> Everything the government does is a subsidy of something or other.

No, a "subsidy" is not the same as a purchase. Purchasing something at whatever the current market price is is just a purchase. Fixing prices at lower than the current market price, and making up the difference in various hidden ways, which is what the government does with fossil fuels, is a subsidy.

> Building out roads nice new roads in every podunk town in America is an enormous subsidy to oil.

No, it's an enormous investment in transportation infrastructure for the benefit of everyone. Which benefits all transportation technologies. Unless you think that hybrid or electric or fuel cell or solar powered vehicles are somehow unable to use the same roads?

> Where on earth do you get the idea that I think this?

What else do you expect me to think when the only thing you propose to fix whatever you claim is wrong is government policy?

> Externalities

Which is a non-answer unless you know, with sufficient confidence based on scientific knowledge (not somebody's beliefs or speculations or hypotheses), the amount of the externality and who can address it at the lowest cost. Which nobody knows for the case of CO2 emissions.

> Governments is justifiable largely on the grounds that it replaces the need for these violent solutions.

First, government solutions are violent: the government can dictate what everybody does only because it can back up what it says with violence if necessary.

Second, you ignore the obvious third alternative: give people a better option in the market. If the government did not subsidize fossil fuels, gasoline would be more expensive and more people would be buying cars that used less, or no, gasoline. No need to use force on anyone. And if there were more entrepreneurs figuring out how to build cars that used less, or no, gasoline, they would get cheaper. That is true even for the SUVs that you apparently abhor: a hybrid SUV can easily get double the gas mileage of a conventional one. But with gas as cheap as it is now due to government subsidies, the added cost of the hybrid simply doesn't pay for itself over the life of the vehicle.

(It's worth noting, btw, that this is even more true because the average "life of the vehicle" in the US is so short due to the availability of cheap financing and leases, which is due to government manipulation of the financial system. If people had to pay higher interest rates on car loans, they would have more incentive to keep cars longer and not buy a new one every year or two just because some shiny new thing came out. Which in turn would mean an initial investment in something like a hybrid would be more likely to pay for itself over the life of the vehicle. Another example of government meddling skewing incentives in a way that does more harm than good. And before you ask, my wife kept her last car for 19 years, and I kept my last car for 14; mine had more than 260,000 miles on it when it finally gave up the ghost. We both plan to keep our current cars as long as possible.)

> So why do you think we need well-supported scientific rationales for new activities?

I have made no such claim. I have never said individuals need well-supported scientific rationales for every new thing they decide to do.

What I have said is that dictating a public policy to everyone requires a well-supported scientific rationale, or at least a much higher standard for one than has been used.

> We can't keep burning carbon the way we are and there's no reason to think we can. There are better reasons to think we can't.

Then we simply disagree. You think this claim has a well-supported scientific rationale. I don't. I think it's a combination of ideological beliefs, speculations, and hypotheses, with no predictive track record to back it up. So I don't think dictating public policy on this basis is justified. If you want to base your own choices on it, go ahead.

Now, if you had said "we can't keep importing fossil fuels from countries like Saudi Arabia the way we are", then I would agree. But the basis for that has nothing to do with CO2 emissions, and everything to do with national security and geopolitical realities.

Or, if you had said "we can't keep burning coal the way we are", I would agree, because burning coal has a huge impact on air quality and respiratory diseases, and mining coal has a huge impact on the environment in the area where it is mined. But again, that has nothing to do with CO2 emissions.

Or we could talk about how it's stupid to burn oil when it has so many important other applications in the chemical industry, or the risk of oil spills.

> pretending that we can continue burning carbon at current rates (without any scientific support for this) is not in any way more justifiable than saying "we need to start burning less carbon".

You're misstating the alternatives. The alternatives for public policy are not "keep burning carbon at current rates" vs. "burn less carbon". The alternatives for public policy are "allow people to make their own decisions about burning carbon" vs. "dictate everyone's carbon burning activities by force". The former does not need a well-supported scientific rationale. The latter does.