Climate is one of the most difficult things to predict. But one thing is certain: man�s effects on it have been enormous and the future will be problematic.

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FIVE

THE NEAR FUTURE

A New World

Can we predict what the future course of evolution may be? It is sometimes tempting to make fanciful conjectures about the nature of future species, but it is also generally nonscientific. Trying to predict the shapes, colors, and appearances of new species would be fantasy, not science. Yet it is possible to make other types of predictions, based on what we know through the study of the evolutionary record.
The first thing we can be sure of is that following the current mass extinction there will be empty ecological niches, and these niches will be filled by newly evolved species. But which species will fill a given niche � here is where a crystal ball is necessary. Stephen Jay Gould has long argued that chance will be the major arbiter in deciding which species will replace a newly extinct taxon. For example, perhaps the extinction of rhinos and elephants will trigger the evolution of some group of antelope species toward gigantism to fill the gap, or perhaps the replacement will come from domestic horses � which it will be is mostly a matter of chance. Yet other evolutionists are not so sure that Gould is correct in this view. Paleontologist Michael McKinney (among others) has argued that the best chances of filling the new niches belong to what he calls supertaxa, species belonging to

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groups that are themselves composed of numerous species. Examples of such groups are the rodents, snakes, and passerine birds � all of which are extremely species-rich. McKinney pointed out that since these groups are generally composed of generalists rather than specialists, their members are abundant � and that the same traits promoting numerical dominance also lead to an ability to diversify rapidly over long periods of time. Another characteristic of this group is small body size.
Second, predicting the makeup of any future biota requires an understanding of what the new range of habitats on Earth will be. While the emergence of humanity as the dominant species on Earth has changed things such as the degree of gene flow between once isolated populations and the commonness of alien invasions, perhaps the biggest change has been in the nature of habitats. Humanity has transformed the Earth�s surface by producing physical habitats that have never existed before. Through the emergence of megacities, the changeover from old-growth to managed agricultural forests, the spread of agricultural landscapes, the fragmentation of native landscapes by roads, changes in the ecology of the oceans due to the disappearance of large fish, mangrove, coral reefs, and seagrass beds, and the chemical makeover of land and water habitats with pesticides and other chemical pollutants, humans will undoubtedly have a marked effect on future evolution. Natural selection will produce new varieties of life to deal with a set of new environmental conditions never before encountered on the planet.

Cities

In the late 1970s I flew from the Yucatan Peninsula to Los Angeles, with a stopover in Mexico City. While Los Angeles was well known to me at that time, I had never been to Mexico City, and looked forward to the experience.
Our plane lifted from the lushly verdant Yucatan on a luminous day, and we flew over a starkly visible Mexico. The flight was not very long, and a vista of mountains and forests passed far beneath us. Eventually I spotted a distant mountain, larger than the others, and as we approached I was filled with wonder. Never had I seen a mountain like this before, perfectly dome-shaped, brown in color, impossibly tall, a vision that enlarged and degenerated into implausibility. Our pilot headed straight toward the summit of this great mount, and just as we were about to crash into it, I realized what it was: the air over Mexico City, a mountain of pollution covering the huge sprawl below. Even in the 1970s Mexico City may have been the world�s largest city, and it was a sure vision of the future,

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of what too many poor humans in one place can produce in what we now call �megacities�.
Very few animals transform their habitat as extensively as Homo sapiens does, and of all our transformations, perhaps none are so visible as the formation of cities. While many of humanity�s changes, such as deforestation and the planting and maintenance of agricultural fields perturb and then change one type of biological system into another, the building of cities is a widespread transformation of the organic into the largely inorganic. Termite nests, prairie dog towns, and a few other examples are slight intimations of this process, but true concrete jungles have of course been altogether unknown prior to ours.
Humans had been building up to large-scale cites for millennia, but the advent of the Industrial Revolution changed the nature of cities forever. Once places where trade was centralized and people lived, cities became, during the nineteenth century and throughout the twentieth century, the places where factories and industries were located. The effect was to bring pollution into the bedroom. Ebenezer Howard, an early urban planner, described these new cites as �ill ventilated, unplanned, unwieldy, and unhealthy�. The French architect Le Corbusier was more poetic in his denunciation: �They are ineffectual, they use up our bodies, they thwart our souls�.
Urbanization is clearly transforming the Earth. At the dawn of the twenty-first century over half of humanity lives in urban areas. By 2030, demographers estimate that twice as many people will live in urban areas as in rural regions. As the twentieth century comes to a close, cities and urban regions occupy about 2% of the Earth�s land surface. Yet they use some 75% of the Earth�s resources, and release a concomitant amount of waste material. They are not only centers of human population, but centers of human production and consumption.
Urbanization has five major effects: it produces an increased demand for natural resources in the area surrounding the city; it obliterates the natural hydrological cycle at the site of the city; it reduces biomass and alters species composition in and around the city; it produces waste products in high concentrations that can alter the environment around the city, and it creates new but altered landforms through landfilling and reclamation. What is �reclaimed�, of course, is generally natural wetlands or lakes. Cities replace natural forests, grasslands, and other vegetation with concrete and brick. These changes vastly affect the flow of water through the site of the city, generally causing water movement to accelerate.
Canadian economist William Rees has brought attention to the concept of the

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�footprint� of cities � the area of land required to supply them with food and timber products, as well as the area (and plant growth) necessary to absorb the carbon dioxide output they generate. One such �footprint�, for London, was calculated by Herbert Girardet of Middlesex University in Great Britain. London was the first of the �megacities�, being the first urban area to attain a population of a million people. Using the analyses pioneered by Rees, Girardet calculated that the footprint size of London is 125 times its surface area. The city covers 159,000 hectares, and its footprint is thus 20 million hectares. This is larger than all of the productive agricultural land in Great Britain put together � for just 12% of Great Britain�s total population.
All of the key activities of cities � transport, heating, manufacturing, the generation of electricity, and the provision of services � rely on a steady and regular supply of fossil fuels. London requires 20 million tons of petroleum each year � and in the process of converting that petroleum to energy, discharges 60 million tons of carbon dioxide into the atmosphere. Every day London disposes of nearly 7,000 tons of household waste.
The urbanization process has been accelerated by economic, political, and biological factors. The liberalization of political systems around the world in the closing decades of the twentieth century resulted in a surge of economic activity that was accompanied by urban growth. Unfortunately, many cities are decaying as they grow, or cannot keep pace with population increases to provide sanitation, food, and chemical-free environments. Thus urban poverty is driving many of the changes found in cities today. Current growth projections suggest that there will be a minimum of 100 �megacities� with populations of over 5 million people by 2030. These environmental anomalies will have profound effects on both local and, ultimately, global climate.
Climate statistics for recent decades have shown that most cites are warmer than the countryside surrounding them. Thus the boundary between the countryside and city forms a steep thermal gradient surrounding an urban heat island. The heating of cities is a product of several factors. The first is the absorption of solar energy. Most roads and many city roofs are made of dark material that absorbs more solar energy than the surrounding countryside. These city surfaces also have a high capacity for heat storage; concrete and tar roof surfaces both store heat by day and release it at night to a far higher degree than does a vegetated land surface. Second, cities are warm because they generate a great deal of artificial heat through their energy output. Finally, the concentration of large numbers of people and

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machines in cities causes marked changes in air quality. The release of huge amounts of carbon dioxide and other greenhouse gases that may not readily dissipate out of the city center itself provides an insulating blanket around the city core.
One of the most salient effects of cities is their formation and accumulation of waste. In earlier times the solution was simple: dump the garbage in the poorer neighborhoods. While this system has changed in more developed areas, it still goes on in poorer countries. Perhaps the most striking example of this practice was in the Philippines. In the 1950s Manila began to dump its escalating volumes of waste in a particularly poor neighborhood. The mini-mountain of trash was named Mt. Smokey because of the haze from burning methane, and it towered 130 feet above sea level in a city built at sea level. Even more striking than its size, however, was the biomass of humans that this heap of garbage sustained. In the early 1990s it was home to some 20,000 people who made a living out of sifting through and recycling the waste and eating its remains; they lived there, fed there, and bred there.
The vast amount of waste material generated by cities is transforming the Earth, and just as surely promoting new strains of evolutionary development. Open pits and piles of waste are breeding grounds for human pathogens, but even more so, they are habitats for legions of insects, birds, and small mammals living off the abundant foodstuffs. It is estimated that New York City receives 20,000 tons of food each day for its human inhabitants. About half is transformed into human energy; the rest becomes human sewage and �wastage�. This vast resource is an evolutionary target of opportunity for the animals that are now exploiting it. It is clear that 10,000 tons of food material a day was not appearing in the small area now known as New York City prior to the presence of the city; its relatively sudden appearance (by the standards of geologic time) is a sure stimulant for exploitation, and might spur evolutionary change as well, so as to optimize that exploitation.
The first consequences of evolutionary change within this system are probably behavioral � the aspect of evolution that is least visible but probably among the fastest to appear through natural selection. The body of a rat or a cockroach is admirably �preadapted� for living in this new habitat, the garbage dump, and may need little morphological, or body plan, adaptation to exploit it. Yet the new challenges of city living have undoubtedly affected the genetically coded behaviors of these animals, and will continue to do so. Since for the most part we view the animals living off our waste and garbage as pests, we do our best to exterminate them. Any behavior that staves off this fate will be selected for and quickly incorporated into the genetic systems of those animals. We may not necessarily see new species

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Some fauna have adapted surprisingly well to inhospitable urban landscapes, and will continue to do so � but they will be fewer and fewer.

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of city-dwelling organisms as the centuries pass (although we may), but there will be consequential evolution taking place nevertheless, much of it behavioral.
Behavior is not all that will be changed; physiological adaptation will certainly be necessary for city inhabitants as well. A consequence of cities and their wastes is the presence of toxins. Even the most expensive methods of waste disposal, such as high-tech incinerators and so-called �sanitary� landfills, generate toxins. The most important among these are heavy metals, chlorine compounds, and dioxin, all found even in incinerator ash. These compounds and many others make their way into groundwater systems and thereby enter the ecosystem of the city. Animals inhabiting cities, and especially those living in areas with high concentrations of toxins, such as sewers, groundwater systems, and at the base of landfills, might undergo adaptation to withstand high levels of otherwise lethal chemicals, high acid or base concentrations, and even the elevated temperatures found in smoldering landfills.
The ultimate changes, however, may be morphological. We may well see the evolution of a bird beak specialized for feeding out of tin cans, or rats developing oily fur to slough off toxic wastewater. Similarly, new breeds of house cats might evolve larger size to deal with more ferocious rats. But might something completely different evolve? Could we see the evolution of an animal specializing in the most obvious resource of all: human beings?

The Future Global Climate

Let us imagine a world of long ago, of very long ago: the world of 750 million years ago. It is a time when the first animals are just appearing. It is also the time of �snowball Earth�.
The discovery that, at several times in its history, the Earth was covered from pole to pole with ice is one of the major geologic finding of the late twentieth century. The just-completed Ice Age pales in comparison to these long-ago times. Ice locked Earth in its grip, covering both land and sea. There was virtually no life on the planet, save for a few oases of warmth next to undersea volcanoes. The discovery that these �snowball Earth� events occurred not just once but repeatedly, albeit long ago, shows but one swing of the messy pendulum we call climate. It is also a lesson in how extreme climate change can be � and perhaps soon will be.
No one disputes that humanity is rapidly changing the composition of the atmosphere, although there is still great debate about whether or not those changes are causing a rise in mean global temperature, also known as global warming.

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Anthropogenic, or human-induced, production of gases such as carbon dioxide, methane, chlorofluorocarbons, sulfur dioxide, and nitrogen oxides have been rising dramatically since the Industrial Revolution. All of these gases have the ability to absorb infrared radiation and reradiate it back to Earth, producing the well-known �greenhouse effect�. To better understand the conditions that will face life in the future, we must better understand what the gas inventory of the atmosphere will be.
As we all know, predicting the weather is a chancy business. Trying to make valid long-range predictions for the next several days is hard enough. Doing the same for the next few thousands of millennia seems impossible. Yet in some ways the long-term view is clearer than the short-term view. Almost all scientific information to date suggests that global warming will be a long-term reality.
Predictions about the possibly of global warming over the next few decades and centuries come from a class of models known as General Circulation Models (GCMs). A starting point of these models is the prediction that the amount of carbon dioxide in the atmosphere will double over the next century. This doubling is sure to have profound ecological effects, including greater temperature increases in mid-latitude temperate and continental interior regions than across the rest of the globe, decreases in precipitation in these same mid-latitude regions, and an increase in severe storm patterns.
Such changes will affect the entire biosphere, but will have their most marked effects on plant communities. Because there is so much paleontological information about how plant species and communities fared during the rapid climate changes accompanying the end of the Ice Age over the past 18,000 years, there is some room for optimism that reasonable projections about oncoming climate changes can be made.
According to paleobotanists, four prime lessons from the near past are applicable to the near future. First, it seems that species, rather than whole communities, respond to climate change. Over the past 18,000 years, the species compositions of various North American forests have changed considerably, yet the forests themselves have persevered. Whole communities and biomes do not respond to climate change, but instead change their species compositions. Second, the responses of individual species to climate change are often accompanied by a time lag. Especially rapid climate change tends to overwhelm many plant species because they are incapable of dispersing rapidly enough to move with the changes. For example, the eastern hemlock tree can disperse at a rate of 20-25 kilometers per century. However, climate patterns can move at a rate of over 300 kilometers per century.

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The net result can be local extinction of a species if climate change is sufficiently fast. The third insight is that patterns of local disturbance will change as climate changes. Fire is one of the principal causes of disturbance in modern forest ecosystems; as climate changes, the pattern and frequency of major forest fires will change as well. Changes in such disturbance patterns may produce a greater change in an ecosystem than the climate change itself. Fourth, it seems that multiple environmental changes can produce unpredictable responses in ecosystems. If enough sources of change come to bear on a given ecosystem, its responses may not be predictable. We may be on the verge of seeing the formation of terrestrial plant communities unlike any that have existed in the past � not through the formation of new species (although that may happen as well), but through novel compositions of groups of species that have no ancient community analogues.
Another factor will be the response of plants to increased levels of carbon dioxide (CO₂) in the atmosphere. Many plants increase their photosynthetic activity and growth rates in response to elevated amounts of CO₂. A result of increased CO₂ levels will therefore be greater global plant productivity, faster growth, and perhaps larger plants of some species. On the other hand, there are distinct differences among plants in their responses to raised levels of CO₂. Some plants (using an enzyme system known as the C₄ metabolic pathway) are already saturated with CO₂ in the present-day atmosphere, and will not respond with faster growth or productivity if CO₂ is elevated globally. A second, more common group of plants (those using the so-called C₃ metabolic pathway) will respond to the increased CO₂ with enhanced growth. There are other determining factors as well. Plants living in high-stress conditions and those from highly disturbed habitats will show little effect, while plant species that are stress-tolerant will do better. Perhaps it will come as no surprise that the winners in a new, CO₂-rich environment may be weedy species.
Although CO₂ levels will have an important effect on plant community compositions and growth rates, by far the single most significant factor affecting plant community composition and growth is water availability. There is an enormous amount of variation among plant species in their ability to withstand drought, so future patterns of precipitation and runoff around the globe will affect the makeup of plant communities most. As global temperature changes affect water distribution across the planet, plants will be forced to adapt to rapidly changing conditions.
In his chapter �Appreciating the Benefits of Plant Biodiversity�, from the final twentieth-century installment of the best-selling series The State of the World,

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botanist John Tuxhill suggests that the first signs of changing carbon dioxide levels are already being observed in tropical rainforests. Tuxhill notes that the �turnover rate� of tropical forests � the rate at which old trees are replaced by younger trees � has been increasingly steadily since the 1950s. As a result, the forests under study are becoming �younger� through increasing domination by shorter-lived trees and woody vines that grow faster than the tall hardwood trees that make up the old climax communities. Such trends will favor a radical changeover in the species composition of the tropical forest. Tuxhill also notes:

Under these conditions, can we expect substantial future evolution in forest communities? Since a very high diversity of plants has already evolved, there are probably many species �preadapted� for the new conditions that are being produced now by global atmospheric change. While one can speculate and dream up new plant species evolving to take advantage of higher carbon dioxide levels, the reality is that very little new evolution may occur within the dwindling forests of the planet.

The Oceans

What separates the current mass extinction from those of the past is that little or no extinction has yet occurred in the oceans, and that changes in temperature, toxicity, and other environmental factors there have been minor compared with those on land. But for how long?
While the oceans have not undergone an equivalent of the megamammal extinction characterizing the last 50,000 years on land, it would be a mistake to assume that some extinction has not occurred. The exploitation of fisheries stocks has not eliminated more than a handful of species, but its effects, from the large-scale disappearance of whales and other marine mammals to the reduction of the large fish species used for human food, have utterly transformed the biological makeup of the oceans and the way in which energy flows through its communities.

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Although the great British zoologist Thomas Huxley opined that �all the great sea fisheries are inexhaustible�, the results of a century of exploitation contradict that statement. The reduction of large carnivores in the sea represents a radical restructuring of the single largest habitat zone on Earth. Perhaps this restructuring will instigate future evolution, but can any outburst of evolution occur while fishing pressure exists?
Humans depend on the oceans for food, raw materials and minerals, and transportation lanes, and the strains of those uses are showing. It is estimated that the proportion of marine fisheries stocks that are overexploited has climbed from almost none in 1950 to between 35% and 60% as the twentieth century came to a close. The most pressing threats to the oceans, according to the 1,600 scientists contributing to the United Nations 1998 Year of the Oceans program, are species overexploitation, habitat degradation, pollution, climate change, and species introductions. As one of these scientists put it, �Too much is taken from the sea and too much is put into it�.
The use of fisheries stocks at the end of the twentieth century was staggering. The world�s human population received 6% of its total protein, and 16% of its animal protein, from the sea, and over a billion people relied on fish for at least 30% of their animal protein supply. Up to 90% of this catch comes from coastal zones (which also supply at least 25% of the Earth�s primary biological productivity).
The major fish stocks showing a marked decline in catch totals include sharks, tuna, swordfish, salmon, and cod. As these stocks decline, new species are exploited in their place. During the 1980s five low-value species � Peruvian anchovy, South American pilchard, Japanese pilchard, Chilean jack mackerel, and Alaskan pollock � accounted for the majority of new landings. Moreover, the efficiency of fishing is now such that formerly prosperous regions of the sea are becoming biological deserts. The once rich Grand Banks off Canada are now bereft of the cod that were once so abundant; the king crab fishery in Alaska has collapsed; the orange roughy fishery of the South Pacific is essentially nonexistent. Trawling � the practice of dragging nets and chains across the ocean bottom � is now so widespread that it is estimated that every bit of the world�s continental shelves is dragged at least once every two years.
The rising human population is also affecting coastal zones. Two-thirds of the world�s largest cites are located on seacoasts, and the environmental effects of these burgeoning human populations are radically changing the seas. The destruction of seagrass beds and mangrove regions to allow human settlement has had a marked

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effect on fish stocks, since these habitats are breeding grounds for many important species.
The seas are also the final resting places of most anthropogenic pollution. River systems dump waterborne waste into the sea; winds carry airborne pollution into the sea; excessive nutrients from nearby cities create dense carpets of algae that ultimately rot in the sea. Such algal blooms take oxygen out of the water and create large �dead zones�. Much of the Gulf of Mexico is now afflicted by such dead zones. Other consequences of nutrient loading in the oceans are an increase in red tides and an increase in paralytic shellfish poisoning. Synthetic organic chemicals also end up in the sea, as do radioactive materials and heavy metals such as mercury.
All of these factors make the oceans one of the most potent cauldrons for future evolutionary change. Yet of all of the ecosystems on Earth, the oceans may see the least amount of species-level extinction and, perhaps paradoxically, the most new evolution. The reasons are several. First, despite our best efforts, even completely overexploited and subsequently �crashed� fisheries stocks do not go extinct. But they do not recover as long as fishing continues � and as long as there is a large human population, there will be overfishing. At the same time, the vast size of the oceans and their lack of native habitability by humans will always provide a buffer for marine creatures. Try as we will to invade them, the oceans will always be far less perturbed than the land. Thus, as new species are formed (beginning, always, as tiny isolated populations), there is less chance that human intervention will immediately stop the new speciation process.
Second, the removal of top carnivores � the species most exploited by humans � will leave a void that will be filled by natural selection and new speciation. Although humans exploit the upper parts of marine food webs, the lower trophic levels are barely touched. Humans do not exploit, for example, the copepods, small worms, and other invertebrates making up the majority of the ocean biomass. New species will evolve to fill the vacuum created by the drastic reduction in numbers of commercial fish stocks.
What will evolve to take the place of the larger fish species? Because fish, according to the fossil record, appear capable of evolving rapidly, it may be that the new species will be other fish. But if large fish species evolve to replace those reduced or rendered extinct by overfishing, the same trend may happen again, and they will become the overexploited. It is more likely that either many small fish species will evolve, or the positions in the upper parts of the marine food chain will be filled by larger invertebrates.

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Agricultural Fields

The largest single habitat type on the surface of the land will soon be agricultural fields. Most of the ancient forests and the drier grasslands and savannas of the Earth have been, or are in the process of being, converted into farms, and this conversion will be a major contributor to new evolutionary events. But if farmers� fields predominate, a second major habitat type increasing in size will be deserts. Quite often, fields turn into deserts through poor agricultural practices and reductions in water availability.
By the late twentieth century the option of expanding grain production by cultivating more land had virtually disappeared. From 1950 to 2000, increases in the harvest of grain came from the conversion of forest and native grassland into grain-producing fields, but this option has been exhausted. The few regions left to exploit include the cerrado of Brazil, a semiarid rangeland in the east-central part of the country, the area around the Congo River in Africa, and the outer islands of Indonesia. At the same time, vast areas currently used for growing grain will be lost to human housing, or through soil erosion and land degradation. The amount of cropland per person on Earth is expected to decline from 0.23 hectares in 1950 to 0.12 hectares in 2000, and to 0.07 hectares by 2050. The area of cropland in India, for example, will not rise, but it will have an estimated additional 600 million people to feed by 2050. By the same time China will need to feed a total of 1.5 billion people.
The winners in the agricultural environment will be insects, rodents, and predators on both. As in the case of domesticated animals, it is likely that a great deal of evolutionary change has already occurred since the inception of agriculture nearly ten thousand years ago, unremarked by early humans. A taxonomist assessing the insect and rodent makeup of the world prior to the start of human agriculture might be surprised at how many species that are common today did not exist then. Rodents are known to have some of the fastest evolutionary rates on Earth; a thousand years is more than sufficient time to create new species, and the ten thousand years since agriculture began may have seen a vast proliferation of small animals living among the crop rows. The same process has surely occurred among insects, perhaps on even a vaster scale than among the rodents. Because animals of this size are not readily observed or perturbed by human mitigation efforts, the surge of evolution is likely to continue. Armies of new ant, beetle, and rodent species seem a probability.

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The Human Population

Ten thousand years ago, there may have been at most 2 to 3 million humans on Earth. There were no cities, no great population centers; humans were rare beasts, scattered in nomadic clans or groups, or at best in settlements of little lasting construction. There were fewer people on the entire globe than are now found in virtually any large American city. By two thousand years ago that number had swelled almost a hundredfold, to 130 million or perhaps as many as 200 million people. The billion mark was reached in the year 1800, and there were 2 billion people by 1930, 2.5 billion in 1950, 5.7 billion in 1995, and approximately 6.5 billion in 2000. At this rate of growth, the human population is expected to exceed 10 billion sometime between 2050 and 2100, assuming an annual increase of 1.6%. While this rate is somewhat reduced from the 2.1% growth rate characterizing the 1960s, it remains a staggering figure.
In 1992 the United Nations published a landmark study calculating potential human population trends, which arrived at several estimates. By 2150, the human population could reach about 12 billion, if human fertility figures fall from their present-day levels of 3.3 children per woman to 2.5 children. If, however, the faster-growing regions of the world continue to increase in population and maintain their current fertility levels, average fertility worldwide will increase to 5.7 children per woman, and the human population could exceed 100 billion people sometime between 2100 and 2200. The latter figure seems beyond the carrying capacity of the planet. Officially, the United Nations uses three estimates for the year 2150: a low estimate of 4.3 billion, a medium estimate of 11.5 billion, and a high estimate of 28 billion.
Predicting future population numbers is a difficult endeavor because of the many variables involved. The definitive work in recent times is Joel Cohen�s 1995 How Many People Can the Earth Support? Cohen�s conclusions are stark:

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There are, of course, many reasons why the higher figures may not be reached. Human disease, such as HIV or some other pathogen, may affect these figures; famine or war could also markedly reduce them. Barring such calamities, our population of approximately 6 billion humans at the turn of the millennium will at least double in slightly more than a century to a century and a half. Once this figure of approximately 12 billion humans is reached, it is assumed that the population will stabilize.
More than 200 years ago, the British scientist Thomas Malthus described the single most intractable problem with human population growth. While our population numbers increase exponentially, human food supply tends to increase on a linear scale as more land is devoted to agriculture. The inescapable conclusion is that the human population will tend to outgrow its food supply. In a related fashion, the human population is likely to outstrip its supply of untainted and unpolluted fresh water.
Water may indeed be the most critical factor in determining the maximum human population that the Earth can support. While the Earth�s stock of water is immense, most of it is salt water held by the oceans. The amount of fresh water is far less � only a small percentage of the total. Moreover, about 69% of that fresh water is locked in glaciers, permanent snow cover, or aquifers more than a kilometer deep, all inaccessible to humans. About 30% is present as accessible groundwater, leaving 0.3% in freshwater lakes and rivers. This totals about 93,000 cubic kilometers of fresh standing water on the Earth�s surface. This water does not stay in place, however: it evaporates into the atmosphere or sinks into groundwater stocks. Thus, a total of between 9,000 and 14,000 cubic kilometers of renewable fresh water is available for human agriculture each year.
Humans use water for more than agriculture. People drink about 2 liters of water per day in temperate climates, and perhaps three times this amount in arid climates. But drinking is the least of human water consumption. In a developing country all household uses � including cooking, consumption, and washing � amount to about 7 to 15 cubic meters of water per year per person. The average person in a developed country uses twice this amount. Yet these figures pale when the amount of water needed to feed each person on Earth is calculated. It takes approximately 200 tons of water per year per person to raise sufficient wheat to maintain that person on a �model-skinny� (a.k.a. starvation) diet. This translates to about 350 to 400 cubic meters of water per person per year � a whopping 300 gallons per day. Eating meat requires even more water. If 20% of the diet comes from animal (meat and dairy) products, about 550 cubic meters of water per person per year are

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Some fauna have adapted to a more aquatic though degraded setting, as in this new freshwater �tree of life�.

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required, whereas the typical American diet requires 1,000 cubic meters of water each year to produce.
Water specialists have calculated maximum, median, and minimum estimates for the amount of water available to agriculture: the maximum figure is 41,000 cubic kilometers of water, the median 14,000, and the minimum 9,000. Assuming that all of that water is used for human food production and consumption, the high figure would sustain a global population of between 25 and 35 billion people on the American diet, and between 100 and 140 billion at near-starvation levels. Yet assuming that all the available water can be used for agriculture is ridiculous; about 80% is actually used for other purposes, especially industrial uses. A more reasonable estimate is that 20% of the total water volume is available for agriculture. With this figure, the world can support at most between 5 and 7 billion people on the American diet, assuming that 41,000 cubic kilometers of water are available, and only 1.1 to 1.6 billion assuming the lowest figure of 9,000 cubic kilometers. Even on a starvation diet, the world can support between 20 and 30 billion people assuming that the maximum projection of water availability is correct, but only between 4 and 6 billion if the minimum figure is correct. Thus the current human population of the Earth may already exceed its carrying capacity based simply on water availability.

Parasites and the New Manna

Seldom has the world seen a more striking transformation: in little more than three hundred years, the majority of the biomass found in terrestrial animals has shifted from many species to only a few. The most prominent of these new winners are humans and the domesticated animals bred to feed them. Since evolutionary forces tend to respond to the presence of new resources, we might expect prodigious amounts of new evolution among human, cow, sheep, and pig parasites.
The evolution of parasites is usually in lockstep with the evolution of new host species. Parasites require particular adaptations for the host bodies they inhabit. Humanity has long been present on Earth, and we have long had our share of parasites. But the huge population increase since the end of the eighteenth century has created large human populations in regions where few people once existed, especially in the more humid and torrid tropical regions, and a consequence of this change has surely been natural selection for more, and more efficient, human

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parasites. While the same trend should be creating new human predators as well as human parasites, the successful evolution of an efficient new human predator is a long-term, and ultimately futile, process: as soon as we humans get wind of any evolutionary change putting ourselves, and especially our children, in harm�s way, we will institute immediate and surely successful eradication efforts. Killing new parasites, however, is a far more difficult endeavor, especially those of very small size, such as microbial forms. It is in this arena that some of the most interesting and fecund new species of the coming biota may be found.

Toxicity

The agencies of humankind are rapidly changing the chemical makeup of the surface of the Earth and its waters and oceans. Most of this chemical change comes from human-induced pollution, the result of municipal, industrial, and agricultural wastes. These wastes can be specifically categorized as nutrients, metals, and synthetic and industrial organic pollutants. All of these pollutants pose challenges to living organisms, and it is certain that future evolution will in some cases be triggered by reaction and adaptation to new levels of these substances.
Nutrients such as nitrates and phosphates are the cause of eutrophication, an explosion of biological activity. Anthropogenic sources of these substances include synthetic fertilizers, sewage, and animal wastes from feedlots. Much of this nutrient pollution makes its way into rivers. A. Goudie and H. Viles, in their book The Earth Transformed, suggest that nitrate and phosphate levels in English rivers have increased between 50% and 400% in the last 25 years alone.
Metals, the second major class of pollutants, occur naturally in soil and water, but their natural concentrations have been vastly increased by human activity. The most toxic to humans are lead, mercury, arsenic, and cadmium. Other metals are poisonous to marine organisms, including copper, silver, selenium, zinc, and chromium.
Since the 1960s, synthetic and industrial pollutants also have been manufactured and released into the environment in large quantities. The synthetic organic compounds currently released into the environment now number in the tens of thousands, and many are hazardous to both terrestrial and aquatic life at even low concentrations. The most dangerous include chlorinated hydrocarbon insecticides (such as DDT), PCBs, phthalates (which are used in the production of polyvinyl chloride resins), PAHs, which result from the incomplete burning of fossil fuels, and DBFs, which are disinfectants. All of these compounds mimic naturally occur-

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ring organic compounds and are readily absorbed by living organisms, producing birth defects, genetic abnormalities, health problems, and death. In humans some of these compounds are implicated in lowered sperm counts.
A further chemical change is brought about by acidification. The well-known phenomenon of acid rain is changing the pH of many terrestrial environments, causing biological problems and even local extinctions among some organisms.

Ozone Depletion

In the 1980s, scientists, and soon thereafter the general public, became aware of the loss of ozone from the upper regions of the atmosphere. Much of the thinning of the ozone layer has been caused by the release of chlorofluorocarbons (CFCs) into the atmosphere. A long-term reduction of ozone would have profound evolutionary effects, as ozone screens out ultraviolet radiation, which is poisonous to living matter given sufficient exposure. If long-term ozone depletion continues, organisms will be forced to evolve new structures or physiological pathways to deal with excess UV radiation.

All of these accumulating chemical changes will require specific physiological adaptations in a host of organisms. Although the most obvious effects of evolution are visible changes in body types, far more evolutionary change takes place at the behavioral and physiological levels. In these cases, evolutionary change is not readily apparent. Yet, in the increasingly toxic environments of Earth, they will remain the most common types of future evolution.

Prophecy

The factors described above can be used to pick the potential evolutionary �winners� of the future: organisms adapted to cities or agricultural fields and capable of living in polluted water or air. Much future evolution may be invisible, taking place among already existing animals through changes in behavior and physiology. Can some vision of our world, even a millennium from now, be imagined? With apologies to H. G. Wells (and to those who require that books about science remain �serious� and dry), here is mine.

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