As part of a carbon tax scheme establishing base prices for fossil fuels will utilize low oil and natural gas prices to raise revenue to address the health and environmental issues associated with their extraction, transportation, and burning; and help incentivize consumers to conserve and companies to innovate.
Where to set the price and what to do with the revenue needs to be debated, however regardless of whether it's used to invest in clean energy, public transportation, or refunded evenly on a per capita basis the important thing is to disincentivize the burning of fossil fuels.
The US implementing a bold carbon tax scheme is what the world needs right now especially as other nations prepare their own commitments ahead of the UN Climate Change Conference so lets set a good example.