Climate system models are an important tool for interpreting observations and assessing hypothetical future. They are mathematical computer-based expression of the thermodynamics, fluid motions, chemical reaction, and radiative transfer of Earth climate that are as comprehensive as allowed by computational feasibility and by scientific understanding of their formulation. Their purpose is to calculate the evolving state of the global atmosphere, ocean, land surface, and sea ice in response to external forcing of both natural causes (such as solar and volcanic) and human causes (such as emissions and land uses), given geography and initial material composition. Such models have been in use for several decades. They are continually improved to increase their comprehensiveness with respect to spatial resolution, temporal duration, biogeochemical complexity, and representation of important effects of processes that can't practically be calculated on the global scale (such as clouds and turbulent mixing). Formulating, constructing, and using such models and analyzing, assessing, and interpreting their answers make climate system models large and expensive enterprises. For this reason, they often associated, at least in part, with national laboratories. The rapid increase over recent decades in available computational speed and power offers opportunities for more elaborate, more realistic models, but requires regular upgrading of the basic computer to avoid obsolescence.
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Climate change models calculate outcomes after taking into account the great number of climate variables and the complex interaction inherent in climate system. Their purpose is the creation of a synthetic reality that can be compared with the observed reality, subject to appropriate averaging of the measurements. Such models can be evaluated through comparison with observations, provided that suitable observations exist. Furthermore, model solutions can be diagnosed to assess contributing causes of particular phenomena. Because climate is uncontrollable (albeit influence able by humans), the models are the only available experimental laboratory for climate. They also are the appropriate high-end tool for forecasting hypothetical climates in the years and centuries ahead. However, climate models are imperfect. Their simulation skill is the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature.
The current norm for a climate system model is to include a full suite of physical representations for air; water, land, and ice with a geographic resolution scale of typically about 250 Km. Model solutions match the primary planetary-scale circulation, seasonal variability, and temperature structures with qualitative validity but still some remaining discrepancies. They show forced responses of the global-mean-temperature that corresponds roughly with its measured history over the past century, though this requires model adjustments. They achieve a stable equilibrium over millennial intervals with free exchanges of heat, water, and stress across the land and water surface. They also exhibit plausible analogues for the dominant modes of intrinsic variability, such as the ENSO (El Nino-Southern Oscillation is a climate pattern that occurs across the tropical Pacific Ocean on average every 5 years)
Although some important discrepancies still remain. At present, climate system models specify solar luminosity, atmospheric composition, and other agents of radiative forcing. A frontier for climate models is the incorporation of more complete biogeochemical cycles (such as carbon dioxide). The greater the sophistication and complexity of an atmospheric model, the greater the need for detailed multiple measurements. Which test whether the model continues to mimic observational reality? Application of climate models to past climate states encompass "snapshots" during particular millennia, but they don't yet provide for continuous evolution over longer intervals.
The Earth has an atmosphere, consisting mostly of nitrogen (78%), oxygen (21%) and number of greenhouse gases, which affect the balance of light and ultraviolet energy coming from the sun and heat energy leaving the Earth, the global energy balance. The greenhouse gases, including carbon dioxide, methane, nitrous oxide, water vapour, absorb energy at some wavelength of the electromagnetic spectrum, but allow energy at other wavelengths to pass through unimpeded. Shorter wavelength sunlight passes through the atmosphere relatively unimpeded, although the ozone layer does absorb a lot of higher wavelength ultraviolet energy. This heats up the Earth's surface, causing it to re-release energy back to the atmosphere. The lower wavelength heat energy given off by the Earth, however, is mostly trapped by greenhouse gases, and is prevented from escaping directly back into space. This trapped heat warms the Earth's surface on average by as much as 33Â°C. The process is called the natural greenhouse effect, and without it, the Earth would be as cold as the moon, about -18Â°C.
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The atmosphere also contains millions of microscopic particles called aerosols, which scatter sunlight. The scattering of sunlight by the atmosphere is important because like the natural greenhouse effect, this can affect the amount of energy stored in the atmosphere, and therefore the Earth's climate.
Changes in the composition of the atmosphere can be a mechanism of climate change. A change in greenhouse gas content of the atmosphere will affect the amount of energy stored in the atmosphere.
For example, if the amount of carbon dioxide in the atmosphere is increased, more heat will be trapped in the atmosphere. This enhanced greenhouse effect raises the Earth's surface temperature. Changes in the amount of carbon dioxide stored in the atmosphere occurred at the end of the last Ice Age. A 50% increase in carbon dioxide levels coincided with a 5Â°C rise in global average surface temperature. Today, mankind through the burning of fossil fuels for energy and transportation, and changing land use, has produced a substantial change in the atmospheric composition over most recent centuries, and it is feared that this continuing change will lead to global warming.
The oceans store an immense amount of heat energy, much more than the atmosphere, and consequently play a crucial role in the regulation of the global climate. As in the atmosphere, surface ocean currents assist in the transfer of heat from low to high latitudes. Warm water moves pole ward whilst cold water returns towards the equator. Energy is also transferred via moisture. Water evaporating from the surface of the oceans stores heat which is subsequently released when the moisture condenses to form clouds and rain.
Heat exchanges also occur vertically within the oceans, between surface water, usually the top 200 meters or so, and the deep water. Seawater in the high latitudes readily sinks, forming deep-water currents. Considerable evaporation of moisture takes place from the warm surface ocean currents as they travel towards the high latitudes. The salt which remains behind in the water after evaporation makes the water heavier or denser. As in the atmosphere the surface and deep-water currents of the world's oceans are inter-linked forming the global ocean circulation. Scientists have proposed that changes in this global ocean circulation influence climate changes over hundreds and thousands of years.
Shifts in the Earth's orbit around the Sun have been shown to be linked to changes in the Earth's climate over hundreds of thousands of years. Scientists now recognize however, that such orbital variations are on their own not enough to account for the shifts in global climate between cold Ice Ages and warm interglacial. Whilst orbital variations may indeed act as a pacemaker to warm-cold climate transitions, additional climate feedback processes have been invoked to explain the large changes in global average temperature of up to 5Â°C. Changes in the composition of the atmosphere, in particular the levels of the greenhouse gas carbon dioxide, have been shown, through reconstruction of palaeoclimatic records (Climate change that predates the instrumental period of direct weather observations is known as palaeoclimate change. Palaeoclimatology provides a longer perspective on climate variability that can improve our understanding of the climate system, and help us to predict future climatic changes as a result of man-made global warming. Evidence for palaeoclimatic change can be obtained by the study of natural phenomena which are climate-dependent. Such evidence comes from palaeoclimatic records), to account for the colder climate of the last Ice Age. A change in ocean circulation however, has been proposed as the trigger mechanism for the transition from a warm interglacial climate to a cold Ice Age 120,000 year ago, and the switch back again 14,000 years ago.
Scientists believe that at the end of the last warm interglacial period 120,000 years ago, high latitude seawater was gradually cooling as a consequence of a reduction in heat received by the Sun due to orbital variations. Colder seawater in the high latitudes will exhibit less deep-water circulation. Sea sediment records have revealed that this change in deep water formation is particularly evident in the Northern Hemisphere. Colder seawater loses less water through evaporation of moisture, and is therefore less salty and lighter. The lighter water finds it more difficult to sink, reducing or even shutting down completely that branch of the global ocean circulation. The warm pole ward-moving surface currents too, are slowed or redirected, and consequently less heat is transferred to the Polar Regions. Less heat, of course, means a colder climate, leading to the growth of ice sheets across the Northern Hemisphere landmasses and the development of a new Ice Age.
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Climate models have suggested that the loss of the warm surface Gulf Stream in the North Atlantic, which keeps the climate of Western Europe mild, could result in a regional drop in temperature of 6 to 8Â°C. In addition, such changes in ocean circulation could occur over relatively short periods, perhaps within 50 to 100 years. Rather paradoxically, scientists have now begun to speculate that global warming could threaten the present course and intensity of the Gulf Stream. In this climate scenario deep water formation is reduced, not by a reduction in moisture evaporation, but by a melting of ice from the Greenland ice cap. The influx of freshwater would reduce the saltiness of the seawater, making it lighter and more difficult to sink. Worryingly, such a pattern of climate change can be seen the palaeoclimatic record 11,000 years ago, at the beginning of the present interglacial warm period. Large volumes of melting ice from the retreating Northern Hemisphere ice sheets flowed into the North Atlantic, shut down the regional ocean circulation and cause a temporary fall in regional temperature of several degrees Celsius in just a few hundred years.
The land nearly 27% of the earth's land surface is covered by forest vegetation.Â Forests play a critical role in the earth's climate system.Â
Forest vegetation and soil affect the global carbon cycle by removing carbon dioxide from the atmosphere through photosynthetic processes.Â Forest vegetation is also known to have higher evapotranspiration rates than other land cover types. A forestation has been viewed as providing a mitigating effect to global warming because of the carbon uptake and enhanced cooling due to increased evapotranspiration.Â
However, the cooling effects of removing greenhouse gases like carbon dioxide from the atmosphere and increasing evapotranspiration rates with expanding forest coverage are often compensated by the warming effect of decreased surface albedo values associated with more forest coverage.Â New research is needed to examine the potential impacts of land cover changes, including a forestation, on the climate system in order to provide the scientific basis for adopting land use decisions that are meant to mitigate global warming.Â
We will talk about the ice. It has huge effect on climate system. The ice around the world includes Antarctica, the Arctic Ocean, and Greenland, northern Canada, northern Siberia and most of high mountains throughout the world. Without the ice around the world the heat would be more.
The impacts of the distributions of land ice and sea ice on planetary climate are extremely important, both regionally and globally. As the boundary between the ocean and atmosphere in the Polar Regions of earth, sea ice plays a critical role as both a leading indicator of climate change and as a key player in the global climate system. Roughly speaking, most of the solar radiation which is incident on snow-covered sea ice is reflected, while most of the solar radiation which is incident on darker sea water is absorbed. The sea ice packs serve as part of earth's polar refrigerator, cooling it and protecting it from absorbing too much heat from sunlight. The ratio of reflected sunlight to incident sunlight is called albedo.
While the albedo of snow-covered ice is close to 1 (larger than 0.8), the albedo of sea water is close to zero (less than 0.1). As more ice is melted, the albedo of the polar oceans decreases, leading to more solar absorption and warming, which in turn leads to more melting, in a positive feedback loop? It is believed that this so-called ice-albedo feedback has played an important role in the marked decrease in Arctic sea ice extent in summer.
The period for which we have instrumental records of climate change, such as observational records of temperature and rainfall, spans only a tiny fraction of Earth History. Furthermore, although we are now concerned with global warming due to mankind's greenhouse gas pollution of the atmosphere, this contemporary climate change should be placed in the context of much longer term changes in climate that have taken place quite naturally. Prehistoric climate change is known to climatologists and Earth scientists as palaeoclimatic change.
The global climate has shifted and varied for billions of year, perhaps since the Earth first had an atmosphere. The oldest palaeoclimatic records have allowed us to reconstruct climate fairly reliably during the last 500 million years. Over this time, the global climate has moved from extensive periods of global warmth to periods of global cold several times, each lasting 100 million years or more. Although today we are concerned about global warming, we do in fact lie in the middle of global ice-house climate, which began 40 million years ago, when the first permanent ice sheets formed on Antarctica. The change from the much warmer global climate which existed during the age of the dinosaurs, when global average temperature was perhaps 10Â°C higher than at present, is thought to have been caused by changes in the distribution of landmasses and the associated changes to energy redistribution throughout the climate system.
Within the long-term global icehouse climate, much shorter-term fluctuations in global climate have occurred. Relatively cold periods known as Ice Ages or glacial, each lasting roughly 100,000 years, are interspersed with much shorter warmer episodes or interglacial, lasting only 10,000 years. We now have a relatively clear record of such climatic fluctuations over the last 2 million years. Currently, the global climate lies within an interglacial. Global average temperature 20,000 years ago towards the end of the last Ice Age was some 5Â°C lower than today, when the north polar ice sheets were expanded to cover a considerably greater area of the continental Northern Hemisphere than is the case today. These glacial-interglacial fluctuations are believed to be driven by changes in the position of the Earth in its orbit around the Sun, and enhanced by climatic feedback processes which involve changes in ocean circulation and the greenhouse gas composition of the atmosphere.
Within the latest present interglacial period, further fluctuations in the global climate can be seen in the palaeoclimatic records and more recently in the instrumental records. During the last 1000 years, the climate has moved from a period of medieval warmth to a "Little Ice Age" between the 16th and 19th centuries, with changes of between 0.5 and 1Â°C in the global average surface temperature. Although it is not clear what has caused these climatic changes, variations in the Sun's energy output, ocean circulation and the occurrence of major volcanic eruptions are believed to play a part.
Most recently, we have entered a renewed period of global warming since the beginning of the 20th century that we suspect is the result of mankind's enhancement of the natural greenhouse effect through the pollution of the atmosphere. Global average temperature is now about the same as it was during the medieval warm period, although still much lower than it was 100 million years ago during the age of the dinosaurs.
Palaeoclimatology, from the Greek word "polios", meaning "ancient", is the study of past climates and past climate change, prior the period of instrumental records. The study of Palaeoclimatology can encompass much of Earth History, or at least that part of it for which reliable palaeoclimatic records are available to reconstruct palaeoclimatic.
Palaeoclimatology may be distinguished from climatology and contemporary climate change, which studies present day climate and climate changes restricted to the most recent period (the last 150 years) since instrumental records of daily weather observations have become available.
To reconstruct palaeoclimate, palaeoclimatologists cannot use direct observations of temperature, rainfall and other climatic variables. Instead they use proxy records of natural phenomena which are climate-dependent. These include analyses of tree rings, ice cores, sea sediments and even rock strata exposed at the Earth's surface which may hold clues to the state of the climate millions of years ago. Climate models run on computers may also be used to test theories about possible mechanisms of palaeoclimate change.