Step 1 Revision
1．Check the homework exercises.
2．Dictate some new words and expressions.
Step 2 Warming up
Ask the students to compare the two sentences below.The second sentence is from the reading passage.Translate them into Chinese and then discuss any difference in meaning and form.Ask the students to explain why “it” is used in the second sentence.
Human activity has caused this global warming.
It_is human activity_that has caused this global warming.
Tell the students there are two more sentences in the text in which “it” is used for emphasis.Find them and then write them down.
________________________________________________________________________
________________________________________________________________________
Suggested answers：
1．It_was a scientist called Charles Keeling，who made accurate measurements of the amount of carbon dioxide in the atmosphere from 1957 to 1997.
2．...it_is the burning of more and more fossil fuels that has resulted in this increase in carbon dioxide.
