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Paleoclimate reconstructions can help us learn the evolution of climate mean state and variability in the past, understand mechanisms for climate change, and test the climate models that are extensively used for future climate projections. However, reconstructions have one major disadvantage that usually they are measurements of proxy variables (e.g., oxygen isotopes in calcium shells of foraminifera) instead of climate state variables (e.g., ocean temperature), which are directly simulated in traditional coupled climate models. This require that the model-data comparisons are indirect in paleoclimate studies and making it extremely difficult to address model-data discrepancies, especially when both models and reconstructions are subject to substantial biases and uncertainties. To overcome this obstacle, my PhD work involves developing a fully coupled water isotope-enabled Community Earth System Model (iCESM) in collaboration with other scientists. The physical climate of the iCESM is one of the best state-of-the-art fully coupled earth system models. In addition to the regular hydrologic cycle, iCESM can explicitly simulate the transport and transformation of water isotopes (e.g., H218O) in its components--the atmosphere, land, ocean, sea ice and river runoff. The iCESM can well-reproduce the major features of water isotopes in observations, including the tropical amount effect and high latitude temperature effect, as well as the continental and altitude effects in precipitation-d18Op in present day observations. The simulated d18O in seawater (d18Ow) also closely resemble the pattern in observations. Moreover, a simulation of the LGM (21,000 ka before present) shows that the model is able to simulate the glacial-interglacial changes of d18Op in ice cores, d18Ow in porefluid reconstructions, and d18Oc in ocean sediments, suggesting the model is suited for paleoclimate studies. With the water-isotope capability of the iCESM, I have investigated the following scientific questions: (1) How and why the water isotope-temperature relationship in Greenland ice cores varies during abrupt climate changes; (2) Whether the El Niño-Southern Oscillation (ENSO) was stronger or weaker at the LGM than the present day. (1) Isotope-temperature relationship. For more than 50 years, water isotope values (e.g., d18O) in ice cores have provided a tremendous amount of information about the Earth climate history during the late Quaternary. Initially based on a "modern analogue method", d18O changes in ice cores were directly interpreted as variations in local temperature, which was challenged later by independent reconstructions. Although it is now becoming clear that the temporal d18O-temperature relationship could vary both spatially and temporally in ice-core records, how the temporal slope could vary during abrupt climate changes and what is causing these changes still remain unclear. In my PhD study, I have quantitatively studied the changes in d18O-temperature relationship over Greenland in response to varied climatic forcings using the iCESM. I found that the temporal slope in Greenland increases significantly with the amount of meltwater discharged into the northern North Atlantic Ocean, due to the reduced moisture from the nearby oceans and the tracer effect from depleted meltwater (e.g., about -30 0 . Otherwise, the d18O-temperature relationships (spatial and temporal) are relatively stable in response to greenhouse gas (GHG), ice sheets and mid-Holocene orbital forcing. It is also found that part of the d18O signal in ice cores during meltwater events can be simply attributed to the tracer effect--the propagation of depleted meltwater in the hydrological cycle--instead of any changes in the climate state. These important findings imply that abrupt temperature changes during meltwater events previously inferred from ice cores could have been significantly overestimated. (2) ENSO variability at the LGM. Despite its paramount importance in climate system, the response of ENSO to anthropogenic global warming is still inconclusive in recent climate models. Studying the ENSO strength in the past can serve as a testbed for these climate models used for future projections and provide us the opportunity to investigate possible relationships between ENSO variability and the mean climate states. But, the ENSO strength at the LGM is inconclusive both in current climate models and paleoclimate reconstructions, including those records using the individual foraminifera analysis (IFA) in the eastern equatorial Pacific (EEP). Here, for the first time, I have directly compared modeled water isotopes in the iCESM with the IFA records. Synthesizing evidence from both models and reconstructions, it is found that ENSO at the LGM is most likely weaker than that of the preindustrial, because of the weakened Bjerknes feedbacks. The iCESM suggests that total variance of the IFA records may only reflect changes in the annual cycle instead of ENSO variability as previously assumed. Furthermore, the interpretation of subsurface IFA records can be substantially complicated by the habitat depth of thermocline-dwelling foraminifera and their vertical migration with a temporally varying thermocline. The model suggests an inverse relationship between ENSO variability and zonal SST gradient, thermocline depth and surface winds in equatorial Pacific, consistent with previous theoretical or observation based studies. Results indicate that ENSO variability could be stronger in response to the future anthropogenic global warming
As anthropogenic emissions of greenhouse gasses continue to alter the Earth's climate, it becomes increasingly vital to understand how the Earth system has responded to high temperatures and pCO2 in the past. The Cenozoic era in particular offers unique insights into climate systems equilibrated with modern to near-future radiative forcing and comparable global geographic boundary conditions. Isotopes in precipitation ([delta] D and [delta] 18O) are one of the more ubiquitous tools used to investigate Cenozoic climate, as they are sensitive to a number of hydro-climatic factors including rainout patterns, terrestrial moisture recycling, evaporative source conditions, and atmospheric mixing by transient eddies. While this sensitivity allows for the potential characterization of large-scale hydrologic dynamics, separating out these disparate effects remains a major challenge for interpreting proxy records. This dissertation aims to address this challenge by using a reactive transport model of isotopes in precipitation to develop a framework for testing hypotheses of past conditions against proxy records. This framework is then applied along with other established methods to investigate a number of Cenozoic climate questions including: (1) migration of the Pacific storm track in response to changes in the structure of tropical Pacific SST's and initiation of Northern Hemisphere glaciation through the Plio-Pleistocene; (2) peak eustatic Pliocene sea level and East Antarctic Ice Sheet stability; and (3) global latent heat transport under Early Eocene hothouse climate conditions.
The field of paleoclimatology relies on physical, chemical, and biological proxies of past climate changes that have been preserved in natural archives such as glacial ice, tree rings, sediments, corals, and speleothems. Paleoclimate archives obtained through field investigations, ocean sediment coring expeditions, ice sheet coring programs, and other projects allow scientists to reconstruct climate change over much of earth's history. When combined with computer model simulations, paleoclimatic reconstructions are used to test hypotheses about the causes of climatic change, such as greenhouse gases, solar variability, earth's orbital variations, and hydrological, oceanic, and tectonic processes. This book is a comprehensive, state-of-the art synthesis of paleoclimate research covering all geological timescales, emphasizing topics that shed light on modern trends in the earth's climate. Thomas M. Cronin discusses recent discoveries about past periods of global warmth, changes in atmospheric greenhouse gas concentrations, abrupt climate and sea-level change, natural temperature variability, and other topics directly relevant to controversies over the causes and impacts of climate change. This text is geared toward advanced undergraduate and graduate students and researchers in geology, geography, biology, glaciology, oceanography, atmospheric sciences, and climate modeling, fields that contribute to paleoclimatology. This volume can also serve as a reference for those requiring a general background on natural climate variability.
This book provides a synthesis of the past decade of research into global changes that occurred in the earth system in the past. Focus is achieved by concentrating on those changes in the Earth's past environment that best inform our evaluation of current and future global changes and their consequences for human populations. The book stands as a ten year milestone in the operation of the Past Global Changes (PAGES) Project of the International Geosphere-Biosphere Programme (IGBP). It seeks to provide a quantitative understanding of the Earth’s environment in the geologically recent past and to define the envelope of natural environmental variability against which anthropogenic impacts on the Earth System may be assessed. A set of color overhead transparencies based on the figures in the book is available free on the PAGES website (www.pages-igbp.org) for use in teaching and lecturing.
This two-volume book provides a comprehensive, detailed understanding of paleoclimatology beginning by describing the “proxy data” from which quantitative climate parameters are reconstructed and finally by developing a comprehensive Earth system model able to simulate past climates of the Earth. It compiles contributions from specialists in each field who each have an in-depth knowledge of their particular area of expertise. The first volume is devoted to “Finding, dating and interpreting the evidence”. It describes the different geo-chronological technical methods used in paleoclimatology. Different fields of geosciences such as: stratigraphy, magnetism, dendrochronology, sedimentology, are drawn from and proxy reconstructions from ice sheets, terrestrial (speleothems, lakes, and vegetation) and oceanic data, are used to reconstruct the ancient climates of the Earth. The second volume, entitled “Investigation into ancient climates,” focuses on building comprehensive models of past climate evolution. The chapters are based on understanding the processes driving the evolution of each component of the Earth system (atmosphere, ocean, ice). This volume provides both an analytical understanding of each component using a hierarchy of models (from conceptual to very sophisticated 3D general circulation models) and a synthetic approach incorporating all of these components to explore the evolution of the Earth as a global system. As a whole this book provides the reader with a complete view of data reconstruction and modeling of the climate of the Earth from deep time to present day with even an excursion to include impacts on future climate.
One of Springer’s Major Reference Works, this book gives the reader a truly global perspective. It is the first major reference work in its field. Paleoclimate topics covered in the encyclopedia give the reader the capability to place the observations of recent global warming in the context of longer-term natural climate fluctuations. Significant elements of the encyclopedia include recent developments in paleoclimate modeling, paleo-ocean circulation, as well as the influence of geological processes and biological feedbacks on global climate change. The encyclopedia gives the reader an entry point into the literature on these and many other groundbreaking topics.
Maintenance of a habitable planet requires connections and balance among Earth's biogeochemical cycles. Further, the strength of the feedbacks and couplings determines the stability of conditions in the surface climate system necessary for the evolution of life. Records of Earth's past climate, paleoclimate records, provide constraints beyond the reach of the instrumental record on the directionality, strength and co-evolution of key Earth system cycles. This includes the geologic carbon cycle, the water cycle and the planet's energy balance. Crucially, the geography, topography and lithology of Earth's continents have two important features that are the focus of this dissertation. First, the continents provide boundary conditions that determine global circulation and hydroclimate patterns that couple Earth's water and carbon cycles (Chapters 1 and 2). Second, the land surface provides a stabilizing negative feedback in the form of silicate weathering fluxes (Chapters 3 and 4, and Appendix E), balancing the long-term carbon cycle through alkalinity and solute delivery to the oceans, and subsequent carbonate burial. In this dissertation I use data, observations and modeling to place mechanistic constraints on how interactions between Earth's surface and long-term biogeochemical cycles maintain balance and habitable conditions in our climate system conducive for the evolution of life. Fundamental to understanding our climate system is predicting the anticipated sign of terrestrial hydroclimate change during periods of climatic change. To this end I have investigated the regional response of hydroclimate change using paleoclimate records from mid-latitude lake systems. First, in Chapter 1, I have developed an inverse model to quantify how a mid-latitude lake system in Asia, the Songliao Basin, responded to a transient warming event during the Cretaceous. This model was also applied to a Holocene ostracod record from Lake Miragoane, Haiti. Second, in Chapter 2, I compile spatial distributions and size estimates of pluvial lake systems in western North America during the Pliocene-Pleistocene. By imposing mass and energy balance constraints (sensu Budyko) I forward modeled lake area distributions to demonstrate that now-arid western North America was wetter during both past colder and warmer periods during the Pliocene-Pleistocene, a result not predicted for future warming scenarios. Geologic observations primarily suggest wetter conditions globally during warmer-than-present periods. Importantly, this observation is a requirement for the operation of the stabilizing negative feedback between silicate weathering and climate. Understanding the factors that control silicate weathering rates is fundamental to constraining the evolution of Earth's carbon cycle over geologic timescales. One challenge for reconstructing past weathering rates is determining the reactivity of Earth's surface. In Chapter 3 I have quantified the reactivity of modern basalt and granite catchments using a process-based solute production model and concentration-discharge weathering relationships. This approach provides mechanisms that link runoff (i.e., terrestrial hydroclimate changes that were the focus of Chapters 1 and 2) with the distribution of global sub-aerial silicate lithologies. In Chapter 4 I utilize an emerging metal isotope system, lithium isotopes, to investigate the terrestrial weathering response to a large perturbation in the carbon cycle during the Cretaceous. We determine that the background-state of the Cretaceous weathering system was more congruent and, hence, more sensitive to perturbations in the carbon cycle than during the Neogene. Finally, in Appendix E I have quantified how step-wise geologic evolution of land plants strengthened the silicate weathering feedback over the Phanerozoic. I have developed a new reactive transport framework for evaluating the relative plant-controlled roles of hydrologic versus thermodynamic mechanisms that influence the coupling between the water cycle and silicate weathering fluxes on a continental scale. The results described in this dissertation constrain links between two connected portions of the exogenic Earth system. I have provided constraints for how the land surface records past changes in regional atmospheric circulation patterns, as well as regional water and energy balances. Further, using reactive transport modeling, I have placed new constraints on the role of plants and lithology in determining the coupling between terrestrial hydroclimate and weathering. Collectively these results suggest that the surface Earth system, our planet's fluid envelope, has been progressively tuned by the advent of continents and the evolution of life.
New York : Wiley, c1985.
Greenhouse gases, global warming, thinning ozone layers—understanding the Earth's climatic changes is one of today's most pressing international concerns. How fast has the climate changed? Where and why is it changing? What is the impact of climate change on our ecosystems, coastal regions, glaciers, forests, and lakes, and even on the evolution of our own species? This introduction to the rapidly emerging field of paleoclimatology explains the patterns and processes in the history of the Earth's climate to answer such essential questions. Using the geologic records of ocean and lake sediment, ice cores, corals, and other natural archives, Principles of Paleoclimatology describes the history of the Earth's climate—the ice age cycles, sea level changes, volcanic activity, changes in atmosphere and solar radiation—and the resulting, sometimes catastrophic, biotic responses. These paleoclimate records provide a baseline against which we can compare modern climate trends. Designed to give a fundamental background—including both history and methodology—to the discipline of paleoclimatology, this book is the first to advance our understanding of how climate change develops, how those changes are detected, and how the climate of the past can shape the climate of the future.
This book, first published in 2002, is a graduate-level text on numerical weather prediction, including atmospheric modeling, data assimilation and predictability.