Human lifespans, when compared to geologic time, are extremely brief, and so it is hard for us to imagine our planet’s climate being much different than it is today. The fact is that over thousands and hundreds of thousands of years the Earth’s climate has experienced wide fluctuations, ranging from the “snowball Earth” of about 635 million years ago to the extremely warm conditions (18°F, or about 8°C warmer, on average, than today) of a 100 million years ago when dinosaurs were flourishing. Over the millennia, there have been many periods of hot and cold. In the past, these have been caused by a variety of factors, ranging from volcanic activity to plate tectonics to the possibility of an asteroid crashing into the planet. Today, scientists fear we are undergoing a new and dramatic climactic change with one important difference: they believe humans are causing it.