Ice ages happened because of a combination of factors working together: slow shifts in Earth’s orbit around the Sun, falling carbon dioxide levels, moving tectonic plates that rearranged ocean currents, and powerful feedback loops that amplified small temperature changes into massive glacial advances. No single cause explains it. The ice ages were the result of these forces converging over millions of years, each one reinforcing the others.
The most recent stretch of recurring ice ages, known as the Pleistocene epoch, began about 2.6 million years ago. Since then, roughly 17 glacial periods have alternated with 17 warmer interglacial periods. We’re currently living in one of those warm intervals, and it’s been going on for about 17,000 years.
Earth’s Orbit Set the Rhythm
The single most important trigger for the cycling between glacial and interglacial periods is the way Earth’s orbit slowly changes shape and orientation over tens of thousands of years. These are called Milankovitch cycles, named after the Serbian scientist who calculated them in the early 1900s. They don’t change how much total energy the Sun puts out. Instead, they change how sunlight is distributed across Earth’s surface and throughout the seasons.
Three orbital changes matter. First, Earth’s orbit around the Sun stretches from nearly circular to slightly more oval and back again over a cycle of about 100,000 years. This is called eccentricity, and it affects how much the Sun-Earth distance varies over the course of a year. Second, the tilt of Earth’s axis wobbles between about 22 and 24.5 degrees over a 41,000-year cycle. Right now it’s at 23.4 degrees, roughly in the middle. A smaller tilt means less intense summers, which is critical because cooler summers allow winter snow to survive year-round and gradually build into ice sheets. Third, Earth’s axis slowly traces a circle like a spinning top, completing one full wobble every 23,000 years or so. This precession determines whether the Northern Hemisphere’s summer happens when Earth is closest to or farthest from the Sun.
When these three cycles align in a way that produces especially cool summers in the Northern Hemisphere, snow accumulates faster than it melts. Over thousands of years, that leftover snow compresses into continent-spanning ice sheets. The 100,000-year eccentricity cycle closely matches the spacing of the most recent glacial periods, while the shorter obliquity and precession cycles create smaller fluctuations within them.
Falling CO2 Primed the Planet
Orbital cycles have been operating for billions of years, but the current pattern of recurring ice ages only started a few million years ago. Something else had to change first to make Earth cold enough for orbital wobbles to push it into glaciation. That something was a long, slow decline in atmospheric carbon dioxide.
One major driver of that decline was the rise of the Himalayas and the Tibetan Plateau. Starting around 40 million years ago, the collision of the Indian and Asian tectonic plates pushed up enormous mountain ranges. As those mountains eroded, a chemical process called silicate weathering pulled CO2 out of the atmosphere. Rainwater absorbs carbon dioxide, forming a weak acid that dissolves silicate rocks. The dissolved carbon eventually washes into the ocean and gets locked into seafloor sediments. Over millions of years, this process drew down enough CO2 to cool the planet significantly. Burial of organic carbon from increased erosion added to the effect.
The numbers tell the story clearly. During full glacial periods, atmospheric CO2 dropped to around 200 parts per million. During warmer interglacials, it rose to around 280 ppm. That 80 ppm difference, modest as it sounds, was enough to help tip the climate between ice-covered and relatively temperate states. CO2 acted both as a long-term precondition (the Himalayan drawdown over millions of years) and as a shorter-term amplifier within each glacial cycle.
Shifting Continents Rearranged the Oceans
Tectonic plates don’t just build mountains. They also open and close ocean passages, redirecting currents that carry enormous amounts of heat around the planet. One of the most consequential events was the closing of the Central American Seaway, the gap between North and South America that once allowed water to flow freely between the Atlantic and Pacific oceans.
The Isthmus of Panama began forming around 13 million years ago and was largely closed by about 2.7 million years ago, right around the time Northern Hemisphere glaciation kicked off. Before the isthmus existed, tropical water circulated between the two oceans, keeping the Atlantic less salty. Once that passage closed, Atlantic surface waters became warmer and saltier, especially in the Caribbean and Gulf of Mexico. That saltier water flowed northward via the Gulf Stream and North Atlantic Current, and when it reached high latitudes and cooled, its density caused it to sink. This strengthened a deep ocean circulation pattern called thermohaline circulation.
Paradoxically, this stronger circulation may have delivered more moisture to northern latitudes, fueling heavier snowfall over Canada and Scandinavia. At the same time, the reorganization of ocean heat transport made high-latitude climates more sensitive to orbital forcing. The closing of Panama didn’t cause the ice ages on its own, but it set the stage for orbital cycles to trigger massive ice sheet growth in the Northern Hemisphere.
Feedback Loops Amplified Everything
None of these initial triggers, orbital shifts, CO2 changes, or ocean reorganization, would have produced mile-thick ice sheets without feedback loops that magnified their effects. The most powerful of these is the ice-albedo feedback.
Fresh snow and ice are extremely reflective. Snow-covered sea ice bounces back more than 80% of incoming sunlight. Once orbital cooling allows ice to start forming and snow to persist through summer, that bright surface reflects solar energy back into space instead of absorbing it. The result is more cooling, which produces more ice, which reflects even more sunlight. During summer, the process can also run in reverse: as ice melts, it exposes darker melt ponds and open water, which absorb more heat and accelerate further melting. This is a straightforward positive feedback that works in both directions, making cold periods colder and warm periods warmer.
CO2 created its own feedback loop. As oceans cooled during glacial onset, they absorbed more carbon dioxide from the atmosphere (cold water holds more dissolved gas than warm water). Lower CO2 meant less greenhouse warming, which caused further cooling, which allowed oceans to absorb even more CO2. The reverse happened during warming: as oceans heated up, they released stored CO2, boosting the greenhouse effect and accelerating the transition out of a glacial period.
Ocean circulation added yet another layer. Research on the Atlantic’s overturning circulation shows that its return flow brings large volumes of cool water to the surface in the tropics, off the coasts of Namibia and Peru. This upwelled water has a pronounced cooling effect on global climate. When the overturning circulation is running strongly, it keeps the tropics and both hemispheres relatively cool, particularly when Northern Hemisphere summer sunlight is at a minimum in the 23,000-year precession cycle. When the circulation weakens or collapses, that cooling disappears and temperatures rise. These circulation shifts helped pace the transitions between glacial and interglacial states.
The Sun’s Role Was Smaller Than You’d Think
It’s natural to assume the Sun itself must be the main driver, but variations in the Sun’s actual energy output play a secondary role compared to orbital geometry. The Sun’s brightness does fluctuate slightly over time, and those fluctuations can influence climate, particularly over centuries. Reduced solar output, along with increased volcanic eruptions, likely contributed to climate shifts over the past thousand years. But the major glacial cycles of the past 2.6 million years track most closely with Milankovitch orbital cycles, not with changes in solar intensity.
The distinction matters because the so-called Little Ice Age, a period of regional cooling from roughly 1300 to 1850, had very different causes from the massive Pleistocene glaciations. The Little Ice Age was driven by reduced sunspot activity (especially during two prolonged quiet periods from 1450 to 1540 and 1645 to 1715), shifts in atmospheric circulation patterns over the North Atlantic, and large volcanic eruptions like Laki in 1783 and Mount Tambora in 1815 that ejected sunlight-blocking ash into the upper atmosphere. These are real climate drivers, but they operate on timescales of decades to centuries, not the tens of thousands of years that define true ice age cycles.
Where We Are Now
We’re currently in an interglacial period that began as the last ice age ended. Paleoclimate records from Nevada’s Devils Hole cave system show that the last four interglacial periods each lasted over 20,000 years, with the warmest, most stable portion running 10,000 to 15,000 years. Based on that pattern, our current warm period could naturally persist for thousands of years more. Some researchers suggest it could last tens of thousands of years, particularly because human-generated CO2 emissions have pushed atmospheric carbon dioxide far above the 280 ppm typical of past interglacials, potentially delaying or preventing the next glacial cycle entirely.