The Energy Balance and Natural Climate Variations
Figure 2 shows the energy balance with incoming solar radiation on the left hand side, and outgoing infrared (longer energy waves, expressed as heat) radiation on the right [IPCC].
The sun drives the climate system by radiating energy to Earth. A stable climate requires a balance between radiation from the Sun entering the atmosphere and radiation going out.
The atmosphere is made up of gases, particles (aerosols) and clouds; these interact to create variability and change within the atmosphere. Nitrogen (N 2), oxygen (O 2), and argon (Ar) do not absorb the Sun’s incoming energy or infrared radiation, but they make up about 98% of the gases in the Earth’s atmosphere.
Carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2O) and ozone (O 3) make up only a minute fraction of the atmosphere (about 0.1%), but they absorb significant amounts of infrared radiation. These naturally occurring greenhouse gases also are produced by human activity.
Water vapor (H 2O) is the strongest natural greenhouse gas, even though it only makes up about 1% of the atmosphere. The amount varies considerably because changing temperatures within the atmosphere cause water to change between vapor, liquid droplets and ice crystals.
Greenhouse gases absorb radiation and give it off again in all directions. The downward radiation warms the Earth’s surface. This process of absorption, emission and warming is called the greenhouse effect. It exists naturally; without this natural warming, the Earth would be too cold for plants and animals (including humans). The greenhouse effect also is amplified by humans’ production of greenhouse gases.
Clouds are important in the Earth’s energy balance and in the natural greenhouse effect. Clouds absorb and emit infrared radiation. So, like the greenhouse gases, clouds contribute to warming the Earth’s surface. On the other hand, clouds are good reflectors of incoming solar radiation -- and thus cool the climate system. The net effect of cloud cover globally in the present climate is a slight cooling. However this effect varies, depending on the clouds’ altitude, type and optical properties.
Dynamic feedbacks and energy transfer processes also affect climate. An equilibrium climate state would have zero average net radiation at the top of the atmosphere. A change in either the solar radiation or the infrared radiation changes the net radiation, causing an imbalance called “radiative forcing.” External forcings, such as solar radiation or the large amounts of aerosols ejected by volcanic eruption into the atmosphere, vary on widely different timescales and cause natural variations in the radiative forcing. These variations may be negative or positive. In either case the climate system reacts to restore the balance.