evidence: For hundreds of thousands of years, the Earth has gone through repeated cycles of warming and cooling. Roughly every hundred thousand years, there has been an ice age,1 driven by changes in the planet’s tilt and orbit that allowed less sunlight to reach the Arctic. Between these ice ages have been periods of warming. The last ice age peaked around 21,000 years ago. This was followed by 10,000 years of warming that brought us to the climate we know now, says David McGee, an associate professor in the MIT Department of Earth, Atmospheric and Planetary Sciences. During that time, global temperatures rose by about 6° Celsius (around 11° Fahrenheit). The amount of carbon dioxide (CO2) in the atmosphere—the main planet-warming greenhouse gas responsible for today’s climate change—rose too, by nearly 50%.