Anthropogenic climate changes, especially global warming, have affected terrestrial, freshwater, and marine ecosystems in the past few centuries. If the Earth's average global temperature continues to increase, these ecosystems will change even more radically in the future.

Long before the appearance of Homo sapiens, periodic momentous climate changes had devastating effects on Earth's ecosystems. For example, 245 million years ago, a colossal effusion of greenhouse gases (GHGs) from massive volcanoes raised average global temperatures by 5œ Celsius, leading to the demise of over 90 percent of living things in the great Permian-Triassic extinction. Humans, in the few million years they have lived on Earth, have not had as cataclysmic an effect on the biosphere as have natural climatic catastrophes. However, over time and particularly since the development of advanced industrialized societies, they have been having an increasingly potent effect on the Earth's climate, which in turn has led to the decline and even the extinction of many species.

The response of the Earth's land and water vegetation to anthropogenic climate change is extremely complex and varied, extending from photosynthesis in relatively simple microscopic plants to the global distributions of highly variegated plant species. Scientists researching climate-induced environmental changes and the effects of those changes have concentrated on a careful selection of flora in the distant and recent past. They have then extrapolated from those focused studies of the past to draw conclusions about present and future time periods, mainly through computer models. Paleobotanists have discovered how some large ecosystems dominated by plants responded to global climate changes in prehistoric ages, but this understanding has not been easily applicable to modern conditions. More secure understanding has resulted from studies of how vegetation responded to the climate changes during the ice ages: Migration proved a means for many species to survive glacial and interglacial climates.

While, in recent centuries, the greatest threat to plant species has been from their unsustainable harvesting by humans and the elimination of their habitats, anthropogenic global warming has also had an influence on certain plant distributions. For example, with a temperature rise of nearly 2œ Celsius in the boreal zone of North America, conifer forests have moved closer to polar regions. Certain computer models relating global warming and plant survival have predicted future changes in the species composition and locations of forests, while still other forests may desiccate, burn, and disappear completely. In tropical rain forests, certain orchids are rare, because they have highly unusual ecological niches. If their environments are climatically transformed, these orchids often prove to be mediocre migrants. Some plant species, unable to tolerate warmer fresh and salt waters, will become extinct, while climate-caused habitat loss will lead to other plant extinctions. Flooding of coastal estuaries and wetlands may further the destruction of plants unable to adapt to the changed conditions.

The abundance, distribution, and ecological niches of animal species will continue to be affected by changing climatic factors. As the climate warmed at the end of the ice ages, some species expanded their ranges while other species migrated to new regions. This postglacial warming resulted in the extinction of many species of large mammals, including the wooly mammoth. Many scientists believe that climate played a role in these extinctions. Analogously, many scientists believe that in the past few centuries, when human activities have contributed to global warming, elevated global temperatures have affected certain animal species.

The declining numbers of amphibians is an important example of this phenomenon. From 1975 to 2000, certain Costa Rican forests warmed significantly, causing shifts in the habitats of birds, reptiles, and amphibians. Some frogs, unable to adapt to these changes, became extinct, while others became endangered or critically threatened. Not all scientists are convinced that climate change is to blame for amphibian declines, however. Some cite pollution and fungus as possible alternate causes. Another often-cited example of a declining species is the polar bear. From 1975 to 2005, Arctic sea ice shrank by about 20 percent, reducing the bears' access to seals, their main prey. In the two decades from 1985 to 2005, polar bear populations fell by over 20 percent.

Because of the many variables contributing to the decline of various animal species, it has been difficult to determine with precision the role that global warming has played in these declines. Nevertheless, certain scientists have used computer models to issue warnings that accelerated global warming will inevitably lead to the extinction of many animal species. In 2004, the United Nations published a report based on the expertise of many scientists that estimated a million species of plants and animals could become extinct by 2050, unless global warming can be stopped. Environmentalists think that the ecosystems most likely to experience the greatest species loss are polar seas, arctic tundra, and coastal wetlands.

Bibliography:

1)         Chivian, Eric, and Aaron Bernstein, eds. Sustaining Life: How Human Health Depends on Biodiversity. New York: Oxford University Press, 2008.

2)         Cowie, Jonathan. Climate Change: Biological and Human Aspects. New York: Cambridge University Press, 2007.

3)         Flannery, Tim. We Are the Weather Makers: The Story of Global Warming. Rev. ed. London: Penguin, 2007.

4)         Woodward, F. I., ed. Global Climate Change: The Ecological Consequences. San Diego, Calif.: Academic Press, 1992.

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