Реферат Global warming 2
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"Global warming" has been introduced by the scientific community and the media as the term that encompasses all potential changes in climate that result from higher average global temperatures. Hundreds of scientists from many different countries are working to understand global warming and have come to a consensus on several important aspects. In general, Global warming will produce far more profound climatic changes than simply a rise in global temperature.
An analysis of temperature records shows that the Earth has warmed an average of
The Earth's climate is the result of extremely complex interactions among the atmosphere, the oceans, the land masses, and living organisms, which are all warmed daily by the sun's energy. This heat would radiate back into space if not for the atmosphere, which relies on a delicate balance of heat-trapping gases - including water vapor, carbon dioxide, nitrous oxide, and methane - to act as a natural "greenhouse," keeping in just the right amount of the sun's energy to support life.
For the past 150 years, though, the atmospheric concentrations of these gases, particularly carbon dioxide, have been rising. As a result, more heat is being trapped than previously, which in turn is causing the global temperature to rise. Climate scientists have linked the increased levels of heat-trapping gases in the atmosphere to human activities, in particular the burning of fossil fuels (coal, oil, and natural gas for heating and electricity; gasoline for transportation), deforestation, cattle ranching, and rice farming.
As the Earth's climate is the result of extremely complex interactions, scientists still cannot predict the exact impact on the earth's climate of these rising levels of heat-trapping gases over the next century. The current best estimate is that if carbon dioxide concentrations double over preindustrial levels, according to the scientific possible scenarios, an atmospheric doubling of carbon dioxide could occur as early as 2050.
Global warming is the increase in the average temperature of Earth's near-surface air and oceans since the mid-20th century. Global surface temperature increased 0.74 ±
An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[9] Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain.
Political and public debate continues regarding global warming, and what actions to take in response. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions.
The twelve warmest years of the twentieth century have occurred since 1980. The Earth’s warmest years since 1861 have been: 1981, 1983, 1987, 1988, 1989, 1990, 1991, 1994, 1995, 1996, 1997 and 1998. 1997 and 1998 were the two warmest years recorded during that period. This lends support to the assumption that the Earth’s climate is warming. However, it may take another decade of continued increases in global temperatures to provide conclusive evidence that the world’s climate is warming as a result of the enhanced greenhouse effect.
Global surface air temperature in 1997 was warmer than any previous year this century, marginally exceeding the temperature of 1995. Part of the current global warmth is associated with the tropical El NiЯo, without which a record global temperature would probably not have occurred.
Global surface temperatures in 1998 set a new record for the period of instrumental measurements, report NASA/GISS researchers who analyzed data collected from several thousand meteorological stations around the world. The global temperature exceeded that of the previous record year, by such a wide margin that the 1998 calendar year is certain to also set a new record. The
Until recently, researchers were uncertain whether Climate developments reflected natural variations in the Earth, or whether in fact human activities contributed to the warming. The latest observed data reveals some striking trends:
- All 10 of the warmest years on record have occurred in the last 15 years.
- The 1990s have already been warmer than the 1980s - the warmest decade on record - by almost
- The global average surface temperature has risen 0.5°-
In 1995, scientists with the Intergovernmental Panel on Climate Change - the authoritative international body charged with studying this issue-reached a conclusion in the Second Assessment Report, which summarizes the current state of scientific knowledge on global warming, also called climate change.
For the first time ever, the Panel concluded that the observed increase in global average temperature over the last century "is unlikely to be entirely natural in origin" and that "the balance of evidence suggests that there is a discernible human influence on global climate."
The Cause
The Earth's climate is the result of extremely complex interactions among the atmosphere, the oceans, the land masses, and living organisms, which are all warmed daily by the sun's energy. This heat would radiate back into space if not for the atmosphere, which relies on a delicate balance of heat-trapping gases - including water vapor, carbon dioxide, nitrous oxide, and methane - to act as a natural "greenhouse," keeping in just the right amount of the sun's energy to support life.
For the past 150 years, though, the atmospheric concentrations of these gases, particularly carbon dioxide, have been rising. As a result, more heat is being trapped than previously, which in turn is causing the global temperature to rise. Climate scientists have linked the increased levels of heat-trapping gases in the atmosphere to human activities, in particular the burning of fossil fuels (coal, oil, and natural gas for heating and electricity; gasoline for transportation), deforestation, cattle ranching, and rice farming.
But Global Warming has received much press in the past decade. There are many questions like these ones. Could the earth’s climate really heat up? What are the causes if such a warming occurs? Is global warming a theory and thrue or false theory at that?
These questions and more are what climate scientists are asking themselves daily. So, there are two sides to every story and both are discussed in the media.
The Impacts
As the Earth's climate is the result of extremely complex interactions, scientists still cannot predict the exact impact on the earth's climate of these rising levels of heat-trapping gases over the next century. But there is striking agreement among most climate scientists about what is likely to occur. Poureful climate models suggest that the planet will warm over the next century at a more rapid rate than ever before recorded. The current best estimate is that if carbon dioxide concentrations double over preindustrial levels, global average surface temperatures will rise between 1.8° and
- severe stress on forests, wetlands, and other natural habitats
- dislocation of agriculture and commerce
- expansion of the earth's deserts
- melting of polar ice caps and consequent rise in the sea level
- more extreme weather events
External forcing
External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radioactive forcing due to changes in atmospheric composition (mainly greenhouse gas concentrations), changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun. Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.
Aerosols and soot
Ship tracks over the Atlantic Ocean on the east coast of the
Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 up to the present. 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.
In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget. 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. 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.
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. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface. The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in
What can influence a climate?
Variations of radius and elongation of a terrestrial orbit. The distance from the Earth to the Sun changes not only on scales of times of an order of 100 million years, but also with the period about 20 thousand years.
Fluctuations of an inclination of a terrestrial axis. The inclination of a terrestrial axis to an orbit plane makes 23,5 ° and hesitates in size 1 ° for tens and hundred thousand years. These changes influence temperature contrast between high and low widths.
Fluctuations of intensity of space beams. Space beams ionise atoms in atmosphere of the Earth. Ions serve as the centres of condensation of water steam and promote formation of clouds that raises albedo the Earth. Intensity of space beams varies at movement of Solar system on the Galaxy.
Change of luminosity of the Sun. Now the quantity of the energy arriving from the Sun, fluctuates very slightly (approximately on 0,1 %). Meanwhile it is impossible to exclude more considerable fluctuations on long intervals of time.
Hotbed gases in atmosphere. Keep infra-red radiation of the Earth, interfering with its leaving in space.
Changes of landscapes. The quantity of disseminated radiation depends on character of a terrestrial surface and vegetation on it and finally albedo the Earth. In particular, essential influence on a landscape is rendered by agriculture and an urbanization.
Falling of asteroids, large volcanic eruptions, nuclear explosions on a surface of the Earth. Emission of aerosols in a stratosphere reduces quantity of the solar energy arriving to the Earth, and the dust in troposphere increases overcast — so-called effect of "nuclear winter». Duration — from several months to tens years.
Due to the enormous complexity of the atmosphere, the most useful tools for gauging future changes are 'climate models'. These are computer-based mathematical models which simulate, in three dimensions, the climate's behavior, its components and their interactions. Climate models are constantly improving based on both our understanding and the increase in computer power, though by definition, a computer model is a simplification and simulation of reality, meaning that it is an approximation of the climate system. The first step in any modeled projection of climate change is to first simulate the present climate and compare it to observations. If the model is considered to do a good job at representing modern climate, then certain parameters can be changed, such as the concentration of greenhouse gases, which helps us understand how the climate would change in response. Projections of future climate change therefore depend on how well the computer climate model simulates the climate and on our understanding of how forcing functions will change in the future.
The IPCC Special Report on Emission Scenarios determines the range of future possible greenhouse gas concentrations (and other forcings) based on considerations such as population growth, economic growth, energy efficiency and a host of other factors. This leads a wide range of possible forcing scenarios, and consequently a wide range of possible future climates.
According to the range of possible forcing scenarios, and taking into account uncertainty in climate model performance, the IPCC projects a best estimate of global temperature increase of 1.8 -
Precipitation is also expected to increase over the 21st century, particularly at northern mid-high latitudes, though the trends may be more variable in the tropics, with much of the increase coming in more frequent heavy rainfall events. However, over mid-continental areas summer-drying is expected due to increased evaporation with increased temperatures, resulting in an increased tendency for drought in those regions.
Snow extent and sea-ice are also projected to decrease further in the northern hemisphere, and glaciers and ice-caps are expected to continue to retreat.
Quite probably that in all event the natural processes having the same nature, as alternation of small and big glacial ages are guilty. Unexpectedly high today's speed of change of temperature can be only short-term episode which will appear subsequently ordinary splash on the schedule. It is impossible to exclude also that in the past already there were such fast fluctuations, — is simple at "record" in glacial cores the information on speed of changes was deformed, for example, at the expense of diffusion.
Results of researches all the same glacial cores have forced to doubt validity of requirements of the Kiotsky report. The matter is that increase of level CO2 in atmosphere frequently did not precede, and followed warming. That is it was not the reason, and a warming consequence. It is easy to explain the mechanism of this phenomenon — at rise in temperature in atmosphere there is the carbonic gas dissolved in water (and here it in 60 times more, than in air) and being in firm breeds. This effect can strengthen, of course, warming, but it is not so obligatory that growth of maintenance CO2 can provoke warming if the climatic system "is not ready" to it.
Another matter if terrestrial atmosphere was in a condition of shaky balance, being it is ready to pass at any moment in a warming mode. In this case any, even small influence — is unimportant, natural or artificial — can quite provoke the beginning of such transition just as the small stone causes an avalanche.
