Emma is one of my regular guest bloggers. I feel really thrilled about the possibility to post one educating post of this great blogger once a month over the next couple of months. Thank you so much, Emma, for sharing these great posts with us! If you would like to check out the previous guest posts, this amazing blogger wrote for me, head over here.
Flagstaff is a weird city for the spring. It snows on and off. You begin to think winter’s over just as we get another 2-3 feet of snow. The temperature rises and drops, rises and drops…for now, we’re still hovering in the fifties and sixties, Fahrenheit.
Sometimes we still drop below forty. And sometimes we soar into the seventies.
It’s funny, how variable this place is. I grew up in a desert and still consider it to be my home. There, the temperature stays in a neat range between sixties (on cold winter mornings) and the low hundreds (on hot summer days). In the mountains nearby, it drops below freezing, but we never spend the winter months there.
Then I take a look at the rest of the world. Flagstaff is in Arizona, United States, northern hemisphere of the globe, so I grew up thinking Christmas was in the winter and my birthday was in the summer. Pretty straightforward, right?
Well, if you’re one of the Momma’s regular readers, chances are you know that’s wrong.
The summer hemisphere enjoys the opposite seasons as the northern hemisphere. It’s fall down there right now, I believe. (Unless I got this post out so late I missed it. Sorry, Momma.)
So how does that even happen?
Okay, for some perspective, let’s take a look at the Earth’s axial tilt.
The Earth sits at about a 23.5 degree incline from the vertical. And technically, vertical is a relative term. There’s no up or down in space, and the plane of the solar system is actually kind of variable. The Earth’s 23.5 degree incline is based on the angle of its orbit around the sun.
If the Earth stood up straight, we’d get intense sunlight at the equator year round, and—whadya know—never have any seasons. Why is that? Because summer in either hemisphere depends on that hemisphere being exposed to the sun’s radiation more than the other.
The image above shows the southern hemisphere in summer and the northern hemisphere in winter. If the Earth were tilted the other way, then the seasons would be reversed.
We know that the seasons will eventually reverse. So…what? Does the Earth wag back and forth like a dog’s tail or something?
This is what we’re going to see over the course of a year.
The earth’s tilt doesn’t change much.
Well, it does…there’s always precession and obliquity to consider—but those are terms I’d rather explain later.
For now, don’t worry about it. The earth’s tilt changes a bit, but very very slowly over thousands of years. And last I checked, seasons don’t last thousands of years.
For the purposes of this explanation, assume that the earth’s tilt is nearly constant. It will remain 23.5 degrees the entire year, and it will stay facing in the same way. As the Earth orbits the sun, the different hemispheres will get different exposures to sunlight.
See, we only changed one variable. Tilt the earth so seasons are possible, then keep the tilt the same and change the location of the sun, and you get the actual seasons.
And yes, yes, I know…the sun didn’t move. We moved. But you get what I mean.
So, as you can see, the seasons are always opposite. Here’s the catch, though. It’s fun. Winter and summer are two extremes. Fall and spring are transitions, though. They are different in that one is transitioning from summer into winter, and the other is transitioning from winter into summer.
Because they’re both transitions from extremes, both hemispheres are getting the same amount of sunlight. The northern hemisphere is getting sunlight back after its winter, and the southern hemisphere is losing sunlight after its summer.
So right now, the seasons are opposite. They always are. But we’re at a point, those of you in the southern hemisphere, where we’re getting equal exposure to the sun.
Okay, now I’m gonna throw a wrench into our plans for complete seasonal equality.
Remember precession and obliquity?
Precession is a sciency term for the Earth’s wobble. It literally wobbles like a top. Over the course of years and years and years. And this, rather than affecting the seasons, affects the position of the north star.
And climate change.
Let’s start out with the north star, because it’s really simple. I’ll show you what happens over the course of about ten thousand years.
Polaris is the current north star, located in Ursa Minor (better known as the Little Dipper).
But about 4000 years ago in 2000 BCE, the north star was Thuban in Draco the Dragon.
And in about twelve thousand years, the north star will be Vega in the constellation Lyra.
So…what on Earth could precession possibly have to do with climate change?
I mentioned before that Earth’s tilt is responsible for making the seasons possible. Obviously, the tilt has a hand in climate. Remember, seasons aren’t weather. Clouds today or tomorrow are weather. Warmer or cooler summers and warmer or cooler winters are climate.
The axial tilt makes the seasons possible. It doesn’t make the seasons happen. But, come on, the orbit makes the seasons happen, and the Earth isn’t going to one day stop orbiting the sun. We can assume that, thanks to the tilt, seasons are gonna keep happening for a while.
As long as the Earth stays tilted in the same direction, it helps the seasons stay constant. But if we change when on Earth’s orbit the axis is tilted in one direction or another—precession—then we’re going to get some climate variability.
Precession isn’t the only aspect of the Earth’s axial tilt that affects climate. Now we’re going to talk about obliquity.
Let’s go back to the top analogy. When you spin a top, it wobbles about and simulates precession. But it doesn’t necessarily wobble about evenly. Uneven force when spinning the top could get it a little unbalanced, as could any imperfections in the material and even simple aerodynamics.
That’s where the metaphor stops being useful, because it doesn’t explain why obliquity happens. But it does happen. In Earth’s case, gravitational forces throughout the solar system keep it from wobbling perfectly. And the tilt varies a bit, between 21.1 and 24.5 degrees.
Enough to change the sunlight distribution across the hemispheres, and enough to aid precession in altering the climate.
This is not responsible for global warming.
Precession and obliquity are just two factors out of three that, together, are known as Milankovitch cycles—or orbital forcing. These motions of the Earth as it moves around the sun have been driving ice ages and interglacials (the warm periods in between) since the dawn of time.
It’s one of the most common misconceptions surrounding global warming. Many skeptics/deniers argue that the climate has changed before, so it must simply be changing again.
Not true. Orbital forcing—including precession and obliquity—has always affected climate and still does. These are not new variables, they’re constants. Human activity is a new variable. And the specific activities humans are doing is a very new variable.
I think that’s enough on seasons and climate for now…until next time!