Natural Factors Affecting Climate
Throughout the history of the Earth, the climate has changed. The ice ages and intervening warm periods are examples. Some changes have been global in scale; others have been regional or hemispheric. There are a number of natural factors that have contributed to changes in the Earth's climate over various time scales. It is important to understand these factors when attempting to detect a human influence on climate.
Changes in Solar Output The amount of energy radiating from the sun is not constant. There is evidence of an 11-year solar cycle (variation in energy output) in the temperature record of the Earth. Changes may occur over longer periods as well.
Changes in the Earth's Orbit Variations in the Earth's orbit occur over long periods. These variations change where and when solar energy is received on Earth. This affects the amount of energy that the Earth reflects and absorbs. Scientists believe these orbital variations are a factor in initiating the ice ages.
The Greenhouse Effect When energy from the sun enters the Earth's atmosphere, about one third of it is reflected back to space. Of the rest, some is absorbed by the atmosphere, but most of it is absorbed by the surface of the Earth. The Earth emits energy at longer wavelengths than it aborbs energy. Some of this energy escapes to space but some of it is absorbed again and re-emitted by clouds and greenhouse gases such as water vapour, carbon dioxide, methane and nitrous oxide. This helps to warm the surface and the troposphere (lowest layer of the atmosphere), keeping it 33 ?C warmer than it would otherwise be. This is the natural Greenhouse Effect, and it is vital to life as we know it.