How has climate changed over time?

In the last 3 million years, climate has gone between tens of thousands of years of cooling followed by a much shorter period of warming. Today's rapid warming is happening 20 times faster than our historical pace. The causes are both natural- and human-based. The consequences of a hotter earth are substantial.
 

Nature impacts climate over time: 

• Land masses changing position through tectonic plate movement
• Changes in ocean currents (also due to plate movement)
• Changes in solar energy received by the earth due to slight changes in earth's orientation and distance from the sun
• Very long volcanic events lasting millions of years
• Short term changes from brief volcanic events and meteorite impacts

The cooling periods created huge sheets of ice that covered much of North America, Europe and Asia, and to a lesser extent the southern hemisphere. The current theory on the cause of ice ages relate to changes in earth's tilt, distance from the sun, and the wobble of its axis. All of these slowly change over tens of thousands of year and correlates very closely to when ice ages occur.
 

Enhanced warming of earth’s lower atmosphere is affecting that natural cycle.

“Models predict that Earth will warm between 2 and 6 degrees Celsius in the next century. When global warming has happened at various times in the past two million years, it has taken the planet about 5,000 years to warm 5 degrees. The predicted rate of warming for the next century is at least 20 times faster. This rate of change is extremely unusual” NASA Earth Observatory