What causes ice ages?
Sun, 11th May 2008
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from the show Repelling Mosquitoes
David Burke, Galway asked:
What causes ice ages and will we have them again and if so when, roughly?
Dave: Thereís various things we call ice ages. Strictly speaking weíre actually in an ice age now. Over the last few tens of millions of years weíve had periods when the north and south poles have been frozen but thereíve been other periods in the Earthís history when they havenít been frozen at all. Over the long period we are in an ice age at the moment. People arenít quite sure why thatís been happening. Itís been suggested it might be something to do with the Himalayas growing and altering he climate patterns in the world. Iím not entirely sure of the reasons for that. On the shorter scale thereís various other things that will affect the climate: one of them is we get cycles of exactly where the Earthís pole is pointing. Sometimes itís almost vertical at which point the seasons are very weak. Some times itís keeled over more.
Chris: Itís the Earthís tilt, isnít it? Itís about 23.5? and it wobbles a bit, doesnít it? The planet sort of wobbles backwards and forwards a little bit on its orbit. I think itís over 30,000 years or something like that.
Dave: Yeah, thereís various different cycles on different forms of vibration.
Chris: Why should that make an ice age?
Dave: If you suddenly have stronger seasons that will affect it, the way water and air flows around the world. Subtle changes like that can affect the temperature of the whole world. That could be creating ice ages.
Chris: Itís a sort of positive feedback loop too, isnít it? Thereís this thing called a Milankovic cycle because where the Earth doesnít go round the sun in a perfect circle itís an ellipse over time you get more of an ellipse than other times. This means that more energy reaches the Earth sometimes from the sun than other times and this means the planet goes into phases of cooling and warming but once youíre into cooling it makes it easier for ice to form. Once you have ice forming it means you can have more ice. More ice reflects more light back, because itís very white, back into space so the Earth cools a bit more and it goes into a positive feedback loop: making it very cold. When something breaks that cycle we warm up again.
Dave: Thatís right and thatís why we should be so worried about human-led climate change because if we make a small effect you never know, there might be a load of other positive feedback. You might find that thereís less snow on the ground so the world heats up even more than it would do just with carbon dioxide.
Chris: Itís not just as simple as carbon dioxide in the air?
Dave: Yeah, basically itís all just horribly complicated.