The primary purpose of carbon tax is to lower greenhouse-gas emissions. The tax charges a fee on fossil fuels based on how much carbon they emit when burned (more on that later). So in order to reduce the fees, utilities, business and individuals attempt to use less energy derived from fossil fuels. An individual might switch to public transportation and replace incandescent bulbs with compact fluorescent lamps (CFLs). A business might increase energy efficiency by installing new appliances or updating heating and cooling systems. A utility company might use wet scrubbers, low NOx-burners or gasification to reduce their emissions (see What is Clean Coal Technology?). And since carbon tax sets a definite price on carbon, there is a guaranteed return on expensive efficiency investments.
Carbon tax also encourages alternative energy by making it cost-competitive with cheaper fuels. A tax on a plentiful and inexpensive fuel like coal raises its per British Thermal Unit (Btu) price to one comparable with cleaner forms of power. A Btu is a standard measure of heat energy used in industry. One Btu is the energy necessary to raise the temperature of one pound of water by one degree Fahrenheit.
And don't forget about all the money raised by the tax. It can help subsidize environmental programs or be issued as a rebate. Many fans of carbon tax believe in progressive tax-shifting. This would mean that some of the tax burden would shift away from federal income tax and state sales tax.
Economists like carbon tax for its predictability. The price of carbon under cap-and-trade schemes can fluctuate with weather and changing economic conditions. This is because cap-and-trade schemes set a definite limit on emissions, not a definite price on carbon. Carbon tax is stable. Businesses and utilities would know the price of carbon and where it was headed. They could then invest in alternative energy and increased energy efficiency based on that knowledge. It's also easier for people to understand carbon tax.
Carbon tax seems straightforward enough, but how is the rate actually determined? At what point is the tax levied? We'll find out the answers to these questions and more in the next section.