Past climate variability: model analysis and proxy intercomparison

This thesis investigates the climate variability of the late Quaternary (21 000 yrs BP to present day) using model simulations and proxy data. The thesis consists of four manuscripts and one appendix. In the first two manuscripts and the appendix I, the extratropical Northern Hemisphere atmospheric circulation in different Quaternary time slices (preindustrial, PI, 1750 AD; Mid Holocene, MH, 6 kyrs BP; Last Glacial Maximum, LGM, 21 kyrs BP) is investigated using different climate models. The contributions of greenhouse gases, ice-sheet topography and albedo on the atmospheric mean climate and its variability are analyzed. In general, the models show no major changes in atmospheric circulation nor in its interannual variability in a climate slightly warmer (MH) than the PI one. In the LGM simulations, the models show decreased sea level pressure interannual variability relative to PI; on the other hand, the interannual variability of surface temperature is increased. The leading mode of sea level pressure variability in the North Atlantic is characterized by a NAO-like behavior in all climate states; however, it represents less total variance and the centers of action are weaker at the LGM. The presence of the Laurentide ice-sheet over North America during the LGM accounts for most of the changes observed in the LGM climate. Finally, the models show that the link between atmospheric and surface climate (temperature and precipitation) variability is altered in a glacial climate compared to the PI. Therefore, assuming present-day climate-proxy relationships when interpreting proxy records may well lead to a misinterpretation of past climates. The results of the first manuscript point out that certain proxies may record seasonal rather than annual climate changes or that they could be tape recorders for climate changes far afield rather than local. These issues are tackled in the second and third manuscripts. In paper III, various marine proxy records from the North Atlantic Ocean that spann the Holocene (10-0 kyrs BP) are compared with each other and with a model simulation of the MH. Sea-surface temperature records based on phytoplankton generally show the existence of a warm early to mid Holocene (9-6 kyrs BP) optimum. In contrast, zooplankton-based temperature records from the North Atlantic and Norwegian Sea show a cool mid Holocene anomaly and a trend towards warmer temperatures in the late Holocene. Model results indicate that while the warming of the sea surface was stronger in summer during the MH compared to the PI due to higher solar radiation at the high latitudes, sub-surface depths experienced a cooling, mirroring the winter sea surface temperatures. These physical changes in the surface and sub-surface characteristics of the water column can explain the discrepancies between the Holocene trends exhibited by phytoplankton- and zooplankton- based temperature proxy records. Paper III addresses the possibility that cave deposits, specifically in South Asia, record non-local rather than local climate changes. Using an atmospheric climate model with embedded stable water-isotope tracers, we propose a novel conceptual model to explain the oxygen isotopic changes recorded in Asian caves during abrupt climate changes (such as Heinrich events). We show the key role of the Indian Ocean in driving δ18O variations in both Indian and Chinese cave deposits: changes of the Indian Ocean surface temperature affects the Indian summer monsoon, which in turn leads to a change in the δ18O signature of the precipitation falling over the Indian subcontinent. This signal is eventually transferred from the Indian Ocean to Chinese caves via recycling of continental precipitation. Therefore, caves in eastern China (e.g., Hulu) do not record changes in the East Asian summer monsoon, as previously thought, but rather changes in the Indian summer monsoon

Tropical cyclone activity affected by volcanically induced ITCZ shifts

Volcanic eruptions can inject a large amount of aerosol particles, which interact with solar radiation and consequently can affect the climate worldwide, hence the intensity and frequency of extreme events for a few years following the eruption. However, only a handful of studies have investigated the impacts of volcanic eruptions on tropical cyclone activity. Through a set of sensitivity modeling experiments, our study demonstrates that volcanic eruptions by shifting the Intertropical convergence zone can impact tropical cyclone activity up to 4 years following the eruption. These results will prove valuable to society, allowing us to better prepare for the consequences of changes in tropical cyclone activity following large volcanic eruptions

The impacts of climate change on tropical-to-extratropical transitions in the North-Atlantic basin 

As tropical cyclones migrate towards mid-latitudes, they can transform into extratropical cyclones, a process known as extratropical transition. In the North Atlantic basin, nearly half of the hurricanes undergo this transition. After transitioning, these storms can reintensify, posing significant threats to populations and infrastructure along the eastern coast of North America. While the impacts of climate change on hurricanes have been extensively studied, there remain uncertainties about its effects on extratropical transitions. This study aims to assess how climate change affects the frequency, location, intensity, and duration of these transitions. To achieve this, high-resolution regional simulations from an atmospheric regional climate model, based on the RCP 8.5 emissions scenario, were used to compare two 30-year periods: the present (1990–2019) and the end of the century (2071–2100). The results indicate a projected decrease in the number of tropical hurricanes, with no significant change in extratropical transition rates. September and October continue to be the primary months for extratropical transitions. However, the season’s peak appears to have shifted from September to October, suggesting that large-scale environmental conditions may become more favorable for extratropical transitions in October in the future. Although a poleward shift in the maximum intensity of tropical hurricanes is detected, the average latitude of the transitions does not change. Our findings suggest that transitioning storms will be more intense in the future, despite a less baroclinic atmosphere due to a stronger contribution from latent heat transfer. However, the risk of reintensification after transition is not expected to increase

Rainfall regimes of the Green Sahara

During the “Green Sahara” period (11,000 to 5000 years before the present), the Sahara desert received high amounts of rainfall, supporting diverse vegetation, permanent lakes, and human populations. Our knowledge of rainfall rates and the spatiotemporal extent of wet conditions has suffered from a lack of continuous sedimentary records. We present a quantitative reconstruction of western Saharan precipitation derived from leaf wax isotopes in marine sediments. Our data indicate that the Green Sahara extended to 31°N and likely ended abruptly. We find evidence for a prolonged “pause” in Green Sahara conditions 8000 years ago, coincident with a temporary abandonment of occupational sites by Neolithic humans. The rainfall rates inferred from our data are best explained by strong vegetation and dust feedbacks; without these mechanisms, climate models systematically fail to reproduce the Green Sahara. This study suggests that accurate simulations of future climate change in the Sahara and Sahel will require improvements in our ability to simulate vegetation and dust feedbacks

Initiation of a Stable Convective Hydroclimatic Regime in Central America Circa 9000 Years BP

Many Holocene hydroclimate records show rainfall changes that vary with local orbital insolation. However, some tropical regions display rainfall evolution that differs from gradual precessional pacing, suggesting that direct rainfall forcing effects were predominantly driven by sea-surface temperature thresholds or inter-ocean temperature gradients. Here we present a 12,000 yr continuous U/Th-dated precipitation record from a Guatemalan speleothem showing that Central American rainfall increased within a 2000 yr period from a persistently dry state to an active convective regime at 9000 yr BP and has remained strong thereafter. Our data suggest that the Holocene evolution of Central American rainfall was driven by exceeding a temperature threshold in the nearby tropical oceans. The sensitivity of this region to slow changes in radiative forcing is thus strongly mediated by internal dynamics acting on much faster time scales

The remote response of the South Asian Monsoon to reduced dust emissions and Sahara greening during the middle Holocene

Previous studies based on multiple paleoclimate archives suggested a prominent intensification of the South Asian Monsoon (SAM) during the mid-Holocene (MH, similar to 6000 years before present). The main forcing that contributed to this intensification is related to changes in the Earth's orbital parameters. Nonetheless, other key factors likely played important roles, including remote changes in vegetation cover and airborne dust emission. In particular, northern Africa also experienced much wetter conditions and a more mesic landscape than today during the MH (the so-called African Humid Period), leading to a large decrease in airborne dust globally. However, most modeling studies investigating the SAM changes during the Holocene overlooked the potential impacts of the vegetation and dust emission changes that took place over northern Africa. Here, we use a set of simulations for the MH climate, in which vegetation over the Sahara and reduced dust concentrations are considered. Our results show that SAM rainfall is strongly affected by Saharan vegetation and dust concentrations, with a large increase in particular over northwestern India and a lengthening of the monsoon season. We propose that this re- mote influence is mediated by anomalies in Indian Ocean sea surface temperatures and may have shaped the evolution of the SAM during the termination of the African Humid Period

Tropical cyclone activity enhanced by Sahara greening and reduced dust emissions during the African Humid Period

Tropical cyclones (TCs) can have devastating socioeconomic impacts. Understanding the nature and causes of their variability is of paramount importance for society. However, historical records of TCs are too short to fully characterize such changes and paleo-sediment archives of Holocene TC activity are temporally and geographically sparse. Thus, it is of interest to apply physical modeling to understanding TC variability under different climate conditions. Here we investigate global TC activity during a warm climate state (mid-Holocene, 6,000 yBP) characterized by increased boreal summer insolation, a vegetated Sahara, and reduced dust emissions. We analyze a set of sensitivity experiments in which not only solar insolation changes are varied but also vegetation and dust concentrations. Our results show that the greening of the Sahara and reduced dust loadings lead to more favorable conditions for tropical cyclone development compared with the orbital forcing alone. In particular, the strengthening of the West African Monsoon induced by the Sahara greening triggers a change in atmospheric circulation that affects the entire tropics. Furthermore, whereas previous studies suggest lower TC activity despite stronger summer insolation and warmer sea surface temperature in the Northern Hemisphere, accounting for the Sahara greening and reduced dust concentrations leads instead to an increase of TC activity in both hemispheres, particularly over the Caribbean basin and East Coast of North America. Our study highlights the importance of regional changes in land cover and dust concentrations in affecting the potential intensity and genesis of past TCs and suggests that both factors may have appreciable influence on TC activity in a future warmer climate.National Science Foundation (U.S.) (Grant AGS-1461517

Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas

Sources and timing of freshwater forcing relative to hydroclimate shifts recorded in Greenland ice cores at the onset of Younger Dryas, ∼12,800 years ago, remain speculative. Here we show that progressive Fennoscandian Ice Sheet (FIS) melting 13,100–12,880 years ago generates a hydroclimate dipole with drier–colder conditions in Northern Europe and wetter–warmer conditions in Greenland. FIS melting culminates 12,880 years ago synchronously with the start of Greenland Stadial 1 and a large-scale hydroclimate transition lasting ∼180 years. Transient climate model simulations forced with FIS freshwater reproduce the initial hydroclimate dipole through sea-ice feedbacks in the Nordic Seas. The transition is attributed to the export of excess sea ice to the subpolar North Atlantic and a subsequent southward shift of the westerly winds. We suggest that North Atlantic hydroclimate sensitivity to FIS freshwater can explain the pace and sign of shifts recorded in Greenland at the climate transition into the Younger Dryas

Hydroclimate in the Pamirs Was Driven by Changes in Precipitation‐Evaporation Seasonality Since theLast Glacial Period

The Central Asian Pamir Mountains (Pamirs) are a high‐altitude region sensitive to climatic change, with only few paleoclimatic records available. To examine the glacial‐interglacial hydrological changes in the region, we analyzed the geochemical parameters of a 31‐kyr record from Lake Karakul and performed a set of experiments with climate models to interpret the results. δD values of terrestrial biomarkers showed insolation‐driven trends reflecting major shifts of water vapor sources. For aquatic biomarkers, positive δD shifts driven by changes in precipitation seasonality were observed at ca. 31–30, 28–26, and 17–14 kyr BP. Multiproxy paleoecological data and modelling results suggest that increased water availability, induced by decreased summer evaporation, triggered higher lake levels during those episodes, possibly synchronous to northern hemispheric rapid climate events. We conclude that seasonal changes in precipitation‐evaporation balance significantly influenced the hydrological state of a large waterbody such as Lake Karakul, while annual precipitation amount and inflows remained fairly constant

Two interglacials: scientific objectives and experimental designs for CMIP6 and PMIP4 Holocene and last interglacial simulations

Two interglacial epochs are included in the suite of paleoclimate simulations in the present phase of the Coupled Model Intercomparison Project (CMIP6). Equilibrium simulations of the mid-50 Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127,000 years before present) are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land-sea contrast and high-latitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments as part of the Paleoclimate Modeling Intercomparison Project (PMIP4), to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically