Global climate has varied since the most primitive atmosphere developed on earth billions of years ago. This variation in climate has occurred on all timescales and has been continuous. The sedimentary rock record reflects numerous sea-level changes, atmospheric compositional changes, and temperature changes, all of which attest to climatic variation. Such evidence, as well as direct historical observations, clearly shows that temperature swings occur in both directions. Past climates have varied from those that create continental glaciers to those that yield global greenhouse conditions. Many people do not comprehend that this means their living climate also varies—it gets warmer or cooler—but typically does not remain the same for extended periods of time. Human history shows us that in general, warmer conditions have been beneficial, and colder conditions have been less kind to society ( Lamb, 1995 ). We currently are living in a not-yet-completed interglacial stage, and it is very likely that warmer conditions lie ahead for humanity, with or without any human interference. Interglacial stages appear to last for about 11,000 years, but with large individual variability. We have been in this interglacial for about 10,000 years.

The sun is the primary source of energy for the climate of the earth. Variations in solar energy reaching the earth’s surface change the climate. Several factors control the influx of solar energy, including (1) variations in the earth’s albedo, (2) variations in earth’s orbit and rotation, and (3) variations in solar energy output. In the short time since 1978, direct measurement of total solar irradiance (TSI) by satellites has shown cyclical variations in solar energy of 0.1% in conjunction with the 11-year sunspot cycle. Indirect evidence from the sun and other sunlike stars indicate that TSI has had significantly greater variation as the the sun goes through various cycles. The correlations between climate and TSI variations are statistically solid. Small variations in TSI initiate indirect mechanisms on earth that yield climate changes greater than that predicted for the TSI change alone. At least three solar variables are known to affect earth’s climate: (1) TSI, which directly affects temperatures; (2) solar unltraviolet radiation, which affects ozone production and upper atmospheric winds; and (3) solar wind, which affects rainfall and cloud cover, at least partially, through control of the earth’s electrical field. Each affects the earth’s climate in different ways, producing indirect effects that amplify small changes in TSI. Individually, they do not cause the entire observed climate changes. Collectively, they appear to be sufficient, especially because solar forcing of earth’s climate is still an emerging science. Undoubtedly, other mechanisms of solar forcing are poorly understood, perhaps even unknown.

Major erathemic climate changes may result from redistribution of oceans and continents through time. When plate tectonic reconstructions portray the presence of near-equatorial currents, greenhouse events are common, but when landmasses exist at the equator, such oceanic circulation patterns are not developed and the transfer of heat to the polar regions stimulates large-scale glaciation. This pattern seems to have operated from the Vendian to the present. Observed tectonic changes between Pleistocene icehouse and Cretaceous greenhouse events provide the basis for development of this hypothesis. As illustrated on Figure 1 in the introduction to this volume, the proposed relationship is a second-order driver and is relatively more important than those phenomena that occur over short periods of time and result in smaller temperature changes. Figure 1 Highly generalized thermohaline circulation in the world ocean that transfers thermal energy (heat) and exerts a major influence on climate (modified from Bigg, 1996 , and Scotese, 1997 ).

The global carbon cycle has been affected by four major perturbations owing to human activities on land, and global temperature change since the year 1700. The cycle has been, and continues to be, forced by global emissions from fossil-fuel burning, land-use change, agricultural fertilization of croplands, organic sewage discharges, and a slight temperature rise. The atmospheric carbon- dioxide (CO 2 ) change in the past 300 years, as computed from our model analysis, agrees well with the observed increase. The anthropogenic perturbations on land have resulted in an increased delivery of carbon to the coastal ocean and changes in its trophic status towards increased net heterotrophy (remineralization of organic carbon exceeding its in situ production). Future increases in emissions of carbon from land, based on the projections of the Intergovernmental Panel on Climate Change (IPCC) and the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC), suggest increases in atmospheric CO 2 to 495 and 435 parts per million by volume (ppmv), respectively, by the mid-twenty-first century The net release or uptake of carbon on land involving phytomass and soil organic carbon depends strongly on patterns of land use. In particular, significant, continuous deforestation of tropical and Russian forests, along with continuous increase in global mean temperature, could lead to a weakening of the hypothesized terrestrial sink for the 1990s during the twenty-first century. Furthermore, an increase in atmospheric CO 2 leads to a lower supersaturation state of coastal ocean water with respect to calcite and aragonite, which may result in lower rates of carbonate storage in shallow oceanic areas. Weakening of the oceanic thermohaline circulation (“the conveyor belt”) may result in a greater transport of atmospheric CO 2 to coastal ocean waters, at the expense of its reduced transport to the surface open ocean.

If we continue along the business-as-usual fossil-fuel-use track, we run the risk, late in the twenty- first century, of triggering an abrupt reorganization of the ocean’s thermohaline circulation. This conclusion is based on evidence stored in Greenland ice, continental-margin sediments, and mountain moraines that tells us that the large and abrupt global climate changes during the last period of glaciation were associated with sudden reorganizations of the ocean’s thermohaline circulation and on simulations carried out in joint atmosphere-ocean models that suggest that raising the greenhouse capacity of the atmosphere to the carbon-dioxide equivalent of 750 ppm would cripple the ocean’s conveyor circulation. However, the recent discovery by Gerard Bond that the 1500-year cycle which paced these glacial disruptions continued in a muted form during times of interglaciation casts a new light on this situation. It leads me to suspect that the large and rapid atmospheric changes of glacial time were driven by a sea-ice amplifier. If so, then, because little sea ice will remain at the time of a greenhouse-induced thermohaline reorganization, perhaps the threat will be far smaller than I had previously envisioned.

The potential of stable isotopic ratios ( 18 O/ 16 O and 2 H/ 1 H) in mid- to low-latitude glaciers as a modern tool for paleoclimate reconstruction is reviewed. To interpret quantitatively the ice-core isotopic records, the response of the isotopic composition of precipitation to long-term fluctuations of key climatic parameters (temperature, precipitation amount, relative humidity) over the given area should be known. Furthermore, it is important to establish the transfer functions that relate the climate-induced changes of the isotopic composition of precipitation to the isotope record preserved in the glacier. This paper will present long-term perspectives of isotopic composition variations in ice cores spanning the last 25,000 years from the mid- to low-latitude glaciers. The δ 18 O records from the far western Tibetan Plateau suggest temperatures as warm as today occurred approximately 3000 years ago. However, δ 18 O records from the Himalayas and the eastern side of the Tibetan Plateau confirm that the twentieth century is the warmest period in the last 12,000 years. In the South American Andes on Huascarán, δ 18 O records suggest temperatures as warm as those of today occurred 5000 years ago. All the tropical glaciers for which data exist are disappearing. The evidence for recent and rapid warming in the low latitudes is presented and possible reasons for this warming are examined. The isotopic composition of precipitation should be viewed not only as a powerful proxy indicator of climate, but also as an additional parameter for understanding climate-induced changes in the water cycle, on both regional and global scales.

ABSTRACT A large amount of hydrographic data obtained in the last three to four decades indicates that temperatures in the deep ocean have been changing globally However, because of the scarcity of older data, it is difficult to trace the ocean thermal history farther back in time. In this study, we examine the possibility of using subseafloor borehole temperature data to estimate the history of the bottom-water temperature (BWT) in the last two to three centuries. The thermal signal associated with BWT fluctuation slowly propagates into the subseafloor rock formation, perturbing the otherwise steady-state temperature field. It is possible to extract this signal and reconstruct the BWT history by inverting the borehole temperature measurements. We make such an attempt using data obtained from a borehole drilled on 669-m-deep seafloor at Ocean Drilling Program Site 1006 in the Straits of Florida. The observed temperature-depth profile in the depth range of 26 through 349 m below seafloor shows significant curvature in the upper 100 m. The BWT history reconstructed from this profile indicates that the long-term average BWT in the early eighteenth century was about 1°C lower than the present value. It decreased to a minimum at about the turn of the century, and then gradually increased to the present value. The pattern of the inferred BWT variation is similar to that of the surface air temperature at Key West, Florida, and the global surface air temperature average.

Abstract Sclerosponges have great potential as seawater temperature recorders. These animals precipitate their skeletons in carbon and oxygen isotopic equilibrium with the surrounding seawater ( Druffel and Benavides, 1986 ). Their skeletons also display chemical properties that vary directly with changes in environmental conditions. Lack of photosynthetic symbionts allows sclerosponges to live below the photic zone, providing the potential to investigate past marine conditions beyond the range of corals. Individual sponges live for several centuries, preserving archives of pre- and postindustrial seawater variations within single specimens ( Hartman and Reiswig, 1980 ). Crosscorrelation of successively older specimens could yield up to 2000 years of marine history. Extracting environmental information can be accomplished by determining elemental characteristics preserved in skeletal growth bands. A method is presented here that utilizes energy dispersive spectroscopy (EDS) to provide inexpensive assessment of magnesium (Mg): calcium (Ca) and chlorine (Cl): calcium (Ca) ratios at high spatial resolution, yielding environmental data with correspondingly high temporal resolution. The relationship between environmental conditions and skeletal characteristics is defined by a spectral transfer function, which can then be applied to skeletal carbonate data from ancient sponges to reconstruct past environmental conditions. Accurate reconstruction of seawater temperature and salinity variations is demonstrated here at submonthly resolution. The technique’s efficiency is ideal for documenting long, high-resolution records of marine paleoenvironments.

Abstract The response of beetles to climate change during the Quaternary Period is reviewed for the purpose of evaluating their future response to global warming. Beetles responded to Quaternary climatic changes mostly by dispersal, which ultimately led to large-scale changes in geographic distribution. Fragmentation and isolation of populations associated with climate change did not result in either higher rates of speciation or extinction, although local extinctions occurred when dispersal routes were blocked by barriers. Studies from archaeological and late Holocene sites indicate that the fragmentation of the natural landscape by human activities had as great an impact on the local diversity of beetle populations as did climate change. Habitat reduction and fragmentation continue today and are making species increasingly vulnerable to extinction. The major difference between the future and past responses of beetles to climate change is that extinction rates are expected to be much higher, independent of whether the causes of climate change are natural or anthropogenic. The question of determining whether global warming has natural or anthropogenic causes is important because of the ethical implications of extinction.

Abstract In the present contribution, we address the relationship between climate and atmospheric carbon-dioxide (CO 2 ) concentration on different timescales, from long-term trends through the Cenozoic to short-term variations in the recent past. The inverse relationship between stomatal frequency of angiosperm leaves and the CO 2 concentration of the ambient air is used as a robust method for quantifying paleoatmospheric CO 2 levels. Short-term, century-scaled CO 2 fluctuations are reflected in the stomatal frequency pattern of early Holocene birch leaves. Changes in paleoatmospheric CO 2 correlate with major environmental and climatic changes, indicated in the terrestrial palynological record and by δ 18 O fluctuations in polar ice. Further evidence for significant perturbations in the global carbon cycle during the early Holocene is revealed by concomitant changes in atmospheric radiocarbon ( 14 C) content. Warm climatic phases during the Cenozoic represent a particularly challenging test of our understanding of stomatal frequency response to past CO 2 concentrations. The principal question is whether an enhanced greenhouse effect was responsible for these periods of increased global temperature. The data available so far indicate that during the late Neogene, when the temperature was significantly increased for the last time in the geological history, the paleoatmospheric CO 2 concentration was close to the present level of about 360 parts per million volume (ppmv). During the peak warmth of the early middle Eocene, however, paleoatmospheric CO 2 concentration was significantly elevated, to about 500 ppmv.

Abstract Existing data indicate that the earth’s climate is probably warming. Politicians and the media typically assume this warming is the result of human activity. This article summarizes previous climate changes to test the validity of assigning causality to human activity. Records of glacial advances and retreats indicate relative summer temperature. Lacustrine and subaerial sediments afford a record of glacier advances and retreats from the Pleistocene to the present time. Palynology offers a record of species succession in response to climate changes. Dendrochronology is another indicator of summer temperature. Isotopic paleontology offers a measurement of temperature at the time of marine sediment deposition, and isotopic evaluation of continental ice is an indicator of temperature at the time of precipitation. Anthropologic sources contain significant climate data, such as information about villages overrun by glaciers, open- ocean iceberg density, or harbors filled with ice. Today, scientists are capable of direct measurement of climatic conditions. These sources record continual changes in climate. Broadly, the temperature changed 15° to 20°C from the Paleocene to the Neogene. Perhaps there was as much as another 10°C change in the Pleistocene. Correlative data from North America, Greenland, and Scandinavia indicate many climate changes were truly global in scope. Although it is difficult to develop precise paleothermometry, qualitative evaluations indicate sudden and dramatic changes in climate. Some are perhaps as great as a change from conditions warmer than today to a full glacial climate in as little as 100 years. The converse can be true. Current data indicate a trend of change that is substantially severe but no greater in rate or magnitude, and probably less in both, than many changes that have occurred in the past.

ABSTRACT Predictions of global climate change are based on large computer-simulation models that are “history-matched” to weather records compiled from the early nineteenth century onward. Climate-change model forecasts would be more convincing if they were based on the natural records of the Holocene (≈10,000 years) and were capable of simulating climate characteristics of this epoch. Temperature records estimated from δ 18 O measurements on ice cores from the Greenland ice cap and the Antarctic could be used to develop models based on geochronological data rather than historically brief weather records. The 20-year average record of δ 18 O values from the Greenland Ice Sheet Project 2 (GISP2) ice core exhibits a long-term trend of declining temperatures over most of the Holocene, except during the last 100 years, when temperatures have increased—a change widely blamed on carbon-dioxide (CO 2 ) emissions from fossil fuels. However, the range in temperatures since the start of the industrial age is typical for the Holocene, and the current rate of increase in temperatures is unusual but not unprecedented. Past periods of consistently increasing (or decreasing) temperatures have not persisted much longer than the current interval, so temperature trends may well reverse in the near future. There are distinct cyclic patterns in temperatures recorded in the GISP2 ice core, including a pronounced sawtoothed 560-year sequence of relatively abrupt change followed by a gradual reversal. The present trend may be the initial phase of such a pattern. In summary, the present climate does not appear significantly different from the past climate at times prior to industrialization.

ABSTRACT The development of the Baltic Sea during the Holocene was mainly controlled by climatically driven eustatic sea-level change and vertical crustal movements. Both factors affect sea-level changes interpreted from sedimentological investigations at coastal locations. Comparison of relative sea-level curves with a eustatic curve reveals the contribution of vertical crustal movements to coastal changes as expressed via a “coast index.” A coast index c(i) for the Baltic region can be derived, by which a location can be allocated to either a “crustal-uplift/subsidence type” or a “climate-controlled type.” Coastal locations investigated along the Fennoscandian Shield belong to the crustal-uplift type and locations along the southern and southwestern coast belong to the climate-controlled type, regardless of whether they are on the East European Platform or the West European Platform. Data on recent vertical crustal movements show a broad, predominantly subsiding zone between the rising blocks of Scandinavia (glacio-isostatic uplift) and the Carpathians (northward drift of the African Plate) to the west of the Tornquist-Teisseyre Zone (TTZ). Movements may additionally be influenced by processes initiated along the North Atlantic Mid-Ocean Ridge. The subsiding belt contiguous to the Fennoscandian Shield is interpreted as a collapsing as- thenospheric bulge originally surrounding the Pleistocene ice shield. The analysis of relative sea-level changes leads to the assumption that the process of collapse reached a steady state during the Late Litorina Stage. Crustal movement data, together with data from modeling of future sea-level change, can be used for calculating scenarios of relative sea-level development, providing a background for long-term planning of human activities in coastal areas.

ABSTRACT Deep-sea and ice-core data show that sea level, and thus global temperature, fluctuated often during the past 1.8 million years. Fossil coral reefs, tidal flats, and beaches are precise indicators of former sea level. Although large fluctuations are indicated by the geologic record, this paper focuses on the younger, well-documented fluctuations recorded by coral reefs and shoreline deposits that accumulated during the past 140,000 years. During this relatively short period, fossil coral species and depositional processes have remained unchanged, and diagenetic alteration of fossils and sediments was minimal. Coral reefs and shoreline accumulations were selected as sea-level indicators for two reasons: 1) They serve as bathtub rings and/or dipsticks for determining former sea levels, and 2) human activity had no influence on the sea in which they grew or accumulated. Emergent coral reefs worldwide suggest that global sea level was at least 6 m above present during isotope stage 5e approximately 120,000 years (120 ka). Drowned coral reefs and oolitic beaches indicate sea level was about 100 m below present during Stage 2 as little as 12 ka. As many as eight sea-level fluctuations occurred between Stages 2 and 5e, each of which was greater than those projected to result from burning fossil fuels. Ice-core data suggest even more fluctuations between stages 5a and 5e. Because the record is written in stone, geologists, especially sedimentologists, should be well qualified to make future predictions. Geologists have, for the most part, been excluded from official decision making.

ABSTRACT Microbial calcification has been identified as a significant source of carbonate sediment production in modern marine and lacustrine environments around the globe. This process has been linked to the production of modern whitings and large, micritic carbonate deposits throughout the geologic record. Furthermore, carbonate deposits believed to be the result of cyanobacterial and microalgal calcification suggest that the potential exists for long-term preservation of microbial precipitates and storage of carbon dioxide (CO 2 ). Recent research has advanced our understanding of the microbial-calcification mechanism as a photosynthetically driven process. However, little is known of the effects of this process on inorganic carbon cycling or of the effects of changing climate on microbial-calcification mechanisms. Laboratory experiments on microbial cellular physiology demonstrate that cyanobacteria and green algae can utilize different carbon species for metabolism and calcification. Cyanobacterial calcification relies on bicarbonate (HCO 3 − ) utilization while green algae use primarily CO 2 . Therefore, depending on which carbonate species (HCO 3 − or CO 2 ) dominates in the ocean or lacustrine environments (a condition ultimately linked to atmospheric partial pressure PCO 2 ), the origin of lime-mud production by cyanobacteria and/or algae may fluctuate through geologic time. Trends of cyanobacteria versus algal dominance in the rock record corroborate this conclusion. These results suggest that relative species abundances of calcareous cyanobacteria and algae in the Phanerozoic may serve as potential proxies for assessing paleoclimatic conditions, including fluctuations in atmospheric PCO 2 .

ABSTRACT Using the technology and experience already gained by the oil and gas industry and in groundwater resource management, sequestration of carbon dioxide (CO 2 ) in geological media is an immediately applicable option for the near- to medium-term mitigation of climate-change effects resulting from the release of anthropogenic CO 2 into the atmosphere. Based on its properties and in-situ conditions, CO 2 can be sequestered as a gas, a liquid, or in supercritical state in depleted oil and gas reservoirs, uneconomic coal beds, deep saline aquifers, and salt caverns. Using CO 2 for miscible flooding of oil reservoirs or for methane production from coal beds has an added economic benefit. The main trapping mechanisms responsible for CO 2 sequestration in geological media are geological, solubility, hydrodynamic, mineral, adsorption, and cavern trapping. Basin- scale criteria for CO 2 sequestration, such as tectonic setting, hydrodynamic and geothermal regimes, hydrocarbon potential and basin maturity, and surface infrastructure, should be used in determining sedimentary basins in the world that are suitable for CO 2 sequestration in geological media. Site-specific criteria, such as particular geological media, in-situ conditions, storage capacity, injectivity and flow dynamics, and sequestration efficiency, need to be applied to identify sites, methods, and capacity for CO 2 sequestration. Continental sedimentary basins in North America, foremost in Texas and Alberta, are the prime candidates for CO 2 sequestration in geological media, followed by circum-Atlantic shelf basins. However, a series of major issues still needs to be addressed before proceeding with full-scale implementation, such as identification of specific sites and their capacities; proper characterization of the sequestration medium and of in-situ conditions; predicting and monitoring the fate of the injected CO 2 ; surface CO 2 capture, transport, and injection; performance assessment; and, finally, general public acceptance. Nevertheless, CO 2 sequestration in geological media is a very promising option for carbon management and is in the stage of development prior to application.

Abstract Records from the Greenland Ice Sheet Project II (GISP2) Greenland ice core are considered in terms of dynamical systems theory and nonlinear prediction. Dynamical systems theory allows us to reconstruct some properties of a phenomenon based only on past behavior without any mechanistic assumptions or deterministic models. A short-term prediction of temperature, including a mean estimate and confidence interval, is made for 800 years into the future. The prediction suggests that the present short-time global warming trend will continue for at least 200 years and will be followed by a reversal in the temperature trend.

ABSTRACT Over the course of the past two decades, I have analyzed a number of natural phenomena that reveal how earth’s near-surface air temperature responds to surface radiative perturbations. These studies all suggest that a 300 to 600 parts per million (ppm) doubling of the atmosphere’s carbon dioxide (CO 2 ) concentration could raise the planet’s mean surface air temperature by only about 0.4°C. Even this modicum of warming may never be realized, however, for it could be negated by a number of planetary cooling forces that are intensified by warmer temperatures and by the strengthening of biological processes that are enhanced by the same rise in atmospheric CO 2 concentration that drives the warming. Several of these cooling forces have individually been estimated to be of equivalent magnitude, but of opposite sign, to the typically predicted greenhouse effect of a doubling of the air’s CO 2 content, which suggests to me that little net temperature change will ultimately result from the ongoing buildup of CO 2 in earth’s atmosphere. Consequently, I am skeptical of the predictions of significant CO 2 -induced global warming that are being made by state-of-the-art climate models and believe that much more work on a wide variety of research fronts will be required to properly resolve the issue.

ABSTRACT An awareness and understanding of the palaeoclimatological history of the planet allows a different construct for today’s concerns about the impact of future climatic changes. In particular, a better appreciation of the magnitude and rate of change during the past few hundred thousand years demonstrates that the changes anticipated during the next few hundred are well within the range experienced during the Pleistocene Era. The planet is now in a period of gradual cooling from the time of the postglacial thermal optimum 6000-9000 years ago. Temperatures are now on an irregular downward path, comparable to the Eemian interglacial, although at present we are experiencing a minor temperature increase as a partial recovery from the “Little Ice Age,” which ended 150 years ago. Climate will always change. The planet is extremely resilient. As the most intelligent species that has colonised the surface, humans clearly have ample capability to adapt. However, our ability to do so will be limited by our political and behavioural patterns. The well-documented evidence from the climate changes in the Northern Hemisphere during the past 1000 years indicates that changes in average global temperatures of 2°C will have significant regional impacts on precipitation and vegetation patterns and lead to further changes in sea level. From the viewpoint of the demands of an increasing global population, these could be managed, given appropriate levels of investment. Warming is definitely easier to cope with than cooling. Stability is not an option.