This xkcd comic plots Earth’s average temperature from 22,000 years ago, near the end of the last ice age glaciation, to the present day. It answers the claim that “the climate has changed before” by showing what those earlier changes looked like.
At the start of the timeline, Earth was about 4°C colder than in the late 20th century, and ice almost a mile thick covered the site of Boston. Over the following thousands of years, the temperature line drifts slowly upward while humans spread, farm and build civilisations.
The line barely moves through recorded history. Then, after fossil fuel carbon dioxide emissions start rising rapidly, it swings sharply to the warm side. The comic ends with projections to 2100 for a best case with immediate action to limit emissions, an optimistic scenario and the current path.
via xkcd: Earth Temperature Timeline