Musk in Paris. Screenshot Tesla and SpaceX CEO Elon Musk gave a speech in Paris on Wednesday at the Sorbonne, and he called in no uncertain terms for a carbon tax.

"We have to fix the unpriced externality," he told the audience, shifting into the wonky quasi-academic mode that he actually appears to enjoy indulging in, when he isn't running two companies and serving as the Chairman of a third, Solar City.

His entire speech hinged on this simple observation: that the addition of carbon to the atmosphere is effectively a worldwide subsidy that's contributing to global warming and preventing humanity from freeing itself from the fossil fuel era.

Musk called this a "hidden carbon subsidy of $5.3 trillion per year," citing the IMF. In response to questions after his speech, he said that a good outcome of the current UN Climate Summit (COP21) taking place in France would be that governments "put their foot down" and use a revenue neutral, gradually applied carbon tax to accelerate the shift from an economy driven by fossil fuels to one driven by sustainable energy.

Musk is convinced that the current fossil fuel era will end — it's just a question of when. In his analysis, the transition will occur simply because we'll run out of carbon-based stuff that we can dig out of the ground and burn. But the existing carbon subsidy, in his estimation, is slowing down progress.

He called this, variously, "the dumbest experiment in history" and "madness."

Musk isn't a newcomer to the idea of a carbon tax. He's been calling for one for years. But the evolution of his businesses and the advent of Tesla Energy, his power-storage undertaking, appear to have sharpened his pitch.

And it's important to note that Musk has never just been about building cars, or going to Mars, or applying solar power more widely. He has a vision for the future that uses those businesses as a means to several important ends: freedom from fossil fuels, making us "multi-planetary," and taking better advantage of what he calls the "big fusion reactor in the sky," the Sun.

An interesting additional aspect of his speech in Paris was his continued embrace of government as part of the solution to climate change. Some high-flying tech entrepreneurs think government is a problem and an obstacle, but Musk has never dodged the accusation that his companies have depended on government action and funding.

"There needs to be a clear message from government in this regard," he said about the implementation of a carbon tax.