President Obama trumpeted new regulation spearheaded by the EPA that will require the nation's fossil fuel burning plants to reduce carbon dioxide emissions by 30% from 2005 levels (which were 10% higher than 2012 levels). According to the Los Angeles Times, this represents, "one of the biggest steps any country has ever taken to confront climate change."
Of course critics predictably either question the consensus view of global warming, arguing that it is not primarily due to anthropogenic carbon emissions; or argue that such regulations will come with very high economic costs by intentionally restricting the use of the cheapest and most efficient fuels. Whether or not there is any merit to those arguments is less relevant (although cost-benefit analysis is important) than the more fundamental question of whether or not this plan, assuming the consensus view of global warming is correct, will significantly effect or reduce atmospheric carbon dioxide concentrations.
Currently, atmospheric carbon dioxide concentrations are near 400 parts per million (ppm). The United States accounts for 19% of the world's carbon emissions, or 76 ppm (0.19 x 400). Fossil fuel burning plants account for 40% of U.S. carbon emissions, or 7.6% of the world's total (.19 x .40 x 100%), or 30.4 ppm (.4 x 76). The regulation would require a 30% reduction from a time when U.S. emissions were 10% higher. So, if the U.S. power plant contribution were currently 10% higher than it is (10% of 30.4 ppm is slightly more than 3 ppm), the total would be about 34 ppm and if that contribution were reduced by 30% (34 x .3) then the U.S. contribution to atmospheric CO2 concentration would be reduced by 10.2 ppm for an overall reduction in atmospheric CO2 concentration of 2.5% (10.2 ppm/400 ppm x 100%). The calculations would be similar, by the way, for raw emissions rather than concentrations as, in 2008, were 35000 teragrams (corresponding to a 350 ppm concentration).
So, assuming every other nation on the planet completely froze atmospheric carbon dioxide emissions, the U.S. effort would decrease atmospheric CO2 concentrations by a whopping 2.5% to 390 ppm. The effect of that on warming would probably be negligible and would certainly be obliterated by the continued CO2 emissions from U.S. sources not due to fossil fuel burning power plants (such as automobiles) and continued contributions from other countries, particularly developing countries such as China and India. Both these sources could be expected to increase and China and India currently account for 30% of the world's carbon emissions. Finally it is important to point out that the most abundant and important greenhouse gas is not carbon dioxide but rather it is water vapour. Water vapour occurs in the atmosphere at concentrations of up to 4 parts per hundred, or 40,000 ppm, which is 100 times the atmospheric concentration of carbon dioxide.
Even assuming the consensus view of global warming is correct, the notion that this negligible reduction in the world's increase in emissions of a trace greenhouse gas will meaningfully impact global warming is absurd. Therefore, if the program has any cost at all, no matter how trivial, it is not worth the zero benefit derived from it.