• The climate system includes the atmosphere, hydrosphere, solid Earth, biosphere, and cryosphere (the ice and snow that exists at Earth's surface). The system involves the exchanges of energy and moisture that occur among the five spheres.

• Current interest in past and future climates is due to several factors. Detailed reconstructions of past climates show that the climate has varied on all time scales from decades to millions of years. Also, research focused on human activities and their effect on the environment has demonstrated that people are inadvertently changing climate. And finally, there is observational evidence that world climate has become more variable.

• Techniques for analyzing Earth's climate history on a scale of hundreds to thousands of years include evidence from sea-floor sediments and oxygen isotope analysis. Sea-floor sediments are useful recorders of worldwide climate change because the numbers and types of organic remains included in the sediment are indicative of the sea surface (the ocean-atmosphere interface) they once lived in. Using oxygen isotope analysis, scientists can use the ¹⁸O/¹⁶O ratio found in the shells of microorganisms in sediment and layers of ice and snow to detect past temperatures. Other methods for determining past climates use buried soils (paleosols), the analysis of the growth rings of trees, the study of pollen contained in sediments, and information contained in historical documents.

  

• Several explanations have been provided to explain climate change. Current hypotheses for the "natural" mechanisms (causes unrelated to human activities) of climate change include (1) plate tectonics, rearranging Earth's continents closer or farther from the equator, (2) volcanic activity, reducing the solar radiation that reaches the surface, (3) variations in Earth's orbit, involving changes in the shape of the orbit (eccentricity), angle that Earth's axis makes with the plane of its orbit (obliquity), and/or the wobbling of the axis (precession), and (4) changes in the Sun's output associated with sunspots.

• Humans have been modifying the environment for thousands of years. By altering ground cover with the use of fire and the overgrazing of land, people have modified such important climatological factors as surface albedo, evaporation rates, and surface winds. Along with water vapor, carbon dioxide is largely responsible for the greenhouse effect of the atmosphere. Therefore, by adding carbon dioxide and other trace gases (methane, nitrous oxide, and chlorofluorocarbons) to the atmosphere, humans are likely contributing to global warming in a significant way.

  

• When any component of the climate system is altered, scientists must consider the many possible outcomes, called climate-feedback mechanisms. Changes that reinforce the initial change are called positive-feedback mechanisms. For example, warmer surface temperatures cause an increase in evaporation, which further increases temperature as the additional water vapor absorbs more radiation emitted by Earth. On the other hand, negative-feedback mechanisms produce results that are the opposites of the initial change and tend to offset it. A good example would be the negative effect that increased cloud cover has on solar energy available to heat the atmosphere.

• Global climate is also affected by human activities that contribute to the atmosphere's aerosol (tiny, often microscopic, liquid and solid particles that are suspended in air) content. By reflecting sunlight back to space, aerosols have a net cooling effect. The effect of aerosols on today's climate is determined by the amount emitted during the preceding couple of weeks, while carbon dioxide remains for much longer spans and influences climate for many decades.

• Because the climate system is so complex, predicting specific regional changes that may occur as the result of increased levels of carbon dioxide in the atmosphere is highly speculative. However, some possible consequences of greenhouse warming include (1) altering the distribution of the world's water resources, (2) a probable rise in sea level, and (3) greater intensity of tropical cyclones.

• Due to the complexity of the climate system, not all future shifts can be foreseen. Thus, "surprises" (relatively sudden, unexpected changes in climate) are possible.

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