Monday, October 11, 2010

This Orange Earth (or, Why It's So Cold)

Southbound. Bags are packed, parkas and boots are on, and I'm loaded aboard a US Air National Guard C17 cargo plane with 38 friends, colleagues, and support staff, winging our way wouth towards Antarctica.

This blog is called "Cold Dirt," and as you'll see, it's all about the cold, frozen, icy, and in places, rapidly thawing soils found in the McMurdo Dry Valleys of Antarctica. Before I dive in to the science of permafrost geology, and glaciology, and all the other research subjects that are the focus of this expedition, I thought it might be worthwhile to unpack these two ideas--"cold" and "Dirt."

Everyone likes to complain about the cold (even Antarctic explorers!), but most folks don't stop to think about why the Earth's poles are as cold as they are. We've become used to seeing pictures from space, or even just plotted on a globe, showing ice caps at the north and south ends of our planet. Why is that?

Almost all of the heat that we feel at the Earth's surface comes from the sun (a little bit trickles up from the warm interior of the Earth, but not enough to feel in most places). The sun beams light towards the Earth, which travels though space, and ultimately arrives at the top of our atmosphere. If you put a solar panel right at the top of the atmosphere to collect that energy, you'd find that every sqaure meter (imagine a solar panel one meter, or about 3 feet, on a side) would pick up about 1250 Joules of energy every second. A Joule is a fiddly unit of energy (your electric bill doesn't count Joules, and for good reason), so it helps to think in Watts (a Watt is one Joule of energy being collected or used every second). A Watt is a measure of power (so a 300 Watt hair drier is more powerful than a 400 Watt hair drier, because every second that it's on, it can turn 100 more Joules of electrical energy into heat for your hair than the 300 Watt drier). Back to the top of the atmosphere, some of that sunlight (solar energy) gets reflected back into space (goobye sunshine!) and some of it shines right down through the atmosphere to the surface. Once it finally reaches the ground, some of that sunlight gets absorbed, warming up the land, ocean, trees, people--everything at the Earth's surface (this is why sunlight feels warm on your skin--you're absorbing energy in the form of solar radiation, and converting it into energy in the form of heat).

That's how the Earth gets warmed: solar energy, in the form of radiation gets absorbed and turned into heat. But why are the poles cold? To understand this, it might  make sense to come down from the top of the atmosphere, and to head into the kitchen or to a  grocery store. Find yourself a nice round orange with a label sticker on it (they usually say, "Florida," or "Sunkist," or even "Apple" if you've got a sarcastic grocer). The orange represents the Earth. The stem-spot on the orange represents the South Pole. For the sake of simplicity, now pick up the orange and hold it out at arm's length, with the south pole pointed down at the floor (if there'a fruitfly buzzing around the orange, somewhere near it's lower half, that represents me, sitting here on this plane). By holding the orange out at arm's length, now you represent the Sun. If you spun around in a circle by pivoting on your toes, you'd be simulating the Earth revolving around the Sun. You'd also look kind of silly.

Now, about that label sticker. On your Orange Earth, the sticker represents a blanket, spread out for a picnic (yes, people picnic in Antarctica, but that's for another post). Hold the orange with your thumb on the south pole, and another finger on the north pole (the opposite side of the orange). Now peel off the sticker, and put it on the side of the orange, right in a spot that's half way around the orange between the north orange pole and the south orange pole. Now the sticker is on the orange's equator. Hold the orange back at arm's length, with the south pole pointed at the floor, and take a look at the sticker. If the sticker is pointed right at you (remember, you're the sun), you can see the whole sticker. All the energy coming out of you (the sun) can be absorbed by that big picnic blanket, rapidly warming it up under the noon sun (if you've every picniced near the equator, you know it can get pretty hot, pretty fast in the direct sunlight). Now peel off the sticker, and put it down near the stem of the orange (the south pole). Now the picnic blanket is in Antarctica. If you hold the orange back out at arm's length, with the south pole (stem) pointed at the floor, what do you see? You can't see the whole face of the sticker! The curvature of the Earth points the sticker away from your eyes (the sun), so that you can only see a little bit of it. Now, the picnic blanket can only absorb sunlight from you, then sun, if the sun's rays (solar radition) can travel from then sun to the ground. Since you can only see a small portion of the sticker, it means that only a small portion of the sticker gets direct sunlight, that can be absorbed, causing the blanket to warm up. The picnic blanket (sticker) near the pole gets less direct sunlight, over a smaller surface area (the part of the sticker you can see), than the picnic blanket at the equaotr. It's going to be a cold lunch in Antarcitca!

The fact that the poles absorb less solar radiation than the equator is only part of the reason that the poles are so cold. The other piece of the puzzle comes from how the two picnic blankets lose heat. Just about everything in the universe loses heat by emiting infrared radiation. Infrared radiation is the kind of radiation that you feel when you stand in front of a fireplace or radiator (makes sense why they call it that). The ground loses heat at night by radiating infrared energy off into space (this is on reason why clear winter nights are so cold). Both picnic blankets lose heat at about the same rate, radiating infrared radiation from their surfaces back out into space. The blankets have a "heat budget" and get hotter when they absorb more solar radiation that they lose through infrared radiation, and getting cooler when they lose more heat than they absorb (the same way a bank account--a personal budget--grows bigger when you spend less money than you earn, and gets smaller when you earn less money than you spend). The polar picnic blanket takes in very little heat from the sun, but loses plenty to space, while the equatorial picnic blanket gets plenty of sunlight, and only loses about as much energy as the polar blanket, helping keep it heat-rich and warm.

The last wrinkle to why the poles are so cold can be figured out by holding your orange Earth out at arm's length and taking a picture of the orange with a camera that has a flash. What do you notice when you look at the picture of the orange? Most of the orange looks about the same in the picture, but the sticker on the orange should look super-bright in the light from the flash. The reason the sticker looks so much brighter than the rest of the orange is because the sticker reflects more of the light from the flash than the orange does (this is becuase the sticker is shiny and light-colored). The fraction of the light that a surface reflects is called its "albedo" (something like concrete that reflects half of the light that gets shined on it has an albedo of 1/2, while something that reflects all the light that's shined on it, like a perfect mirror, has an albedo of 1). Fresh snow has a very high albedo (more than 9/10, or almost 1). Reflected sunlight never gets absorbed by the ground, meaning it never gets to be part of the ground's energy budget, and can't warm it up. All of the snow and ice at the Earth's poles means that they have a high albedo. The poles reflect a lot of their tiny share of sunlight back into space, so that it never gets a chance to help warm the ground.  

So that's the "cold" part of "Cold Dirt." I'll talk more about the "Dirt" once I reach my science site. For now, though, I've got a few more hours of flying time, before we land on the ice runway at McMurdo Station (edit: made it in on the second pass! Hello from McMurdo!). Time for a quick nap in the parka, and dreams of fresh oranges.


  1. "Made it on the second pass"... what happened on the first pass?

  2. There was fairly thick fog and snow at the McMurdo sea ice runway (a 10,000 ft. long runway carved out of the Ross Ice Shelf), and a very low "ceiling" (the height to clouds too thick to see though). When we finally broke through the clouds, the pilot had a hard time getting lined up with the runway--we swung left, and then right, and then back left again--feeling like a pendulum bob in an over-wound clock. The pilot decided to try again, so with a roar from the engines, we started to climb back from ~100 feet altitude. Flights have "boomeranged" (headed back to New Zealand, to try again the next day) because they couldn't safely land, so everyone was feeling pretty anxious. Fortunately, we made a wide swing back around, and the pilot got the plane safely to the ground (and all of us to the galley in time for dinner).