Q: Why don't orbital changes alone explain ice ages?
A: Because they don't make enough difference in the amount of solar radiation reaching Earth.
Q: Then why do ice ages occur?
A: The key to past cycles, and to many of the changes that may be in our future, is feedbacks--that is, processes that amplify or reduce climate effects.
Earth's past can be a window into its future
For the past 11,600 years--a span that includes all of recorded human history--our climate has been relatively stable. But those years were unusual. In the two million years before that, enormous ice sheets grew and shrank, and sea levels rose and fell.
The causes of those amazing shifts are complex, and they had nothing to do with humans. But these events left a geological trail that is both unmistakable and sobering. Evidence from the past can teach us a lot about what we may expect--or hope to avoid--in the coming years. For instance: ice sheets melt much faster than they form; and sea levels can rise fast, too. Big changes can happen, and they have happened. Such changes, in a world now crowded with people and complex civilizations, could be catastrophic.
Emptying Ice Sheets into the Sea
In the past, some episodes of sea-level rise have been startlingly fast--more than a meter (39 inches) of rise per century. But scientists are unsure whether such a rapid rise could occur in the future because our understanding of ice sheets is incomplete.
The English Channel is an arm of the Atlantic Ocean separating the islands of Great Britain from northern France. But some 21,000 years ago, when vast ice sheets held enough water to lower the oceans 120 meters (about 400 feet), today's channel was merely a valley through which flowed a powerful river draining much of continental Europe.
Melting before their eyes
Sometimes, finding evidence of melting ice is a matter of simple observation. Here, scientists watch as rushing water tumbles through a crevasse to the bottom of an ice sheet. By lubricating the bottom of ice sheets, increased meltwater may dramatically increase the rate at which glaciers move and ultimately break apart.
Why did ancient ice come and go?
Scientists have given us a partial answer to this fascinating question: glacial ages arise from changes in Earth's orbital patterns taking place over tens of thousands of years. These shifts change the amount of sunlight falling on the Northern Hemisphere during the summer. When these shifts combine in a particular way, ice sheets may expand or retreat--but these cycles don't explain the current melting of ice sheets.
Eccentricity: Over a period of 96,000 years, Earth's orbit around the Sun changes from nearly circular to more oval, and back.
Tilt: The tilt of Earth's axis of spin relative to its orbital plane changes from 21.5° to 24.5° and back again every 41,000 years.
Wobble: Earth's axis of spin wobbles, or rotates, just like a spinning top, in a phenomenon known as precession.
Adding It Up
The cycles of tilt, precession and--to a lesser extent--eccentricity likely affect climate by changing the warmth and length of summers at high latitudes in the Northern Hemisphere, a region that's sensitive to summer sun. Because there is so much land there, snow and ice build up in the winter. More of that snow and ice disappears when summers are long and warm than when they are short and cool--and the difference affects global climate. That's because snow and ice reflect solar energy, and the more that survives the summer, the cooler Earth stays.