Showing papers on "Global warming published in 1997"

Arctic Environmental Change of the Last Four Centuries

Abstract: A compilation of paleoclimate records from lake sediments, trees, glaciers, and marine sediments provides a view of circum-Arctic environmental variability over the last 400 years. From 1840 to the mid-20th century, the Arctic warmed to the highest temperatures in four centuries. This warming ended the Little Ice Age in the Arctic and has caused retreats of glaciers, melting of permafrost and sea ice, and alteration of terrestrial and lake ecosystems. Although warming, particularly after 1920, was likely caused by increases in atmospheric trace gases, the initiation of the warming in the mid-19th century suggests that increased solar irradiance, decreased volcanic activity, and feedbacks internal to the climate system played roles.

UK birds are laying eggs earlier

Abstract: The evidence for global climate change and for its underlying anthropogenic causes is gathering rapidly. Over the past 11 years the active growing season of plants has advanced by roughly 8 days in northern latitudes1. This evidence for increased photosynthetic activity is supported by the positive trend in the amplitude of the seasonal cycle in atmospheric CO2 (ref. 2). The phenology of animal populations should also be affected by climate change, but to date there has been little evidence of this. Here we report that long-term trends in the seasonal distributions of laying dates of birds in the United Kingdom show a tendency towards earlier laying, consistent with the changes reported in growing season.

Climate change, hurricanes and tropical storms, and rising sea level in coastal wetlands

Abstract: Global climate change is expected to affect temperature and precipitation patterns, oceanic and atmospheric circulation, rate of rising sea level, and the frequency, intensity, timing, and distribution of hurricanes and tropical storms. The magnitude of these projected physical changes and their subsequent impacts on coastal wetlands will vary regionally. Coastal wetlands in the southeastern United States have naturally evolved under a regime of rising sea level and specific patterns of hurricane frequency, intensity, and timing. A review of known ecological effects of tropical storms and hurricanes indicates that storm timing, frequency, and intensity can alter coastal wetland hydrology, geomorphology, biotic structure, energetics, and nutrient cycling. Research conducted to examine the impacts of Hurricane Hugo on colonial waterbirds highlights the importance of long-term studies for identifying complex interactions that may otherwise be dismissed as stochastic processes. Rising sea level and even modest changes in the frequency, intensity, timing, and distribution of tropical storms and hurricanes are expected to have substantial impacts on coastal wetland patterns and processes. Persistence of coastal wetlands will be determined by the interactions of climate and anthropogenic effects, especially how humans respond to rising sea level and how further human encroachment on coastal wetlands affects resource exploitation, pollution, and water use. Long-term changes in the frequency, intensity, timing, and distribution of hurricanes and tropical storms will likely affect biotic functions (e.g., community structure, natural selection, extinction rates, and biodiversity) as well as underlying processes such as nutrient cycling and primary and secondary productivity. Reliable predictions of global-change impacts on coastal wetlands will require better understanding of the linkages among terrestrial, aquatic, wetland, atmospheric, oceanic, and human components. Developing this comprehensive understanding of the ecological ramifications of global change will necessitate close coordination among scientists from multiple disciplines and a balanced mixture of appropriate scientific approaches. For example, insights may be gained through the careful design and implementation of broad-scale comparative studies that incorporate salient patterns and processes, including treatment of anthropogenic influences. Well-designed, broad-scale comparative studies could serve as the scientific framework for developing relevant and focused long-term ecological research, monitoring programs, experiments, and modeling studies. Two conceptual models of broad-scale comparative research for assessing ecological responses to climate change are presented: utilizing space-for-time substitution coupled with long-term studies to assess impacts of rising sea level and disturbance on coastal wetlands, and utilizing the moisture-continuum model for assessing the effects of global change and associated shifts in moisture regimes on wetland ecosystems. Increased understanding of climate change will require concerted scientific efforts aimed at facilitating interdisciplinary research, enhancing data and information management, and developing new funding strategies.

Potential effects of climate changes on aquatic systems: laurentian great lakes and precambrian shield region

Abstract: The region studied includes the Laurentian Great Lakes and a diversity of smaller glacial lakes, streams and wetlands south of permanent permafrost and towards the southern extent of Wisconsin glaciation. We emphasize lakes and quantitative implications. The region is warmer and wetter than it has been over most of the last 12000 years. Since 1911 observed air temperatures have increased by about 0.118C per decade in spring and 0.068C in winter; annual precipitation has increased by about 2.1% per decade. Ice thaw phenologies since the 1850s indicate a late winter warming of about 2.58C. In future scenarios for a doubled CO2 climate, air temperature increases in summer and winter and precipitation decreases (summer) in western Ontario but increases (winter) in western Ontario, northern Minnesota, Wisconsin and Michigan. Such changes in climate have altered and would further alter hydrological and other physical features of lakes. Warmer climates, i.e. 2 CO2 climates, would lower net basin water supplies, stream flows and water levels owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0. 2t o 2 .5 m lower. Human adaptation to such changes is expensive. Warmer climates would decrease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Temperature simulations for stratified lakes are 1‐78C warmer for surface waters, and 68C cooler to 88C warmer for deep waters. Thermocline depth would change (4 m shallower to 3.5 m deeper) with warmer climates alone; deepening owing to increases in light penetration would occur with reduced input of dissolved organic carbon (DOC) from dryer catchments. Dissolved oxygen would decrease below the thermocline. These physical changes would in turn aAect the phytoplankton, zooplankton, benthos and fishes. Annual phytoplankton production may increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nutrient inputs would be expected. Zooplankton biomass would increase, but, again, many complex interactions are expected. Generally, the thermal habitat for warm-, cool- and even cold-water fishes would increase in size in deep stratified lakes, but would decrease in shallow unstratified lakes and in streams. Less dissolved oxygen below the thermocline of lakes would further degrade stratified lakes for cold water fishes. Growth and production would increase for fishes that are now in thermal environments cooler than their optimum but decrease for those that are at or above their optimum, provided they cannot move to a deeper or headwater thermal refuge. The zoogeographical boundary for fish species could move north by 500‐600 km; invasions of warmer water fishes and extirpations of colder water fishes should increase. Aquatic ecosystems across the region do not necessarily exhibit coherent responses to climate changes and variability, even if they are in close proximity. Lakes, wetlands and streams respond diAerently, as do lakes of diAerent depth or productivity. DiAerences in hydrology and the position in the hydrological flow system, in terrestrial vegetation and land use, in base climates and in the aquatic biota can all cause diAerent responses. Climate change eAects interact strongly with eAects of other human-caused stresses such as eutrophication, acid precipitation, toxic chemicals and the spread of exotic organisms. Aquatic ecological systems in the region are sensitive to climate change and variation.

An evaluation of climate/mortality relationships in large U.S. cities and the possible impacts of a climate change.

Abstract: A new air mass-based synoptic procedure is used to evaluate climate/mortality relationships as they presently exist and to estimate how a predicted global warming might alter these values. Forty-four large U.S. cities with metropolitan areas exceeding 1 million in population are analyzed. Sharp increases in mortality are noted in summer for most cities in the East and Midwest when two particular air masses are present. A very warm air mass of maritime origin is most important in the eastern United States, which when present can increase daily mortality by as many as 30 deaths in large cities. A hot, dry air mass is important in many cities, and, although rare in the East, can increase daily mortality by up to 50 deaths. Cities in the South and Southwest show lesser weather/mortality relationships in summer. During winter, air mass-induced increases in mortality are considerably less than in summer. Although daily winter mortality is usually higher than summer, the causes of death that are responsible for most winter mortality do not vary much with temperature. Using models that estimate climate change for the years 2020 and 2050, it is estimated that summer mortality will increase dramatically and winter mortality will decrease slightly, even if people acclimatize to the increased warmth. Thus, a sizable net increase in weather-related mortality is estimated if the climate warms as the models predict.

Twentieth-century sea surface temperature trends

Abstract: An analysis of historical sea surface temperatures provides evidence for global warming since 1900, in line with land-based analyses of global temperature trends, and also shows that over the same period, the eastern equatorial Pacific cooled and the zonal sea surface temperature gradient strengthened. Recent theoretical studies have predicted such a pattern as a response of the coupled ocean-atmosphere system to an exogenous heating of the tropical atmosphere. This pattern, however, is not reproduced by the complex ocean-atmosphere circulation models currently used to simulate the climatic response to increased greenhouse gases. Its presence is likely to lessen the mean 20th-century global temperature change in model simulations.

Coastal Ecosystem Processes

Abstract: Introduction Beaches and Tidal Flats Introduction Food Chains, Energy, and Carbon Flow Nitrogen Cycling Linkages to Physical Processes Mangroves and Salt Marshes Introduction Global Trends in Plant Biomass and Primary Production Factors Limiting Plant Production and Growth Food Webs and Decomposition Processes Nitrogen Flow Outwelling Seaweed and Seagrass Ecosystems Introduction Standing Crop and Primary Productivity Photosynthesis and Whole-Plant Carbon Balance Limiting Factors The Role of Grazers Detritus and Mineralization Processes Ecosystem Budgets Carbon Balance: Export and Links to Adjacent Systems Coral Reefs Introduction Sources of Carbon Production The Fate of Organic Matter Nitrogen and Phosphorus: Cycles and Limitation The Coral Factory: Carbon and Energy Budgets Systems-Level Perspectives: Models and Budgets The Role of Coral Reefs in the Tropical Biosphere The Coastal Ocean I The Coastal Zone Introduction The Coastal Ocean Defined What Is an Estuary? Hydrographic Classification of Coastal Systems Coastal Plain Estuaries, Tidal Lagoons, and Bays (Types IV, V, and VI) Coastal Lagoons (Type VII) River-Dominated Systems (Types I, II, and III) The Coastal Ocean II The Shelf Proper and Shelf Edge Introduction Shelf-Sea Fronts Along- and Across-Shelf Gradients Processes at the Shelf Edge Nutrient Cycles and Global Change in the Coastal Ocean Global Estimates of Fishery Yields to Humans Degradation and Conservation A Glimpse at the Global Problem Eutrophication Habitat Modification and Destruction Restoration Attempts: Problems and Progress Sustainability: Implications for Management Conservation: Tools and Impediments Global Climate Change: Coastal Implications A Final Remark References Index

Tundra plants and climate change: The International Tundra Experiment (ITEX).

Abstract: The International Tundra Experiment (ITEX) was established in late 1990 at a meeting of arctic tundra ecologists as a response to predictions that the human-enhanced greenhouse warming would occur earliest and most intensely at high latitudes. The initial objective of ITEX was to monitor phenology, growth and reproduction in major circumpolar vascular plant species in response to climate variations and environmental manipulations at sites throughout the tundra biome. The manipulations involve passive warming of tundra plots in open-top chambers (OTCs), and manipulating snow depth to alter growing season length. Standard protocols were developed for measurements, experimental design and statistical analyses, and published in an ITEX Manual. The standard methods ensure comparable data are collected at all sites. This special issue of Global Change Biology is based on papers developed from the 6th ITEX Workshop, held at the University of Ottawa, Ottawa, Canada, 7–11 April 1995. The papers compare short-term responses (1–3 years) of common species to climate variations and manipulations at ITEX sites. The OTCs increase mean near-surface temperatures by 1–3°C during the growing season, simulating predictions from global circulation models. All species investigated responded to the temperature increase, especially in phenology and reproductive variables. However, these short-term responses were individualistic, and no general pattern in type or magnitude of response was noted for functional types or phenology class. Responses were generally similar among sites, although the magnitude of response tended to be greater in high Arctic sites. Early snowmelt increased carbon:nutrient ratios in plants. Sustained growth and reproductive responses to warming will depend on nutrient supply, and increased carbon:nutrient ratios in litter could buffer nutrient cycling, and hence plant growth. Ongoing, long-term research at ITEX sites, linked to other global change initiatives, will help elucidate probable effects of climate change at the ecosystems level in arctic and alpine tundra.

Effects of climate change on the freshwaters of arctic and subarctic north america

Abstract: Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring. © 1997 John Wiley & Sons, Ltd.

Interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry

Abstract: The relative timings of the last deglacial warming in the Southern and Northern hemispheres are not well constrained, but are a crucial component in understanding the mechanisms of deglaciation1. A clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high-resolution temperature records that can be tied to the absolute calendar timescale. Moreover, the quantification of tropical temperatures during the last glacial cycle is controversial2–8. Here we apply the alkenone method of sea surface temperature reconstruction9,10 to several high-resolution sediment cores recovered from the tropical Indian Ocean between 20° N and 20° S. The inferred initial sea surface temperature warming ∼15,000 calendar years ago at 20° S is in phase with Northern Hemisphere sea (this study) and air11 temperature changes, but lags Antarctic warming12–14 by several millennia. This finding, along with the results of recent modelling studies15,16, provides strong support for the idea that changes in the ocean's global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation.

The Role of the Sun in Climate Change

Abstract: The luminosity of the sun governs the temperatures of the planets. Yet the solar forcing, or driving, of climate, primarily due to changes in solar radiation, has never been well documented. Recent satellite measurements have shown that solar radiation varies as a function of time and wavelength, a concept that has been hypothesized for the past two centuries and has recently become a major topic with all the attention paid to global warming. This book reviews the physics of the concept of solar forcing, from its beginnings in the early 1800's and apparent success in the 1870's, to its near demise in the 1950's and recent resurgence. Since its emphasis is on solar variations as a driver for climate change, with only a brief discussion of other mechanisms, the book will be of most interest to students in climate studies.

Elevation Dependency of the Surface Climate Change Signal: A Model Study

Abstract: Results are presented from a present-day and a doubled CO2 experiment over the Alpine region with a nested regional climate model. The simulated temperature change signal shows a substantial elevation dependency, mostly during the winter and spring seasons, resulting in more pronounced warming at high elevations than low elevations. This is caused by a depletion of snowpack in doubled CO2 conditions and further enhanced by the snow–albedo feedback. This result is consistent with some observed temperature trends for anomalously warm years over the Alpine region and suggests that high elevation temperature changes could be used as an early detection tool for global warming. Changes in precipitation, as well as other components of the surface energy and water budgets, also show an elevation signal, which may have important implications for impact assessments in high elevation regions.

Observed Hemispheric Asymmetry in Global Sea Ice Changes

Abstract: From November 1978 through December 1996, the areal extent of sea ice decreased by 2.9 ± 0.4 percent per decade in the Arctic and increased by 1.3 ± 0.2 percent per decade in the Antarctic. The observed hemispheric asymmetry in these trends is consistent with a modeled response to a carbon dioxide–induced climate warming. The interannual variations, which are 2.3 percent of the annual mean in the Arctic, with a predominant period of about 5 years, and 3.4 percent of the annual mean in the Antarctic, with a predominant period of about 3 years, are uncorrelated.

Observations and Predictions of Arctic Climatic Change: Potential Effects on Marine Mammals

Abstract: Recent analyses have revealed trends over the past 20-30 years of decreasing sea ice extent in the Arctic Ocean coincident with warming trends. Such trends may be indicative of the polar amplifications of warming predicted for the next several decades in response to increasing atmospheric CO2. We have summarized these predictions and nonuniform patterns of arctic climate change in order to address their potential effects on marine mammals. Since recent trends in sea ice extent are nonuniform, the direct and indirect effects on marine mammals are expected to vary geographically. Changes in the extent and concentration of sea ice may alter the seasonal distributions, geographic ranges, patterns of migration, nutritional status, reproductive success, and ultimately the abundance and stock structure of some species. Ice-associated seals, which rely on suitable ice substrate for resting, pupping, and molting, may be especially vulnerable to such changes. As recent decreases in ice coverage have been more extensive in the Siberian Arctic (60 E-180 E) than in the Beaufort Sea and western sectors, we speculate that marine mammal populations in the Siberian Arctic may be among the first to experience climate-induced geographic shifts or altered reproductive capacity due to persistent changes in ice extent. Alteration in the extent and productivity of ice-edge systems may affect the density and distribution of important ice-associated prey of marine mammals, such as arctic cod, Boreogadus saida , and sympagic ("with ice") amphipods. Present climate models, however, are insufficient to predict regional ice dynamics, winds, mesoscale features, and mechanisms of nutrient resupply, which must be known to predict productivity and trophic response. Therefore, it is critical that mesoscale process-oriented studies identify the biophysical coupling required to maintain suitable prey availability and ice-associated habitat for marine mammals on regional arctic scales. Only an integrated ecosystems approach can address the complexity of factors determining reproductivity and cascading trophic dynamics in a warmer Arctic. This approach, integrated with monitoring of key indicator species (e.g., bowhead whale, ringed seal, and beluga), should be a high priority.

Third Millennium BC Climatic Change and Old World Collapse

Abstract: Around 4000 years ago the advanced urban civilizations of Egypt, Mesopotamia and India suddenly collapsed. Recent discoveries now strongly suggest that the collapse was linked to climate change. This volume presents recent findings and reviews relevant information. The present theory is that a major shift of the precipitation pattern affected many parts of the world at approximately the same time, with disastrous effects on the populations of Asia, Africa and Eastern Europe. Can a similar climate shift with a serious adverse imapct on society happen again? In a world facing global warming, there could be many lessons to learn from the experiences of ancient societies.

Potential changes in the distribution of dengue transmission under climate warming

Abstract: The purpose of the present paper is to document an initial attempt to quantify the influence of warming temperatures on the intensity and distribution of dengue transmission throughout the world using an expression of vectorial capacity modified to reflect the role of temperature on development and survival of the vector and virus. We rearranged the traditional vectorial capacity expression (the mean number of potentially infective contacts made by a mosquito population per infectious person per unit time) to develop an equation for the critical density threshold, an estimate of the number of adult female vectors required to just maintain the virus in a susceptible human population. In this expression, temperature influences adult survival, the lengths of the gonotrophic cycle and the extrinsic incubation period of the virus in the vector, and vector size, a factor that indirectly influences the biting rate. Before making projections for warming scenarios of current climate plus 2 or 4 degrees C, we validate our technique by successfully comparing model projections and the observed spatial, temporal, and altitudinal distribution of dengue using current climate in five cities that are endemic or have had epidemics in the past. Our results indicate that the current warming projection of the International Council of Scientific Unions and the Intergovernmental Panel on Climate Change of 2 degrees C by the end of the next century can be expected to result in a potential increase in the latitudinal and altitudinal range of dengue; the potential duration of the transmission season will also increase in temperate locations as well. We discuss how an increase in temperature-related transmission intensity can be expected to lower the average ages of primary and secondary infections and thereby significantly increase the proportion of secondary infections occurring among infants and adolescents, the ages especially susceptible to dengue hemorrhagic fever and shock syndrome.

Widespread effects of climatic warming on freshwater ecosystems in north america

Abstract: Freshwaters in different regions show many similarities and differences in their responses to climatic warming. Bases for comparison include reports from regional committees, long-term records for several sites where climate has warmed in the past two decades and other human alterations to freshwaters that simulate some of the expected results of climatic warming, such as reservoir construction. Palaeoecological studies of freshwaters under climatic warming and differences in communities under different climatic regimes are also considered. Major changes in the physical, chemical and biological characteristics of lakes occur. Many of the changes to lakes and streams are the result of strong effects of climatic warming on terrestrial catchments. Inputs from catchments can be either dampened or amplified by in-lake processes, in some cases causing counter-intuitive responses, such as the acidification of streams but alkalinization of lakes in areas where supplies of base cations are limited. Consideration of land-water interactions and interactions between climatic warming and other human stresses are important for the accurate prediction of the effects of climatic change.

Multi-fingerprint detection and attribution analysis of greenhouse gas, greenhouse gas-plus-aerosol and solar forced climate change

Abstract: A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single fingerprint is optimal for the detection of climate change, further tests of the statistical consistency of the detected climate change signal with model predictions for different candidate forcing mechanisms require the simultaneous application of several fingerprints. Model-predicted climate change signals are derived from three anthropogenic global warming simulations for the period 1880 to 2049 and two simulations forced by estimated changes in solar radiation from 1700 to 1992. In the first global warming simulation, the forcing is by greenhouse gas only, while in the remaining two simulations the direct influence of sulfate aerosols is also included. From the climate change signals of the greenhouse gas only and the average of the two greenhouse gas-plus-aerosol simulations, two optimized fingerprint patterns are derived by weighting the model-predicted climate change patterns towards low-noise directions. The optimized fingerprint patterns are then applied as a filter to the observed near-surface temperature trend patterns, yielding several detection variables. The space-time structure of natural climate variability needed to determine the optimal fingerprint pattern and the resultant signal-to-noise ratio of the detection variable is estimated from several multi-century control simulations with different CGCMs and from instrumental data over the last 136 y. Applying the combined greenhouse gas-plus-aerosol fingerprint in the same way as the greenhouse gas only fingerprint in a previous work, the recent 30-y trends (1966–1995) of annual mean near surface temperature are again found to represent a significant climate change at the 97.5% confidence level. However, using both the greenhouse gas and the combined forcing fingerprints in a two-pattern analysis, a substantially better agreement between observations and the climate model prediction is found for the combined forcing simulation. Anticipating that the influence of the aerosol forcing is strongest for longer term temperature trends in summer, application of the detection and attribution test to the latest observed 50-y trend pattern of summer temperature yielded statistical consistency with the greenhouse gas-plus-aerosol simulation with respect to both the pattern and amplitude of the signal. In contrast, the observations are inconsistent with the greenhouse-gas only climate change signal at a 95% confidence level for all estimates of climate variability. The observed trend 1943–1992 is furthermore inconsistent with a hypothesized solar radiation change alone at an estimated 90% confidence level. Thus, in contrast to the single pattern analysis, the two pattern analysis is able to discriminate between different forcing hypotheses in the observed climate change signal. The results are subject to uncertainties associated with the forcing history, which is poorly known for the solar and aerosol forcing, the possible omission of other important forcings, and inevitable model errors in the computation of the response to the forcing. Further uncertainties in the estimated significance levels arise from the use of model internal variability simulations and relatively short instrumental observations (after subtraction of an estimated greenhouse gas signal) to estimate the natural climate variability. The resulting confidence limits accordingly vary for different estimates using different variability data. Despite these uncertainties, however, we consider our results sufficiently robust to have some confidence in our finding that the observed climate change is consistent with a combined greenhouse gas and aerosol forcing, but inconsistent with greenhouse gas or solar forcing alone.

Global warming, irreversibility and learning

Abstract: A number of economists have argued that the literature on the irreversibility effect implies that current abatement of greenhouse gas emissions should be greater when there is the possibility of obtaining better information in the future about the potential damages from global warming than when there is no possibility of obtaining better information. In this paper we show that even the simplest model of global warming does not satisfy either of Epstein's (1980) sufficient conditions, so it is not possible to use Epstein's analysis to tell whether the irreversibility effect applies to models of global warming. We derive an alternative sufficient condition for the irreversibility effect to hold.

On Modification of Global Warming by Sulfate Aerosols

Abstract: There is increasing evidence that the response of climate to increasing greenhouse gases may be modified by accompanying increases in sulfate aerosols. In this study, the patterns of response in the surface climatology of a coupled ocean–atmosphere general circulation model forced by increases in carbon dioxide alone is compared with those obtained by increasing carbon dioxide and aerosol forcing. The simulations are run from early industrial times using the estimated historical forcing and continued to the end of the twenty-first century assuming a nonintervention emissions scenario for greenhouse gases and aerosols. The comparison is made for the period 2030–2050 when the aerosol forcing is a maximum. In winter, the cooling due to aerosols merely tends to reduce the response to carbon dioxide, whereas in summer, it weakens the monsoon circulations and reverses some of the changes in the hydrological cycle on increasing carbon dioxide. This response is in some respects similar to that found in s...

Cryostratigraphy, paleogeography, and climate change during the early Holocene warm interval, western Arctic coast, Canada

Abstract: Botanical and cryostratigraphic records from northwest Canada indicate that the climate of the early Holocene was considerably warmer than today: tree line was over 100 km farther north; and a thaw unconformity, dating from 8000 14C years BP, formed at the base of an active layer 2.5 times thicker than at present. Numerous thermokarst-lake basins formed in the preceding millennia. Both the botanical and cryostratigraphic indices described are products of summer conditions. Previous reconstructions of early Holocene climate have not assessed the significance of paleocoastal location on the seasonality and extent of apparent climate warming. At present, there is a steep gradient in growing-season conditions between cooler sites on the Beaufort Sea coast and warmer, inland locations. Winter conditions are more uniform because both sea and land are snow-covered. Coastal retreat in the region has been rapid, due to sea level rising over a gently sloping shelf containing readily erodible sediments. The coastlin...

Global Change and Arctic Terrestrial Ecosystems.

Abstract: Global warming is likely to have the greatest impact at high latitudes, making the Arctic an important region both for detecting global climate change and for studying its effects on terrestrial ecosystems. The chapters in this volume address current and anticipated impacts of global climate change on Arctic organisms, populations, ecosystem structure and function, biological diversity, and the atmosphere.

Lake Bonneville fluctuations and global climate change

Abstract: Lake Bonneville, the largest late Pleistocene closed-basin lake in the North American Great Basin, fluctuated widely in response to changes in climate. The geochemistry and mineralogy of endogenic calcium carbonate deposited in deep water, and stratigraphic studies of shore-zone deposits, provide evidence of millennial-scale lake-level fluctuations that had amplitudes of about 50 m between 30 and 10 ka. Falling-lake events occurred at 21, 18.5–19, 17.5, 16–15.5, 14–13, and 10 ka (radiocarbon years) synchronously with the terminations of Heinrich events H1 and H2 and other smaller scale iceberg-rafting events (a, b, c, and Younger Dryas) in the North Atlantic Ocean. The Lake Bonneville results thus support other climate records that suggest that late Pleistocene millennial-scale climate change was global in extent. The size and shape of the Northern Hemisphere ice sheets, which determined the mean positions of storm tracks, may have been the primary control on late Pleistocene water budgets of Great Basin lakes.

Global warming and active-layer thickness: results from transient general circulation models

Abstract: The near-surface thermal regime in permafrost regions could change significantly in response to anthropogenic climate warming. Because there is only a small lag between these two processes, the impact of warming on the active layer can be investigated using relatively simple climate-driven models. A formulation attributable to Kudryavtsev was used to study the potential increase of active-layer thickness in the permafrost regions of the Northern Hemisphere, where warming is predicted to be more pronounced than elsewhere. Kudryavtsev's solution was validated using contemporary data, and successfully reproduced the actual depths of frost and thaw at widely spaced locations in North America and Eurasia. Modern climatic data and scenarios of climate change for 2050, derived from three transient coupled ocean-atmosphere general circulation models (GCMs), were used in conjunction with the thaw-depth solution to generate hemispheric maps showing contemporary active-layer thickness for several soil types and moisture conditions, and its relative changes over the next century. The simulations indicate a 20–30% increase of active-layer thickness for most of the permafrost area in the Northern Hemisphere, with the largest relative increases concentrated in the northernmost locations.

How Dry is the Tropical Free Troposphere? Implications for Global Warming Theory

Abstract: The humidity of the free troposphere is being increasingly scrutinized in climate research due to its central role in global warming theory through positive water vapor feedback. This feedback is the primary source of global warming in general circulation models (GCMs). Because the loss of infrared energy to space increases nonlinearly with decreases in relative humidity, the vast dry zones in the Tropics are of particular interest. These dry zones are nearly devoid of radiosonde stations, and most of those stations have, until recently, ignored the low humidity information from the sondes. This results in substantial uncertainty in GCM tuning and validation based on sonde data. While satellite infrared radiometers are now beginning to reveal some information about the aridity of the tropical free troposphere, the authors show that the latest microwave humidity sounder data suggests even drier conditions than have been previously reported. This underscores the importance of understanding how these low humidity levels are controlled in order to tune and validate GCMs, and to predict the magnitude of water vapor feedback and thus the magnitude of global warming.

Geochemistry of corals: Proxies of past ocean chemistry, ocean circulation, and climate

Abstract: This paper presents a discussion of the status of the field of coral geochemistry as it relates to the recovery of past records of ocean chemistry, ocean circulation, and climate. The first part is a brief review of coral biology, density banding, and other important factors involved in understanding corals as proxies of environmental variables. The second part is a synthesis of the information available to date on extracting records of the carbon cycle and climate change. It is clear from these proxy records that decade time-scale variability of mixing processes in the oceans is a dominant signal. That Western and Eastern tropical Pacific El Nino-Southern Oscillation (ENSO) records differ is an important piece of the puzzle for understanding regional and global climate change. Input of anthropogenic CO2 to the oceans as observed by 13C and 14C isotopes in corals is partially obscured by natural variability. Nonetheless, the general trend over time toward lower delta18O values at numerous sites in the world's tropical oceans suggests a gradual warming and/or freshening of the surface ocean over the past century.

Global climate change: the potential effects on health.

Abstract: Excess carbon dioxide, methane, and other gases which trap heat are accumulating in the troposphere, the earth's lower atmosphere, because of the scale and type of human economic activity Climate scientists predict that the resultant increase in the troposphere's “radiative forcing” will warm the earth's surface1 2 3 Indeed, in its recent second assessment report, the Intergovernmental Panel on Climate Change—a multidisciplinary scientific body established by the United Nations in 1988 to advise governments—concluded that on balance an anthropogenic influence upon the global climate was now “discernible” 1 The intergovernmental panel forecasts an increase in the average world temperature of 10-35°C over the coming century1 This forecast is necessarily uncertain because the sensitivity of climate to atmospheric change is imperfectly understood and because future trends in gaseous emissions and modulating processes (for example, the cooling effects of industrial aerosol emissions) cannot be foreseen accurately Nevertheless, the expected rate of climate change over the coming century would be far greater than any natural change in world climate since the advent of agriculture 10 000 years ago Anthropogenic climate change signifies that for the first time the aggregate global impact of humankind exceeds the physical and ecological limits of the biosphere4 The potential consequences of this and other global changes (including stratospheric ozone depletion, loss of biodiversity, worldwide land degradation, and depletion of aquifers) are wide ranging We can expect that climate change will affect the health and wellbeing of human populations in diverse ways This greatly extends the temporo-spatial scale of environmental health beyond our usual concern with localised and immediate exposures to toxic or infectious agents4 A major research task, therefore, is the application of current knowledge to forecasting probable health effects The primary objective is to provide indicative forecasts of an important consequence that will …

Vegetation-induced warming of high-latitude regions during the Late Cretaceous period

Abstract: Modelling studies of pre-Quaternary (>2 million years ago) climate implicate atmospheric carbon dioxide concentrations1, land elevation2 and land–sea distribution3–5 as important factors influencing global climate change over geological timescales. But during times of global warmth, such as the Cretaceous period and Eocene epoch, there are large discrepancies between model simulations of high-latitude and continental-interior temperatures and those indicated by palaeotemperature records6,7. Here we use a global climate model for the latest Cretaceous (66 million years ago) to examine the role played by high- and middle-latitude forests in surface temperature regulation. In our simulations, this forest vegetation warms the global climate by 2.2 °C. The low-albedo deciduous forests cause high-latitude land areas to warm, which then transfer more heat to adjacent oceans, thus delaying sea-ice formation and increasing winter temperatures over coastal land. Overall, the inclusion of some of the physical and physiological climate feedback effects of high-latitude forest vegetation in our simulations reduces the existing discrepancies between observed and modelled climates of the latest Cretaceous, suggesting that these forests may have made an important contribution to climate regulation during periods of global warmth.

The potential for feedback effects induced by global warming on emissions of nitrous oxide by soils

Abstract: About 65% of all emissions of nitrous oxide, N2O, are from soils, and are caused by aerobic nitrification and anaerobic denitrification. Tropical forest soils are probably the most important single source, followed by cultivated soils. Emission rates in natural systems are related to the rate of N mineralization from organic matter, and N deposition; in agricultural systems they are related to the quantities of N used as fertilizers and, where relevant, to recent land use change. The global budget for N2O is not well balanced, and sources may still be underestimated. Direct evidence of a positive feedback of global warming on N2O emissions comes from studies of air in ice cores. One of the projected effects of future global warming is a lowering of water tables in northern peatlands; experiments suggest that this would lead to increased emissions, but that the effect on total emissions would be small. The results of many experiments with non-peatland soils indicate that the effect of temperature on soil emissions is generally positive, and that the rate of increase may be very steep when denitrification is the principal process involved. Process-level modelling suggests that the reason is increased soil respiration, which causes an increase in anaerobic volume in which denitrification can take place, in addition to the increased denitrification rate per unit anaerobic volume brought about directly by the rise in temperature. These results imply that generally a positive feedback on emissions from soils is likely. However, in some environments, a large proportion of total annual emissions can occur during freeze–thaw cycles; such cycles may become more or less frequent, depending on the climatic zone, and this may result in either a positive or negative feedback effect due to global warming. Models of global and regional trends give very conflicting predictions of the direction and the magnitude of climatic impacts on fluxes, but the prediction of a positive feedback seems to be the more soundly based.

Uncertainties in Projections of Human-Caused Climate Warming

Abstract: Increased atmospheric greenhouse gases resulting from human activities have produced a sharp increase in the potential to warm the climate to levels that may have serious implications. Predictions of future climate changes in response to these greenhouse gas increases depend almost completely upon physically based mathematical models of the climate system. The strengths and weaknesses of these models to project various aspects of climate change are evaluated in terms of a "betting odds" approach to provide guidance to the policy deliberation process.