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Blowin' in the Wind, Issue #019 Milankovitch cycles climate effects and ice age - June 1, 2005
June 01, 2005

Milankovitch cycles climate change

Have you ever heard of Milankovitch cycles climate shifts and their long term effects?

Climate change from greenhouse gases? Not exclusively. This article is part of a series about NATURAL causes of climate change. See the other one (so far) at

Why were there ice ages thousands of years ago and not now? Could it ever happen again?

A Milankovitch Cycle (mil-ANK-o-vich) occurs and reoccurs constantly and we are in the middle of three now. We just don't notice it because it takes many thousands of years for a complete cycle to finish.

We look at the three independent, simultaneous cycles now:

1) How far do we get from our beloved sun?

You may know that the earth is closer to the sun at certain times of the year than at others. In the Northern Hemisphere's summer, the earth is actually about 3% further from the sun, a point called aphelion, than in our winters (nearly 150 million kilometres).

No big deal, you may say, summer here has not been cancelled yet due to this phenomenom. At least not in our lifetimes. The resulting change in incoming radiation intensity varies by only about 6%.

What you may NOT know is that this percentage, a proxy for orbit eccentricity, changes over time - a period of thousands of years. Some time from now the solar radiation may change by 25% or more throughout the year due to distance variation, when the orbit becomes long and skinny in shape. And there may come a time when there is little temperature difference between summer and winter.

Maybe that's what happened in the past, during previous freeze-overs. By the way, if the planet were travelling in a perfect circle, there would be no change in distance and eccentricity would be set to zero.

This cycle of the Milankovitch hypothesis takes nearly 100,000 years.

2) Your summer the same as mine?

Do you know what the Tropic of Cancer and Capricorn represent? Anywhere on or between those lines, the sun appears directly overhead at least once per year. And nowhere else. They appear on maps and globes at latitudes of 23.5 degrees north and south. But these positions also change.

They depend on the earth's tilt (or obliquity if you want to get fancy). The tilt goes from 22.1 to 24.5 degrees and back again once every 42,000 years. So what? A higher number means more temperature difference between summer and winter, while a lower number means a greater difference between the equator and the poles instead. This could be when an ice age happens.

One scientist said that warmer winters lead to more snow, because of higher dewpoint and moisture capacity in the air. This part of the cycle has cooler summers, eventually not allowing that snow to melt completely. And it builds up over thousands of years. That seems reasonable.

3) When is it summer anyway?

Referring back to item #1 above, we are closest to sun during N-hemisphere's winter, but that is not always the case. Once every 23000 years, it gradually flips to the earth being closest to the sun (perihelion) during S-hemisphere's winter and back again. The earth actually wobbles, and instead of Polaris, Vega becomes the new North Star.

Can't rely on anything these days.

What does it all add up to?

These three cycles are collectively known as the Milankovitch Cycles.

See these cycle durations? They are neither equal nor divisible by each other. That means it may take millions of years for them to line up again as they are now. Enough time for continents to drift several kilometres. This changes the pattern of land and water a bit and you can see that world's climate will never be exactly the same again.

We also have not discussed feedback cycles. Here's an example: Snow on the ground causes the temperatures to be colder and colder temperatures increases the likelihood that future precipitation will be snow, and so on. That can cause a big build-up after a while.

There are a lot of things which depend on each other. More about this in future issues.

Thanks for reading!!!

Let others know where you get good information from. There is now a list of weather articles for you to read and spread around, including HTML code for making li'nks. The list is on this page.
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