There are many climate feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish (‘negative feedback’) the effects of a change in climate forcing. For example, as rising concentrations of greenhouse gases warm Earth’s climate, snow and ice begin to melt. This melting reveals darker land and water surfaces that were beneath the snow and ice, and these darker surfaces absorb more of the Sun’s heat, causing more warming, which causes more melting, and so on, in a self reinforcing cycle. This feedback loop, known as the ‘ice-albedo feedback’, amplifies the initial warming caused by rising levels of greenhouse gases.
The amount of light that is reflected by a material is called the albedo. Really dark, black materials have an albedo of 0 (no reflection at all) if they are perfectly black and perfectly white materials have an albedo of 1.0 (total reflection). The table below lists some representative albedos for a range of materials that cover the Earth’s surface. Most of these albedos are sensitive to the angle of incidence of the sunlight; this is especially true for water. When the Sun is at an angle of 40° and higher relative to the horizon, the albedo of the water is fairly constant, but as the angle decreases from 40°, the albedo increases dramatically, so that it is about 0.5 at a Sun angle of 10° and 1.0 at a sun angle of 0°.
ALBEDO OF EARTH MATERIALS
|Substance||Albedo (% reflectance)|
|Water||0.06 – 0.1|
|Ice & Fresh Snow||0.9|
|Grass lands||0.18 – 0.25|
|Deciduous forest||0.15 – 0.18|
|Coniferous forest||0.09 – 0.15|
|Rain forest||0.07 – 0.15|
Detecting, understanding and accurately quantifying climate feedbacks have been the focus of a great deal of research by scientists unravelling the complexities of Earth’s climate.
Adding more of a greenhouse gas, such as CO2, to the atmosphere intensifies the greenhouse effect, thus warming Earth’s climate. The amount of warming depends on various climate feedback mechanisms. For example, as the atmosphere warms due to rising levels of greenhouse gases, its concentration of water vapour increases, further intensifying the greenhouse effect. This in turn causes more warming, which causes an additional increase in water vapour, in a self-reinforcing cycle. This water vapour feedback may be strong enough to approximately double the increase in the greenhouse effect due to the added CO2 alone.
Additional important climate feedback mechanisms involve clouds. Clouds are effective at absorbing infrared radiation and therefore exert a large greenhouse effect, thus warming the Earth. Clouds are also effective at reflecting away incoming solar radiation, thus cooling the Earth. A change in almost any aspect of clouds, such as their type, location, water content, cloud altitude, particle size and shape, or lifetimes, affects the degree to which clouds warm or cool the Earth.
Some changes amplify warming while others diminish it. Much research is in progress to better understand hown clouds change in response to climate warming, and how these changes affect climate through various feedback mechanisms.
Another feedback is the melting of permafrost in boreal regions, resulting in the release of methane, a potent greenhouse gas, and CO2 from soil organic matter. Recent studies in Siberia, North America and elsewhere have documented the melting of permafrost.
An important feedback is the release of carbon from ecosystems due to changing climatic conditions. The dieback of high-carbon ecosystems, such as the Amazon, due to changes in regional precipitation patterns, has been predicted from some models, but it has not yet been observed. Laboratory studies have indicated accelerated decomposition of soil organic matter in temperate forests and grasslands due to temperature and precipitation changes, or the CO2-induced enhancement of decomposition by mycorrhizae.
Source material: IPCC 4th Report, 2007 Technical Summary, FAQ andUnited Nations Environment Programme, 2007, Fourth Global Environment Outlook: environment for development Assessment Report, http://www.unep.org/geo/geo4/report/GEO-4_Report_Full_en2.pdf.