WHEN MOST OF US think about ice ages, we imagine a slow transition into a colder climate on long time scales. Indeed, studies of the past million years have shown a repeatable cycle of Earth's climate going from warm periods (interglacial, as we now experience) to glacial conditions, with periods related to changes of the tilt of the rotation axis of Earth (41,000-year period); in the orientation of the elliptical orbit of the Earth around the sun (23,000-year period), called precession of the equinoxes; and in the eccentricity of the elliptical orbit (100,000 years). Ice age conditions generally occur when all of the above conspire to create a minimum of summer sunlight on the arctic regions of the planet, although the ice age cycle is global in nature and occurs in phase in the Northern and Southern hemispheres. It profoundly affects distribution of ice over lands and ocean: atmospheric temperatures and circulation: trod ocean temperatures and circulation, both at the ocean surface and at great depth.
The orbital forcing mechanism was first pointed out by James Croll in the 19th century and developed more fully by Milutin Milankovitch in 1938. Over longer durations, it has been speculated that cycles in the Earth's motion about the center of the galaxy are important. Yet, humans are short-lived, and perhaps only academics and museums are interested in long-term changes in the past. Since the end of the present interglacial and the slow march to the next ice age may be several millennia away, why should we care? In fact. won't the buildup of carbon dioxide (C[O.sub.2]) and other greenhouse gases possibly ameliorate future changes? Indeed, various groups advocate the benefits of global warming, such as the Greening Earth Society in the U.S. and the Subtropical Russia Movement. Some in the latter group even advocate active intervention to accelerate the process, seeing this as an opportunity to turn much of cold, austere northern Russia into a subtropical paradise.
Evidence has mounted that global warming began in the last century and that man may be in part responsible. Both the Intergovernmental Panel on Climate Change (IPCC) and National Academy of Sciences have concurred. Computer models are being used to predict climate change under different scenarios of greenhouse forcing, and the Kyoto Protocol has advocated active measures to reduce C[O.sub.2] emissions which contribute to warming. Thinking is centered around slow changes to our climate and how they will affect humans and the habitability of our planet. Yet, this thinking is flawed, since it ignores the well-established fact that Earth's climate has changed rapidly in the past and could do so in the future. The issue centers around a paradox--that global warming could instigate a new Little Ice Age in the Northern Hemisphere.
Evidence for abrupt climate change is readily apparent in ice cores taken from Greenland and Antarctica. One sees clear indications of long-term changes discussed above, with CO2 and proxy temperature changes associated with the last ice age and its transition into our present interglacial period of warmth. In addition, there is a strong chaotic variation of properties with a quasi-period of around 1,500 years, We say chaotic because these millennial shifts look like anything but regular oscillations, Rather, they resemble rapid, decadal-long transitions between a cold and warm climate, with lengthy interludes occurring in one of the two states.
The best-known example of these events is the Younger Dryas cooling of about 12,000 years ago, named for the arctic wildflower remains identified in northern European sediments. This event began and ended within a decade, and, lot its 1,000-year duration, the North Atlantic region was about five degrees colder. The lack of periodicity and the present failure to isolate a stable forcing mechanism a la Milankovitch have prompted much scientific debate about the cause of the Younger Dryas and other millennial scale events. …