changing climates increase the uncertainties of life for all organisms. A long-term warming trend would alter the distribution of life on the planet as colder habitats shrink and warmer ones expand. Some species would become more common, and others would become rarer. We cannot predict with any precision which species will become extinct—or when. Plants and animals that are highly adapted to already extreme (hot, cold, or dry) climates are most likely to be the first and most drastically affected.
We consider a species to be extinct once all known individuals of that type have died. Many interacting factors affect the survival of individual organisms, and therefore the persistence of their species. In general, extinction results when a species' requirements and abilities no longer match the resources and hazards in its environment. For animals, these factors include food, water, and shelter from predators and weather extremes. For plants, they include water and nutrients and the action of herbivores and pollinators. Sometimes a factor is critically important, like rainfall in a desert. It determines whether enough individuals will survive that a species can persist. If that "limiting factor" changes in some way, survival rates may rise or fall. If they fall far enough, extinction results. Climate is a major limiting factor for life on Earth. When it changes, life on Earth also changes. A continuing trend of global warming, cooling, or drying will lead to extinctions that might otherwise not occur as soon. We still know little about the precise climate limits or thresholds of most species. Because it is also difficult to predict precisely what the climatic conditions will be like in any given place at any given time in the future, it is even harder to predict which species will become extinct as a direct result of climate change, and when it will happen. In addition, because species interact and rely on each other in many ways, climate change produces many sometimes indirect or complex effects among them. This adds further layers of uncertainty to predictions about extinctions. For the most part, we can make only very general predictions about climate change and extinctions. This uncertainty leads many climate scientists and ecologists to conclude that humans would be wise to avoid or resist contributing to the uncertain risks of climate change whenever it lies within our power to do so.
climate, biogeography, and extinction
Climate change is complex. Tracking the local effects of regional or global change requires a great deal of data. Much of this information is now collected via remote sensing devices like radar and satellite-mounted cameras. So much data is collected that they can only be compiled into a usable form with very high-speed computers. However, those technologies are very recent. Scientists began collecting accurate and extensive climate measurements in the 18th century, recording data by hand. Naturalists like Alexander von Humboldt and H.C. Watson first correlated climates and species distributions in the early 19th century. Thus began the study of biogeography.
Long before there were biogeographers, it was evident that different kinds of plants and animals occupied different kinds of places. Biogeography added mathematical precision to the folk knowledge that temperatures were lower at higher elevations and higher latitudes and that mountain ranges received more precipitation to windward than to leeward and were warmer on the sunnier slopes facing the equator. More climate and biogeographical data became available at the same time that cartography and species inventories were improving. All were necessary for accurately describing what lived where and for predicting what sorts of species would live in various places. Repeated inventories, measurements, and mapping were needed to show whether and how bio-geography was changing.
Among the first patterns understood by biogeog-raphers was that average temperatures on the earth's surface changed with latitude. Temperatures tended to be low in polar regions and higher near the equator. At the same time, they saw that temperatures near sea level tended to be warmer than temperatures at higher elevations. They found that even near the equator, the tops of very tall mountains (such as the Andes) were as cold as the poles and discovered that the plants and animals of polar and alpine locations were very similar. As observations accumulated, bio-geographers were able to begin mapping the ranges of different species. Climate measurements helped biologists determine the limits of heat, cold, and precipitation that various species could tolerate.
It was long debated whether species actually could become extinct. It was not until large, easily observed birds like the dodo and the great auk could no longer be found and the fossilized remains of large, otherwise unknown animals were being discovered, that the fact of extinction was established. Not until the third quarter of the 19th century did it become clear that extinctions might regularly follow as the unintended consequences of intensive human activities. Climate changes traceable to human activities were hardly recognized for over another century, during which our population doubled twice over. During the roughly same period, our major technologies changed from being mostly animal, wind, and water powered to being combustion powered, using wood and fossil fuels.
Conditions on the Earth's surface and in its atmosphere have undergone many changes over time. Some of these changes were quite drastic and had proportionally drastic effects on living things. We can say with some confidence that the Earth's climate has sometimes been much warmer than it is today, and we know that at other times it has been much colder. This much can be inferred partially from recorded history but more reliably from fossils and other geological evidence. Scientists have proposed many plausible explanations for these climate changes, but since the events cannot really be modeled in detail or rep
licated for study, they can only agree about general effects, rather than local specifics.
Paleontologists and others who study the evolutionary history of life on earth have concluded that most of the species that ever inhabited the planet are now extinct. They have also estimated, as a sort of rule of thumb, that any given species, on average, persists for about a million years. Estimates of the total number of species that have existed on the planet range from tens of millions to hundreds of millions. Some of these species are known from the fossil record to have persisted much longer than a million years, and others for much shorter periods. Estimates and averages are only as good as the actual data and methods used to make them. Even if the data we have to work with are reliable, the fossil record is far from complete, and different methods of analysis continue to yield different estimates.
Because our planet is changeable, or dynamic, extinction seems to be normal and inevitable for species, much as death is inevitable for individuals. At the same time, however, evolution also generates new species from some of the old ones, as the average characteristics of a population change and "adapt" to emerging conditions. Overall, there is still life on Earth because the rate at which species evolve has exceeded the rate at which they become extinct.
No one is credibly predicting that all life on earth would end, and all species would go extinct, as a result of human-caused global warming, but many scientists are concerned that any continuing trend in climate change would increase the rate of extinctions, changing life as we know it and perhaps making life more difficult or less interesting for humans as a result. Many people want to preserve life as we know it and to prevent extinctions of other species caused by human activities. Global warming is one of many environmental changes human activities may bring about. The combined effects of human activities, along with those of geological and even cosmic events, are complex. Among the extinctions that occur during the foreseeable future, we will probably be able to blame very few solely, or even mostly, on human-caused climate change. However, if apparent trends continue, climate change will probably contribute in some way—large or small—to almost any extinction that occurs.
It is difficult to distinguish extinctions caused mostly or mainly by climate change from those caused by other factors such as directly converting habitats to human uses. Conservation biologists have long considered habitat destruction to be the most likely cause of extinctions. Habitat has been described many ways, but it generally means an environment in which enough individuals of a particular species can survive and reproduce to keep their population from decreasing to zero. In other words, each species has a habitat, and each needs a persisting habitat to continue as a species.
Some habitats are more complicated than others, but all habitats can be thought of as having two general kinds of components. Biotic components are living things: all the other organisms that somehow affect the life of a plant or animal. Abiotic components are factors like terrain, minerals, water, sunlight, and temperature. Climate change can directly affect some of the abiotic components of a habitat. When particular places become warmer or cooler, or drier or wetter, the ability of any particular species to persist in that place also changes.
Some abiotic habitat components, such as temperature and humidity, will vary daily or seasonally. Organisms have to be able to tolerate the extremes of night and day, summer and winter, and wet and dry seasons. When the climate of a place changes to the point that one of an organism's tolerances is exceeded, a habitat literally ceases to exist.
Because of the shape of the Earth, less sunlight reaches the poles than the tropics. Habitats are limited by the climatic effects of latitude. If we could look down from space at the North Pole and see all the way to the equator, but still recognize all the land plants and animals, we would see that similar kinds of organisms are roughly arranged in a series of bands or zones centered on the pole, like a target. Working out from the center, each zone is slightly warmer than the one immediately inside it. When the average global temperature falls, the polar center of the target expands and the hottest equatorial zones shrink or even disappear. This happened during the Ice Ages, when glaciers covered much of the Northern Hemisphere. Animals and plants had to change, migrate, or become extinct. When the average global temperature rises, the polar center shrinks, and each climate zone moves toward it. The icy center may even disappear, and the next zone takes its place. Meanwhile, entirely new, hotter zones may appear at the equatorial edge.
Because the atmosphere is less dense in the mountains than at sea level, all habitats are also limited by the climatic effects of elevation. Higher elevations are colder. Seen from directly above, a tall mountain has bands of similar plants and animals, just as the whole planet does. A general trend in climate change means that these bands move down and up the mountain just as the latitude bands move toward and away from the poles.
climate-related causes of extinctions
Extinctions can occur gradually or suddenly. Large numbers of extinctions have sometimes occurred during relatively short periods of time. These "mass extinctions" resulted when a catastrophic event such as an asteroid impact suddenly made large areas of the earth's surface, or its oceans, uninhabitable. The effects produced by such catastrophes probably included sudden, drastic climate changes, but not enough evidence has been found to say with certainty how great these changes were or exactly how long they lasted.
Changing climates affect the survival prospects of individual organisms. As a result, changing climates affect the survival and reproduction rates of whole populations and species. Populations may rise or fall as climates change. Some increases or decreases will be dramatic and obvious. Others will be almost unno-ticeable to us. Almost all such population changes will result from combinations of many small changes, rather than a few catastrophic ones.
As average global temperatures increase (or decrease), populations will migrate to follow shifts in local conditions. Some organisms can do this quickly and easily. Many animals already migrate to follow seasonal changes in food and water supplies. There are rare exceptions, but many individuals, such as rooted plants, cannot move at all. Their populations can migrate only as seeds are dispersed and new individuals germinate and survive in newly suitable locations. Meanwhile, the old individuals, trapped in increasingly unsuitable locations, gradually die out. When populations cannot shift to new locations quickly enough, species may become extinct. Extinctions also follow when no new locations become available or when potentially suitable locations exist but cannot be reached in time. We can easily imagine scenarios that include the extinction of plant species unable to disperse to new habitats. Because the phenomenon is so complex, scientists have been reluctant or unable to publish firm, reliable estimates of the numbers of species that could become extinct as a result of climate change, or to predict when such extinctions will occur.
Climate change is most likely to directly produce species extinctions in already extreme, barely surviv-able environments. These are the very cold, hot, wet, dry, or chemically unusual places in which only relatively few types of highly specialized organisms can exist. Where such extreme conditions are climate induced, even small temperature changes can be highly significant. Organisms in extreme environments are likely to be living near the limits of physiological possibility. When extreme environments become more extreme, some organisms die. When extreme environments become too extreme, nothing can survive in them—but that is only part of the story.
When extreme environments become more moderate, more species can move into them, leading to increased competition for living space and other resources. They may become "too moderate" for specialists that have lost, or perhaps never evolved, ways to compete or escape in highly diverse and densely populated environments. In other words, given a trend of global warming, hot, dry environments may become hotter and drier, crossing some survival threshold of survival for desert-adapted species. Individuals of those species will have to emigrate or die. However, some hot, dry environments might become wetter, or cooler, or both, even as average global temperatures are rising. This will allow species adapted to the new, more moderate conditions to immigrate and to compete with, prey on, or infect the existing populations in unprecedented ways.
direct effects of climate change on species extinctions
The direct effects of climate change are most likely to affect organisms of polar regions, mountaintops, and equatorial areas. Under a general trend of increasing temperatures, the very coldest climates—the Arctic and alpine tundras—could disappear, and along with them would likely disappear at least some of the species adapted to tolerate them. As the warmer habitats move toward the poles and up the mountains, their species will follow. Those that cannot migrate or disperse as fast as their potential habitats are shifting will either have to evolve new climatic tolerances or die out.
When abiotic factors change, some habitats may contract, even to the point of disappearing altogether. Others may expand, and new ones may appear. Overall, they can be imagined as flowing slowly across the landscape, expanding in some directions while retreating from others, sometimes forming and seemingly evaporating like puddles. The most obvious response for organisms that can move is to follow the changes in habitat or to find and occupy the new habitat. As long as enough individuals of a species can somehow keep up with these movements, their species may persist.
An expected direct effect of climate change with the potential for causing species extinction is a rise in sea level caused by the melting of polar and alpine glaciers. Large areas of low-lying coastal lands would be inundated by rising sea levels. In effect, some areas of terrestrial habitats would be converted to areas of aquatic habitats. Some low-lying oceanic islands would disappear, and along with them any land plants and animals that might be found nowhere else. Whether as a result of habitat inundation or other effects, species with very restricted ranges, called endemics, are likely to be more significantly affected by climate change than those with larger ranges.
Every species has different abiotic tolerances, so the edges of their potential habitats, based on moisture or temperature, rarely correspond exactly. Instead, these habitat edges usually overlap. Not only do they overlap, but climate change will affect each one differently, so different species habitats will move, grow, or contract at different rates. As we have seen, not all species are equally mobile. This means that two species may experience different direct, abiotic affects in the same place. These differences create the possibility of many indirect effects of climate change as species interact in new ways and places.
Most effects on species resulting from any continuing climate change trend will be indirect. All animals rely on other species as sources of food. Many plants rely on insects and other animals to pollinate them or disperse their seeds. When different species come to depend on each other in predictable ways, their relationship is called a symbiosis. Symbioses range from pure exploitation, where only one species benefits, to cooperation or mutualism, where both species benefit. In many cases, such as those of internal parasites or intestinal bacteria, one organism actually becomes the entire habitat of another. Far more often, individuals of different species have no obvious interactions at all but do influence each other in much more subtle ways, such as by preying on another species' competitors, or its predators, or its pollinators, or by spreading its disease organisms.
The possibilities for changing species interactions are seemingly endless, but we can describe only a few examples here. Individuals of predatory species might find themselves able to range farther north, or higher into the mountains, where they will encounter potential prey species that have never seen them before. These prey animals may lack defensive or escape behaviors, and their populations may be significantly reduced. This does not mean that tropical cats like jaguars will be decimating caribou herds. Most of the land animals in the world are insects, as are most of the predators. We are hardly aware of predation at the insect level, but it is cumulatively enormous and enormously influential.
Most insects are unable to regulate their body temperatures except by seeking shelter. Flying insects have to meet minimum temperature requirements before their muscles work efficiently, allowing them to lift off. However, flying insects are highly mobile. Once aloft, they are often carried great distances by winds, sometimes to places where they normally cannot survive. However, if climates warm and their habitats move and expand, insects are likely to arrive in any newly suitable locations pretty quickly. If these pioneering insects are herbivores, they may find plants that have evolved no defenses against them. This could hasten the demise of individual plants and reduce populations that were physiologically capable of tolerating the direct effects of warmer temperatures.
Many plants rely on insects for pollination. Some plant populations could be reduced if predatory insects begin to survive in areas formerly unavailable to them because of climate factors and begin preying on the local pollinators. If pollinators become too scarce, plant reproduction could be reduced to levels that cannot maintain a population. Both the plants and the pollinators could be affected.
Polar ice caps and alpine glaciers are composed of accumulating snow. If they melt, the resulting water is fresh, not salty. There is not enough fresh water in these sources to significantly dilute the world's oceans and change the fundamental chemistry of seawater. However, fresh water is less dense than salt water, and until the two mix, fresh water entering the oceans actually floats as a surface layer. The addition of massive amounts of cold, fresh water to the Arctic, North Atlantic, and north Pacific oceans, and to the south polar regions of the Atlantic, Pacific, and Indian oceans, would affect the way currents flow and nutrients circulate in these areas. This would affect the types and distribution of plankton, and thus all the many levels of oceanic food webs in those areas. Reduced plankton production would ultimately mean less prey for aquatic predators of polar seas such as polar bears, penguins, and some toothed whales, seals, and sea lions. Added to the direct effects of reduced pack ice, such changes could lead to the extinction of animals highly specialized for life under cold polar conditions that would no longer exist.
Hot deserts have fewer rivers, lakes, and ponds, but many of them have springs and small water courses that support endemic aquatic species including fishes, amphibians, reptiles, and many invertebrates and microorganisms. If these hot deserts become even hotter or drier because of climate change, these "oases" could literally dry up. In the process, numerous rare aquatic species that cannot move to other habitats (even if they existed) would become extinct in the process.
Aquatic species endemic to small tributary streams in any watershed face various new conditions when a region becomes drier or wetter. Neither trend is automatically beneficial. If it becomes drier, the smaller tributaries become ephemeral or intermittent, forcing fully aquatic species downstream into larger, more permanent waters, where they may encounter more (and larger) predators, at least for a time. If the region becomes wetter, the small tributaries will become larger, and the larger predators may move upstream. In high, steep terrain, the physical characteristics of the newest small tributaries may make them unsuitable for colonization.
In wet tropical areas, the effect of climate change will most easily be seen if it results in changes to the flow of atmospheric moisture to the region and, as a result, to the seasonality and overall amount of precipitation. At its simplest, a rainforest with less rain will gradually become another kind of forest, having fewer species requiring high moisture or seasonal inundation by floodwaters and more that tolerate drier conditions. As in all cases, if suitable habitat disappears or appears only at an unreachable distance, some species could become extinct. The complexity and diversity of tropical forests is such that not only some tree species, but also their dependent animals (and, in turn, their own dependent animals), could become extinct in the process. Our knowledge of the flora and fauna of these regions is insufficient to support any precise estimate of the number of species present, much less the number that could be affected by any particular degree of climate alteration.
climate-related extinctions involving other human activities
In anticipation of a continued warming trend in polar regions, various countries are already positioning themselves to take advantage of ice-free Arctic seas and increasingly temperate high latitudes. Others are bracing for possible desertification in tropical grass and scrublands. Areas likely to experience intensified human use may have higher likelihood of species extinctions.
Increasing commercial ship traffic in Arctic waters would produce the same sorts of side effects that shipping has elsewhere. Leaks and spills of fuel and cargo oil would affect the biota of littoral zones. Ballast water exchange would further redistribute aquatic species, leading to new predation and competition among aquatic species without prior experience of each other.
Newly ice-free Arctic lands would potentially become available for mining, oil and gas development, and allied manufacturing activities. This will require an influx of workers and equipment, along with creation of the physical, economic, and cultural infrastructure needed to support them. Each activity entails a direct conversion of some existing habitat to human use. This could fragment the habitats of migratory birds such as snow geese and affect survival rates for large mammals such as caribou and musk oxen.
More ship traffic, mining, and oil exploration would encourage more permanent human settlements to service, and be serviced by, these industries, leading to a greater likelihood of chemical pollution and of sewage and solid waste management issues. Human population centers would encourage the establishment of human commensals and inquilines, such as dogs and cats, rats and mice, cockroaches and house-flies. Each potentially adds a new challenge to the persistence of Arctic species.
Under a continued warming trend, farmers in Europe, Asia, and North America would experience the same northward and upward shift in habitat bands affecting uncultivated plants and wild animals. For example, grain production will likely be possible farther north, in the Canadian "Prairie Provinces." This would require "sodbusting" of existing grasslands or logging of forests to convert them into farms, reducing or eliminating their habitat value to most wildlife. All the world's major crops—corn, soybeans, wheat, rice, and cotton, along with most every other valued plant—would become economically viable in new areas while becoming impractical in others where they have been traditionally grown. The net effects on agricultural production are hard to estimate, as are the potential effects on other species.
SEE ALSo: Agriculture; Animals; Antarctic Circumpolar Current; Arctic Ocean; Atlantic Ocean; Biology; Botany; Cetaceans; Climate Zones; Conservation; Desertification; Deserts; Ecosystems; Geography; Glaciers, Retreating; History of Climatology; Ice Ages; Indian Ocean; Land Use; Marine Mammals; Modeling of Ocean Circulation; Modelling of Paleoclimates; Oceanic Changes; Pacific Ocean; Penguins; Phytoplankton; Plants; Polar Bears; Rainfall Patterns; Sea Level, Rising; Upwelling, Coastal; Upwelling, Equatorial.
BIBLIoGRAPHY. Miguel B. Araujo, et al., "Reducing Uncertainty in Projections of Extinction Risk from Climate Change," Global Ecology & Biogeography (v.14/6, November 2005); Thomas J. Crowley and Gerald R. North, "Abrupt Climate Change and Extinction Events in Earth History," Science (v.240/4855, May 20, 1988); Malte C. Ebach and Raymond S. Tangney, eds., Biogeography in a Changing World (CRC Press, 2007); Susan Joy Hassol, Impacts of a Warming Arctic: Arctic Climate Impact Assessment
(Cambridge University Press, 2004); Robert L. Peters and Thomas E. Lovejoy, Global Warming and Biological Diversity (Yale University Press, 1992); Charles L. Redman, ed., The Archaeology of Global Change: The Impact of Humans on Their Environment (Smithsonian Books, 2004); Chris D. Thomas, et al., "Extinction Risk From Climate Change," Nature (v. 427, January 2004); Richard L. Wyman, ed. Global Climate Change and Life on Earth (Routledge, Chapman and Hall, 1991).
Matthew K. Chew Arizona State University
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