For the first time since 2013, the US Environmental Protection Agency has issued renewable fuel standards for the nation, upping the amount of ethanol in our gasoline supply. In 2016, renewable fuels—mostly corn ethanol—must make up 10.10 percent of the national fuel supply, or 18.1 billion gallons. The EPA also issued final renewable fuel standard for 2014 and 2015, showing that next year's target is a slight increase over the past two years. In 2014—the last year that the Energy Information Administration has calculated total US gasoline consumption (136.8 billion gallons), the total percentage of renewable fuels was 9.2 percent, or 16.3 billion gallons.

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Almost all of this ethanol will make its way into our cars in the form of E10 gasoline, which is a blend of 10 percent ethanol and 90 percent gasoline. E10 is widespread throughout the US, and mandated in a number of states (mainly throughout the midwest). The ethanol acts as an oxygenator and anti-knocking agent for the fuel, replacing the groundwater pollutant MTBE (which itself replaced tetraethyl lead). E10 is slightly less energy dense than "regular" gasoline and so cars' fuel economy will be three to four percent lower when using the fuel. This is offset by slight decreases in CO emissions (as well as the intended reduction in greenhouse gases).

In 2016 the overall percentage of renewable fuels will be just over 10 percent, leading to criticism from the oil industry warning about damage to our cars' engines and fuel systems. At higher concentrations, ethanol-gasoline blends can be corrosive to some metals and materials used for hoses, gaskets, and seals; generally blends above E10 (E15 and E85) should only be used by "flex-fuel" vehicles that have been designed to tolerate the increased ethanol levels. Neither have much popularity in the US though, being confined mainly to corn-producing states in the midwest.

A quick look at the numbers reveals that the oil industry's fears are actually misplaced, however. Although 2016 will see just over 10 percent of the fuel supply become renewable, just over a tenth of that will be biomass-based diesel (1.6 percent, or 1.9 billion gallons). The biofuel with the greatest effect on carbon emissions is cellulosic ethanol, which uses inedible feed stocks like corn stover or wood chips. The EPA requires cellulosic ethanol to have at least a 60 percent reduction of lifetime emissions compared to gasoline, but in 2016 the requirement for this fuel is just 230 million gallons (0.13 percent).

We're doing better with what the EPA terms "advanced biofuels" (which use sugarcane, bionaptha, or biobutanol) that have a 50 percent reduction in lifetime emissions, at 3.6 billion gallons (2 percent) in 2016. However, the overwhelming majority of ethanol in the US fuel supply still comes from corn, for which the EPA requires at least 20 percent less lifetime carbon emissions. Whether any of these figures accurately reflect real-world impacts on carbon emissions is unclear.

Congress tasked the EPA with setting targets for renewable fuels in the 2007 Energy Independence and Security Act, although the final 2016 figure of 18.1 billion gallons is almost 20 percent less than the bill's original figure of 22.2 billion gallons.