How much ice? We don't snow!

Last week, I got an awesome question in my inbox from Stella, a 10-year-old from Shoreline, WA:

"At its peak, how much of the earth was covered in ice?"

First of all, STELLAr question! Second, there are a couple of answers to this question depending on how sure you want scientists to be about the answer. I’ll start with the answer that we’re pretty darn sure about.

Ice during the “Last Glacial Maximum”

The “Last Glacial Maximum” is the most recent time that the earth experienced a worldwide ice age, which was between 26,000 and 20,000 years ago [Clark et al., 2009]. For reference, humans had started making pottery by this time, but we wouldn’t start feeding ourselves through agriculture for another 10,000 years or so.

Areas covered by ice during the LGM in grey. Data from [Ray and Adams, 2001] 

At the Last Glacial maximum, about 1/4 of the Earth's land was covered by ice (compared to about 1/10 now). That might not sound like that much, but the ice in some places (like in modern day Canada) was 2.5 miles thick! So much water was frozen on land that sea level fell. Scientists think that sea level fell so much that the area between Siberia and Alaska was dry, creating a land bridge that allowed humans to migrate from Asia to North America for the first time. When the ice melted and sea levels rose again, flooding the land bridge and cutting off North America from Asia.

Sea levels from the Last Glacial Maximum to modern day. Animation from NOAA.

Glaciologists, scientists who study glaciers, look for many things to see where in the world used to be covered in ice, like rocks left behind by glaciers (called glacial erratics, shown below), or areas that have been shaped by glaciers flowing over them (e.g. drumlins, shown below). Because we can see these things and directly observe them, we’re really sure about how much of the Earth was covered in ice at the Last Glacial Maximum.

Glacial erratics in Yosemite National Park. Erratics are rocks that have been carried by glaciers and then dropped when the glacier melts. Erratics often end up in locations where they look like they don’t belong. Photo credit to Robert H. Goun.

The feature shown above is called a drumlin. Drumlins form underneath flowing ice and are made of debris, like gravel and sand, carried by the glacier. This is one of many types of features that tells us about where in the world was covered by ice during the Last Glacial Maximum.

Snowball Earth?

There is a hypothesis (still being argued about in scientific communities) that sometime before 650 million years ago, when only simple life existed only in the oceans, the Earth was entirely frozen over! We don't know for sure, but it is possible that the Earth was completely covered in ice at this time. Dinosaurs showed up on earth about 200 million years ago, so 650 million years ago was a really, really, really long time ago. More research is always happening so maybe in our lifetimes we will know one way or the other!

Artist rendition of the snowball Earth. Image courtesy of NASA.

Thanks for asking such a cool question, Stella!
Do you have a question about climate, glaciers, or why science matters? Email me at or contact me through the “Contact” page!

Clark, P. U., A. S. Dyke, J. D. Shakun, A. E. Carlson, J. Clark, B. Wohlfarth, J. X. Mitrovica, S. W. Hostetler, and A. M. McCabe (2009), The Last Glacial Maximum, Science, 325(5941), 710–714, doi:10.1126/science.1172873.

Ray, N., and J. Adams (2001), A GIS-based vegetation map of the world at the last glacial maximum (25,000-15,000 BP), Internet Archaeol., 11.


Popular posts from this blog

Go with the flow

Three-quarters empty or one-quarter full?

Seeing the City Through the Trees