前程百利小编带来关于温室效应的托福阅读材料。大气能使太阳短波辐射到达地面，但地表受热后向外放出的大量长波热辐射线却被大气吸收，这样就使地表与低层大气温作用类似于栽培农作物的温室，故名温室效应。环境问题频现，引发人类关注。希望本文中的相关背景知识可以帮助到正在备考的考生。
The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet’s lower atmosphere and surface. It was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.Existence of the greenhouse effect as such is not disputed even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.
Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).The major greenhouse gases are water vapor which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO2) which causes 9–26 percent; methane (CH4) which causes 4–9 percent; and ozone (O3) which causes 3–7 percent.Clouds also affect the radiation balance but they are composed of liquid water or ice and so are considered separately from water vapor and other gases.
Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere leading to increased radiative forcing from CO2 methane tropospheric ozone CFCs and nitrous oxide. The concentrations of CO2 and methane have increased by 36% and 148% respectively since the mid-1700s. These levels are much higher than at any time during the last 650000 years the period for which reliable data has been extracted from ice cores.Less direct geological evidence indicates that CO2 values this high were last seen about 20 million years ago. Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change particularly deforestation.
CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic sociological technological and natural developments. Accordingly the IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios ranging from 541 to 970 ppm by the year 2100.Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal tar sands or methane clathrates are extensively exploited.
The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence but substantial ozone depletion did not occur until the late 1970s.Tropospheric ozone contributes to surface warming.
以上就是前程百利小编为大家带来的托福阅读材料，平时的阅读练习是托福考试阅读备考的重要一环。希望大家利用好每份材料，尽可能多地从中获取知识。预祝大家托福考试取得好成绩。
