be forced by 6-hourly wind and surface pressure fields from LIG simulations of IPCC-type climate models. Due to non-linear effects, tides and surge levels will be influenced by changes in mean sea level. Therefore, a key input variable is map of regional mean sea levels during LIG. However, there is still considerable uncertainty on sea level high-stands and regional patterns during the LIG. Using output from a Glacial Isostatic Adjustment model (GIA), we will model tides and surges for a set of plausible scenarios of relative sea levels and assess sensitivities.
During the last deglaciation (21 - 7 kaBP), the gradual retreat of Northern Hemisphere ice sheet margins produced large proglacial lakes. While the climatic impacts of these lakes have been widely acknowledged, their role on ice sheet grounding line dynamics has received very little attention so far. Here, we show that proglacial lakes had dramatic implications for the North American ice sheet dynamics through a self-sustained mechanical instability which has similarities with the known marine ice sheet instability albeit providing fast retreat of large portions of the ice sheet over the continent. Systematically reproduced in the latest stage of the deglaciation, this mechanism could provide a physical origin for the debated melt water pulse 1B. Echoing our knowledge of Antarctic ice sheet dynamics, they are another manifestation of the importance of grounding line dynamics for ice sheet evolution.
Proxy-model comparisons show large discrepancies on volcanic aerosols’ hydrological effects in the Asian monsoon region (AMR). This was mostly imputed to uncertainties of the single model used in previous studies. Here, we compared two groups of CMIP5 multi-model ensemble mean (MMEM) with the tree-ring-based reconstruction Monsoon Asia Drought Atlas (MADA PDSI), to examine their reliability on reflecting hydrological effects of the volcanic eruptions in 1300-1850 CE. Time series plots indicate that MADA PDSI and MMEMs agree on the significant drying effects of volcanic perturbation over the monsoon-dominated subregion, while mismatches exist over the westerlies-dominated subregion. Comparisons on spatial patterns suggest that MADA PDSI and MMEMs agree better in one year after the volcanic eruption than in the eruption year, and in subregions with more available tree ring chronologies. MADA PDSI and CMIP5 MMEMs agree on the drying effect of volcanic eruptions in western-East Asia, South Asian summer monsoon and northern East Asian summer monsoon (EASM). Model results suggest significant wetting effect in southern EASM and western-South Asia, which agrees with the observed hydrological responses to 1991 Mount Pinatubo eruption. Analysis on LME model simulations show similar hydrological responses. These results suggest that CMIP5 MMEM is able to reproduce volcanic eruptions’ hydrological effects in southern AMR.
Modern investigations have shown that oxygen and carbon isotopes of land snail shells are useful indicators of climate and vegetation in monsoonal region. However, stable isotope study on snail fossil shells in strata has been seldom done, and the reliability of those indicators needs further verification. Moreover, intra-shell stable isotope analysis of individual snail is rather scarce, and seasonal variation of the glacial-interglacial monsoonal climate remains unclear. In this context, we performed δ18O and δ13C analyses on fossil shells of cold-aridiphilous Cathaica pulveratrix and sub-humidiphilous Metodontia yantaiensis from the loess section over the last two glacial cycles at Beiyao site in southern Chinese Loess Plateau. The δ18O of fossil shells reflected monsoonal rainfall amount and more rainfall during MIS3 and MIS7. Meanwhile, the δ13C of fossil shells indicated relative abundance of C3/C4 plants and more C4 biomass during MIS3 and MIS7. The δ18O and δ13C of the two species from the same horizon are significantly different, reflecting differences in their growing season and/or physiological habits. Intra-shell variations of stable isotopes showed that climatic seasonality was relatively strong during the glacial periods whereas seasonality became weakened during the interglacials. Our findings provide an environmental background for explaining past human activities at the Beiyao site. The investigation of stone artifacts showed that ancient human activities were relatively strong during MIS3 and MIS7. During these stages, the warm and humid climate with smaller seasonal contrast was favorable for the regional expansion of human activities.
We investigate hydrology during a past climate slightly warmer than the present: the Last Interglacial (LIG). With daily output of pre-industrial and LIG simulations from eight new climate models we force hydrological model PCR-GLOBWB, and in turn hydrodynamic model CaMa-Flood. Compared to pre-industrial, annual mean LIG runoff, discharge, and 100-year flood volume are considerably larger in the Northern Hemisphere, by 14%, 25% and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, Pearl, and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible.
The cyclical pattern of glaciations (about every ~100,000 years in the past million years in isotope records) suggests an external forcing, such as changes in the amount of sunlight (insolation) reaching areas on the cusp of glaciation (e.g., ~65° latitude) during the season when ice is most likely to melt (summer). Alternatively, ice ages could be caused by insolation triggering changes within the Earth’s climate system that alter carbon dioxide and ice sheet size, leading to globally synchronous climate change. The key to testing these hypotheses is knowing the timing and magnitude of past temperature change. Mountain glaciers tell a slightly different story about past climate change. Strand et al. (2022, https://doi.org/10.1029/2022PA004423) contribute a new moraine chronology from an understudied region in Mongolia showing that the glacier reached a maxima 10,000 years prior to the lowest CO2 level and peak in global ice volume, and that massive glacier retreat was underway millennia prior to any of the proposed climate drivers. Their moraine chronology is in agreement with those from the Southern Alps of New Zealand, which is located in the opposite hemisphere and maritime climate rather than continental. Here, I review the results from Strand et al. (2022) in the context of ongoing advances in the methods and moraine chronologies from the last glacial cycle. The issue remains that we do not have a clear understanding of the cause of ice ages and the strength of feedbacks within Earth’s climate system.
Globally consistent natural evidence on past climate evolution is indispensable for climate model evaluations and forecasts. However, it has rarely been investigated quantitatively whether large sets of globally distributed pollen records with limited dating resolution can be statistically linked. This could facilitate the identification of global in contrast to regional climate change signals on millennial to orbital time scales. We consider a global set of time-irregular pollen records for a joint analysis of spatial similarity on different time scales during the last glacial. Making use of measures suitable for irregular time series and by application of a spatio-temporal stochastic model, we examine significant commonality between pollen records. We quantitatively assess the resulting paleo-climate networks while respecting the spatially heterogeneous and sparse proxy archive layout. The network configurations are compared to synthetic proxy networks, which mimic different real-world record impairments. We find strong commonalities of well resolved Chilean, North Pacific and European records on orbital to millennial time scales. They reveal partly inverted deglaciation signals for westward exposed coastal tree vegetation. Such signals are consistently observable for several mid-latitude records, probably indicating equatorward shifts of westerly circulation structures during the last glacial. Surrogate data suggests that a notable part of total records might be insufficiently resolved to detect statistically significant record similarity at least when classical correlation-based measures are utilised. We compare the results to temperature and precipitation signals in PMIP3 models.
Equilibrium climate sensitivity (ECS) is the global temperature change expected after doubling atmospheric CO2 concentration. This Commentary reviews how Sherwood et al. (2020) used Bayesian statistics and evidence from climate-process physics, historical observations, and earlier proxies to reduce the likely range of ECS from 1.5-4.5 K to 2.6-4.1 K. They may have overestimated ECS by adding non-equilibrium short-term adjustments to the radiative forcing of greenhouse gases and by underestimating the effect of solar irradiance and aerosols. Two alternative periods during the Holocene show that forcing by agents other than CO2 was significant and requires further research.
Great Salt Lake (UT) is a hypersaline terminal lake in the US Great Basin, and the remnant of the late glacial Lake Bonneville. Holocene hydroclimate variations cannot be interpreted from the shoreline record, but instead can be investigated by proxies archived in the sediments. GLAD1-GSL00-1B was cored in 2000 and recently dated by radiocarbon for the Holocene section with the top 11 m representing ~7 ka to present. Sediment samples every 30 cm (~220 years) were studied for the full suite of microbial membrane lipids, including those responsive to temperature and salinity. The ACE index detects the increase in lipids of halophilic archaea, relative to generalists, as salinity increases. We find Holocene ACE values ranged from 81-98, which suggests persistent hypersalinity with <50 g/L variability across 7.2 kyr. The temperature proxy, MBTʹ5Me, yields values similar to modern mean annual air temperature for months above freezing (MAF = 15.7°C) over the last 5.5 kyr. Several GDGT metrics show a step shift at 5.5 ka before which temperature estimates are unreliable due to the shift in lake ecology and likely shallow depth. The step change in lake conditions at 5.5 ka and additional variations within the late Holocene are compared to regional climate records. We find evidence for a dry mid-Holocene in GSL, corroborating other records.
Increased flood risks have been projected in the Kabul River Basin, but with large uncertainties. To place future changes in a long-term perspective, we produce a 501-year precipitation reconstruction for the basin using seven tree-ring chronologies of Cedrus deodara, Picea smithiana, and Pinus gerardiana from the Hindukush Mountains, a monsoon-shadow area. The reconstruction proves robust over rigorous cross-validations (R2 = 0.62, RE = 0.61, CE = 0.53). The full reconstruction (1517–2018) shows heterogeneous changes in the precipitation distribution: there is a weak increasing trend in the median annual precipitation, no apparent trend in the 50-year maximum precipitation, and, importantly, a steadily decreasing trend in 50-year minimum precipitation. In other words, our reconstruction shows that drought risks have been increasing over the past five centuries. Drought risks, compounded with projected flood intensification, pose significant threats for the transboundary river. Future water management decisions should factor in past long-term climate variability.