U-shaped valleys, moraines, and grooves abraded into bedrock are direct evidence that glaciers have come and gone. They are likewise evidence that Earth’s climate has been dramatically different in the past. Consider that during 90% of the last 600 million years, world temperatures have averaged 72°F; today they average 58°F, significantly cooler, right? Yet, during ice ages, world temperatures were cooler still, probably below 50°F.
The last time Earth’s climate was dramatically different from what it is today was during the last glaciation, an interval when Earth’s global-ice cover greatly exceeded what it is now. The last glaciation, which culminated about 20,000 years ago, was the most recent of a succession of glaciations, or ice ages, that punctuate our geologic past. But what exactly constitutes an ice age? Why do they happen?
An ice age is characterized by conditions that, from time to time, cause continental ice sheets to develop in nonpolar regions.
Our definition says “from time to time” because during an ice age there may be intervals when continental ice sheets shrink dramatically or even disappear, even though relatively cool climatic conditions prevail. Such periods are known as interglacial intervals, but interglacials are properly parts of an ice age because the glaciers return.
Only two ice sheets, having continental-size dimensions and thicknesses, exist today. The one in Antarctica lies at a pole, and the other one in Greenland lies at such high latitudes that it can be considered a polar region. Other glaciers in nonpolar, high mountain areas are not large enough to be considered ice sheets. Hence, according to our definition, the two existing ice sheets do not qualify the present as an ice age.
It may be difficult to convince a native Greenlander or someone recently returned from Antarctica that we are not living in an ice age. That is why our definition specifies nonpolar regions. For most of us who live in nonpolar regions, the present environment is not one of ice-age conditions. We have either seen the end of the Ice Age or are living through an interglacial interval within it.
Why do some periods of geologic time, like the Pleistocene, show frequent ice ages, while others, like the Jurassic show none at all?
If you have reviewed the section “Where are Glaciers Found?” you have the tools for coming up with a reasonable explanation for the occurrence and reoccurrence of ice ages during the last 2.3 billion years. Namely, glaciers tend to form and persist in two kinds of situations: high latitudes and high altitudes. The Theory of Plate Tectonics has enabled us to recognize and understand important global changes in geography that affect ice ages.
As landmasses and ocean basins have shifted position, occasionally they have assumed an arrangement that is optimal for widespread glaciation in high latitudes. Where evidence of ancient ice sheet glaciation is now found in low latitudes, we invariably find evidence that such lands formerly were located in high latitudes.
Word of Warning: Although this explanation appears adequate for the pattern of glaciation during and since the late Paleozoic, information about earlier glacial intervals is very fragmentary and more difficult to evaluate.
Glaciers can form and survive away from the poles, even at or close to the equator, if high altitudes are available. The creation of mountain ranges, through the collision of Earth’s crustal plates, is necessary for glaciers to form and persist at low latitudes.
Shifting crustal plates causes ocean basins to open and close. Changes in the shapes and locations of ocean basins causes changes in oceanic circulation, which alters the transport of heat and moisture from the equator to the poles. When the poles are accessible to wide-ranging ocean currents, the result may be a lesser temperature differential between equator and poles and a failure to build up significant ice in polar regions. One the other hand, if the poles are isolated from the major ocean currents, then they tend to cool and perhaps spawn ice ages. This was certainly true for the Ordovician and Permian-Pennsylvanian glaciations. It may also be true today, where access to the Arctic Ocean has been nearly cut off by the encircling continents. In addition, the presence of Antarctica astride the South Pole prevents currents from reaching that pole as well.
Rocks of glacial origin (tillite) and associated polished and striated rock surfaces have helped to identify ancient glaciations throughout the geologic past. The earliest recorded ice age dates to about 2.3 billion years ago, in the middle Precambrian. Evidence of other ice ages has been found in late Precambrian, early Paleozoic (Ordovician), and late Paleozoic (Permian-Pennsylvanian) times.
The geologic record of ice ages is fragmentary and not always easy to interpret, but evidence from such low-latitude regions as India, equatorial Africa, southeastern South America, and Australia suggests a glacial past. Throughout geologic history, Earth’s land areas have had very different relationship to one another than they do today. For example, during the late Paleozoic glaciation, continents that are now at low latitudes were assembled into a supercontinent at the South Pole. This continental configuration helped to bring on an ice age by providing a base for the accumulation of snow and the growth of a polar ice cap.
For the better part of two million years the advances and retreats of glaciers have defined the Pleistocene Epoch of geologic time, so much that the terms Pleistocene and Ice Age are often used interchangeably. Until only a few decades ago, it was thought that Earth had experienced only four glacial episodes during the Pleistocene Epoch. This assumption was based on studies of ice sheet and mountain glacier deposits on land. It had its roots in early studies of the Alps where geologists identified stream terraces they thought were related to four ice advances. This traditional view was discarded when studies of deep-sea sediments disclosed a long succession of glaciations during the Pleistocene. About 30 glacial episodes are recorded, rather than the traditional four.
The implication is clear: whereas seafloor sediments provide a continuous historical record of climate changes, evidence of glaciation on land generally is incomplete and interrupted by many unconformities.
Historians and climatologists have called the period between about A.D. 1500 and 1900 the “Little Ice Age” because much of the northern hemisphere experienced temperatures far colder than average. The cooling brought a buildup of glacier ice in the mountains and in the arctic. Pack ice clogged sea lanes around Iceland and Greenland. Glaciers in the Alps advanced dramatically, overrunning alpine meadows. In the northern Rocky Mountains, glaciers advanced out of their cirques and spilled down mountainsides forming large moraines.
Little Ice Age conditions persisted until the middle of the 19th century when a general warming trend caused mountain glaciers to retreat. In Glacier National Park, for example, the last episode of glacier advance came in the late 18th and early 19th century. That cold spell ended sometime before 1850, and the park’s glaciers and snowfields have been mostly retreating ever since. Around 1850, the park had about 150 glaciers; it has 37 now.
In the “Past Ice Ages” section, we noted that the arrangement of landmasses in polar regions, the formation of mountain ranges raising to high altitudes in nonpolar regions, and the isolation of the poles from major ocean currents have set the stage for the onset of ice ages and the formation of glaciers. The cause of the cyclic pattern of glacial and interglacial intervals, however, is yet to be explained. In short, what causes ice ages?
See the Milankovitch Cycles knowledge center.
Climatic fluctuations on the scale of centuries or decades were responsible for the Little Ice Age and similar episodes of glacier expansion. Such fluctuations, however, are too brief to be caused either by movements of continents or variations in Earth’s orbit. This realization requires us to seek other explanations for their cause. Two explanations have received special attention.
The world’s climatic system is driven by solar energy, so it is natural that we should look to the sun as the cause of an ice age, as we saw with changes in Earth’s tilt and orbit around the sun. There is another appealing idea associated with the sun, that is, variations in the sun’s energy output.
The energy output of the sun has varied so little that we speak of a solar constant. Yet, measurements suggest that the sun’s energy output fluctuates over time. Reliable observations of the solar output of energy have been available only since 1979. Before this, the observation of sunspots provided an indication of differences of output. Short-term sunspot cycling, lasting 11 to 14 years, produce small changes in solar output that have been linked to variations in temperature and precipitation on Earth. At present, however, there has been no clear demonstration that solar variations are responsible for climatic changes on the scale of the Little Ice Age, and studies have been inconclusive.
Volcanism can have either a positive or negative influence on glaciation. Volcanic gases, such as carbon dioxide and water vapor, emitted into the atmosphere inhibit the transmission of outgoing energy from Earth’s surface, contributing to a greenhouse type of warming. This is generally regarded as a negative influence.
Explosive volcanism, injecting fine dust and sulfate particles high into the atmosphere, is generally regarded as a positive influence, because the particles act as crystallization nuclei for snowflakes, and they shield Earth from incoming solar radiation, promoting cooling and reducing wastage of snow. In 1815, for example, the major eruption of the volcanic island of Tambora in the East Indies lowered average surface temperature in the northern hemisphere by about 1.3°F. This may not seem like much, but this seemingly small decrease in temperature may have caused the summer of 1816 to be the coldest on record in many places. In New England, for instance, midsummer frosts occurred, destroying crops and promoting the name, “The Year Without a Summer.”
Open Volcanism knowledge center.