"Greenhouse" (Warm) Climates

Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:16 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: G “GREENHOUSE” (WARM) CLIMATES Definition and origins of the term “greenhouse” The term “greenhouse” is defined as follows from a climatological standpoint in the Oxford English Dictionary (OED) online, based on the textbook by Trewartha (1937, p. 25): “The phenomenon whereby the surface and the lower atmosphere of a planet are maintained at a relatively high temperature owing to the greater transparency of the atmosphere to visible radiation from the sun than to infra-red radiation from the planet.” It is now becoming clear that although Trewartha (1937) may have been the first person to use the term “greenhouse effect,” the concept and discussion of ancient cold / warm climates began in the previous century. Although many sources cite Jean Baptiste Joseph Fourier as the originator of the greenhouse effect (Fourier, 1827), this has been called into question. The most significant attack on the abuse of Fourier’s work has been published by Fleming (1999), who states that most of the citations regarding Fourier are “unreliable, misdirected and anachronistic,” and possibly stem from one of the most cited pieces of work on global warming (Arrhenius, 1896) that originally misquoted Fourier (1827). Although Fourier (1824, 1827) made analogies to the greenhouse effect, his work was focused more on terrestrial temperatures and ultimately the theory of heat (Fleming, 1999). Charles Babbage in 1847 may have been the first person to propose the greenhouse effect (Steel, 1992) in the following text taken from The Works of Charles Babbage (Campbell-Kelly, 1989), “It is on this principle that greenhouses act as traps for catching and imprisoning the sun’s rays: probably some very slight difference in the composition of the glass of which they consist may considerably alter this power. . . If the solids of the fluids on the surface, or if the atmospheres of distant plane[t]s possess the properties of reflection, radiation and absorption in certain degrees, it is by no means impossible that some of the most remote of them may be hotter than those which are much nearer to the central body. . . When however the heat communicated from the sun is confined and prevented from escape, and so forced to accumulate, very high temperatures are attained. . . Under the varied circumstances of climate, which might arise from differences in the reflecting, the radiating, the absorbing, and the conducting power of the moon’s surface, as well as from different degrees of central heat, the presence of water upon its surface might produce very different effects.” A major advance in understanding climates and climate change was published by John Tyndall (1861) who conducted physical experiments demonstrating that vapors and gases are able to absorb and subsequently emit radiative heat. Some thirty-five years later, Arrhenius (1896) took the findings of Tyndall and others and showed that the Earth’s heat budget could be dramatically affected by the abundance of carbonic acid (a reaction product of water and CO2) in the atmosphere. Arrhenius (1896) went on to conclude, with the assistance of geologist Arvid Gustaf Högbom, that changes in the total abundance of carbonic acid in the atmosphere could have been effective enough to cause glacial / interglacial cycles. Based on his model, Arrhenius made calculations as to the effect of increased (and decreased) carbon dioxide on temperature, and his results are very similar to modern sophisticated models. Chamberlain (1897) used the findings of Arrhenius and Högbom in order to understand the development of glaciations in the geological record through a drawdown in carbon dioxide and hence mountain uplift and, consequently, increased weathering. Although many researchers highlight Chamberlain as one of the fathers of the global carbon cycle model, Berner (1995) demonstrated that a great deal of Chamberlain’s work was actually based on Högbom’s work. However, it was Chamberlain who further developed the CO2 theory of climate change, which forms the basis of many paleoclimatic investigations of the geological record (see Berner, 1995; Fleming, 2000). Arrhenius subsequently published a general audience book entitled ‘Worlds in the Making’, wherein he discusses the effects of greenhouse gases in the atmosphere and their contributions towards climate change (Arrhenius, 1908). In this book, he gave an account of the “hot-house” theory, hence the birth of the concept of “greenhouse” (warm) climates. Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:17 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: 2 “GREENHOUSE” (WARM) CLIMATES Figure G51 The mean annual radiation and heat balance (modified from Berner and Berner, 1996). Numbers in brackets represent percentages that are either absorbed or re-radiated. The greenhouse effect and the climate system The Earth is a unique dynamic system, with close links and feedbacks between the hydrosphere, atmosphere, biosphere, lithosphere, and outer space. Changes in these links and feedback mechanisms can dramatically affect the magnitude of exchange between the components of this dynamic system that ultimately drive climate and determine whether the environment remains in a warm or cool mode (Figure G51). Although the importance of the atmosphere as a geological agent was recognized long ago, it has been somewhat neglected in studies of past climate change until fairly recently. It is now widely accepted that greenhouse gases significantly affect the Earth’s climate both on short- and long-term cycles, especially the global carbon cycle (Berner, 1999). Many greenhouses gases can cause both warming and cooling, due to the absorptive and radiative properties of those gases (Khalil, 1999). The most common greenhouse gases that are considered when discussing climate change on glacial / interglacial cycles and short- and long-term geological cycles are provided in Table G3. One of the key components in Figure G51 is the amount of incoming radiation versus that which is being emitted back to space. In principle, if the budget of incoming versus outgoing radiation were equal and all components within the climate system were static then the climate would also remain static. However, if radiation input is less than output, the Earth climate system would become cooler, whereas if radiation input is greater than output, the Earth would become warmer: this is essentially the “greenhouse effect.” Due to the absorptive and radiative properties of the greenhouse gases listed in Table G3, their relative abundance in the atmosphere can significantly affect the global temperature of the Earth. High abundances of CO2 and H2Ovap in the atmosphere will increase re-radiation and thus prevent heat from escaping. Of great interest to society at present is CO2, mainly because of the effects of the rise of industrialization and burning of fossil fuels and thus an increased abundance of atmospheric greenhouse gases. However, in recent years carbon-isotope ratios in the deep geologic record have been applied to infer methane dissociation Table G3 Common greenhouse gases Greenhouse gas Formula Carbon dioxide Methane Water vapor Nitrous oxide Sulfur dioxide Ozone CO2 CH4 H2O N2O SO2 O3 events and subsequent global warming (Hesselbo et al., 2000; Dickins, 2001; Beerling et al., 2002). The abundance of greenhouse gases in the atmosphere is not the only factor influencing the global climate on geological timescales. The schematic diagram in Figure G52 includes the long-term and short-term carbon cycle (Jenkyns, 2003), primarily identified through our ability to obtain high-resolution records in the deep geologic record. This figure illustrates the majority of pathways in the short- and long-term carbon cycle, and in particular considers the most relevant processes that imprint on and affect the geoclimatic record. Long-term events can be referred to as greenhouse (warm) episodes, while shortterm events act as transient climate events. More detailed analyses of biotic feedbacks in paleoclimatology are given in Woodwell and Mackenzie (1995), Hay et al. (1997), Berner (1999) and Huber et al. (2001). Paleoclimates in the geologic record Understanding the causes and consequences of global climate change is one of the most important issues of modern science, and includes the investigation of climatic variations in the geologic record (paleoclimatology). Fueled by concerns about the impact of human activities on the physical and biological environment of the Earth, an increasing amount of research has been devoted to untangling records and mechanisms of past and present climate variation. The Intergovernmental Panel of Climate Au2 Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:18 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: “GREENHOUSE” (WARM) CLIMATES 3 Figure G52 A diagrammatic illustration depicting the pathways in the short- and long-term global carbon cycle that would be expected during a greenhouse event (e.g., volcanism, methane release). Shaded components represent increases, whereas those without shading represent decreases. Note that this illustration does not depict all pathways and, thus, negative and positive feedback mechanisms. See text for additional sources. Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:19 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: 4 “GREENHOUSE” (WARM) CLIMATES Change (IPCC, 2001) constitutes a large international assemblage of scientists and data that attempts to investigate the effects of modern climate change (in part induced by anthropogenic activities) through modeling projections of future climatic scenarios (see http: / / www.ipcc.ch / ). Although largely focused on recent and future climate change until recently, the fourth assessment of the IPCC includes many research areas that attempt to uncover the causes of ancient climatic variability, including its magnitude, variability, frequency, and impacts on the Earth system. Our understanding of ancient climates, let alone rapid climate change in the geologic record, is still in its infancy even 100 years since the work of Arrhenius and others. This is reiterated by Huber et al. (2001), who write in the preface of Warm Climates in Earth History: “In spite of the prevalence of warm climates in Earth history and the potential practical significance of understanding them, the fundamental causes, nature, and mechanics of warm climates are still poorly understood. . . Perhaps most importantly, though, integrated studies have revealed consistent areas of disagreement between empirical and theoretical analyses. Increasing temporal and geographic resolution in models and proxy data will only improve the ability to use deep time as a testing ground for our understanding of the causes, nature, and mechanics of globally warm climates.” Many methods can be used to determine whether a time period in the geological record experienced a warm or cool climate, for example, the presence or absence of climate-sensitive sediments (e.g., evaporites, coals, corals, bauxites), geochemical data (e.g., negative and positive excursions in oxygen- and carbon-isotope ratios of foraminifera), faunal distributions (e.g., the presence of crocodiles at high latitudes), floral physiology (e.g., stomatal indexes, leaf margin analysis), and many other techniques. It is beyond the scope of this entry to discuss all the periods in the geologic record; however, a summary of warm climate modes will be provided with appropriate references for further study (Figure G53). More specific discussion of certain key intervals that illustrate rapid climatic warming will also be provided. Figure G53 depicts the climate modes of the Phanerozoic as determined through paleoclimatic analysis (Frakes et al., 1992) and oxygen isotopic evolution of the ocean (Veizer et al., 1999; Shaviv and Veizer, 2003; Royer et al., 2004). The warm (greenhouse) climate mode, as defined by Frakes et al. (1992), is a climate that is “globally warm, as indicated by the abundance of evaporites, geochemical data, faunal distributions, etc., and with little or no polar ice.” On the other hand, Fischer (1986) defines a greenhouse mode as a period that has low latitudinal temperature gradients, high mean ocean temperatures, a sluggish ocean, and marine anoxia. Notwithstanding variations in the definition of a greenhouse mode, there are four major warm modes in the Phanerozoic that will be discussed in detail below (note, that this discussion considers the broader nature of greenhouse modes with only minor discussion on transient climate events). What is apparent in Figure G53 is a lack of co-variation between reconstructed and modeled atmospheric CO2 levels with warm and cool climate modes; such a finding was reported by Shaviv and Veizer (2003), and vehemently debated by Royer et al. (2004). Although this apparent disparity may invoke the conclusion that CO2 is not a controlling factor on temperature in geologic time (Boucot and Gray, 2001; Shaviv and Veizer, 2003; Wallmann, 2004), the effect of CO2 and many other environmental factors (e.g., carbon burial, seawater pH, sea level, volcanism, weathering, Figure G53 Climate modes through the Phanerozoic based on paleoclimatic analysis (Frakes et al., 1992) and oxygen isotopic evolution of the ocean (Shaviv and Veizer, 2003; Royer et al., 2004). Climate modes: C = cool; W = warm (see definitions in Frakes et al., 1992). Top panel: Temperature curves represent present global average minus the reconstructed temperatures: the modern global average temperature in the geologic record would have a value of zero (see Royer et al., 2004 for details). The three temperature curves are: (1) an uncorrected curve (Veizer et al., 1999) [solid line], (2) a pH-adjusted curve based on modeled CO2, and (3) pH-adjusted proxy CO2 data (Royer et al., 2004) [dashed and gray line respectively]. The horizontal dashed line is based on the Paleogene glaciation in the Cenozoic (Zachos et al., 2001), which would give a temperature of ca. +2.2 ! C above the present global average. If this horizontal dashed line is used as an estimate for warm (above) and cool (below) climates, then correspondence with the climate modes of Frakes et al. (1992) is much better. C = Ediacaran; e = Cambrian; O = Ordovician; S = Silurian; D = Devonian; C = Carboniferous; P = Permian; T = Triassic; J = Jurassic; K = Cretaceous; Pg = Paleogene; N = Neogene. Bottom panel: Phanerozoic atmospheric CO2 concentrations based on the GEOCARB III model (dashed line) and proxy CO2 data (after Royer et al., 2004). cosmic rays) coalesce to generate paleoclimatic cycles. It is apparent, however, that during periods of major glaciations (e.g., Early Carboniferous–Late Permian and Early Eocene– Present), atmospheric CO2 levels are low, and that other cool climate modes are only less warm periods (Royer et al., 2004). In general, less scientific research has been focused on Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:20 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: “GREENHOUSE” (WARM) CLIMATES the cause and effects of “greenhouse” (warm) climates in the geological compared with “icehouse” (cool) climates. Early earth Our interpretation of the early Earth climate is at best poor, and as Lyons (2004) eloquently states: “a universal theme in studies of the early Earth is that big stories are told with little data and lots of speculation.” This applies particularly to our understanding of the faint young Sun paradox (Longdoz and Francois, 1997), which states that the power of the sun was only 70% of what it is today, and thus other mechanisms for global warmth were required to prevent the Earth from remaining frozen. The major factor causing warmer early Earth temperatures is attributed to exceptionally high levels of greenhouse gases, such as CH4 (Pavlov et al., 2003) and CO2 (Hessler et al., 2004; Ohmoto et al., 2004; Pierrehumbert, 2004; Fowler et al., 2003). Early cambrian to late ordovician periods Our understanding of this warm episode is somewhat skewed, as continental reconstructions suggest that landmasses occupied low latitudes (Golonka, 2002). The use of paleontology in the Cambrian as an indicator of warmth is difficult due to the great number of unoccupied ecological niches and limited biological diversification (Frakes et al., 1992). Therefore, other proxies of climatic warmth must be utilized. The earliest part of the Cambrian is poorly understood, primarily because of a lack of pristine marine carbonate available for paleotemperature reconstructions. An isotope curve indicates that by the Middle–Late Cambrian, paleotemperatures far exceeded present temperatures (Figure G53), which coincided with extremely high atmospheric CO2 levels at that time (Royer et al., 2004), possibly as a result of intense volcanism over the Precambrian–Cambrian transition (Doblas et al., 2002). Karhu and Epstein (1986) suggest Cambrian paleotemperatures as high as 50 ! C, although it is not yet fully understood whether changes in the oxygen-isotope composition of seawater (dw) may have exacerbated the reconstructed paleotemperatures, which far exceeded normal marine faunal tolerances. However, paleotemperatures during the Ordovician became more reasonable at "30–38 ! C (Veizer et al., 1999; Brand, 2004), although a suggested Ordovician dw value of –3% would have lowered the temperatures further to "27 ! C (Wallmann, 2001; Shields et al., 2003). Other evidence for a greenhouse climate during this interval includes the presence of massive evaporites at low latitudes (Alvaro et al., 2000; Baikov, 2004), carbonate expansion to high latitudes (Frakes et al., 1992), lateritic and bauxitic profiles (Sturesson, 2003), and increased phosphorite accumulation (Meng et al., 1997). The presence of oolitic ironstones has also been suggested as an indictor of warmth during a marine transgressive phase (Frakes et al., 1992; Yapp, 2004). During the Ordovician, abundant black-shale deposition supports a transgressive sea-level phase, accompanied by high oceanic productivity (Easto and Gustin, 1996). Late silurian to early carboniferous periods After the Late Ordovician glaciation, the Late Silurian still experienced relatively cool temperatures, although by the beginning of the Devonian, temperatures begin to rise sharply, reaching a peak in the Middle Devonian before decreasing again (Figure G53). By comparison to the Cambrian–Ordovician greenhouse episode, this time interval was relatively cooler and coincided with 5 decreasing atmospheric CO2 levels (Wallmann, 2001), increasing volcanism, and tectonic activity during the Caledonian and Arcadian Orogenies (Torsvik and Cocks, 2004). The peak paleotemperature in the Middle Devonian also coincided with large evaporite deposits in west Canada, Siberia, and North Africa (Frakes et al., 1992). Carbonate deposition increased during the Devonian, bioherms were dominant and reef systems extended latitudinally up to "60! (Frakes et al., 1992; Dubalatov and Krasnov, 2000; Edinger et al., 2002). A humid environment is believed to have been initiated in the Late Silurian, when the Coal Age began (Calder and Gibling, 1994), and is concurrent with the first record of fossil charcoal, suggesting wildfires (Glasspool et al., 2004). During the Devonian, marine faunas became more cosmopolitan (Babin, 2000), soils were well-developed latitudinally in the terrestrial environment, and plant communities diversified, with the occurrence of the first trees (Retallack, 1997). The most prominent feature depicting an arid climate was the widespread deposition of red beds, such as the Old Red Sandstone and Catskill Formation. As with the previous greenhouse mode, phosphorite accumulation increased (Martin, 1995), as did upwelling and oceanic productivity (Caplan and Bustin, 2001), culminating in major marine anoxia during the Frasnian–Fammenian interval of the Devonian sea-level rise (see Rackian and House, 2002). However, the Late Devonian–Early Carboniferous climate switched dramatically from a greenhouse to icehouse world, which is attributed to intensified silicate weathering and organic carbon burial (Averbuch et al., 2005). Late permian to middle jurassic periods One of the most intensely studied greenhouse intervals occurred during the Permian–Triassic boundary (Kidder and Worsley, 2004). This boundary interval is believed to have coincided with the onset of a greenhouse climate with widespread marine anoxia (Grice et al., 2005), a global coal gap (Retallack et al., 1996), the disappearance of Glossopteris (Spalletti et al., 2003), and a massive increase in phosphate accumulation, classically described by the Phosphoria Formation, USA (Knudsen and Gunter, 2002). The Carboniferous–Permian record had atmospheric CO2 levels that were comparable to present concentrations, although by the Middle Permian they began to rise to levels of "2,000 ppmV, peaking at the Permian–Triassic boundary. Paleotemperatures rose sharply during this interval, from cool to warm, but peaked during the Early Triassic (Figure G53). Extensive evaporite deposits occurred in the Triassic and Early Jurassic, although dominantly found in the Late Triassic with scarce bauxite and laterite deposits during the Triassic (Frakes et al., 1992). A number of paleoproxies for the Triassic suggest that it was a period with climates alternating between arid and humid: (a) paleosols from the Middle Triassic of Argentina (Tabor et al., 2004), (b) foliar physiognomy and tree-rings from Antarctica (Cúneo et al., 2003), (c) sediment analysis from the Late Permian–Early Triassic indicating fluctuating precipitation regimes with a peak in the Early Triassic (Wopfner, 2002), and (d) clay mineral analysis from Europe (Ruffell et al., 2003). On the basis of tree-ring widths, Pires et al. (2005) suggested that the climate became more uniform by the Late Triassic, which is also indicated by clay-mineral analysis (Ruffell et al., 2003). However, the Late Triassic was heavily influenced by monsoonal conditions, as implied by the strong orbital cyclicity recorded in the Newark Basin (LeTourneau and Olsen, 2003). By the Late Triassic, a number of bolide impacts and extensive volcanism altered the Earth system, possibly resulting in a climate with less fluctuations but growing warmth (see Tanner et al., Comp. by: RPrabhuGalleys0000835385 Date:16/5/08 Time:16:29:21 Stage:First Proof File Path://ppdys1108/ Womat3/Production/PRODENV/0000000005/0000007785/0000000016/0000835385.3D Proof by: QC by: 6 “GREENHOUSE” (WARM) CLIMATES 2004). One of the major episodes that occurred at the Triassic– Jurassic boundary, which has been attributed to climate change, is the emplacement of the Central Atlantic Magmatic Province (Hames et al., 2003), which contributed to a transient climate event and possibly the turnover or extinction of many floral and faunal groups (Pálfy et al., 2000; Hesselbo et al., 2002). The Early Jurassic also witnessed the diversification and latitudinal spread of floras (Krassilov, 2003) and the global distribution of black shales, which occurred most prominently during the early Toarcian (Hesselbo et al., 2000). The early Toarcian oceanic anoxic and transient climate event has been attributed to the massive dissociation of methane gas hydrates (clathrates), as recorded by a rapid negative d13C excursion in both the marine and terrestrial carbon reservoirs (Hesselbo et al., 2000). A similar explanation has also been proposed for the Permian–Triassic boundary (Krull et al., 2004). Paleotemperatures rose significantly over the Toarcian, as recorded by belemnite oxygen-isotope ratios (Jenkyns et al., 2002; van de Schootbrugge et al., 2005). However, this is not apparent in Figure G53. Both the early Toarcian oceanic anoxic event and Permian–Triassic boundary event are related to periods of major volcanism, such as the Karoo-Ferrar continental flood basalts (Duncan et al., 1997) and Siberian Traps (Renne et al., 1995), respectively. These volcanic outpourings are potentially the original source of greenhouse gases and consequent global warming episodes (Huyn and Poulsen, 2005). Au1 Mid-cretaceous period to early cenozoic era The initiation of this greenhouse (warm) mode has been revised from that given in Frakes et al. (1992) and is now placed at the Barremian–Aptian boundary 125 Myr ago, where it records a major shift in climate, oceanic structure, and environment (Gröcke, 2002; Jenkyns, 2003; Erba, 2004). Based on obtaining pristine marine microfossil records from Ocean Drilling Program cores, our understanding of the Cretaceous and Cenozoic climate is much greater (e.g., Jenkyns, 2003). Significant Pacific Ocean large igneous province formations began at the Barremian– Aptian boundary with the formation of the Ontong Java Plateau (Larson and Erba, 1999) and consequent increasing atmospheric CO2 levels and warming (Huber et al., 2002; Leckie et al., 2002). Major sea-level transgression and regressions occurred during the Cretaceous periods, although the ocean remained sluggish, with high productivity resulting in significant deposition of black shales (Poulsen et al., 2001; Wilson and Norris, 2001; Leckie et al., 2002). The Cretaceous oceanic anoxic events record a significant temperature increase with the maximum occurring during the Cenomanian–Turonian event (Norris et al., 2002; Poulsen, 2004). However, this event also records a decrease in atmospheric CO2 levels (Kuypers et al., 1999). Extreme Cretaceous warmth is also shown by Antarctic and Arctic floral and faunal compositions and the prevalence of opportunism (Herman and Spicer, 1996; Tarduno et al., 1998; Huber, 2000; Francis and Poole, 2002). Whether volcanism and/or methane dissociation were prevalent during the Cretaceous (e.g., the Aptian oceanic anoxic event) and Cenozoic (e.g., Paleocene-Eocene Thermal Maximum) is still a matter of debate (Bains et al., 1999; Gröcke et al., 1999; Dickins, 2001; Beerling et al., 2002). However, a number of paleoclimatic indicators (e.g., oxygen isotopes, black shales, rapid carbon-isotope excursions) suggest sustained warmth with several transient climate events (e.g., Aptian, Cenomanian–Turonian oceanic anoxic events, and the Paleocene-Eocene Thermal Maximum). Recent evidence also indicates that the Cretaceous warmth was interrupted by rapid short-lived cooling (icehouse) episodes (Stoll and Schrag, 2000; Gale et al., 2002; Pirrie et al., 2004). Summary Our understanding of greenhouse (warm) climates in geologic time has been limited since a primary focus of deep time research to date has been on icehouse (cool) climates, which are more easily recognized in the sedimentological record. A concerted effort is required to integrate multi-proxy approaches to estimate ancient temperatures and atmospheric CO2 levels more accurately. In addition, based on the modern focus on rapid climate change in today’s society, there is a greater need to identify and research transient climate events in deep time. Studies of the Initial Eocene Thermal Maximum, Toarcian, Aptian, and Cenomanian / Turonian oceanic anoxic events and the Permian–Triassic boundary and Frasnian– Fammenian boundary are needed. Our knowledge of early Earth climates (bar Snowball Earth episodes) is limited due to a lack of reliable proxies indicative of greenhouse modes. Four major greenhouse (warm) modes have been identified in the Phanerozoic (Figure G53), and within these intervals, several (or many) transient climate events conducive with rapid, extreme warming can be identified. Equally important is that within these greenhouse (warm) modes, rapid icehouse (cool) climates are also becoming recognized, thus suggesting that the Earth’s climate could flip rapidly in deep geologic time. Darren R. Gröcke Bibliography Álvaro, J.J., Rouchy, J.M., et al., 2000. Evaporitic constraints on the southward drifting of the western Gondwana margin during Early Cambrian times. Palaeogeogr. Palaeoclim. Palaeoecol., 160, 105–122. Arrhenius, S., 1896. On the influence of carbonic acid in the air and upon the temperature of the ground. Phil. Mag. J. Sci. Series 5, 41, 237–276. [For more information on Svante Arrhenius see the special issue in AMBIO, 26 (1) in 1997.] Arrhenius, S., 1908. Worlds in the Making. New York, USA: Harper & Brothers. 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