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What forces Climate?

The Earth's climate changes when the amount of energy stored by the climate system is varied. The most significant changes occur when the global energy balance between incoming energy from the Sun and outgoing heat from the Earth is upset. There are a number of natural mechanisms that can upset this balance, for example fluctuations in the Earth's orbit, variations in ocean circulation and changes in the composition of the Earth's atmosphere. In recent times, the latter has been evident as a consequence not of natural processes but of man-made pollution, through emissions of greenhouse gases and aerosols. By altering the global energy balance, such mechanisms "force" the climate to change. Consequently, scientists call them "climate forcing" mechanisms.

Global, annual-mean radiative forcings (Wm-2) due to a number of agents from the period from pre-industrial (1750) to present (2000).
Click on the image for more information

What is the "Carbon Cycle"?

The Carbon Cycle is a complex series of processes through which all of the carbon atoms in existence rotate. The same carbon atoms in your body today have been used in countless other molecules since time began. The wood burned just a few decades ago could have produced carbon dioxide which through photosynthesis became part of a plant. When you eat that plant, the same carbon from the wood which was burnt can become part of you. The carbon cycle is the great natural recycler of carbon atoms. Unfortunately, the extent of its importance is rarely stressed enough. Without the proper functioning of the carbon cycle, every aspect of life could be changed dramatically.

CMDL Groups studying the Carbon Cycle:

Carbon Cycle Greenhouse Gases
Halocarbons & other Atmospheric Trace Species
Ozone and Water Vapor (impact of water vapor)

What is the Greenhouse Effect?

The Sun, which is the Earth's only external form of heat, emits solar radiation mainly in the form of shortwave visible and ultraviolet (UV) energy. As this radiation travels toward the Earth, the atmosphere absorbs 25% of it, and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface. The Earth releases a lot of energy it has received from the Sun back to space. However, the Earth is much cooler than the Sun, so the energy re-emitted from the Earth's surface is much weaker, in the form of invisible longwave infrared (IR) radiation, sometimes called heat.
Greenhouse gases like water vapor, carbon dioxide, methane and nitrous oxide trap the infrared radiation released by the Earth's surface. The atmosphere acts like the glass in a greenhouse, allowing much of the shortwave solar radiation to travel through unimpeded, but trapping a lot of the longwave heat energy trying to escape back to space. This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is the Earth's natural greenhouse effect and keeps the Earth 33°C warmer than it would be without an atmosphere, at an average 15°C. In contrast, the moon, which has no atmosphere, has an average surface temperature of -18°C.

CMDL Groups studying the Greenhouse Effect:

Carbon Cycle Greenhouse Gases

What Role does Radiation play in Determining the Earth's Climate?

Energy or solar radiation enters the atmosphere as sunshine. The sky reflects a portion of this radiation, the rest is absorbed by the Earth and is eventually released again as thermal radiation. Both man-made and naturally occurring events can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airborne ash resulting from volcanic activity reduce the solar resource by increasing the scattering and absorption of solar radiation. Some scientists think of the radiation budget in terms of a balance. If the Earth gets more energy from the Sun, the Earth heats up and emits more thermal energy. This brings the radiation budget into balance. If the Earth emits more of this thermal energy than it absorbs from solar radiation, the Earth cools off. As it cools off, the Earth emits less energy. This change also brings the radiation budget back into balance.

CMDL Groups studying the role of radiation in the Earth's climate:

Solar & Thermal Atmospheric Radiation
Aerosols

Part of the text for this page was derived from the Encyclopedia of the Atmospheric Environment.

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