In case you haven't been following, Bob Murphy wrote a shameful op-ed on the proposed Cap-and-Trade scheme, which prompted quite a bit of criticism from me and "TokyoTom" (good summary). If Bob were merely claiming that politicians screw things up, none of us would have objected. Unfortunately, he said a lot more than that. The focus of this post will be on his claim that carbon emission caps "don't reflect scarcity".
Now, as you see in the exchange, Bob tries to claim that what he really meant was that if the carbon cap were too low, or somehow not correct, that wouldn't reflect scarcity, but otherwise it would. If you follow the exchange, you'll see how I showed that there is no possible way, based on the phrasing of his argument, that he could claim the op-ed meant that. Nevertheless, he has repeatedly gotten considerably sympathy from others (not me) with this last-ditch attempt to salvage himself from having to apologize for his op-ed, by arguing that, so long as government doesn't precisely set the cap to what the perfect, pure, austere free market would, the cap still would not reflect scarcity and he was technically correct. I will now show how even this claim is wrong, by starting from a simple case, and working up to the claim Bob made.
Economic scarcity refers to the situation where "not all of society's goals can be pursued at the same time; trade-offs are made of one good against others." Now, let's see where this takes us.
Problem #1: I want to hit Bob. Bob does not want me to hit him. Does scarcity exist?
Answer: Yes, because it's impossible to satisfy the social goals of both me hitting Bob and Bob not being hit by me.
Problem #2: I attempt to hit Bob. Bob retreats to his house and locks himself inside. I attempt to bypass the locks. Does the difficulty of getting to Bob reflect scarcity?
Answer: Yes. Since my goal comes at the cost of Bob's, Bob will take measures to ensure I do not reach mine. The lock, a manifestation of Bob's desire not to be hit, therefore reflects scarcity.
Problem #3: Having such difficulty getting past Bob's locks, I instead try to act out my anger against him by sending 100 locusts down his chimney. I would have sent more, but my insurer restricted me to having only 100 locusts at any given time. Does this restriction on my ability to carry locusts reflect scarcity?
Answer: Yes. In deciding the max it will allow me to carry while maintaining coverage, the insurer must consider how badly I can hurt others with a given number of locusts, since hurting others can cause me to be liable for damage. The limit, being a mechanism by which the conflict with the desires of others not to be hurt manifests, therefore reflects scarcity.
Bonus answer: Note that locusts are not guaranteed to hurt Bob, but the more I send, the more likely that is. So the limit on how many locusts I can have only reduces the harm to Bob in a probabilistic sense. However, the limit still reflects scarcity.
Problem #4: Same situation, but in an alternate universe. There is an intrusive government that passes laws in an attempt to minimize conflict between its subjects and thus maximize its looting; insurers of the type above don't exist. It has decreed that people may own 146 locusts, but no more. In trying to buy more than 146 locusts, a red flag goes up, and I am prevented from buying any more. Does this difficulty in buying locusts reflect scarcity?
Answer: Of course. Whatever criticism of government you might make, its decrees ultimately rule in favor of some goals and against some others. Therefore, when it hinders one goal (such a stopping locust attacks) in preference to another (such as the goals of locust-lovers in keeping collections), this is a manifestation of scarcity.
Problem #5: What if the government's limit were 23 instead? Or 100? Or 0?
Answer: Yes, it still would reflect scarcity. No matter how closely or poorly it approximates what limits would result from market processes, the above reasoning applies.
Problem #6: Scientists reveal that emitting substance X increases the probability of catastrophic damage. The governments of the world then place an overall emission cap of C. As a result of the cap, the price of doing things that result in X emission goes up. Do the higher prices from the cap reflect scarcity?
Answer: Yes, for the same reasons as in #4: the higher prices ultimately result from the (probabilistic) conflict with the goals of others. Yep, even substance X is CO2.
Problem #7: If Bob responded to the above line of reasoning (about scarcity) by saying that the proposed scheme is not a Blicknorg , is that responsive?
Answer: Don't be ridiculous; that's just changing the topic.
Long story short, the caps do reflect scarcity.
 And how would you ever learn what cap a free market would set? Why, you first have to price all resources, including and especially the atmosphere. Anyone want to take a wild, wild guess as to the ratio of the words that Bob has spent:
a) advocating that atmospheric property rights be clearly delineated,
to the words Bob has spent
b) demanding that government NEVER do a SINGLE thing to in any way define such rights?
I'll give you a hint: it's somewhere between "zero" and "can I have what you're smoking?"
 Actually, in the discussion, what Bob actually tried to do to refute the solid argument that caps would reflect scarcity, was claim that the caps are not a "market solution", rather than a Blicknorg. However, since he refused to ever clarify what exactly that meant, despite being asked several times, he might as well have said Blicknorg.