Scientists drilling in the Earth's polar regions are hoping to find some small clues to a big question: Are we headed into another ice age or are we racing irredeemably toward a period of global warming?
The answer is far from certain. Nonetheless, arctic expeditions are deicing a great deal of new information from cores obtained at two drilling sites in Greenland. Another expedition is underway in Antarctica, and a fourth is to begin there later this year.
Scientists know, for example, that for the past 800,000 years, the advance and retreat of global ice ages has been relatively constant: approximately 100,000 years per cycle, with only 10,000 or so years accounting for interglacial periods. The current interglacial period, known as the Holocene era -- which encompasses human civilization as we know it -- has actually lasted a little more than 10,000 years. Hence, glaciologists say, another ice age is overdue.
On the other hand, evidence from the permanent ice caps shows that there are far more greenhouse gases in the atmosphere right now than at any time within the past several hundred thousand years. Furthermore, those gases, primarily carbon dioxide and methane, have long "residence times" -- up to 1,000 years. Even if worldwide fossil-fuel combustion ceased immediately, it could take many centuries before atmospheric components returned to pre-industrial proportions.
Richard Alley, a glaciologist at Penn State University involved with the ice-core drilling, says one goal of climate research is to discover the "on-off switch" -- the planetary mechanism that starts and stops the ice ages. That is easier said than done. The processes that account for global climate change are complex and interactive. "The more detail you look at, the more structure you see," says Alley.
One significant factor is the periodic long-term variations in the Earth's orbit, rotation and axis tilt, which seems to coincide with the advent of an ice age. Then there is the albedo, or reflectivity, of the ice caps, which affects the amount of solar radiation the Earth absorbs. Scientists also place great importance on the role of ocean currents (see sidebar).
Still another factor is the growth and behavior of glaciers. Their great thickness (the Greenland cap is about two miles deep; Antarctica's are nearly three miles deep) traps enough heat to form underground pools of fresh water that can act as lubricants and cause gigantic chunks of ice to slip suddenly into the ocean. The ice breaks up into vast armadas of icebergs, affecting water temperature and sea levels.
Scientists drilling into the glaciers have recovered length after length of ancient snowfalls that reveal a wealth of information about Earth's ice cap's and their effect on climate. Proportions of two oxygen isotopes contained in the ice indicate average air temperature. The amount of sea salt reveals windiness, as does the presence of desert dust. There can be residue from forest fires and volcanic eruptions, as well as evidence of the level of solar radiation. Atmospheric gases are trapped in microscopic bubbles in the ice.
Such information is especially valuable when the annual layers are relatively undisturbed, as they are in the Greenland cores, which go back about 110,000 years. (The cores from eastern Antarctica, some of which reach back a half-million years, have been subjected to snowdrifting.)
Beginning later this year, scientists will drill at a site called Siple Dome, a 3,000-foot-deep ice ridge in western Antarctica. …