No one knows for certain when the next ice age will begin, but geological evidence indicates that it could start within two thousand years.
During the past 2 million years, ice ages have been coming and going according to a fairly regular schedule. They seem to last about 100,000 years or so, and are usually separated by short interglacial periods lasting about eight thousand to twelve thousand years.
We are living in an interglacial period that began when the last ice age ended, roughly ten thousand years ago. The historical model would therefore suggest that the next ice age is probably already on its way, and should arrive in a couple thousand years.
The term ice age has come to refer to the peaking of the ice sheets during the Pleistocene Epoch. The Pleistocene epoch began about 1.5 million years ago; it was a time of regularly reappearing ice ages. The end of the most recent ice age, about ten thousand years ago, marked the close of the Pleistocene and the start of the modern, Holocene Epoch, the interglacial epoch during which the human race has flourished.
At the height of the last ice age the average global temperature was about five degrees Celsius cooler than it is today.
One third of the Earth’s surface, including northern Europe, Canada, Greenland, Antarctica, parts of the United States, and areas of Australia, New Zealand, and South America, was blanketed in ice sheets up to ten thousand feet thick. The huge glaciers contained 5 percent of the world’s water, which caused sea levels to be lowered by as much as four hundred feet.
The rising and falling of sea levels that comes with global glaciation has helped scientists chart the succession of ice ages. Coral grows only in warm waters close to the surface of the ocean. When the sea level dropped with the last ice age, coral dropped with it.
As the glaciers melted and the sea began to rise again, new coral grew on top of the old. Each level of coral growth marks what was once the surface of the sea. By dating coral growth scientists have been able to put together a fairly detailed chronology of the last ice age.
Another way scientists have been able to date ice ages is by examining the fossil record. The presence of leafy plants, for example, indicates a warm climate. By studying and dating vegetable fossils, scientists can tell what the global climate was like during a certain time.
One thing they have learned is that during the interglacial period prior to the last ice age, the Earth was warmer than it is today. The strongest evidence for this belief comes from rock sediments from the ocean floor. The levels of carbon dioxide in such sediments indicate that there was more carbon dioxide in the atmosphere during the previous interglacial period than there currently is.
High atmospheric carbon dioxide levels create a greenhouse effect, whereby radiation from the sun is trapped inside the Earth’s atmosphere, causing the global temperature to rise. During the last century, the greenhouse effect has caused global warming at an annual rate of 0.3 to 0.4 degrees Celsius. Sometime during the next century the global temperature should reach as high as it was during the last interglacial.
When global temperature rises, the polar ice caps melt. If the ice caps were to thaw, they would surge toward the ocean. As a result, massive icebergs would be discharged into the sea. The flow of ice into the sea would raise sea levels around the globe, which would cause total inundation of coastal areas, which in turn would cause more ice to spill into the water.
As all this ice entered the ocean, ocean temperatures would drop, causing more sea ice to form. Since ice reflects rather than absorbs heat, the extra sea ice would radiate heat back out of the atmosphere, causing temperatures to drop further. More ice would then form. And so on. Once the ice gets going it pretty much takes care of itself.
In this way, some scientists speculate, one unlikely and unwholesome effect of global warming will be to bring about the next ice age.