The detailed causes of the recent warming remain an active field of The scientific consensus is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era， and the observed warming cannot be satisfactorily explained by natural causes This attribution is clearest for the most recent 50 years， for which the most detailed data are The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and Existence of the greenhouse effect as such is not The question is instead how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of some greenhouse Recent increases in atmospheric carbon dioxide (CO2) The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring， and declines during the Northern Hemisphere growing season as plants remove some CO2 from the Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F)， without which Earth would be On Earth， the major greenhouse gases are water vapor， which causes about 36–70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO2)， which causes 9–26 percent; methane (CH4)， which causes 4–9 percent; and ozone， which causes 3–7 Human activity since the industrial revolution has increased the concentration of various greenhouse gases， leading to increased radiative forcing from CO2， methane， tropospheric ozone， CFCs and nitrous The atmospheric concentrations of CO2 and methane have increased by 31% and 149% respectively since the beginning of the industrial revolution in the mid- These levels are considerably higher than at any time during the last 650，000 years， the period for which reliable data has been extracted from ice From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over the past 20 Most of the rest is due to land-use change， in particular CO2 concentrations are expected to rise due to ongoing burning of fossil fuels and land-use The rate of rise will depend on uncertain economic， sociological， technological， and natural The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios， ranging from 541 to 970 ppm by the year Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100， if coal， tar sands or methane clathrates are extensively
范文一 Nowadays， online education has become very popular among Those who want to learn something， whether they are young or old， all can have their classes on different Some can even get a certificate or diploma after completing the required Therefore， the number of people receiving this kind of education is on the Why is it so popular? The reasons are as For one thing， the quick development of the Internet has provided many people a chance to attend class given by famous schools and For another， compared with having classes at school， online education saves our time， energy and money， especially to those who are too busy to study full The most important reason is that the schedule is flexible so that we can have a free choice to study what we need badly and decide when， where and how to As far as I'm concerned， I appreciate this new form of education and also have got some benefits from It's indeed a helpful complement to the traditional education 范文二 My View on Online Education We live in an information Everyday we are confronted with large amounts of information from a variety of channels such as TV， newspaper and But the most fast and convenient source may be Internet， by which we cannot only keep pace with society， but also enrich our This long-distance education or on-line education has been widely accepted and Why does it become a rage? There are mainly two On the one hand， on-line education can provide choice lectures and classes given by famous teachers or schools across the This will tremendously broaden our view and diversify our knowledge On the other hand， in this fast-pace society， people cannot afford so much time to attend regular Therefore， the flexible schedule of on-line education brings much convenience for their For my part， I've benefited a lot from this form of It opens a window for me and is and will be my life-long 范文三：My View on Online Education Being online is no longer something new or fresh in our To some extent， it has become part of our daily We can do a lot of things online， such as searching for information and communicating with friends far But recently another helpful online activity has become very “in” That is online Why could online education be so popular within such a short period of time? Among other reasons， the quick development of the Internet should be an essential one， which makes all our dreams of attending class in the distance Another underlying reason is the quick development of both society and Today， modern science and technology is developing with fantastic To catch up with this development we all feel an urgent and strong desire to However， due to the great pace of modern society， many people are too busy to study full time at Online education just comes to their Personally， I appreciate this new form of It’s indeed a helpful complement to the traditional educational It can provide different learners with more flexible and versatile ways of Best of all， with online education， we can stick to our jobs and at the same time study and absorb the latest
This is the VOA Special English Economics RBusinesses are still slow to hire and unemployment remains near ten But an economic research group reported this week that the recession in the United States ended in June of last It was the longest since the nineteen James Stock is on the committee that dates the rises and falls of the business cycle for the National Bureau of Economic R The Harvard professor says this recession was in some ways a lot like others since World War TJAMES STOCK: "I think what I’ve been struck by is how these numbers basically look like all the other recessions we've had in the postwar "But one difference was the length -- eighteen Many recessions since the nineteen forties lasted less than a Another difference was the The economy shrank by more than four About eight million people lost their Professor Stock thinks job growth is likely to increase in the coming months based on current levels of JAMES STOCK: "I think that we are at a point right now where productivity has increased so much that for output to continue to grow， we will be seeing increases in "Some economic signs have Manufacturing， industrial production and consumer spending have made small But a Labor Department report shows that new claims for unemployment insurance rose unexpectedly last On Monday， President Obama took questions at a meeting shown on CNBC One questioner wondered if today's young Americans still could hope for a good QUESTIONER: "What I'm really hoping to hear from you is several concrete steps that you're going to take moving forward that will be able to re-ignite my generation， re-ignite the youth who are beset by student And I really want to know， is the American Dream dead for me?"BARRACK OBAMA: "Absolutely Look， we still have the best universities in the We’ve got the most dynamic private sector in the We've got the most productive workers in the There is not a country in the world that would not want to change places with "But the economy could change the balance of power in Congress in the November Critics say the president's economic stimulus programs have only put the country deeper in Economists disagree about the extent to which the spending has But many agree that the president's policies avoided a worse Or， as M Obama put it at the United Nations on Thursday: "The global economy has been pulled back from the brink of a "In Washington， central bank policy makers said inflation remains In fact， they said it may be too low to support their aims for price and wage And， Lawrence Summers， director of the president's National Economic Council， said he will leave at the end of the year to return to Harvard UAnd that's the VOA Special English Economics Report， written by Mario R I’m Steve E
