小题1:B
小题2:C
小题3:A
小题4:D
小题1:从第一段第四行“But it is now gradually agreed that the amount of carbon dioxide in the atmosphere will double from 0.03% to 0.06% in the next 50 years and that temperatures worldwide will rise by 2°Celsius.”可知现在只有0.03%的carbon dioxide在大气中，所以carbon dioxide在大气中只占a small portion（一小部分）。
小题1:从第四段第二行开始“n power stations, in factories and in our cars, we are burning more and more fossil fuels( coal, oil and natural gas ) and this produces huge quantities of CO2---18 billion tons of it enter the atmosphere every year. Added to this, the destruction of forests means that less CO2 can be converted into oxygen by plants. So, the amount of CO2 in the atmosphere is increasing every year.”可知有两个原因，一是more fossil fuels are burned，二是more forests are destroyed，所以应选C。
小题1:从第一段In the 1960s Professor Bolin predicted that the “greenhouse effect”, caused by an increase in the amount of carbon dioxide (CO2) in the atmosphere, would lead to important changes in the Earth’s climate.可知是指对气候的影响，直接体现在温度上，所以选A.
小题1:从第三段第二行When living creatures breathe out or when things are burned, CO2 enters the atmosphere. Until recently all of this was absorbed by plants, which converted it back into oxygen.可知是在解释how does the “greenhouse effect”operate，即温室效应怎样起作用，所以选D符合划线单词的意思.