Could reversing global warming start an ice age?

According to one school of thought, a warming planet is one that's less likely to wind up in an ice age. Because the Earth is always going through warming and cooling cycles, and we've been in one of the warming cycles for about 12,000 years now, scientists say it's inevitable that we'll hit another big chill sometime in the next 10,000 to 100,000 years. If that happens, much of the world -- including Europe and North America -- would be covered in a thick sheet of ice.

According to some researchers, the heat trapped in the Earth's atmosphere from the greenhouse effect will offset this cooling -- essentially preventing the Earth from entering another ice age [sources: Science Daily, Cosmos]. Though averting an ice age sounds like good news, the researchers caution that global warming isn't any picnic, either. It could lead to other drastic and unpleasant effects on the planet (think rising sea levels and dwindling global food supplies).

Another school of thought makes the opposite prediction: Global warming might actually lead to another ice age. According to this theory, warming temperatures disrupt ocean currents -- particularly the Gulf Stream, the flow that redistributes warm water from the Gulf of Mexico to northern Europe. As the Gulf Stream makes its deposits of warm water along the coasts of Great Britain and northwestern Europe, it keeps the temperatures there warmer than they would be otherwise.

The worry is that, when Arctic ice melts as a result of global warming, huge amounts of fresh water will pour into the North Atlantic and slow down the Gulf Stream. A study of circulation in the North Atlantic has discovered that there already has been a 30 percent reduction in currents flowing north from the Gulf Stream [source: Pearce]. A slowed Gulf Stream could potentially lead to dramatic cooling in Europe.

Will either of these scenarios really happen? It's hard to say for sure. Climate experts haven't even come to a consensus about the cause and effects of global warming, let alone whether it might prevent or trigger the next ice age.

The question of whether reversing global warming might lead to an ice age could be irrelevant if it never happens. According to a study by the National Oceanic and Atmospheric Administration (NOAA), the changes in ocean surface temperature, rainfall, and sea level that have already occurred are irreversible for a thousand years after carbon dioxide emissions are completely stopped [source: NOAA]. That means no matter how much we curb our emissions today, we may not be able to undo the damage that has already been done anytime soon.

The one thing scientists do seem to agree on is that another ice age is not likely to occur for thousands of years -- not even remotely close to any of our lifetimes.