Earth's climate has always been changing. The graph below, generated from 800,000 years of ice core data, shows how temperature has fluctuated throughout history.

The cooler periods on this graph correspond to ice ages or glacial periods, when much of the northern hemisphere was covered by thick sheets of ice. The warmer regions of the graph indicate interglacial periods.

Although the planet's climate has always fluctuated, the rate of change has become more dramatic since the Industrial Revolution. This suggests that recent changes have anthropogenic origins.