In my last blog, I said that the only thing that heats Earth is the sun. That is not quite true. In the spirit of full disclosure, residual radioactivity in Earth’s core does supply some heat, mostly to hydrothermal vents and volcanoes. A lot of additional energy is provided by the hot air of politicians’ speeches. Still, “It’s the sun.”
Why, then, might Earth be warming? Is the energy we get from the sun changing? Is Earth doing something different with that energy? Answer: yes, to both.
Again, this isn’t rocket science or nuclear physics. But it does require that you pay attention and think a little bit.
Earth’s North Pole points toward a star we have cleverly named the North Star or the Pole Star. At some point, we may become more clever and name it the North Pole Star.
Earth’s axis is tilted (about 23.44 degrees at present) with respect to its orbital plane, so that part of the year the northern hemisphere is tilted toward the sun (summer) and part of the year the northern hemisphere is tilted away from the sun (winter). The southern hemisphere, not to be outdone, has the same seasons, but reversed.
Because the North Pole Star is so far away, it seems that the North Pole always points toward it. But while this is going on, Earth acts like a child’s gyroscope. In its travels about the sun, the place in the sky toward which the North Pole points rotates in a circle that takes about 26,000 years to complete. The North Pole Star will be a different star in not so many years. In fact, that’s happened several times in recorded history. This movement is (gyroscopic) precession.
In addition to precession, the axis of Earth “wobbles” in about an 18-year cycle (“nutation”) meaning that every 18 years or so the tilt toward the sun in summer is a tiny bit greater, and the tilt away from the sun in winter is a tiny bit less. Nine years afterwards, the tilt toward the sun in summer is a tiny bit less, and so on.
The orbit of Earth is an ellipse—a slightly squeezed circle. The amount of squeeze is measured as the eccentricity of the orbit. And that changes slightly in a 413,000 years-or-so cycle. Other aspects of the orbit with different periods combine to produce a cycle of about 100,000 years.
The combination of precession and orbital eccentricity then combine to create a roughly 21,000-year cycle. What this boils down to is that different hemispheres (northern, southern) get different amounts of light and heat from the sun and that amount changes in a 21,000 year cycle.
Because there is more land in the northern hemisphere than the southern, and because land absorbs sunlight better than water, today Earth absorbs more heat from the sun than it did about 10,500 years ago, and more than it will in 10,500 years.
In about 10,500 years, the “summer” and “winter” tilt will be reversed, and June and July will be winter months in the northern hemisphere, and December and January will be summer months. Australia may see a White Christmas—if we haven’t destroyed Earth before that.
Around 1920, an astronomer/geophysicist, Milutin Milanković merged these cycles mathematically and plotted them on graph paper. His hypothesis was that these cycles affected the amount of energy reaching and being absorbed by Earth, and therefore, affected global temperatures. A study of ice cores has validated this hypothesis, although there remain some questions that have yet to be answered. Milanković’s cycles are important, but are not the entire answer.
It’s still the sun, but it’s also sunspots.
We’ll look at those in the next post.
Gyroscopic precession: https://www.youtube.com/watch?v=ty9QSiVC2g0 [accessed 2016-11-18]
For an understandable explanation of Milanković cycles, and some excellent images, see https://en.wikipedia.org/wiki/Milankovitch_cycles
Registered Curmudgeon, scientist, skeptic, humanist, and writer.