根据短文内容，回答下列问题
Have you ever been inside a greenhouse? A greenhouse is a glass building
where plants are grown. It's very warm, because the glass wails 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 beat, which rises up towards the sky.
Then gases in the sky are like a greenhouse's glass walls. They keep heat from
going into space, and then earth stays warm. For the past 10,000 years, temperatures
of the earth have not changed much. But, for the past 100 years or so, scientists
have noticed the earth seems to be warming up more than usual. We call this
global warming.
