Earth’s climate in the past
The Earth was formed about 4.5 billion years ago. It's hard to say exactly what the Earth's daily weather was like in any particular place on any particular day thousands or millions of years ago. But we know a lot about what the Earth's climate was like way back then because of clues that remain in rocks, ice, trees, corals, and fossils. These clues tell us that the Earth's climate has changed many times before. There have been times when most of the planet was covered in ice, and there have also been much warmer periods. Over at least the last 650,000 years, temperatures and carbon dioxide levels in the atmosphere have increased and decreased in a cyclical pattern. You can see this pattern in the graph below.
Figure : These graphs are based on the Vostok ice core from Antarctica. They do not include the most recent increases in carbon dioxide and temperature caused by humans. Notice the strong connection between carbon dioxide and temperature. Source: EPA's Climate
These two graphs show how the amount of carbon dioxide and the Earth's temperature have changed over the last 650,000 years. Both graphs show a similar up-and-down pattern. People didn't cause the climate change that occurred thousands or millions of years ago, so it must have happened for other natural reasons.
Changes in Earths orbit: The shape of the Earth's orbit around the sun naturally changes over time, and so does the way the Earth tilts toward the sun. Many of these changes happen in cycles that repeat over tens of thousands of years. These changes affect how much of the sun's energy the Earth absorbs, which in turn affects the Earth's temperature. Over at least the last few million years, these cycles likely caused the Earth to alternate between cold and warm periods. About 20,000 years ago, ice sheets covered large parts of North America, where they extended as far south as where Chicago is now. In some places, this ice was a mile deep! For the last few thousand years, we've been in a relatively warmer period.
Figure : This map shows an outline of the ice sheets that covered a large part of North America about 20,000 years ago (Source: Adapted from NASA (2011))
Changes in Sun’s energy: The sun goes through sunspot cycles every 11 years or so. During times when there are sunspots, dark spots—some as big as 50,000 miles wide—move across the surface of the sun. When this happens, the sun gives off slightly more energy, which makes the Earth a bit warmer. The sun also goes through longer term changes that affect how much energy it gives off.
Photosynthesis: The Earth's first billion years were very different from the conditions today. The sun was cooler then, but the planet was generally warmer. That's because there were a lot of greenhouse gases, like carbon dioxide and methane, in the atmosphere. Also, the atmosphere back then contained very little oxygen. It was a very different world—a world without people or the kinds of plants and animals that thrive in today's climate. But photosynthesis, which became common about 2 billion years ago, changed all that. During photosynthesis, plants take carbon dioxide out of the atmosphere and replace it with oxygen. Photosynthesis permanently changed the atmosphere by adding more oxygen to the air while reducing the amount of greenhouse gases.
Volcanic eruptions: When volcanoes erupt, they spew more than red hot lava! They also add carbon dioxide to the atmosphere, along with dust, ash, and other particles called aerosols. At certain times during the history of the Earth, some very active volcanoes added a lot of carbon dioxide to the atmosphere, causing the planet to get warmer. However, most of the time, including today, the major effect from volcanoes is actually cooling the Earth because aerosols block some sunlight from reaching us. If an eruption is big enough to launch these particles high into the atmosphere, it can lead to slightly cooler temperatures around the world for a few years.