You may have missed this one -- a lot of people have -- but geologists take seriously the notion that around 700 million years ago, the entire earth was frozen over. This is generally called the "Snowball Earth" theory. All of or most of the ocean surface was covered with ice, and glaciers covered the continents. The climate at the equator was much like that of Antarctica today. The latest piece of evidence is the Canadian rocks below, which formed when Canada was near the equator but contain glacial deposits.
The period of the snowball earth was just before the "Cambrian Explosion," the sudden diversification of animal life into the phyla we know today. The life that did exist at the time, including blue-green algae, seems to have come through the "Cryogenian" period unscathed.
Why did it happen? We don't really know, but it may have been some combination of reduced solar energy with the distribution of the continents, which were all around the equator (land reflects more sunlight than the ocean). How did it end? Because of the build-up of greenhouse gases in the atmosphere, mainly carbon dioxide. Volcanoes are constantly releasing carbon dioxide into the air, but now most of it is removed by plants or by being incorporated into carbon-containing rocks like limestone. If the earth was covered with ice, plant life would have been greatly reduced and to some extent cut off from the atmosphere, and contact between the air and rocks would also have been reduced. According to some calculations, carbon dioxide levels must have built up to 350 times what they are today to melt the glaciers. Once they started melting they would have disappeared fairly quickly, in less than a thousand years. There is some evidence that the earth went through at least three cycles of freezing and thawing before this period ended. The final melting of the snowball may have helped trigger the Cambrian explosion.
The lesson, if there is one, is that our planet is a changeable thing. At times it has been much hotter than today, at times much colder. We only dimly understand why, but we do know that carbon dioxide is involved. By pumping more and more of it into the air, we are performing a big experiment with the only home we have.