The Earth’s climate is always changing. In the planet’s long history, there were Ice Ages when much of the land was covered with glaciers. There were hot steamy times when plants and animals that now live only in the tropics lived on almost every continent. So global warming—and global cooling—is nothing new.

If change is natural, what’s different this time?

Today’s global warming is different for two reasons:

1) The cause of the change.
2) How fast it is happening.

In the past, climate changes happened because of natural cycles. But this time, humans are the cause. People have been adding extra carbon dioxide to the atmosphere since the Industrial Revolution. This is the time in the late 1700s and early 1800s when people started using machines instead of doing things by hand—and started burning fossil fuels for energy to run the machines.