What has happened to the forests of the U.S.A. in the last 350 years? In 1620. about half the U.S.A. was covered by forests. In 1850, about a third was covered by forests. Today the forests have almost gone. But too many trees are still being cut down. There is a lot of sand left. China doesn't want to follow the U. S. A.'s example. So China has built a new Great Wall across the northern part of China, This time, it is a "Great Green Wall" of trees, millions of trees. The Great Green Wall is 7,000 kilometers long, and between 400 and 1,700 kilometers wide. The Green Wall will stop the wind from blowing the earth away. It will stop the sand from moving towards the rich farmland in the south. But more "Great Green Walls" are still needed, and not only in China. They must be built all over the world.

(1)How much of the U.S.A. was covered by forests 230 years after 1620?

(2)What is the Great Green Wall?

(3)How can we stop the wind from blowing the earth away?