The concentration of carbon dioxide in the atmosphere is measured as parts per million. One hundred parts per million is 0.01% of the atmosphere.

Carbon dioxide and other greenhouse gases usually precede major climate shifts. As greenhouse gas levels rise, more of the sun's energy is trapped in the atmosphere. Average global temperatures rise after the CO₂ levels rise (usually between one and four thousand years afterward). The same pattern occurs during period of falling levels of CO₂ and temperature.

Present-day levels of carbon dioxide are higher than any time in the past 400,000 years. This extremely high increase, almost 32%, has only occurred in the last 150 years. Humans have caused the increase by burning fossil fuels and deforestation (removing a major absorber, or sink, of greenhouse gases).

From roughly 235,000 years ago to roughly 130,000 years ago, between the two high points: A combination of changes in Earth's orbit, axis tilt, and atmosphere create a cycle of warming and cooling that lasts roughly 100,000 years from warmest point to warmest point.

35,000 years ago: Neanderthal humans become extinct.

30,000 to 12,000 years ago: The Beringia land bridge between North America and Asia is accessible on foot.

12,000 years ago: The last ice age, called the Wisconsin Glaciation in North America, ends.

9,000 years ago: People in the Fertile Crescent (Nile Valley and Mesopotamia) first begin farming.

6,000 years ago: People first begin to write. Earliest recorded history.