The climate is controlled by the flow of energy to and from the earth.
The earth is kept warm by the energy it continuously receives from the sun.
It avoids getting too hot by simultaneously giving off energy as well. We
can understand the climate, and how it might be affected by volcanic activity,
in terms of these two processes.
The flow of energy between the sun and the earth and between the earth and outer space is strongly affected by the atmosphere. Gases and clouds in the atmosphere absorb, emit, and reflect radiation. Each gas absorbs at a number of different wavelengths. The presence of atmospheric gases and clouds makes the passage of radiation to and from the earth's surface much more complicated than it would be otherwise. Furthermore, air movements in the atmosphere can also transport energy.
On average, the energy 'budget' is balanced both at the top and at the bottom of the atmosphere. At the top, the total incoming radiation is equal to the total outgoing radiation. At the bottom, the total energy absorbed by the earth's surface is equal to the total energy transported away. In other words, the total heating equals the total cooling. Both these budgets are kept in balance because the rate of cooling increases as the temperature does. Any variations in total heating result in temperature changes that rapidly bring the budget back into balance.
The energy budget is altered when the atmosphere's composition is changed
- by, for example, volcanic gases that escape into the atmosphere from an eruption. The result can
be a change in climatic temperatures. Increased amounts of gases such as
carbon dioxide make the atmosphere absorb long-wavelength radiation from
the surface more strongly and also emit more radiation back down towards
the surface. The energy budget at the surface is therefore upset. For the
balance to be restored, some warming must
occur.