Although the history of glaciation during the Pleistocene epoch (2 million to 10,000 years ago) is well established, we do not know with complete certainty why glaciation takes place. For over a century, geologists and climatologists have struggled with this problem, but it remains unsolved.

It is long known that Earth’s orbit around the Sun changes periodically, cyclically affecting the way solar radiation strikes the Earth, but the idea that these changes affect climate was first advanced by James Croll in the late 1800s. Later, Milutin Milankovitch elaborated the theory with calculations that convincingly argued that the cycles, now known as Milankovitch cycles, could cause climatic variations.

The Milankovitch cycles emerge from the way three cyclic changes in Earth’s orbit combine. One characteristic of Earth’s orbit is its eccentricity, the degree to which the orbit is an ellipse rather than a circle. Changes in the eccentricity of Earth’s orbit occur in a cycle of about 96,000 years. The inclination, or tilt, of Earth’s axis also varies periodically, moving between 22 degrees and 24.5 degrees. The tilt of Earth’s axis, toward the Sun at some times of the year and away from the Sun at other times, is responsible for the annual cycle of seasons. The greater the tilt, the greater the contrast between summer and winter temperatures. Changes in the tilt occur in a cycle 41,000 years long. Also, Earth wobbles as it spins, like a slightly unsteady top. The wobble cycle is completed once every 21,700 years. Changes in eccentricity, tilt and wobble do not affect the total amount of solar radiation Earth receives in a year, but they do affect how evenly or unevenly this radiation is disturbed over the course of a year. According to the Milankovitch theory, about every 40,000 years the three separate cycles combine in such a way that the difference between summer and winter temperatures is at a minimum. At this point winter temperatures are milder but so too are summer temperatures. As a result, less ice is melted in the summer than is formed in the winter, so glaciers build up and a period of glaciation results.

Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a detailed chronology of the Pleistocene temperature changes was determined that could test the predictions of this theory. A correspondence between Milankovitch cycles and climate fluctuations of the last 65 million years seems clear. Furthermore, studies or rock samples drilled from the deep-sea floor and the fossils contained in them indicate that the fluctuation of climate during the past few hundred thousand years is remarkably close to that predicted by Milankovitch.

A problem with Milankovitch’s explanation of glaciation arises from the fact that the variations in Earth’s orbit, and hence the Milankovitch cycles, have existed for billions of years. Thus we might expect that glaciation would have been a cyclic event throughout geologic time. In fact, periods of glaciation are rare. So there must be another factor acting together with the Milankovitch cycles that causes periods of glaciation. Once this additional factor makes the temperature low enough, the cyclic variations of the Milankovitch cycles will force the planet into and out of glacial epochs with a fixed regularity.

Many hypotheses have been proposed for the additional cooling factor. Some suggest that variations in the Sun’s energy output could account for the ice ages. However, our present understanding of the Sun’s luminosity holds that it should have progressively increased, not decreased, over the course of Earth’s history. Still others argue that volcanic dust injected into the atmosphere shields Earth from the Sun’s rays and initiates an ice age. However, no correlation has been found between volcanic activity and the start of the last ice age. An increasingly attractive theory holds that decreases in atmospheric carbon dioxide starts the cooling trend that leads to glaciation. Carbon dioxide traps solar energy reflected from the Earth’s surface. If carbon dioxide levels decrease, less heat is trapped and Earth’s surface cools. Recent studies of the carbon dioxide content of gas bubbles preserved in the Greenland ice cap do in fact show that high carbon dioxide levels are associated with warm interglacial periods, and low levels with cold glacial periods.

尽管更新世时期（200万年至1万年前）的冰川作用历史已经确立，但我们并不完全明确为什么发生冰川作用。一个多世纪以来，地质学家和气候学家一直在努力解决这个问题，但仍未解决。 众所周知，地球围绕太阳的轨道周期性变化，周期性地影响太阳辐射对地球的撞击方式，但是这些变化影响气候的想法最早是在19世纪晚期詹姆斯克罗尔提出的。后来，米兰科维奇详细阐述了这一理论，令人信服地论证了这个周期，认为现在称为米兰科维奇旋回，可能导致气候变化。 米兰科维奇旋回从地球轨道三次周期性变化的结合中出现。地球轨道的一个特点是它的偏心率，轨道是一个椭圆而不是一个圆。地球轨道偏心率的变化发生在大约96,000年的周期内。地球轴线的倾斜度或倾斜度也会周期性变化，在22度和24.5度之间移动。地球轴线在一年中的某些时间朝向太阳并在其他时间远离太阳的倾斜是导致季节的年度周期的原因。倾斜程度越大，夏季和冬季的温差就越大。倾斜的变化发生在41,000年的周期中。另外，地球在旋转时摇摆不定，就像一个不稳定的陀螺。摆动周期每21，700年完成一次。偏心率，倾斜度和摆动的变化不会影响一年内地球接收到的太阳辐射总量，但它们的确会影响一年中这种辐射受到干扰的均匀或不均匀。根据米兰科维奇理论，大约每40,000年，就会以这样一种方式结合在一起，使得夏季和冬季之间的温差最小。此时冬季气温较轻，但夏季气温也较低。因此，夏季融化的冰量少于冬季形成的冰川，因此冰川就会积聚并形成冰川期。 米兰科维奇20世纪20年代和30年代提出了气候循环的观点，但是直到20世纪70年代才确定了更新世时期温度变化的详细的年表，从而可以检验这一理论的预测。米兰科维奇周期与过去6500万年气候波动之间的对应关系似乎很清楚。此外，从深海地层钻探的岩石和岩石样品及其中所含的化石表明，过去几十万年的气候波动与米兰科维奇所预测的非常接近。 米兰科维奇对冰川作用的解释存在一个问题，因为地球轨道的变化以及米兰科维奇旋回已经存在了数十亿年。因此，我们可以预计，在整个地质时期，冰川将是一个循环事件。事实上，冰川期很少见。因此，必须有另一个因素与米兰科维奇周期共同作用，从而导致冰期的发生。一旦这个附加因素使得温度足够低，那么 米兰科维奇旋回的周期性变化将以固定的规律进入和离开冰川时代。 对于额外的冷却因素，已经提出了许多假说。一些人认为，太阳能量输出的变化可以解释冰的年龄。然而，我们现在对太阳光度的理解认为，在地球的历史进程中，它应该逐渐地增加而不是减少。还有一些人认为，注入大气中的火山尘埃使地球免受太阳射线的照射，并开启了冰河时代。然而，在火山活动和上一个冰河世纪开始之间没有任何关联。一个越来越有吸引力的理论认为大气中二氧化碳的减少会导致冰川的冷却趋势。二氧化碳捕获从地球表面反射的太阳能。如果二氧化碳水平降低，热量就会减少，地球表面就会冷却。最近对格陵兰冰盖中保存的气体气泡的二氧化碳含量的研究实际上表明，高二氧化碳水平与温暖的间冰期相关联，而在寒冷的冰河时期则是低水平的。