Feedback

The Earth's climate responds to changes in the amount of energy stored by the climate system, and in particular when the global energy balance between incoming energy from the Sun and outgoing heat from the Earth is upset. When "climate forcing" upsets the balance of the Earth's climate in this manner, it responds not only to the process forcing the climate to change, but also to feedback effects which can either augment or diminish the original influence.

Today, the Earth is warming up. Global warming, it is believed, is largely the result of man-made pollution of the atmosphere with excess greenhouse gases. Globally, the average surface temperature has increased by about 0.6C during the last 100 years. In the polar regions however, regional warming has been considerably greater. In some parts of Antarctica and northern Russia, temperatures have increased by about 2C in only 50 years. Many scientists think that such rapid warming in these ice-covered parts of the world is a consequence of the ice-albedo feedback effect.

Ice, being white, reflects a lot of sunlight and keeps the surface colder for longer. Highly reflective surfaces like ice have high albedos. As an enhanced greenhouse effect warms the surface of the Earth, some of the ice at high latitudes melts, exposing either bare ground or ocean, both of which have lower albedos (or reflectivities) than ice. With a lower albedo, the exposed surfaces reflect less sunlight, with more sunlight being absorbed. This causes a further rise in surface temperature, and in turn a further melting of ice. Here, the climatic response to primary greenhouse heating acts as a secondary climate forcing that augments the initial climate change.

Negative feedback occurs when the response to primary changes acts in the opposite direction to that of the initial climate forcing. Negative feedback reduces the climate response to initial causes of climate change. The formation of clouds in a greenhouse-heated world may cause a negative feedback. A warmer atmosphere will contain more moisture, and consequently more clouds. Clouds reflect a lot of sunlight and may help to reduce the amount of global warming due to man-made greenhouse gas emissions.

Over the longer term, changes in the shape of the Earth's orbit around the Sun are believed to influence natural global climate variations that are evident in the palaeoclimatic records over tens and hundreds of thousand of years. It is clear however, that climate feedback effects have augmented the differences in global climate between the Ice Ages and the warmer interglacial periods, including changes in ocean circulation and atmospheric composition.