Re: Has another ice age started? Is global warming holding it at bay

Hi Ben,

Good question! The relationship between human activities and Earth's climate is really complicated, and no one understands it very well. I think the key to sorting out your question is to think about time scales, that is, how long things take to happen.

First let's talk about ice ages. Ice ages are cycles where the amount of ice grows and grows, and then melts again. Each cycle seems to take about 100,000 years! The last time the ice started growing was about 100,000 years ago. The ice reached its maximum amount about 20,000 years ago: at that time, the ice from the Arctic reached all the way down to the door of my office on Cape Cod, MA, USA. (I don't know about ice in Australia; the Southern Ocean may have gotten in the way.) If the next cycle is just like the last one, the ice will start growing again, but will take tens of thousands of years before any big changes are noticeable. If that's what's happening, you could say that technically, yes, another ice age has started. But if you meant, "would we be covered with ice right now if it weren't for global warming?", the answer is probably no.

The big question is, will the next cycle be like the last one? No one knows for sure, and it might be fun for you to find out more about what causes ice ages and then make an educated guess yourself. The best info on ice ages I could find on the Web is right here on the MSN; you can find it by doing a search for the phrase "ice age". It works best if you choose the "exactly" button.

So what about the ice on the Thames? I didn't know about ice fairs 150 years ago, but I do know they had them at the beginning of the 1600's. The period from 1500-1700 is sometimes called the "little ice age" because it was unusually cold in Europe, but now we should compare time scales. The "little ice age" lasted a couple hundred years; a real ice age cycle lasts about 100,000 years. So the Thames ice fairs probably were not a sign that the glaciers were on their way.

Ok, now global warming. You're absolutely right that lots of greenhouse gasses have been released since the industrial revolution, and you're also right that the temperature doesn't seem to have risen. But this depends on what you mean by the temperature! Scientists calculate that the average temperature has probably risen by 1 degree C over the past century or so. But you don't notice the average temperature in your daily life, you notice the highs and lows and the temperature on your way home from school. Try this: for two weeks, write down the time and temperature when you wake up, at noon, and when you go to bed, and then graph temperature vs. time. When you've made your graph, ask yourself if the second week was warmer than the first. It's not an easy question!! Do you look at the hottest temperature in each week? Or the average over the whole week? Or the average temperature at noon? Or the average temperature of each day? Chances are the difference between the two weeks will be much smaller than the difference from morning to noon, or from day to day. The same is true from weeks to years to centuries to hundreds of centuries, and that's why it's so hard to notice--or even define!--global warming.

It gets more complicated, because you measured the temperature in one city three times a day. Is that enough information to judge the average weekly temperature of Australia? Probably not, though its enough to guess with. Likewise, scientists only have enough data to estimate the earth's temperature over time. (The data, by the way, comes from everything from ancient tree rings to air bubbles trapped in ice cores!) But is our data accurate enough to measure a 1 degree rise in 150 years? Some scientists say "yes", but they have to use some fancy statistical math to prove it.

Ok, let's assume Earth has warmed a little since the industrial revolution. Now that you know about time scales, ask yourself if 150 years of warming is even noticeable to a 100,000-year-long ice age. But what if global warming also continues for a long time? Then who knows? Again, you should do some research and develop your own theory. There's at least one good web site with lots of information about global warming and climate change. Several people have also asked the MSN about global warming, and you can look at those questions and answers by searching for "global warming" with the "exactly" button.

There's a whole lot that no one knows about climate and greenhouse gasses, so keep thinking about it!

Dan Goldner (goldner@mit.edu)
MIT/Woods Hole Joint Program in Oceanography