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%.

“There's a lot of similarities there, of course, with our situation with modern global warming,” says McGee. 

Ice sheets melted and seas rose, just as they are now. Driven by changing temperatures and rain and snow patterns, entire habitats like deciduous forests moved towards the poles, while the places those forests left behind began to look more like warmer locales. Some species migrated, or went extinct. These trends, too, are being echoed today.

But there are also key differences between the last significant period of warming and the modern day. First, today’s warming breaks from the historical cycle. In fact, before human-caused warming began, scientists believe the Earth was roughly due to enter a cooling cycle (although research to confirm this is ongoing).2

The cause of today’s climate change is also different from the planetary forces that set off the breaks between ice ages. In past cycles, changes to the Earth’s rotation kicked off warming by increasing the sunlight reaching icy parts of the Earth. As ice melted, the Earth became less reflective, and retained more of the sun’s heat. That warming led carbon dioxide to move from the ocean into the atmosphere, prompting more warming.

But today, the cause is reversed: by burning fossil fuels, we have put large amounts of CO2 into the atmosphere very quickly, and that has spurred warming.

The speed of climatic change today is also more or less unprecedented. The amount of CO2 that humans have added over just the last hundred years is comparable to the amount that was added over 100 centuries after the last ice age. In other words, in the modern day, atmospheric carbon has risen about 100 times faster than when humans emerged from the last ice age. That difference, McGee says, is part of why current climate change is so alarming. 

“At the end of the last ice age, ecosystems had a good deal of time to adapt to the warming as it occurred,” he says. “Right now, they have much less time because warming is happening a lot faster.”

Past periods of warming certainly caused instability, says McGee. “The end of the last ice age wasn't a smooth ride. Sure, it was slow and natural: it occurred over 10,000 years, which is a long time. Even so, that 10,000 years was punctuated by really dramatic shifts.” But the current pace of climate change is likely to be even more disruptive. 

Humans have based entire societies around the current climate. Where we grow food, build cities, and set up infrastructure like bridges and roads are all intertwined with the environment as it looked over recent history. Now that environment is changing—quickly. “It's very destabilizing,” says McGee.

Thank you to several readers for sending in related questions, including Chuck Whinney of Sea Isle City, New Jersey, Ward Deharo of Milford, Massachusetts, and Gerald Smart of the United Kingdom.