Climate Changes

The global mean climate changes because the amount of net energy that enters the climate system changes in time. The Earth receives energy from the sun. Part of this energy is transformed into heat. The total amount of solar energy that is transformed into heat is not always the same because:

• The amount of energy that the Sun radiates is not always the same
• The orbit of the Earth around the Sun is not always the same
• The composition of the atmosphere is not always the same
• The reflection of sunlight at the Earth's surface (albedo) is not always the same
• The amount of cloud cover is not always the same

The Earth also receives energy from the atmosphere in the form of infrared radiation. Without the atmosphere, the Earth would only receive solar radiation and the temperature on Earth would be much lower. The warming of the Earth as a result of the infrared radiation of the atmoshere to the Earth is called the greenhouse effect. This greenhouse effect does not always have the same intensity because:

• The amount of cloud cover is not always the same
• The concentration of greenhouse gases in the atmosphere is not always the same

Furthermore the Earth loses energy to space in the form of infrared radiation. This amount of radiation depends on the temperature. The global mean temperature adjusts in such a way that the mean amount of energy that enters the Earth-atmosphere system (in the form of solar radiation) balances to the amount that leaves the atmosphere (in the form of infrared radiation).

These processes mentioned above do not operate independent of each other but can strengthen or weaken each other. If the temperature of the atmosphere rises, the amount of watervapor in the atmosphere increases which results in an increase of the amount of infrared radiation to the Earth (the greenhouse effect is strengthened). This will reinforce the initial temperature rise. Such a reinforcement is called a positive feedback. In this example one speaks of the watervapor feedback. Another important feedback is the snow albedo-feedback. When the temperature of the atmosphere falls, more snow remains on the Earth's surface. More sunlight is reflected and the initial temperature drop will be reinforced. This last feedback is an essential link in the onset of ice ages. A rather uncertain feedback is the cloud feedback. Clouds reflect the sunlight and thus have a cooling effect on the climate. Night clouds however diminish the amount of infrared radiation to space and thus have a warming effect on the climate. Moreover this effect on the infrared radiation strongly depends on the temperature and thus the height at which the cloud is situated. Because it is by no means clear how the cloud distribution responds to temperature changes, there is a lot of uncertainty about the nature and strength of the cloud feedback and it behaves very different in the different climate models.

Because of atmospheric and ocean currents there is a constant redistribution of heat over the Earth's surface. Changes in these currents can cause large climate fluctuations in certain regions. The variations in the mean temperature of a certain region are generally much bigger than the variations of the global mean temperature.

In the climate simulations of this project we prescribe the following so called climate forcings:

• the amount of energy that the sun radiated from 1940 to 2000
• the amount of volcanic dust in the atmosphere between 1940 and 2000
• the amount of sulfate aerosoles in the atmosphere between 1940 and 2000
• the concentration of greenhouse gases between 1940 and 2080

For the first three forcings we set the values for the years after 2000 to the year 2000 values. For the concentration of greenhouse gases in the years after 2000, of which CO2 is the most important, we use the 'business as usual' scenario, in which the world continues to burn fossil fuels undiminished and as a result, the concentration of CO2 continuously increases.

The plots below show the prescribed solar constant (the first climate forcing) and the concentration of CO2 (part of the fourth climate forcing).