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
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
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.
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
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
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.