The behavior of individual governments will be critical in determining whether the world moves forward with clean-energy technologies. Much was said about how the agreement sent a strong “signal” to investors, and indeed, Paris was swarming with corporate chieftains and Silicon Valley heavyweights. But the strength of that signal will depend heavily on whether governments are willing to promote such investments while removing the tax subsidies that favor dirtier fossil fuels — perhaps to the point of embracing carbon taxes.