I thought I’d cover another science topic today, after this week’s fill of politics. Most of you have probably heard mention of Milanković Cycles (the spelling usually Anglicized to Milankovitch Cycles) in relation to the study of the earth’s climate. Today I’ll explain what they are, and why they’re relevant to the CAGW debate.
Those of you with training in the physical sciences will regard what follows as old hat, and are free to continue your darts match in the corner. Everyone else, grab a drink and gather round.
Milutin Milanković (1879-1958) was a Serbian civil engineer who, having been interned by the Austro-Hungarian army at the beginning of the First World War and given a job in the library of the Hungarian Academy of Sciences, found himself with a lot of free time on his hands. Bored, he started wondering about the variations of the earth’s orbit about the sun, and what effect they might have on the earth’s climate (now that’s really bored).
If you’ve done any high-school physics, you’ll know that the earth’s orbit around the sun isn’t quite circular; since Johannes Kepler’s 1609 Astronomia Nova (a reprint of which sits, rather pretentiously, on my bookshelf) we’ve known the orbits of the planets tend to be slightly elliptical (an ellipse is a flattened circle, with two focal points as opposed to a circle’s centre. The sun sits at one of these foci. You can wiki it all here).
What you probably weren’t told in high-school physics is that the shape of the earth’s orbit isn’t quite a perfect ellipse, nor does it remain constant. Situated in a wildly chaotic universe, it’s subject to numerous fluctuations; some periodic and predictable, some not. Most are too small to be of any importance, but Milanković identified the three most significant ones:
Eccentricity. This is a measure of how flattened an ellipse is. An ellipse with an eccentricity of zero is actually a circle. The earth’s orbital eccentricity varies between about 0.005 and 0.05, predominantly in two superimposed oscillations of 100,000 and 400,000 years. The 100,000 year one is the most important Milankovitch cycle, as we’ll get to later.
This is important because, firstly, at some times the earth is closer to the sun, and at others further away, and secondly, because it skews the lengths of the seasons (as measured by the astronomical equinoxes and solstices). These variations have the effect of changing the insolation at any given point on the earth’s surface (insolation being a term coined very late one night many years ago, when an extremely drunk physicist stood up on a pub table and attempted to pronounce incident solar radiation. Why can’t modern physicists hold their booze? But I’m straying into another thread).
Obliquity. Not only does the earth’s elliptical orbital shape vary, but so does its axial tilt. You’re all aware, I’m sure, that the earth’s axis of rotation is tilted about 23½ degrees from the perpendicular, with respect to the ecliptic (plane of orbit). This is what gives the northern and southern hemispheres opposite seasons; why, in other words, I’m freezing my arse off as I sit here in Tasmania typing this, while most of you are luxuriating in a supposed barbecue summer. Why hasn’t anyone told people on the Californian coast?
This 23½ degree figure changes over time; it actually oscillates between about 22° and 24½°, with a period of about 41,000 years. Which means, I guess, if Club Med’s tropical resorts are around long enough, someone’s eventually gonna sue them for false advertising.
Precession of the axes. Ever watch a spinning top? As the top slows, the axis of rotation itself starts revolving about the vertical. The earth’s axis does much the same thing, as this diagram illustrates:
What this means is that the equinoxes move a little west every year, with respect to the fixed stars; therefore at some times the northern summer occurs at the perihelion (closest approach of earth to sun), so northern summers are more “insolated” (read hotter) than southern summers; at other times vice versa. A complete such cycle of the earth’s axis takes about 26,000 years. Gravitational interactions with Jupiter and Saturn, though, cause a precession of the orbit’s ellipse itself (apsidal precession), meaning the effect you see in the diagram above actually takes closer to 21,000 years. Those of you with permanent time-share holiday accommodation, consider yourselves warned.
So, what does all this have to do with Global Warming?
If you’ve been following the preceding discussion, you’ll see that the amount of solar radiation hitting the earth’s surface has varied over history, being a function of solar output, distance from the earth to the sun and, at any particular point on the earth’s surface, the angle to the sun measured by the tilt of the earth’s axis. As we’re able to calculate historical insolation (we believe) to a fair degree of accuracy, we can then compare the effects of the Milankovitch Cycles to the history of the earth’s climate, and draw some conclusions about their effects. Look what happens when we put the graphs of the Milankovitch Cycles, together with their derived insolation (solar forcing) next to the history of glaciation, as inferred from the Vostok ice cores:
Note particularly the 100,000 year oscillation (blue line), compared to glaciation. Pretty obvious, isn’t it? What it amounts to is this: when the earth’s orbit is nearly circular, the seasons are more even; currently there is about a 6% difference in insolation between aphelion (furthest point on the orbit from the sun) and perihelion (nearest point). Conversely, when the earth’s orbit is more elliptical, that difference can be in the order of 25%. Furthermore, something called the Stefan-Boltzmann Law magnifies the emissivity (heat radiated back out to space) as temperature difference to the fourth power; meaning, for example, that a drop of temperature from 22°C to 12°C will result in a percentage change in grey-body emissivity of… anyone?…
Izen! Bring your crayons over here and work it out on the board for us while we talk, there’s a good fellow?
It’s more complicated than that, of course, but you can see why many scientists believe that long-term trends in the earth’s climate are driven primarily by the Milankovitch Cycles. Try fighting them.
Yeah, yeah, OK Ozboy, but what about the exploding chook shed? When are you going to get to that bit?
Oh, all right, very well, if you insist—but it’s not a very pleasant tale. You see, the evangelists of Global Warming keep telling me that the Milankovitch cycles are irrelevant in explaining more recent shifts in the earth’s climate; that in fact, my little corner of the world is set to become hotter, and drier—and it’s all my own fault, apparently. Never mind that last year my rain gauge recorded over 1,200 millimetres—nearly double the local long-term average.
This year had been drier however, about 30% less rain at this date then we would have had normally. Maybe it’s all true, I thought. Maybe, maybe I really am contributing to Global Warming, drought, famine and misery. Gosh… I’ve been wrong all along! Quick—what can I do to fight climate change? To save the planet? What protest marches can I join? What—
And at that point, God unzipped, and took a leak on my head.
At least, that’s what it felt like. One hundred and fifty millimetres (six inches) of rain in a single day: a record in my lifetime. Just walking about on my property became difficult, as boots would sink up to the ankles in mud and become stuck.
About a hundred metres away from my house, there’s an old chook shed. It was built from rough-cut logs hewn locally, and roofed with corrugated sheet iron. Mrs Oz and I had done some work on it recently, and were planning to move our chooks (sorry, Aussie slang: chickens) into it by the end of this year. Next to the shed, a few metres up a gentle slope, stood a tree. A rather big tree, even by Tasmanian standards. Both shed and tree have stood since the property was first gazetted, over 40 years ago. Both had endured everything the elements have thrown at them over the decades, but 14,000 tons of water dropped on the Ozboy estate in a single day was more than even our sandy and exceptionally well-draining soil could cope with; hypersaturated, it gradually took on greater and greater fluid properties, until on Wednesday night we were awoken with an almighty BANG…
My chook shed, as I’m sure John Cleese would eulogize, is no more. It has expired. It has ceased to be.
God is subtle, quipped Einstein, but never malicious.
There are days, however, when this chronicler, for one, believes neither.