Ice ages are natural climate phenomena characterized by long periods of global cooling and extensive ice sheet formation. These glacial periods have occurred multiple times throughout Earth's 4.6 billion year history, alternating with warmer interglacial periods. The timeline shows temperature fluctuations over the past 800,000 years, revealing the cyclical nature of ice ages. We are currently living in an interglacial period that began about 11,700 years ago.
Milankovitch cycles are the primary astronomical cause of ice ages. These cycles, discovered by Serbian mathematician Milutin Milankovitch, describe how Earth's orbital variations affect climate over thousands of years. Three main cycles control the amount and distribution of solar radiation reaching Earth. Eccentricity changes Earth's orbital shape from circular to elliptical over 100,000 years. Obliquity varies the axial tilt between 22.1 and 24.5 degrees over 41,000 years. Precession causes Earth's axis to wobble like a spinning top over 23,000 years. These combined cycles determine when and where ice ages occur.
Each Milankovitch cycle affects Earth's climate through specific orbital changes. Eccentricity operates on a 100,000-year cycle, changing Earth's orbital shape from nearly circular to more elliptical, which varies the Earth-Sun distance throughout the year. Obliquity works on a 41,000-year cycle, with Earth's axial tilt varying between 22.1 and 24.5 degrees, affecting the intensity of seasonal contrasts. Precession occurs over 23,000 years as Earth's axis wobbles like a spinning top, changing the timing of seasons relative to Earth's position in its orbit. These combined cycles alter the distribution and intensity of solar radiation reaching different parts of Earth's surface, setting the stage for ice age formation.
Orbital changes directly affect the distribution of solar radiation on Earth's surface. Insolation, or incoming solar radiation, varies by latitude and season, and is critical for ice formation. The key process begins when reduced summer insolation in northern latitudes prevents complete melting of winter snow. This happens particularly at latitudes between 60 and 65 degrees north, which are most sensitive to orbital changes. When summer radiation is insufficient to melt all the accumulated winter snow, ice begins to build up year after year. Over thousands of years, this gradual accumulation leads to the formation of massive ice sheets that can cover entire continents.
Ice sheet formation follows a gradual step-by-step process over thousands of years. It begins when winter snowfall consistently exceeds summer melting, causing snow to persist year-round. Over several years, this accumulated snow compresses into firn, a granular form of ice. Further compression over decades transforms firn into dense glacial ice, which appears blue due to light scattering. As the ice continues to accumulate, it can reach thicknesses of up to 3 kilometers and begins to flow outward under its own weight, eventually covering entire continents. The ice-albedo feedback mechanism accelerates this cooling process, as the white ice surface reflects more sunlight back to space, further reducing temperatures and promoting additional ice formation.