Saturday, July 31, 2010

Learning about the Climate Part One: Long Term Change

Disclaimer: I did not receive payment from any political or corporate institution to write this post. Written content can be found included in lecture material at Massey University, Palmerston North. Well okay, minus the smart comments. They’re mine.

So, Climate Change. Welcome to part one of a two part miniseries.

A lot of people do not understand it. In this miniseries I am going to explain how it works as well as we know, because we do not know everything. Yes, anthropological climate change will be discussed, but I will lay out the evidence for you to make your own mind up. Because I’m a smart cookie, I know not everyone is like me and doesn’t necessarily get off on science, so I’ll try to make this as easy and entertaining a read as possible. I promise pretty pictures.

The first thing you need to understand about climate change is there is both long term climate change and short term climate change (under which falls anthropological change theory). The second thing you need to understand is the causes are like dominos arranged in an erratic jumble. If one climate system is altered, the others will be affected by the alteration.

In part one, I will talk about long term change.

Orbital forcing is the main driver behind long term climate change, but it does not explain everything. Orbital forcing is how the earth travels around the sun.

In laymans terms, the Earth is a lump of rock rocketing around a nuclear fusion reactor and is held in place by gravity.

Well, sort of held in place.

The lump of rock we call home does not have a smooth orbit at all. We actually rocket through space on an ellipse, and we do a lot of wobbling on our tilted axis. I’m glad we’re a little unstable, because then we get seasons and I’m not left in eternal winter freezing my backside off at the bottom of the globe.

The sun is not even at the centre of the ellipse. It is off to the side.

There are three parts to orbital forcing, starting with the procession of the equinoxes.

As the Earth cruises round on its ellipse, it receives varying amounts of solar radiation. The sun is rather warm, and remember how I just said the sun is not at the centre of the ellipse? The point closest to the sun is called the perihelion. The perihelion is where we receive the most solar radiation.

Also, because the Earth is tilted on its axis (currently 23.5 degrees. Yes, I said currently. Who wants to talk about polar shift?), this radiation is not distributed evenly. At the current time, Northern Hemisphere summer occurs at the aphelion, which is the opposite of the perihelion. So for me down here in the Southern Hemisphere, I spend winter freezing and summer frying.

Now, because the Earth wobbles on its axis, the direction of the tilt varies. After 10.5ka (1ka is 1000 years) it is completely reversed. Basically, it is the Northern Hemisphere’s turn to fry in summer and freeze in winter. After 21ka the cycle is back to where we are today.

The second part to OF is the aforementioned tilt. The tilt of the Earth varies between 21.5-24.5 degrees. The greater the tilt, the greater the difference between summer and winter. This runs on a 41ka cycle (there and back again).

The third part, and this really is the fun part, is the orbit changes. Yes, we are a little rock flying through space on gravity. But, the shape of our orbit cycles from being an ellipse to almost circular every 100ka. Being an ellipse, we have more pronounced contrasts between summer and winter.

These three parts are glued together. Play with one and you’ll play with the others. Deep sea core samples suggest our climate has been dominated by the 100ka cycle. Our period is called the Quaternary period, and it has been around for 2.7 million years. The time between glacial periods has been roughly 100ka for the bulk of the Quaternary. The 41 and 21ka cycles have either amplified or moderated the effects, depending on where we were at the time.

I did want to provide you with an animation of orbital forcing, but Havard are being very stingy. Nevertheless, if you want to download a 74MB animation follow this link:

And here is a page with some diagrams:

And here is another pretty picture looking at the relationship between orbital forcing and glacial periods:

As you can see, there is a relationship between glacial periods and orbital forcing.

However, orbital forcing only accounts for 77% of the variations occurred during the quaternary. Records indicate climatic cycles have shifted. 800,000 years ago we were on a 41ka cycle and then we shifted to our current 100ka cycle. There are other elements involved.

What elements are these?

Oh boy, here we go.

Disposition of continental landmass, tectonic activity, oceanic circulation, extent of ice cover and possibly variation in carbon dioxide, methane and dust. This is where things get tricky, because once again these guys are all dominos who like to knock each other over. For example, when the Panama Isthmus (the link between the Americas) closed, oceanic circulation was altered, which affected the climate. Also, the continents have drifted towards the poles.

But really, scientists are still working out how the other 23% works. What are the causes and what are the effects?

If you’re interested, take a look at the temperature trend of the quaternary period:

That is an oxygen isotope record. Basically, peaks represent a warm planet and troughs mean it’s cold.

You’ll notice the trend is we’re getting colder, and have greater variation between warm and cold periods. We’re also nearing the peak of a warm period. It is also interesting to note we get warm fast and cold relatively slowly.

Long term climate change theories still have a lot of gaps to fill in. Right now, some new ideas include connections between the magnetic field and climate (i.e. Venus has a crap one and the solar wind steals all its hydrogen, oxygen, etc and leaves it to belch carbon and sulphur dioxide into its atmosphere. We on the other hand, have a good magnetic field and we keep our nice molecules.) and Solar Resonant Diffusion.

That is all for today kiddies. Next I will cover short term change, hopefully while keeping my backside firmly in Switzerland as promised.

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