Global warming refers to an increase in the earth's temperature. This temperature change in turn causes a change in climate. A warmer earth may lead to changes in rainfall patterns, a rise in sea level, and different impacts on plants, wildlife, and humans.

When scientists talk about global warming, they are mainly talking about what is caused by humans. Finding out how we impact the environment makes it possible for us to change our behaviours. Scientists think that the things we do that send gasses into the air are making our planet warmer. During the Industrial Revolution, we began changing our climate through agriculture and industry. Both agriculture and industry release huge amounts of gasses into the air. These gasses then help trap the sun's radiation in the earth's atmosphere. Because of this, the earth is warming up. The warmer the earth gets, the more water vapor is released into the air. Water vapor is a gas that keeps heat, which makes the global warming even worse.

The Industrial Revolution was the time when people started using machines to make life easier. Industry sends out gasses by burning fuels. The Industrial Revolution made it easier for humans to start their lives in any place on Earth. People started cutting down forests which recycle gasses that keep heat in the atmosphere. This makes it even harder to get rid of the gasses we create.

Global warming is a big problem that must be solved soon. If everybody joins together, there is a chance that we can stop the process.

(Adapted from TOEFL Junior Reading)

What does the word "they" in the second paragraph refer to?