Geography
Glacial Cycles and Sea Level Changes
Quick fact
At the peak of the last glacial period, about 20,000 years ago, global sea level was roughly 120–130 meters lower than today, enough to connect Asia and North America via the Bering Land Bridge.
Why this is interesting
You know sea levels are rising today—but did you know that during the last ice age, the ocean was over 120 meters lower, exposing land bridges between continents?
Read the full explanation
Understanding Glacial Cycles and Sea Level Changes
Imagine a giant bathtub that slowly fills and drains as ice ages come and go. During a glacial period, colder temperatures cause snow to accumulate into massive ice sheets on land (like in Canada and Scandinavia). This locks up huge amounts of water, so the ocean volume shrinks and sea level falls. When the climate warms into an interglacial, those ice sheets melt, returning water to the ocean and raising sea level. The process is not fast—it takes thousands of years—but the total change is enormous: tens to over a hundred meters.
A deeper explanation
The engine behind glacial cycles is Earth's orbital variations—Milankovitch cycles—which alter the amount and distribution of sunlight reaching the planet. Over tens of thousands of years, these subtle changes trigger feedbacks: more snow cover reflects more sunlight (albedo effect), cooling the climate further and allowing ice sheets to grow. As ice accumulates, the crust beneath it is pushed down (isostatic depression), and when the ice melts, the land slowly rebounds. Meanwhile, the ocean itself also responds: warmer water expands (thermal expansion) slightly, but the dominant sea level change comes from the exchange of water between ice sheets and the ocean. This mechanism explains why ancient shorelines are now underwater or high above current sea level. It matters because the same processes are active today: as human-caused warming melts glaciers and ice sheets, sea level rises—but understanding ancient cycles helps us calibrate how fast and how much change is possible.