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Milankovirch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. for some time this theory was considered untestable, largely because there was no sufficiently precise chronology of the ice ages with which the orbital variations could be matched.
To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various tines in the Earth’s past. A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18. when an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. the degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.
As an indicator of shifts in the Earth’s climate, the isotope record has two advantages. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. Second, it is a more continuous record than that taken from rocks on land. Because of these advantages, sedimentary evidence can be dated with sufficient accuary by radiometric methods to establish a precise chronology of the ice ages. The dated isotope record shows that the fluctuations in global ice volume over the past several hundred thousand years have a pattern: an ice age occurs roughly once every 100,000 years. These data have established a strong connection between variations in the Earth’s orbit and the periodicity of the ice ages.
However, it is important to note that other factors, such as volcanic particulates or variations in the amount of sunlight received by the Earth, could potentially have affected the climate. The advantage of the Milankovitch theory is that it is testable; changes in the Earth’s orbit can be calculated and dated by applying Newton’s laws of gravity to progressively earlier configurations of the bodies in the solar system. Yet the lack of information about other possible factors affecting global climate does not make them unimportant.
米兰科维奇早在二十世纪初就提出冰期是由地球环绕太阳的轨道变化引起的。原先人们认为这一理论是无法证实的，主要是因为没有足够准确的冰期年表可以和轨道变化规律做比照。
要建立冰期年表就需要测定以前地球上各个不同时期的陆冰相对存量。最新发现使这种测定成为可能：一个特定时期的相对陆冰量可以从海洋沉积物中氧的同位素O16 和O18 的比例推算出来。在水里，几乎所有的氧都是O16 ，但是，在一千个水分子中，会有几个与O18 结合。冰期开始的时候，大陆冰盖逐渐增大，这使得从海洋蒸发最终又回到海洋的水稳步减少。当水份从海洋表面蒸发时，相对较重的O18 会留下来，所以海水中的O18 会越来越多。通过对这一时期海洋沉积物的分析就可以测定O18的含量，因为这些沉积物是由海洋微生物的碳酸钙骨骼构成的，而这些骨骼又是获取周边海域的氧原子构建的。沉积物样本中O18和O16 的比例越高，这些沉积物形成时期的陆冰面积就越大。
作为地球气候转变的标志，这种同位素记录有两大优势。其一，它是全球性的记录，从不同大洲取来的沉积物样本中同位素比例几乎没有区别。其二，与从岩石和土壤中获得的记录相比，它更具有连续性。因为有这些优势，才能通过辐射测量法测定沉积物样本的年代，从而建立准确的陆冰年表。确定年代的同位素记录显示出在过去几十万年中全球冰川变换的规律：冰期大约每100000年出现一次。这些数据还显示出地球轨道变化与冰期周期性之间的紧密联系。
然而，要注意的是火山微粒和地球所吸收阳光量的变化也可能是影响气候变化的潜在因素。米兰科维奇理论的优势在于它是可测定的。用牛顿重力定律可以计算出地球轨道变化并确定更早的太阳系天体布局年代。缺少其它影响地球气候的可能因素并不意味已经确定的因素不重要。
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