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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization

 

 

 

Britannica Internet Guide Selection
See: Information from Encyclopædia Britannica about global warming

 

More Questions and Answers from the Education page of the U.S. Global Change Research Program.

 

 

 

 

 

 

Why Can't Ecosystems Just Adapt?

Climate change has the potential to alter many of the Earth's natural ecosystems over the next century. Yet, climate change is not a new influence on the biosphere, so why can't ecosystems just adapt without significant effects on their form or productivity? There are three basic reasons.

First, the rate of global climate change is projected to be more rapid than any to have occurred in the last 10,000 years. Second, humans have altered the structure of many of the world's ecosystems. They have cut down forests, plowed soils, used rangelands to graze their domesticated animals, introduced non-native species to many regions, intensively fished lakes, rivers and oceans, and constructed dams. These relatively recent changes in the structure of the world's ecosystems have made them less resilient to further changes. Third, pollution, as well as other indirect effects of the utilization of natural resources, has also increased since the beginning of the industrial revolution. Consequently, it is likely that many ecosystems will not be able to adapt to the additional stress of climate change without losing some of the species they contain or the services they provide, such as supplying sufficient clean water to drink, food to eat, suitable soils in which to grow crops, and wood to use as fuel or in construction.

For millions of years, species have been shifting where they grow and reproduce in response to changing climate conditions. Over the next century, global warming could result in approximately one-third of the Earth's forested area undergoing major transitions in species composition. From the fossil record we have an indication of the maximum rate at which various plant species have migrated to more suitable areas; from 0.04 km/yr (about 0.03 miles/yr) for the slowest to 2 km/yr (about 1.3 miles/yr) for the fastest. However, the projected rate of surface temperature change in many parts of the world could require plant species to migrate at faster rates (1.5 to 5.5 km/yr or about 1 to 3.5 miles/yr). Thus, many species may not be able to move rapidly enough to prosper. These changes in vegetation and ecosystem structure may in turn give rise to additional releases of carbon into the atmosphere, further accelerating climate change.

Moreover, as the old vegetation dies in areas most affected by climate change, such as forests in northern latitudes, it is likely to be replaced by fast growing, often non-native species. These species commonly yield less timber, provide lower quality forage for domesticated animals, supply less food for wild animals, and furnish poorer habitat for many native animals. The prevalence of pest species, such as weeds, rats, and cockroaches, may also increase.

Humans actively and productively use and manipulate large portions of the land surface of the Earth, whether it be for agriculture, housing, energy, or forestry. These practices have created a mosaic of different land uses and ecosystem types, resulting in fewer remaining large and contiguous areas of a single type of habitat than existed in the past. Therefore it will often be difficult for plants and animals to move to a location with a more suitable climate even if a species was able to migrate quickly enough. This was not the case thousands of years ago, when ecosystems last experienced rapid climate change. Now, many of the world's ecosystems are essentially trapped on small islands, cut off from one another, only capable of travel over a limited and shrinking number of bridges. As this increasingly occurs, more species are likely to be stranded in an environment in which they cannot survive and/or reproduce.

Further complicating the response of many of the Earth's terrestrial and aquatic ecosystems to climate change is the prevalence of stress from other disturbances associated with resource use. In the case of trees, for example, many species are already weakened by air pollution. Increased concentrations of carbon dioxide in the atmosphere will raise the photosynthetic capacity of many plants, but the net effect on ecosystem productivity is unclear, particularly when combined with higher air temperatures or where soil nutrients are limiting.

Among the ecosystems that are most likely to experience the most severe effects from climate change are those that are at higher latitudes, such as far northern (Boreal) forests or tundra, as well as those where different habitat types converge, such as where grasslands meet forests, or forests give way to alpine vegetation. Coastal ecosystems are also at risk, particularly saltwater marshes, mangrove forests, coastal wetlands, coral reefs, and river deltas. Many of these ecosystems, already under stress from human activities, may be significantly altered or diminished in terms of their extent and productivity as a result of future climate change.


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