QUICK CRASH COURSE ON THE PROBLEM
Molecules that consist of chains of carbon and hydrogen can store significant chemical potential energy. This makes them valuable sources of fuel because if you can get them over an initial energy barrier, they will enter a chain reaction with the oxygen in their surroundings, making CO2 and H2O as well as lots of heat that drives more of the same reactions. Natural gas, coal, and oil contain hydrocarbons that can undergo these reactions, so we burn them en masse and capture some of the heat to push vehicles around or to boil water to turn turbines. Unfortunately, CO2 , one of the products of these reactions that we are carrying out by the quadrillion, is a "greenhouse gas," which means that higher concentrations of it cause our atmosphere to retain more heat -- that's what global warming is. In addition, about half of that CO2 enters the ocean, where it reacts with water to form carbonic acid, causing the ocean to become less basic. Warming the planet will likely cause massive ecological stress for all the biomes on earth's surface, adversely affect agriculture for billions, and flood the most densely populated places on the planet; acidifiying the oceans will kill keystone species in marine food chains, leading to mass die-offs and an intensification of the ongoing mass extinction.
THE CARBON TAX IS ONE PROPOSAL FOR SLOWING THESE EFFECTS
Because of how dangerous global warming and ocean acidfication are, and because they are both caused by elevated CO2 concentrations in the atmosphere, some people have proposed a tax on the carbon content of fuels. The hope is that by making fuels that produce CO2 more expensive to purchase, there will be more economic incentive to acquire energy using renewable, carbon-neutral sources. Several variations on this type of tax (e.g. taxing items like vehicles that are known to produce a lot of CO2) have been employed around the world; in fact, the U.S. is one of the only industrialized nations on Earth that hasn't implemented some form of carbon tax.
DOES IT WORK?
Global CO2 emissions continue to increase on a year-to-year basis, and the effects of global warming continue to become more and more plainly manifest around the world. These facts would seem to count against the effectiveness of a carbon tax considering how widely implemented it is, but the reality is that the taxes that have been applied up to this point have been piecemeal and relatively toothless. A strong international effort to aggressively tax carbon, simultaneously raising government revenue and crippling the fossil fuel industry, could still conceivably make a difference in the severity of the effects we face. Are these challenges insurmountable? Possibly, but if we don't use every possible avenue to combat the awful human price that is already beginning to be paid, we will be complicit in the deaths of billions.