to the students who, in last term's evaluations, criticized me for drinking my in-class coffee from environment-unfriendly disposable cups instead of reusable mugs.
What is the "greenest" way to drink coffee around the office?
If you use a disposable cup, it's going to linger a long while on this Earth—polystyrene isn't biodegradable at all, and for all practical purposes, you shouldn't expect a paper cup to degrade very fast in a landfill, either.[...]
Pound-for-pound, petroleum-based polystyrene is a pretty bad material—it takes twice as much energy to produce a gram of polystyrene as it does to produce the same quantity of ceramic. But you'll need at least 70 times as much energy to produce a ceramic mug as you will to manufacture a polystyrene cup, and probably even more to produce a stainless steel mug.
How could that be? Simply speaking, it's all about mass: A polystyrene cup is much lighter than a permanent mug. That means it requires far less material, so the fact that it's made from petroleum is more than made up for by the greater mass of the mugs. [...]
Washing your mug will add to its energy burden. Research from the early 1990s suggests that each time you clean a mug in the dishwasher, it takes about as much energy—and would probably produce as many emissions—as it takes simply to produce a new polystyrene cup[...] As the Lantern has pointed out before, washing the mug by hand may not absolve you, either— although you can help your case by using cold water.
And, NB: in an office without a useful dishwashing sink, the likely prospect is of washing the mug multiple times-- say, rinsing it once in hot water in the rest room, and then taking it home regularly to run through the dishwasher. In other words, it's highly tricky to figure out the math here.
Update: Chris Lawrence's and Nick Troester's reactions make me worry that the tone of this post came out wrong-- I was only amused by the commentary in evaluations, and it built on some running jokes in class. One anonymous benefactor gave me a stainless steel McGill coffee mug to try to reform my sinner's ways. It was all in good fun.