Climate is defined as the typical behavior of the atmosphere, the aggregation of the weather, and is generally expressed in terms of averages and variances of temperature, precipitation and other physical properties. The greenhouse effect, the ability of certain gases like carbon dioxide and water vapor to effectively trap some of the reemission of solar energy by the planet, is a necessary component to life on Earth; without the greenhouse effect the planet would be too cold to support life. However, human activities are increasing the concentration of carbon dioxide and several other greenhouse gases, resulting in concerns about warming of the Earth by 1-5°C over the next century (IPCC, 1996a). Recent increases in global averaged temperature over the last decade already appear to be outside the normal variability of temperature changes for the last thousand years. A number of different analyses strongly suggest that this temperature increase is resulting from the increasing atmospheric concentrations of greenhouse gases, thus lending credence to the concerns about much larger changes in climate being predicted for the coming decades. It is this evidence that led the international scientific community through the Intergovernmental Panel on Climate Change (IPCC, 1996a) to conclude, after a discussion of remaining uncertainties, "Nonetheless, the balance of the evidence suggests a human influence on global climate". More recent findings have further strengthened this conclusion. Computer-based models of the complex processes affecting the carbon cycle have implicated the burning of fossil fuels by an ever-increasing world population as a major factor in the past increase in concentrations of carbon dioxide. These models also suggest that, without major policy or technology changes, future concentrations of CO2 will continue to increase, largely as a result of fossil fuel burning. This chapter briefly reviews the state of the science of the concerns

Table 1.1 Summary of trends in observed climatic variables (WMO, 1998; Harvey, 2000). Note that NH implies Northern Hemisphere and SH implies

Southern Hemisphere


Analysis Period

Trend or Change

Surface air temperature and sea surface temperature (SST) Alpine glaciers

Extent of snowcover in the NH Extent of sea ice in the NH Extent of sea ice in the SH

Length of the NH growing season Precipitation

Heavy precipitation Antarctic snowfall Global mean sea level

1851-1995 Last century


1973-1994 1973-1994

1981-1991 1900-1994

1910-1990 Recent decades Last century

Implies warming of 0.6-1.0 °C in alpine regions

10% decrease in annual mean Downward since 1977 No change, possible decrease between mid 1950s and early 1970s

12 ±4 days longer

Generally increasing outside tropics, decreasing in Sahel Growing in importance 5-20% increase 1.8 ± 0.7 mm/year about climate change that could result from fossil fuels and other human related emissions.

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