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Climate can be simply defined as the average weather. Natural climate variability results from fluctuations in the atmosphere. These fluctuations can vary in length from hundreds of millions of years to one year, in addition to seasonal changes. Earth has had 100,000-year glacial-interglacial cycles when the climate was alternately cooler (glacial) and about as warm as the present (interglacial). Global surface temperatures have typically varied by 5º to 7º C through these cycles, and in some middle and high latitude regions up to 10º to15º C with accompanying large changes in ice volume and sea level. Since the end of the last ice age about 10,000 years ago, average global surface temperatures have probably varied little more than 1º C. Some of these fluctuations, including the Little Ice Age in the last millennium, lasted several centuries and had major ecological and cultural impact.

Today, when people speak of climate change, they focus on changes in climate that result from human activity. When scientists initially became interested in climate variables they began to study the atmosphere. They observed that atmospheric processes are strongly linked to the areas of Earth’s surface that are covered by dry land, ocean, or significant ice. When they investigated these areas further, they identified another link with areas of vegetation and other living systems on the land and in the ocean. These five components—atmosphere, land, ocean, ice, and biosphere—collectively form the “climate system.”

The driving force behind weather and climate is energy from the sun. About one- third of the solar radiation that reaches Earth is reflected back. The rest is absorbed by the atmosphere and Earth’s surface. The energy absorbed from solar radiation must be balanced by outgoing radiation from Earth (called terrestrial radiation). Factors known as climate-forcing agents can change the balance between the solar radiation energy absorbed by Earth and that emitted by Earth in the form of infrared radiation.

Except for solar radiation, the most important climate forcing agents arise from the greenhouse effect on outgoing infrared radiation. Water vapor, the main greenhouse gas, is not influenced by human activities, but emissions resulting from human activities are substantially increasing the atmospheric concentrations of other greenhouse gases, including carbon dioxide, methane, chlorofluorocarbons (CFCs), and nitrous oxides. These increases enhance the greenhouse effect, which involves the trapping of outgoing heat and subsequent increase in the temperature of Earth’s surface.

For 1000 years before the Industrial Revolution, the amount of greenhouse gases remained relatively constant. With the development of agriculture and increased industrialization, the abundance of greenhouse gases (except water vapor) increased significantly. The combustion of fossil fuels and post-industrial deforestation have caused a 26% increase in carbon dioxide concentration in the atmosphere. Methane concentrations have more than doubled because of rice production, cattle rearing, biomass burning, coal mining, and ventilation of natural gas. Nitrous oxide has increased by about 8% since pre-industrial times, likely as the result of human activity, especially agriculture. Chlorofluorocarbons, used as aerosol propellants, solvents, refrigerants, and foam-blowing agents, have been present in the atmosphere only since their invention in the 1930s.

Aerosols resulting from volcanic eruptions block incoming solar radiation and can lead to cooling of Earth’s surface, which could help counteract greenhouse warming. Human activities, mainly production of sulfur emissions, can also increase aerosol content in the lower atmosphere, which could lead to increased cloud reflectivity and regional cooling.
It is difficult to determine how much climate will change as a result of increases in greenhouse gases because as climate begins to warm, several processes can amplify or reduce the warming. Currently the major feedbacks identified are from changes in water vapor, sea ice, clouds, and the oceans. Climate change predictions are based on mathematical models called General Circulation Models (GCMs), that combine our knowledge of physical processes and the interactions between various systems like the ocean, atmosphere, and land (e.g., see; Climate specialists describe the predicted changes in climate in terms of variability of weather and frequency of extremes. In central North America, warming is expected to vary from 2º to 4º C in winter and 2º to 3º C in summer. Precipitation increases are predicted to range from zero to 15% in the winter and a decrease of 5–10% in the summer.

The conclusion that global temperature has been rising is strongly supported by the retreat of most of the world’s mountain glaciers since the end of the nineteenth century. During the same period, global sea level has risen by an average of one to two millimeters per year. Climatologists agree that global mean temperature alone is an inadequate indicator of greenhouse-gas-induced climate change. A better way to understand the patterns of climate change, including man-made climate change, is the subject of ongoing research.

Several questions about climate change and global warming remain to be answered. These questions include: (1) the response of clouds and cloud formation to increased greenhouse gases; (2) the exchange of energy between the oceans and the atmosphere; (3) the chemical reactions of greenhouse gases and how they influence climate change ; and (4) the response of polar ice sheets to warming and how that affects predictions of sea level rise.

Two other variables that influence climate are deforestation and the hydrologic cycle. Deforestation has the potential to affect climate through the carbon and nitrogen cycles, which alter atmospheric concentrations of carbon dioxide. Changes in the hydrologic cycle involving precipitation, evaporation, and runoff also affect climate. Deforestation and desertification influence the amount of solar energy absorbed at Earth’s surface (this is termed albedo or reflectivity).

Ecosystem processes such as photosynthesis and respiration are dependent on climatic factors and carbon dioxide concentrations in the short term, so ecosystems will change in structure and composition as the climate changes. Because different organisms respond differently to these changes, some will flourish, while others will decline. The climate system represents a complex network of linked processes that help to both moderate and accelerate large-scale change, so it is difficult to predict the effect on living systems and particularly the rates of change that may occur over the near future.

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