From Ice Ages to Now: The 2.6-Million-Year History of Earth's Glacial Cycles

The glaciers retreating today are not anomalies — they are the remnants of a much more extensive ice age world. Eighteen thousand years ago, at the Last Glacial Maximum, ice sheets covered much of North America, northern Europe, and northern Asia. Sea levels were 120 metres lower than today. The Amazon basin was cooler and drier. Much of what is now ocean was dry land. Understanding the geological history of Earth's glacial cycles is essential context for understanding why what is happening today is unprecedented — and alarming.

The Milankovitch Cycles — Earth's Orbital Pacemaker

Ice ages are ultimately driven by subtle variations in Earth's orbit and axial tilt — the Milankovitch cycles, named after the Serbian mathematician Milutin Milankovitch who calculated them in the 1920s. Three orbital parameters cycle over tens of thousands of years: the eccentricity of Earth's orbit (100,000-year cycle), the obliquity or tilt of Earth's axis (41,000-year cycle), and the precession or wobble of Earth's spin axis (26,000-year cycle). These variations alter the distribution of solar radiation across Earth's surface, triggering ice ages when northern hemisphere summers receive less sunlight — allowing winter snow to persist and accumulate.

What Ice Cores Reveal

Ice cores — cylinders of ice drilled from glaciers and ice sheets — are among the most valuable archives of Earth's climate history. Bubbles of ancient air trapped in the ice preserve samples of past atmospheres, allowing scientists to measure CO₂, methane, and other greenhouse gas concentrations going back 800,000 years. The ice itself records temperature through the ratio of oxygen isotopes. Dust layers record past volcanic eruptions. The result is a detailed, continuous record of climate variability over hundreds of thousands of years — a record that makes the current rate of greenhouse gas increase and temperature rise starkly exceptional.