英文阅读,Have you ever been inside a greenhouse?A greenhouse is

英文阅读,
Have you ever been inside a greenhouse?A greenhouse is a glass building where plants are grown.It's very warm,because the glass walls let the sunlight in but keep the heat from getting out.The earth is like being inside a large greenhouse.Light from the sun warms land,water,and air.In turn,the warmed-up land,water,and air give off heat,which rises up toward the sky.Then gases in the sky are like a greenhouse's glass walls.They keep heat from going to space,and the earth stays warm.For the past 10,000 years,tempertures of the earth have changed much.But,for the past 100 years or so,scientists have niticed the earth seems to be warming up more than usual.We call this global warming.
1.What is a greenhouse?Why is it very warm inside?
2.Why are the land,water and air on the earth warm?
3.What happens to the heat given off by the land,water and air?
4.According to some scientists,when did global warming begin?