Our Changing Atmosphere
我们所改变的大气
According to the National Academy of Sciences，the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century，with accelerated warming during the past two decades．There is new and stronger evidence that most of the warming 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．
根据美国国家科学院的说法，在过去的一个世纪里，地球表面温度上升了大约1华氏度，在过去的20年里加速变暖。有新的更有力的证据表明，过去50年的大部分变暖都可归因于人类活动。人类活动通过温室气体的积累改变了大气的化学成分——主要是二氧化碳、甲烷和一氧化二氮。
Energy from the sun drives the earth's weather and climate，and heats the earth's surface； in turn，the earth radiates energy back into
space．Atmospheric greenhouse gases，( water vapor，carbon dioxide，and other gases) trap some of the outgoing energy，retaining heat somewhat like the glass panels of a greenhouse．Without this natural \"greenhouse effect，\" temperatures would be much lower than they are now，and life as known today would not be
possible．Thanks to greenhouse gases，the earth's average temperature is a more hospitable 60°F．However，problems may arise when the atmospheric concentration of greenhouse gases increases。
太阳释放的能量形成了地球的气候和气候，并温暖地球的表面；而地球也转而将能量向太空辐射。大气中的温室气体（水蒸气、二氧化碳和其它气体）捕获住一些外溢的能量，形成像温室玻璃板一样的效果保留住热能。如果没有这种自然的“温室效应”，气温将会比现在低得多，今天已知的生命将不可能存在。幸亏有温室气体的存在，地球才会保持平均华氏60多度的宜人温度。然而，当温室气体的大气浓度增加时，问题可能会出现。
Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increase concentration of carbon dioxide．What has changed in the last few hundred years is the additional release of carbon dioxide by human activities．Fossil fuels burned to run cars and trucks，heat homes and businesses，and power factories are responsible for about 98% of US carbon dioxide emissions，24% of methane emissions，and 18% of nitrous oxide emissions．Increased agriculture，deforestation，landfills，industrial production，and mining also contribute a significant share of emissions．In 1997，the United States emitted about one-fifth of total global greenhouse gases．
科学家们普遍认为，化石燃料和其他人类活动的燃烧是二氧化碳浓度增加的主要原因。在过去的几百年里，人类活动增加了二氧化碳的排放。化石燃料燃烧来驱动汽车和卡车，为家庭和企业供暖，给工厂提供动力，它们负责了美国大约98%的二氧化碳排放，24%的甲烷排放，以及18%的一氧化二氮排放。农业、森林砍伐、垃圾填埋场、工业生产和
采矿叶的增加也造成了相当大的排放。1997年，美国排放的温室气体约占全球总量的五分之一。
The 20th century’s 10 warmest years all occurred in the last 15 years of the century． Of these， 1998 was the warmest year on record． The snow covers in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased． Globally， sea level has risen 4～8 inches over the past century． Worldwide precipitation over land has increased by about one percent． The frequency of extreme rainfall events has
increased throughout much of the United States．
二十世纪的10个最温暖的年份都发生在本世纪的最后15年。其中，1998年是有记录以来最热的一年。北半球的积雪和北冰洋的浮冰减少了。在全球范围内，海平面在过去的一个世纪里上升了4到8英寸。世界范围内的陆地降水增加了大约1个百分点。在美国的大部分地区，极端降雨事件的频率都在增加。
Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change． Scientists expect that the average global surface temperature could rise 1～4.5°F(0.6～2.5℃) in the next fifty years， and 2.2～10°F(1.4～5.8℃) in the next century， with significant regional
variation． Evaporation will increase as the climate warms， which will increase average global precipitation． Soil moisture is likely to decline in many regions， and intense rainstorms are likely to become more frequent． Sea level is likely to rise two feet along most of the US coasts． Calculation of climate change for
specific areas are much less reliable than global ones， and it is unclear whether regional climate will become more variable．
越来越多的温室气体聚集在一起便有可能加快气候变化的速度。科学家们预计，未来50年全球平均地表温度将上升1～4.5°F(0.6～2.5℃)，下世纪将上升2.2～10°F(1.4～5.8℃)，区域变化显著。随着气候变暖，蒸发作用也会加剧，从而造成全球平均降水量的增加。在许多地区，土壤湿度很可能会下降，强烈的暴雨很可能会变得更频繁。美国海岸大部分地区的海平面可能会上升两英尺。对特定地区气候变化的估计会远不如对全球气候变化的估计那么可靠，我们也不是很清楚局部地区的气候是否会变得更反复无常。
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