Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: The purpose of this paper is to provide an overview of paleoceanography and paleoclimatology as a framework for other papers dealing with The Earth System: Geochemical Perspectives. An introduction to both paleoceanography and paleoclimatology is followed by examples of the temporal changes through the Phanerozoic. The important and interactive role of the biosphere is emphasized. Many important changes in the Earth system have affected the coupled ocean–atmosphere system and many, in turn, have been reflected in biotic events. Many such changes can be tracked through time using geochemical signatures as proxy indicators. Whereas the scale of past paleoceanographic and paleoclimatic changes have been generally appreciated for some time, the recognition of periodic rapid change in the ocean/climate state and the ability to study and measure these precisely is only a recent accomplishment. The potential for such rapid change in the ocean/climate/biosphere of the Earth system raises concerns for events in the near future that may be forced by anthropogenic activities that enhance natural variability.
ABSTRACT: This paper reviews the principal results from paleoclimate studies and includes background material slanted toward climate modelers. The inferred temperature history of the last 4.6 billion years indicates major changes in the components of the earth’s climate system. A secular change in global insolation receipt is due to a 20-30% increase in solar luminosity since the formation of the earth. A CO2 -H2 O greenhouse effect may have offset the lower luminosity during early earth history. Inferred fluctuations of global temperature have occurred over a broad range of time scales. On time scales of 106 -108 years, paleogeographic factors (e.g., continental drift and sea level changes) have contributed significantly to temperature changes associated with transitions between nonglacial and glacial states. Preliminary modeling efforts indicate that additional factors (e.g., CO2 , changes in atmospheric circulation) must also be considered in order to explain the origin of nonglacial climates. The origin of polar ice caps may result from ocean circulation changes that were caused by plate tectonic processes. Fluctuations of ice volume on a time scale of 10³-105 years correlate with insolation variations caused by orbital perturbations. Feedback interactions within the land-sea-air-ice system (e.g., ocean circulation changes and bedrock dynamics) have been responsible for a significant modulation of the orbital signal. Ice ages may be due to orbitally induced temperature changes superimposed on a global cooling of terrestrial origin.
ABSTRACT: Variations of the atmospheric CO2 level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO2 degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO2 level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO2 level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO2 level (4–5 PAL) maintained by the enhanced CO2 degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO2 level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.
ABSTRACT: Over geological time, photosynthetic carbon fixation in the oceans has exceeded respiratory oxidation of organic carbon. The imbalance between the two processes has resulted in the simultaneous accumulation of oxygen in, and drawdown of carbon dioxide from, the Earth's atmosphere, and the burial of organic carbon in marine sediments1–3 . It is generally assumed that these processes are limited by the availability of phosphorus4,5 , which is supplied by continental weathering and fluvial discharge5–7 . Over the past two million years, decreases in atmospheric carbon dioxide concentrations during glacial periods correlate with increases in the export of organic carbon from surface waters to the marine sediments8–11 , but variations in phosphorus fluxes appear to have been too small to account for these changes12,13 . Consequently, it has been assumed that total oceanic primary productivity remained relatively constant during glacial-to-interglacial transitions, although the fraction of this productivity exported to the sediments somehow increased during glacial periods12,14 . Here I present an analysis of the evolution of biogeochemical cycles which suggests that fixed nitrogen, not phosphorus, limits primary productivity on geological timescales. Small variations in the ratio of nitrogen fixation to denitrification can significantly change atmospheric carbon dioxide concentrations on glacial-to-interglacial timescales. The ratio of these two processes appears to be determined by the oxidation state of the ocean and the supply of trace elements, especially iron.
Fletcher, B. J., Brentnall, S. J., Anderson, C. W., Berner, R. A., Beerling, D. J. (2008). Atmospheric carbon dioxide linked with Mesozoic and early Cenozoic climate change. Nature Geoscience 1 (1): 43-48
ABSTRACT: The relationship between atmospheric carbon dioxide (CO2 ) and climate in the Quaternary period has been extensively investigated, but the role of CO2 in temperature changes during the rest of Earth's history is less clear. The range of geological evidence for cool periods during the high CO2 Mesozoic 'greenhouse world' of high atmospheric CO2 concentrations, indicated by models and fossil soils, has been particularly difficult to interpret. Here, we present high-resolution records of Mesozoic and early Cenozoic atmospheric CO2 concentrations from a combination of carbon-isotope analyses of non-vascular plant (bryophyte) fossils and theoretical modelling. These records indicate that atmospheric CO2 rose from ~420 p.p.m.v. in the Triassic period (about 200 million years ago) to a peak of ~1,130 p.p.m.v. in the Middle Cretaceous (about 100 million years ago). Atmospheric CO2 levels then declined to ~680 p.p.m.v. by 60 million years ago. Time-series comparisons show that these variations coincide with large Mesozoic climate shifts, in contrast to earlier suggestions of climate–CO2 decoupling during this interval. These reconstructed atmospheric CO2 concentrations drop below the simulated threshold for the initiation of glaciations on several occasions and therefore help explain the occurrence of cold intervals in a 'greenhouse world'.
PUBLISHER's DESCRIPTION: The changes in the Earth’s climate over the past 600 million years, from the Cambrian to the Quaternary, come under scrutiny in this book. The geological evidence for ancient climates is examined, such as the distribution of climate-sensitive sediments. The Earth’s climate has changed many times throughout the Phanerozoic. Thus in this book the climate history has been divided into Warm and Cool modes, intervals when either the Earth was in a former ‘greenhouse’ state with higher levels of atmospheric CO2 and polar regions free of ice, or the global climate was cooler and ice was present in high latitudes. The studies presented here highlight the complex interactions between the carbon cycle, continental distribution, tectonics, sea level variation, ocean circulation and temperature change as well as other parameters. In particular, the potential of the carbon isotope records as an important signal of the past climates of the Earth is explored. This book will be useful to all students and researchers with an interest in palaeoclimates and palaeoenvironments.
ABSTRACT: Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, have punctuated the last glacial period (~100-10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a 'warm' circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard-Oeschger events. For a large freshwater input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.
INTRODUCTION: Quaternary records of climate change from terrestrial sources, such as lake sediments and aeolian sediments, in general agree well with marine records. But continuous records that cover more than the past one million years were essentially unavailable until recently, when the high-sedimentation-rate site of Lake Baikal was exploited. Because of its location in the middle latitudes, Lake Baikal is highly sensitive to insolation changes and the entire lake remained uncovered by ice sheets throughout the Pleistocene epoch, making it a valuable archive for past climate. Here we examine long sediment cores from Lake Baikal that cover the past 12 million years. Our record reveals a gradual cooling of the Asian continental interior, with some fluctuations. Spectral analyses reveal periods of about 400 kyr, 600 kyr and 1,000 kyr, which may correspond to Milankovitch periods (reflecting orbital cycles). Our results indicate that changes in insolation were closely related to long-term environmental variations in the deep continental interior, over the past 12 million years.
ABSTRACT: The Earth system processes most of the chemical components of its atmosphere and oceans in geologically short periods of time. It does this in a regulated way, one that maintains a remarkably constant surface environment. This we know primarily from the fossil record of uninterrupted, complex life on Earth that extends over the last billion years.
We are only beginning to understand the feedbacks that control the chemistry of the oceans and atmosphere. Numerical models of biogeochemical cycles are most stable when there is feedback between the amount of a chemical component in the ocean or atmosphere and its transfer to or from that reservoir. Coupling of subsystem, especially of those operating on different time scales, enhances stability.
An example of the role of feedback in stabilizing Earth's chemical environment is the mechanism of control of atmospheric oxygen. There appears to be no strong relationship between oxygen level and oxygen consumption. However, oxygen production may be a function of oxygen level; the burial rate of organic carbon (oxygen production) in marine sediments may be sensitive to bottom water oxygenation levels. Also, combustion may be an effective mechanism of transferring nutrients (namely phosphorus) from efficient, terrestrial ecosystems to less efficient, marine ecosystems. When O2 rises, fires become more frequent and P is transferred to the ocean, stimulating marine organic carbon burial but depressing global burial rates. Global O2 production rates decline, as does the O2 level: a negative feedback.
Models of ocean chemical composition are presently incapable of reproducing the temporal constancy indicated by geological observations. These models do not incorporate ion-exchange equilibria as important processes in marine geochemical cycles. When included, these equilibria significantly damp the fluctuations in ion ratios calculated by the extant models.
Large, D. J., Jones, T. F., Briggs, J., Macquaker, J. H. S., Spiro, B. F. (2004). Orbital tuning and correlation of 1.7 My of continuous carbon storage in an early Miocene peatland. Geology 32 (10): 873-876
ABSTRACT: Peatland is an important terrestrial carbon reservoir that contains >25% of soil carbon and accounts for 25%-38% of natural methane emissions. Most of this carbon is contained in postglacial boreal peat. Our understanding of the carbon cycle within this reservoir and its links to the atmosphere is therefore restricted to periods of <10 k.y. A record of the longer-term behavior of the peatland carbon reservoir under nonglacial conditions does, however, exist in thick lignite deposits formed over periods of >1 m.y. Spectral analysis of varying lignite color reveals that 120 m of early Miocene lignite from the Gippsland Basin, Australia, contains a 1.7 m.y. record of orbitally paced climate oscillations dominated by the response to obliquity. Use of the regular orbital signal indicates that the average long-term rate of peatland carbon accumulation recorded in the lignite is 27.5 g.m-2 yr-1 . This rate is constant over periods of >100 k.y. and is independent of shorter-term, <10 k.y., fluctuations in climate and hydrology. Matching the lignite record to the theoretical insolation curve indicates that the lignite formed between 22.5 and 20.8 Ma. Contemporaneous long-term changes in lignite color and the13 C/12 C ratios of marine foraminifera may relate to changing peatland methane flux and thus point to a link between terrestrial and marine carbon dynamics.
ABSTRACT: A variance spectrum of climatic variability is presented that spans all time scales of variability from about one hour (10-4 years) to the age of the Earth (4 × 109 years). An interpretive overview of the spectrum is offered in which a distinction is made between sources of variability that arise through stochastic mechanisms internal to the climatic system (atmosphere-ocean-cryosphere) and those that arise through forcing of the system from the outside. All identifiable mechanisms, both internal and external, are briefly defined and clarified as to their essential nature. It is concluded that most features of the spectrum of climatic variability can be given tentatively reasonable interpretations, whereas some features (in particular the quasi-biennial oscillation and the neoglacial cycle of the Holocene) remain fundamentally unexplained. The overall spectrum suggests the existence of a modest degree of deterministic forms of climatic change, but sufficient nonsystematic variability to place significant constraints both on the extent to which climate can be predicted, and on the extent to which significant events in the paleoclimatic record can ever manage to be assigned specific causes.
Kathryn Moran, Jan Backman, Henk Brinkhuis, Steven C. Clemens, Thomas Cronin, Gerald R. Dickens, Frédérique Eynaud, Jérôme Gattacceca, Martin Jakobsson, Richard W. Jordan, Michael Kaminski, John King, Nalan Koc, Alexey Krylov, Nahysa Martinez, Jens Matthiessen, David McInroy, Theodore C. Moore, Jonaotaro Onodera, Matthew O'Regan, Heiko Pälike, Brice Rea, Domenico Rio, Tatsuhiko Sakamoto, David C. Smith, Ruediger Stein, Kristen St John, Itsuki Suto, Noritoshi Suzuki, Kozo Takahashi, Mahito Watanabe, Masanobu Yamamoto, John Farrell, Martin Frank, Peter Kubik, Wilfried Jokat, Yngve Kristoffersen (2006). The Cenozoic palaeoenvironment of the Arctic Ocean. Nature 441 (7093): 601-605
ABSTRACT: The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm 'greenhouse' world, during the late Palaeocene and early Eocene epochs, to a colder 'icehouse' world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ~14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (~3.2 Myr ago) and East Antarctic ice (~14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (~45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at 49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (~55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.
ABSTRACT: The late Paleocene thermal maximum (LPTM) is associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle. We have developed a new orbital chronology for Ocean Drilling Program (ODP) Site 690 (Weddell Sea, Southern Ocean) by using spectral analysis of high-resolution geochemical records. The LPTM interval spans 11 precessional cycles yielding a duration of 210 to 220 k.y. Thed13 C anomaly associated with the LPTM has a magnitude of about -2.5‰ to -3‰; we show that about -2‰ of the excursion occurs within two steps that each were less than 1000 yr in duration. The remainder developed through a series of steps over 52 k.y. The timing of these steps is consistent with a series of nearly catastrophic releases of methane from gas hydrates, punctuated by intervals of relative equilibria between hydrate dissociation and carbon burial. Further, we are able to correlate the records between ODP Sites 690 and 1051 (western North Atlantic) on the scale of 21 k.y. cycles, which demonstrates that the details of thed13 C excursion are recognizable between distant sites. Comparison of cycle records at Sites 690 and 1051 suggests that sediment representing the interval 30 k.y. just prior to and at the onset of the LPTM are missing in the latter location. This unconformity probably resulted from slope failure accompanying methane hydrate dissociation within 10 k.y. of the start of the LPTM.
ABSTRACT: The last 500 million years of the strontium-isotope record are shown to correlate significantly with the concurrent record of isotopic fractionation between inorganic and organic carbon after the effects of recycled sediment are removed from the strontium signal. The correlation is shown to result from the common dependence of both signals on weathering and magmatic processes. Because the long-term evolution of carbon dioxide levels depends similarly on weathering and magmatism, the relative fluctuations of CO2 levels are inferred from the shared fluctuations of the isotopic records. The resulting CO2 signal exhibits no systematic correspondence with the geologic record of climatic variations at tectonic time scales.
ABSTRACT: The character of continental-interior paleoclimate at mid-latitudes, especially the aspect of temperature, has been a major source of debate in modeling and paleontologic communities over the past few years. A recent climate modeling study provides new insight into the issue of climatic conditions of early Eocene North America. Model cases with six times the present level of CO2 , or with doubled present CO2 and a large lake in western North America, produced results most similar to proxy paleoclimate interpretations. It is significant that the North American continental-interior climate responded as strongly to the existence of the lake as to the atmospheric CO2 level. The large lake deflects the winter-freeze line poleward of the region containing most paleoclimate data site locations, producing above-freezing winter temperatures and providing a possible solution to the minimum-temperature difference that exists between models and data. The effect of the lake upon regional climate is significant and proves to be critical to reproducing the early Eocene climate of North America.