Archive for September, 2007

Why does climate change?

September 18, 2007

Sevinch asks:

how do environments change over time, and what causes them to change?

There are a lot of factors that contribute to environmental changes. The first critical concept is to accept that our environment is not static – it is always changing. The idea that climate change can be stopped is kind of like thinking that the Earth can be stopped – so that it stays in the same place in its orbit. It just ain’t gonna happen.

So, now that we have accepted that climate is variable instead of static, why does it change. The whole concept of “climate” encompasses a lot of different components – some of them are fairly stable, like the amount of sunlight that hits the earth, and some are a lot more variable like the detailed make-up of the atmosphere. Lets start with some of the more stable ones.

The amount of sunlight that hits the earth is pretty constant, but even that has some variability. As the sun’s activity changes, the amount of energy (light) that it emits also changes. Every 11 years, there is an increase in sunspot activity, which means that there is  a change in the amount of energy that the sun emits, which (of course) changes the amount of energy that hits the earth. The amount of the suns energy that is absorbed by the earth is also variable – a lot of the energy is reflected back out into space. The exact amount is determined by the reflectivity of the earth. The amount of energy that is reflected back out determined the albedo, or how bright the earth looks from space. Once again, this number is fairly constant, but still has variability. An increase in cloud cover, snow cover, or pretty much anything that changes the color of the surface of the earth will change the albedo, thus changing the amount of energy that the earth absorbs from the sun.

Now, lets look at some of the more variable components. The detailed make-up of the atmosphere plays a huge role in climate. Small changes in the amounts of certain elements or compounds can have huge effects on the ability of the atmosphere to retain energy. As the suns energy enters the earths atmosphere, it is either reflected back into space, absorbed by the atmosphere, or passes through the atmosphere to reach the surface of the earth. The atmosphere can only hold so much energy – it is stored as wind, heat, and in  a whole series of complex molecules that hold onto the energy or use it to form new compounds. As the chemical composition of the atmosphere changes, its capability to hold and store energy also changes, which in turn changes the temperature of the atmosphere.

So, how does the composition of the atmosphere change? Lots of ways. As the biota (all of the living things put together) on the earth changes, so does what they dump into the air. There was no free oxygen in the air until green plants started dumping it into the air (oxygen is a waste product of plant metabolism). Because the biota of the earth is also always changing, the content of the atmosphere is too.

Finally, that light from the sun can reach the surface of the earth. When it hits the ground (or a tree, or a building, or whatever), it has a chance to be reflected back through the atmosphere into space, or it can be absorbed by whatever it hits. If it is absorbed, it will either warm up what it hits, or (in the case of green plants) be used to drive metabolism. As the surface of the earth changes, what the sunlight hits will change, which will change the chances of it being absorbed or reflected. A good man-made example is paving – roads, parking lots, etc. Most paving is black, and is really good at absorbing the sun’s energy. So if, for example, we plot a great big parking lot down in the middle of Alaska, in the winter that parking lot will be a whole lot warmer than the surrounding snow because it will reflect a lot more light than the white snow surface does. One parking lot in the middle of Alaska won’t make much difference to the world environment, but a big area of pavement like any large city can (and does) make a difference. When you add up all the surface area of the world that is paved, you get a pretty good amount of extra energy being absorbed.

A natural (in some cases) case of surface change that effects the absorption of sunlight is changes in forest and plains. Large areas of wilderness undergo natural cycles between being grasslands/plains and forests. Forests tend to be darker colored, and are thus better at trapping the energy from the sun. As these natural cycles take place, they can drive climate change – but the relationship is complex – they not only drive climate change, but they are effected by climate change.

Finally, there can be sudden huge events that can trigger climate change. There are two excellent examples of this that we know about. The first is the asteroid that hit the earth and “killed off all the dinosaurs”. Is wasn’t the asteroid that killed the dinosaurs, it was the climate change that the impact caused. Similarly, when Krakatoa erupted, there was a “year of no summer”, and the climactic effects of the eruption lasted for decades. The reasons for the two changes in climate are the same: the event did the same major thing: they dumped a whole slew of dust into the air.

This dust changed the way that sunlight interacted with the earths atmosphere. A lot more light was reflected back out into space, and the light that wasn’t reflected was a lot more likely to be absorbed into the atmosphere. This means that the overall amount of energy that is trapped by the atmosphere/earth together is lowered (because more is reflected). IN the case of the asteroid, this was enough of a change to trigger a major long-term climate change. In the case of krakatoa, the change was much more short-lived, but still noticeable. Another effect of this is that all of the plant life on the surface that is relying on the sun’s energy to stay alive suddenly are starving – not as much sunlight is getting to them. Lots of plants die off. But wait: those plants are critical for keeping the oxygen in the air, so when they die, the balance is knocked even further out of whack. Then because there is less oxygen, the atmospheres ability to hold energy changes, and so on and so on.

Fortunately, the system tends to be self correcting – when it gets knocked out of whack, it self-corrects and swings back the other way – if it is too cold, the system reacts by becoming more efficient at trapping energy from the sun. But then it gets too warm, so it self-corrects again. Things keep self correcting back and forth in smaller and smaller swings until something else comes along and knocks it all out of whack again.

How to Make Charcoal

September 5, 2007

Timmy asks:

When I make a fire, I end up with little lumps of charcoal, but most of the wood burns to ash. How do they make charcoal without wasting all the wood?

The little bits of charcoal that are left in your fireplace or campfire are made one of two ways. The first way is that the fire goes out, and as it cools, it leaves little bits of unburned wood. The little bits are burned on the outside, but the inside is still partially unburned. To understand this, we have to look at how wood burns. There are a few stages:

  1. As the wood heats up, gasses are released from the cells in the wood. These gasses burn up, and make a lot of the yellow flames that we see in a fire.
  2. Once the wood has released the gasses, what is left is very dry, but solid. When these dry hard bits are exposed to high heat (like the heat produced when those gasses burn), they also burn, releasing more gas, heat, and finally leaving behind ash.

Sounds simple, no? Well, it is – sort of. The confusing bit is that once a fire is lit, all of these things happen at the same time. Fortunately, because the fire on the outside of the wood uses up all of the oxygen, the wood will burn from the inside out. If you have a fire and put it out before a log has burned up, you can cut open the log. You will find that the outside of the log is covered with charcoal that is full of cracks and fissures. This is where the phase 2 burning is taking place. The easy to burn gasses have been released, and the underlying structure is partially burned.

A bit deeper in, you will find charcoal that doesn’t have many cracks in it. This is the part of the log that has released most of the easy to burn gasses, but not all, and the structural part of the wood hasn’t started to burn yet. It hasn’t started to burn either because the heat hasn’t penetrated that deep yet (wood conducts heat very slowly), or because there is no oxygen to allow the structural part of the wood to burn (remember this: it’s important).

Even deeper in, you’ll find wood that hasn’t begun to burn at all.

The part that we’re interested in is the middle bit – the part where the charcoal doesn’t have a lot of cracks in it. We know that we can make charcoal like this by carefully controlling the temperature – getting the log hot enough to release all those easy to burn gasses, but not hot enough to actually burn the underlying structure. This is really tough to do, because if the log hot enough to release the gasses, it is hot enough to ignite the gasses, which will then heat up the underlying wood, and burn it too (this is why fire works instead of just going out).

A much easier way to turn the log into charcoal is to light it on fire, and get it good and hot, then to remove all of the oxygen. If the fire was big enough, the heat that is left over will continue to “cook” the logs, letting them release the easy to burn gasses. If you’re really fancy, you can allow in just enough oxygen to let some (but not all) of the gasses burn. This can generate enough heat to make sure the fire stays hot enough to keep cooking the wood until all of the gasses have been released, but the underlying structure doesn’t burn. This will create whole logs of charcoal.

That’s how charcoal was made in the old days. They’d make a great big pile of logs. I mean BIG: whole trees – lots of them. Then they’d coat the whole pile with a thick layer of mud (or clay), leaving a bunch of big holes around the bottom to let in air, and a hole in the top to act like a chimney. Light it on fire and the fire will bake the mud/clay into a rock-like wall, so that it won’t cave in as the wood burns. When the fire is going really good (you can tell by the color of the smoke coming out of the chimney – when the fire is burning well, there will be no smoke), start plugging up the air holes around the bottom. Watch the smoke. As it turns into a thick white smoke, you know that you have started starving the fire – there isn’t enough air getting in to burn all of the gas that is being released from the wood. The unburned gasses make the smoke white. By adjusting the amount of air that is allowed in, you can make sure that there is enough oxygen to let some of the gas burn (to keep it hot in there), but not enough to actually burn the charcoal that is left. Once the wood has released all of the gasses, you plug up all the holes. What is left of the fire will go out because there isn’t anything left burning – no oxygen.

Charcoal factories used to spend weeks or months building these wood piles, and they would often take weeks to burn. Sometimes they would take over a month to go out and cool completely once the holes were blocked off. Once the whole mess has cooled off, you bust open the baked mud walls, and you’ve got a big old pile of charcoal. In huge pieces – sometimes almost as big as the logs that went in. These are cut up (or broken up) into whatever size you need. Voila. Charcoal.

Ask Dr. Science. He knows more than you do.

September 5, 2007

Hey all. Time for a great new Blog service. Yep. I’m going to put that 15 years of graduate school to work for you. Got a question about something in the sciences or technology? shoot me a question (submit a comment), and I’ll apply all that great academic knowledge and ivory tower experience to answer your question. Remember: We’re trained professionals. Don’t try this at home. Objects in mirror are closer than they appear. Time to get another Timmy.
So whadaya waiting for? Send me a question already!

–Dr. Science