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    Mr. Mag
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    Global warming

    مُساهمة  Mr. Mag في السبت نوفمبر 06, 2010 8:18 am

    Global warming


    Global warming is the increase of the Earth's average temperature since the mid-20th century. Global warming may become a problem for the world and has already been linked to floods and other types of storms.

    Today, the atmosphere of the world is about a half a degree Celsius warmer than it was two hundred years ago. Many people want to know why the Earth's climate is changing. Most say that the changes are caused by people (by cars and factories, for example). Some say the change is not the fault of humans.

    The average temperature at the surface of the Earth has gone up by 0.6 Celsius since the late 19th century[1]. There are several theories (ways) which try to explain this increase. Most scientists think that the warming of the last 50 years is believed to be a result of increases in the greenhouse effect caused by human-generated carbon dioxide (CO2). A few think that changes are because of differences in the amount of heat from the sun and other natural causes.

    A "theory" in science is not just an idea. It is a proposed answer which has been tested and agreed on by many scientists, yet has not been confirmed as a actual objective of existence, and is in debate. Climate theories are hard to test because they usually depend on computer models, which are themselves theories. Today, most climatologists agree that the theory of anthropogenic global warming has survived challenges and testing against data from many sources. Anthropogenic global warming is a better "fit" to what scientists see in the data.

    Climate models show that temperatures will probably increase by between 1.4 °C and 5.8 °C from 1990 to 2100 [2]. Much of the uncertainty in this increase is from not knowing how much CO2 and other greenhouse gasses cars, factories and other polluters will put into the air. There is also some doubt in some details of the climate models. Climate commitment studies predict that even if levels of greenhouse gases were to stay the same, the temperature would still increase by 0.5 °C over the next one hundred years. This is because of the gasses that people have already put into the atmosphere.

    Although talk about global warming is often about temperature, global warming may cause changes in other things as well, including the sea level, rain, storms and other weather patterns. These may affect human activity via floods, droughts, heat waves, changes to farm productivity, etc. These changes will most likely also affect crops, animals and insects.

    A recent UN report indicates livestock generate more greenhouse gases around the world than the transport.[3] A senior UN official and co-author of the report Henning Steinfeld said "Livestock are one of the most significant contributors to today's most serious environmental problems."[4





    Greenhouse effect

    The greenhouse effect is caused by an atmosphere containing gases that absorb and emit infrared radiation. Greenhouse gases trap heat within the surface-troposphere system, causing heating at the surface of the planet or moon. This mechanism is fundamentally different from that of an actual greenhouse, which works by isolating warm air inside the structure so that heat is not lost by convection. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.

    The black body temperature of the Earth is 5.5 °C. Since the Earth reflects about 28% of incoming sunlight, in the absence of the greenhouse effect the planet's mean temperature would be far lower - about -18 or -19 °C instead of the much higher current mean temperature, about 14 °C.

    Global warming, a recent warming of the Earth's surface and lower atmosphere, is believed to be the result of a strengthening of the greenhouse effect mostly due to human-produced increases in atmospheric greenhouse gases.

    Basic mechanism

    The Earth receives energy from the Sun mostly in the form of visible light and nearby wavelengths, largely in the range 0.2 - 4 μm, corresponding to the Sun's radiative temperature of 6,000 K.[11] About 50% of the sun's energy is absorbed at the Earth's surface and the rest is reflected or absorbed by the atmosphere. The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as the Idealized greenhouse model, show this heat being lost as thermal radiation. The reality is more complex: the atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for "window" bands), and most heat loss from the surface is by sensible heat and latent heat transport. Radiative energy losses become increasingly important higher in the atmosphere largely because of the decreasing concentration of water vapor, an important greenhouse gas. It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid-troposphere, which is effectively coupled to the surface by a lapse rate.

    Within the region where radiative effects are important the description given by the idealized greenhouse model becomes realistic: The surface of the Earth, warmed to a temperature around 255 K, radiates long-wavelength, infra-red heat in the range 4 - 100 μm.[11] At these wavelengths, greenhouse gasses that were largely transparent to incoming solar radiation are more absorbent.[11] Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers. In order to maintain its own equilibrium, it re-radiates the absorbed heat in all directions, both upwards and downwards. This results in more warmth below, while still radiating enough heat back out into deep space from the upper layers to maintain overall thermal equilibrium. Increasing the concentration of the gases increases the amount of absorption and re-radiation, and thereby further warms the layers and ultimately the surface below.[7]

    Role in climate change

    Strengthening of the greenhouse effect through human activities is known as the enhanced (or anthropogenic) greenhouse effect.[14] This increase in radiative forcing from human activity is attributable mainly to increased atmospheric carbon dioxide levels.[15]

    CO2 is produced by fossil fuel burning and other activities such as cement production and tropical deforestation.[16] Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm [17] in 1960 to about 389 ppm in 2010. The current observed amount of CO2 exceeds the geological record maxima (~300 ppm) from ice core data.[18] The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first described in 1896 by Svante Arrhenius, has also been called the Callendar effect.

    Because it is a greenhouse gas, elevated CO2 levels contribute to additional absorption and emission of thermal infrared in the atmosphere, which produce net warming. According to the latest Assessment Report from the Intergovernmental Panel on Climate Change, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".[19]

    Over the past 800,000 years,[20] ice core data shows unambiguously that carbon dioxide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm.[21] Paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.[22]HYPERLINK \l "cite_note-22"[23]

    Bodies other than Earth

    In our solar system, Mars, Venus, and the moon Titan also exhibit greenhouse effects.[27] Titan has an anti-greenhouse effect, in that its atmosphere absorbs solar radiation but is relatively transparent to infrared radiation. Pluto also exhibits behavior similar to the anti-greenhouse effect.[28]HYPERLINK \l "cite_note-28"[29]

    A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.[30] A runaway greenhouse effect involving carbon dioxide and water vapor is thought to have occurred on Venus.[31]



    Aerosols and soot

    Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.[38] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James E. Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been driven mainly by non-CO2 greenhouse gases.[39]

    In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.[40] Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[41] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight.[42]

    Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[43] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[44] The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.[45]

    Climate models

    The main tools for projecting future climate changes are mathematical models based on physical principles including fluid dynamics, thermodynamics and radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact combinations of models for different parts of the Earth. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes.[55] Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes.[56] Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.[57]

    Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.[58]HYPERLINK \l "cite_note-58"[59]HYPERLINK \l "cite_note-59"[60] Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[2]

    Models are also used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.[3]

    The physical realism of models is tested by examining their ability to simulate current or past climates.[61] Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.[33] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic shrinkage has been faster than that predicted.[62]

      الوقت/التاريخ الآن هو الخميس ديسمبر 08, 2016 6:03 pm