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Wallace S. Broecker, Newberry Professor of Earth & Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia University
Suggestions that the ongoing greenhouse buildup might induce a shutdown of the ocean's thermohaline circulation raise the question as to how the Earth's climate would change if such an event were to occur. The answer preferred by the popular press is that conditions akin to those that characterized the Younger Dryas – the last kiloyear cold snap – would return. But this extreme scenario is an unlikely one, for models suggest that in order to force a conveyor shutdown, Earth would have to undergo a4 to 5 degree Celsius greenhouse warming. Hence, the conditions at the very onset of the shutdown would be very different from those that preceded the Younger Dryas. Thus, it is unlikely that new climate conditions would be nearly so severe. Unfortunately, because no atmosphere model to date has been able to create the observed large and abrupt changes in the climatic state of the Earth's atmosphere, we lack even the crudest road map. However, as was the case for each of the abrupt changes recorded in Greenland's ice, if the conveyor were to shut down, climate would likely "flicker" for several decades before locking into its new state. The consequences to agricultural production of these flickers would likely be profound.

Edward Brook, Department of Geology, Washington State University
Cores through the polar ice sheets provide a remarkable archive of past climate conditions and a context for understanding human impacts on climate. They document climate change on time scales as short as a few years and as long as hundreds of thousands of years. Studies of the isotopic composition of the ice, dust content, other impurities, and trapped gases provide a detailed description of major glacial-interglacial changes in Earth's climate and show clearly the link between changes in climate and biogeochemical cycles over the past 400,000 years. Ice core records also document significant shorter term ('millennial scale') climate change in exquisite detail, showing that the relatively slow pace of glacial-interglacial climate change is not the only mode of variability the Earth is capable of. With respect to the modern situation, records of greenhouse gases are perhaps most significant. These direct measurements of past atmospheric conditions show that current levels of greenhouse gases represent a significant increase above natural levels since the early 1800s, and that both the current concentrations and rates of increase are unprecedented in the past 400,000-plus years.

Scott A. Elias, Fellow, Institute of Arctic & Alpine Research, University of Colorado
The global warming predicted for the next century is not without precedent in recent history. Much of the world experienced temperatures averaging at least 3 degrees Centigrade warmer than those of today during the last interglacial period. Furthermore, the rapid rate of climate change predicted for the 21st century likely also occurred repeatedly in the ancient past. Multiple lines of evidence show that major climatic oscillations at the end of the last glaciation took place within a few decades at most.

Through at least 17 glacial-interglacial cycles over the last two million years, beetle populations responded to large-scale climatic change by shifts in distribution. Few if any extinctions are seen in the Pleistocene fossil beetle record, in contrast to the mammalian fossil record. Looking to the future, however, it is clear that a major environmental factor will come into play that did not exist in the Pleistocene: anthropogenic habitat destruction. Previous (ancient) large-scale climate changes took place against a backdrop of primeval ecosystems. Because of their exceptional dispersal abilities, insects were able to colonize new regions with relative ease. This may not be the case in a world where natural habitats are vanishing concurrent with global warming.

Peter Frumhoff, Director, Global Resources, Union of Concerned Scientists
The primary cause of climate change is the release to the atmosphere of greenhouse gases that result from the burning of fossil fuels. Effective policies to reduce the threat and severity of climate change must therefore reduce these emissions, particularly in the United States and other industrialized countries. But another important source of greenhouse gas emissions is the ongoing clearing and fragmentation of forests, particularly in tropical countries, a process that is also a globally significant driver of biodiversity loss. Effective policies to conserve and restore these forests can thus help meet both climate change and biodiversity conservation objectives. I will assess emerging opportunities to finance forest conservation and restoration in the tropics through the new Clean Development Mechanism of the climate treaty's Kyoto Protocol.

Tony Janetos, Senior Vice President for Programs, World Resources Institute
Many studies of the relationship between climate change and biodiversity have focused on whether or not rapid changes in climate would lead to reductions in diversity. By and large, the results have suggested that rapid changes in climate would have adverse consequences for biological diversity in today's landscape. My focus is on a different aspect of the relationship between these two issues: the degree to which these two environmental issues are intrinsically linked in both their underlying causes and possible solutions.

Many of the driving forces that are endangering the Earth's climate and diversity of life are the same and are due to similar factors in our needs to provide the basic necessities of life. The need to increase agricultural productivity, the need to provide the most basic resources for economic development, and the desires to achieve equitable benefits from the use of natural resources are major contributors both to climate change and to losses of biodiversity. Therefore, the ways in which societies might seek to address these issues also have important common features. One of the many challenges for effective policy action on these critical issues is in fact not to treat them as separate, but to look for actions that achieve synergies.

Douglas Hill, Consulting Systems Engineer, Regional Plan Association
With global warming, the New York metropolitan region faces worsened summer hot spells, more frequent coastal flooding, and water shortages. Present trends in energy consumption, such as the growing popularity of big, inefficient automobiles, exacerbate the growth in emissions of carbon dioxide. In the long run, the metropolitan area may suffer more from the indirect effects of climate change. Reacting to greenhouse effects may divert capital needed to restore the region's aging infrastructure, contributing to greater inefficiencies in doing business, and less cost-competitiveness in global markets. With its dependence upon international business, the region may also suffer disproportionately from depressed economic conditions overseas due to climate change. As the traditional gateway for immigrants, New York may become overburdened with environmental refugees. With oppressive summer heat, recurring flooding, and strangling traffic congestion, the metropolitan region will become a less attractive place to live. In the near term, there are opportunities to reduce greenhouse gas emissions and take measures to adapt to climate change that will be beneficial regardless of its future severity.

Kent E. Holsinger, Professor of Biology, Director, Center for Conservation and Biodiversity, University of Connecticut
Humans have had an enormous impact on our planet. Not only are levels of atmospheric carbon dioxide expected to double in the next 50 years, human activities now dominate most of the world's ecosystems. Global changes such as habitat destruction and the spread of invasive exotics pose the most immediate threats to the survival of most plant species. The threat posed by global warming is less immediate, but it may be equally severe. Extrapolating patterns of plant movement in response to past climate change suggests that existing nature reserves are mostly too small and too loosely connected to ensure long-term survival of species with narrow ranges. Conservation planners have barely begun to consider the impacts of climate change on design and implementation of strategies to conserve plant biodiversity.

Michael W. Klemens, Director, Metropolitan Conservation Alliance, Wildlife Conservation Society
The fauna and flora of the New York metropolitan region evolved in an ever-changing environment. During repeated Pleistocene glaciations, species retreated southward; during inter-glacial periods, plants and animals reinvaded ice-free areas. These oscillations responded to climatic changes in a time scale measured in millennia.

Four hundred years of European colonization caused large-scale habitat changes. Some large mammals were extirpated; many of the lower vertebrates expanded or contracted their ranges. Forest species became rare as land was cleared, yet persisted in small woodlots and steep ravines. When farming moved westward in the nineteenth century, grasslands reverted to forest, forest species re-established themselves, and grassland species became rare. These short-term pulsations occurred over time scales of 50-100 years.

Given such resiliency, does global warming pose a threat to biodiversity? Global warming is not the threat – it is the condition of our landscape that diminishes the resiliency of plants and animals to respond to change. I have coined the term "post-Eisenhowerian Era" to call attention to the radical changes that the 1950's brought to our region's (and our nation's) landscape. Within thirty years, the Eisenhower Interstate Highway System created a fragmented, dysfunctional landscape, first by the direct effect of highway construction, then by spasms of sprawl that moved humans out of compact cities and towns into the hinterlands.

The post-Eisenhowerian landscape is so fragmented that it is a de facto archipelago, subject to the constraints of island biogeography. The changes produced by global warming could have been accommodated through the 1940's. The "hard fragmentation" of post-Eisenhowerian landscapes reduces the ability of most species to migrate in response to habitat change. Solutions to this challenge include re-engineering "hard fragments" into "soft," maintaining biotic corridors in rural and suburban areas, and reinvesting in our cities and towns.

Ross D. E. MacPhee, Chairman and Curator, Mammalogy, American Museum of Natural History
The "late Quaternary extinctions" (LQE) witnessed, over a 50,000-year period, the loss of hundreds of mammalian and perhaps thousands of bird species. The LQE epoch is broadly analogous to recent times in that it was a period of marked climatic change. But were any of the LQE caused by climate change, and, if so, how could we tell? Recent evidence suggests that, in the New World, most losses occurred within a 400-year envelope ca. 12,000 years agoexcept in the West Indies, where losses began to occur some 5,000 years later. The West Indian extinctions precisely correlate with the earliest evidence of human presence. Extremely rapid losses immediately subsequent to human first contact are now known to have been the rule in Polynesia, Madagascar, Australia, and elsewhere. The present consensus seems to be that there is no case in which "climate change" by itself explains any LQE, whereas human first contact can be used to explain virtually all of them.

Although at present the principal consequence of "global warming" is climatic, its root cause is conceived to be anthropogenic. However, if the late Quaternary analogy is meaningful, a 2-5 degree C change by itself is not going to result in widespread extinction among terrestrial vertebrates because wider fluxes than that occurred without causing losses (e.g., late Pleistocene Australia). If losses do occur in future, they will be largely the result of other human impacts that have already removed much of the resilience in earth's biotas.

Adam Markham, Director, Climate Change Program, World Wildlife Fund
The evidence is mounting that we are experiencing a worsening trend of global warming resulting mainly from emissions of carbon pollution to the atmosphere from the burning of fossil fuels such as coal and oil. Global warming poses a major threat to many of the world's wildlife species and their habitats. Tropical cloud forests, alpine meadows, prairie wetlands, low-lying coastal areas and coral reefs may be especially sensitive to change. Ecosystems near the poles, including Arctic tundra and boreal forest, are deemed to be at particular risk because of the greater expected warming at higher latitudes. Computer models used to project possible future climate scenarios indicate that more than a third of the world's existing protected areas are likely to experience significant change in a warming world.

Indications already exist that the warming trend of the last few decades is beginning to have an impact. Glaciers are melting world-wide and sea-ice is thinning in the western Arctic. Many species, including some butterflies, plants and seashore animals, are being recorded moving to higher latitudes or altitudes in response to the warming. With an earlier onset of spring in the northern hemisphere, migratory birds are arriving to breed earlier in the Great Lakes region and in northern Europe. Each species will react differently and respond to climate change at different rates, so ecosystems will not be able to move wholesale in response to change. Some ecosystems may be lost entirely, and others will be much reduced in extent. Global warming presents a major challenge to conservationists, and should be given priority in planning to protect biodiversity into the next millennium and beyond. Because the most vulnerable systems will often be those that are most under stress or fragmented already, current efforts to preserve habitat should be increased in order to help buffer the impacts of climate change.

Paul E. Olsen, Storke Memorial Professor of Earth & Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia University
Extinction is the fate of nearly all species. However, the geological record shows six intervals of concentrated extinction that stand as landmarks in life's history. The oldest two are so ancient that it is hard to gauge their magnitude or cause. The next three are: the end-Permian mass extinction 250 million years ago, the largest of all the extinctions; the end-Triassic event 202 million years ago, after which dinosaurs became the dominant land animals; and the end-Cretaceous mass-extinction 65 million years ago, which marked their demise. These three extinctions appear to have been caused by dramatic environmental change. This is known in greatest detail and certainty for the end-Cretaceous event, which was apparently caused by the impact of a giant asteroid or comet. An asteroid is also implicated for the end-Triassic event; however, all three mass extinctions also appear synchronous with giant, terrestrial volcanic eventsa coincidence too grand to be accident. In contrast, the most recent mass-extinction event has occurred during the last 50,000 years and appears to be on-going. Unlike the extrinsic, non-biological origin of the previous events, this mass-extinction coincides with the spread of a new species, Homo sapiens, and its associated technology.

Dorothy Peteet, NASA/Goddard Institute for Space Studies/Lamont-Doherty Earth Observatory of Columbia University
How quickly has climate changed in the past? Did climate change happen simultaneously around the world? One way to investigate such questions is through study of the remains of plants in lakes and bogs. Seeds, needles, pollen and other plant parts are very well preserved in lake muds because of the lack of oxygen in the sediments. From careful analysis and precise timing of many sediment records, we now know that since the last ice age (21,000 years ago), the climate has experienced at least one major climatic reversal to cold conditions. Our research over the last decade shows that the climate flipped rapidly in the northeastern US, where a warm mixed boreal-hardwood forest was replaced within 100 years by a cold boreal forest. The flip back to warm conditions was even more rapid, occurring within 50 years. Investigations on Kodiak Island, Alaska, reveal that the climate there also reversed rapidly at the same time, and a lush green coastal environment changed to a cold, dry tundra for about a thousand years. Do other continents reveal this change? Our final goal is to understand why these dramatic changes happened in order to better understand our climate system.

James Porter, University of Georgia
Corals, like most tropical marine organisms, are much closer to their upper lethal temperature than to their lower lethal temperature. As a result, global warming has brought a variety of unwanted stresses to coral reefs. Some reefs in the Florida Keys are experiencing a substantial loss of coral cover and biodiversity due to coral bleaching (the loss of their beneficial symbiotic algae caused by elevated water temperatures), and to disease (caused by a host of new pathogens). During extensive surveys throughout the Keys there has been a quadrupling of the number of stations exhibiting disease and a tripling of the number of coral species afflicted by disease. One locality, the deep (20m) reef at Carysfort Light, has experienced a 62% reduction of living coral cover during the three-year survey. Similar stresses from other marine environments create a chaotic pattern of response as natural environments respond to global climate change.

John G. Robinson, Vice-President and Director, International Conservation, Wildlife Conservation Society
The nature of conservation has changed with the realization that the "Nature" we seek to conserve is itself constantly changing and variable. No longer can we assume that natural systems are ordered and stable. This creates a challenge to those who seek to conserve natural systems within parks and reserves. How do you manage dynamic natural communities within parks, and for what do you manage? The challenge is compounded if climate change introduces additional pressure on parks, affecting the survival of individual species and the composition of the whole biological community. This presentation explores the utility of landscape approaches to conserve biological systems, which are themselves dynamic, in a world whose overall climate is also changing.

Cynthia Rosenzweig, Research Scientist and Head, Climate Impacts Group NASA/Goddard Institute for Space Studies
The New York City region is the quintessential urban agglomeration in the United States, with nearly 20 million people, or 7 percent of the nation's population. Currently, it is the fourth largest urban center in the world. How this region will respond to the challenges of global change may be seen as a bellwether for other large cities. The urgency stems from the rapid rate of global urbanization and growth in number of large-scale urbanized regions world-wide. The number of cities with 1 million or more residents is projected to increase from 782 in 1990 to 1657 in 2015.

Three interacting elements of New York City react and respond to climate variability and change: people (socio-demographic conditions), place (physical systems), and pulse (decision-making and economic activities). To understand these elements, five sector studies are projecting climate change impacts: Coastal Zone, Infrastructure, Water Supply, Public Health, and Institutional Decision-making. Each study is assessing potential climate change impacts through the analysis of the current conditions in the region, lessons and evidence derived from past climate variability, scenario predictions, critical issues, equity of impacts, potential non-local interactions, and policy recommendations.

Peter O. Thomas, Senior Conservation Officer, U.S. Department of State
In March 1999, a Department of State report concluded it was likely that anthropogenic global warming contributed to the extensive coral bleaching and mortality that occurred simultaneously throughout the disparate reef regions of the world in 1998. The geographic extent, increasing frequency, and regional severity of mass bleaching events are a likely consequence of a steadily rising baseline of marine temperatures. Even under the best of conditions, many of these coral reef ecosystems will need decades to recover. Human populations dependent on reef services face losses of marine biodiversity, fisheries, and shoreline protection. Trends of the past century suggest that coral bleaching events may become more frequent and severe as the climate continues to warm, exposing coral reefs to an increasingly hostile environment. This global threat to corals places greater urgency on our efforts to manage the entire range of human-induced threats to reefs, including climate change. Coral reefs are projected to be among the most sensitive ecosystems to long-term climate change; their response may foreshadow climate impacts on other ecosystems. Natural resource management agencies must consider their role in drawing attention to the predicted impacts of global climate change on the resources they manage. Even ecosystems granted well-enforced legal protection such as parks, sanctuaries, or sustainable areas may be made vulnerable by global climate change.

David Wilcove, Environmental Defense Fund
Global climate change occurs against a backdrop of other human-created stresses on biodiversity. Foremost among these are the loss and fragmentation of natural ecosystems and the spread of invasive species. To counteract the potential impacts of climate change, scientists have suggested that reserves be linked together by habitat corridors; that the reserve system be expanded to incorporate a wider range of latitudes; and that, wherever possible, reserves be situated along altitudinal gradients, so that species can move in response to changing climate. Unfortunately, ongoing habitat destruction, coupled with the spread of alien species, is likely to limit the possibility of following any of these recommendations. Given differences in topography and land ownership, efforts to create reserve networks capable of withstanding global climate change are likely to be more successful in the western United States than in the East.

Scott Wing, Research Curator, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution
There are no recent precedents for the rapid warming anticipated over the next 100 years, but there are examples in the geological past. Studying the fossil record is the one way we can investigate the effects of global climate change without waiting for it to happen. Fossils can be used both to test predictions made by computer models of global climate, and to gauge the effect of climatic warming on biological diversity and ecological structure.

Fifty-five million years ago at the beginning of the Eocene epoch there were forests and crocodilians at 80 degrees north, and palm trees grew in Montana, even though North America was farther north than it is today. Scientists have simulated Eocene climatic conditions with the same general circulation models used to predict future climate change. The model-based winter temperatures are consistently too cold, however, unable to match the equable climatic conditions that fossils document. This raises the question: if they can't predict the past, how accurate are their projections for the future?

We can also use the fossil record to study the effects of temperature increase on ecosystems. Geologists have identified a short interval of very rapid warming almost exactly 55 million years ago. Although the causes are still poorly understood, paleontologists have seen the results in everything from mass extinction of unicellular marine species to the intercontinental migration of mammals. The past is an important testing ground for our models of climate dynamics and the response of organisms to climate change.

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