...s have been ell documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities.Its ell accepted by scientists that greenhouse gases trap heat in the Earths atmosphere and tend to arm the planet. By increasing the levels of greenhouse gases in the atmosphere, human activities are strengthening Earths natural greenhouse effect. The key greenhouse gases emitted by human activities remain in the atmosphere for periods ranging from decades to centuries.A arming trend of about 1F has been recorded since the late 19th century. arming has occurred in both the northern and southern hemispheres, and over the oceans. Confirmation of 20th-century global arming is further substantiated by melting glaciers, decreased sno cover in the northern hemisphere and even arming belo ground.hats Likely but not CertainFiguring out to hat extent the human-induced accumulation of greenhouse gases since pre-industrial times is responsible for the global arming trend is not easy. This is because other factors, both natural and human, affect our planets temperature. Scientific understanding of these other factors most notably natural climatic variations, changes in the suns energy, and the cooling effects of pollutant aerosols remains incomplete.Nevertheless, the Intergovernmental Panel on Climate Change IPCC stated there as a discernible human influence on climate and that the observed arming trend is unlikely to be entirely natural in origin. In the most recent Third Assessment Report 2001, IPCC rote There is ne and stronger evidence that most of the arming observed over the last 50 years is attributable to human activities.In short, scientists think rising levels of greenhouse gases in the atmosphere are contributing to global arming, as ould be expected but to hat extent is difficult to determine at the present time.As atmospheric levels of greenhouse gases continue to rise, scientists estimate average global temperatures ill continue to rise as a result. By ho much and ho fast remain uncertain. IPCC projects further global arming of 2.2-10F 1.4-5.8C by the year 2100. This range results from uncertainties in greenhouse gas emissions, the possible cooling effects of atmospheric particles such as sulfates, and the climates response to changes in the atmosphere.The IPCC states that even the lo end of this arming projection ould probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes ould include considerable natural variability.hat are the Big UnknonsScientists have identified that our health, agriculture, ater resources, forests, ildlife and coastal areas are vulnerable to the changes that global arming may bring. But projecting hat the exact impacts ill be over the 21st century remains very difficult. This is especially true hen one asks ho a local region ill be affected.Scientists are more confident about their projections for large-scale areas e.g., global temperature and precipitation change, average sea level rise and less confident about the ones for small-scale areas e.g., local temperature and precipitation changes, altered eather patterns, soil moisture changes. This is largely because the computer models used to forecast global climate change are still ill-equipped to simulate ho things may change at smaller scales. iSee the U.S. Climate section for more detail on climate models.sSome of the largest uncertainties are associated ith events that pose the greatest risk to human societies. IPCC cautions, Complex systems, such as the climate system, can respond in non-linear ays and produce surprises. There is the possibility that a armer orld could lead to more frequent and intense storms, including hurricanes. Preliminary evidence suggests that, once hurricanes do form, they ill be stronger if the oceans are armer due to global arming. Hoever, the jury is still out hether or not hurricanes and other storms ill become more frequent.More and more attention is being aimed at the possible link beteen El Nio events the periodic arming of the equatorial Pacific Ocean and global arming. Scientists are concerned that the accumulation of greenhouse gases could inject enough heat into Pacific aters such that El Nio events become more frequent and fierce. Here too, research has not advanced far enough to provide conclusive statements about ho global arming ill affect El Nio.Living ith UncertaintyLike many pioneer fields of research, the current state of global arming science cant alays provide definitive ansers to our questions. There is certainty that human activities are rapidly adding greenhouse gases to the atmosphere, and that these gases tend to arm our planet. This is the basis for concern about global arming.The fundamental scientific uncertainties are these Ho much more arming ill occur Ho fast ill this arming occur And hat are the potential adverse and beneficial effects These uncertainties ill be ith us for some time, perhaps decades.Global arming poses real risks. The exact nature of these risks remains uncertain. Ultimately, this is hy e have to use our best judgement guided by the current state of science to determine hat the most appropriate response to global arming should be. An IntroductionAccording to the National Academy of Sciences, the Earths surface temperature has risen by about 1 degree Fahrenheit in the past century, ith accelerated arming during the past to decades. There is ne and stronger evidence that most of the arming over the last 50 years is attributable to human activities. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly ho earths climate responds to them. Go to the Emissions section for much more on greenhouse gases.Our Changing AtmosphereEnergy from the sun drives the earths eather and climate, and heats the earths surface in turn, the earth radiates energy back into space. Atmospheric greenhouse gases ater vapor, carbon dioxide, and other gases trap some of the outgoing energy, retaining heat somehat like the glass panels of a greenhouse.ithout this natural greenhouse effect, temperatures ould be much loer than they are no, and life as knon today ould not be possible. Instead, thanks to greenhouse gases, the earths average temperature is a more hospitable 60F. Hoever, problems may arise hen the atmospheric concentration of greenhouse gases increases. Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15. These increases have enhanced the heat-trapping capability of the earths atmosphere. Sulfate aerosols, a common air pollutant, cool the atmosphere...
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