reduced their black carbon emissions from fossil fuel sources by a factor of 5 or more since 1950. Thus, the technology exists for a drastic reduction of fossil fuel related black carbon." Jacobson believes that "[g]iven proper conditions and incentives, [soot] polluting technologies can be quickly phased out. In some small-scale applications (such as domestic cooking in developing countries), health and convenience will drive such a transition when affordable, reliable alternatives are available. For other sources, such as vehicles or coal boilers, regulatory approaches may be required to nudge either the transition to existing technology or the development of new technology." Hansen states that "technology is within reach that could greatly reduce soot, restoring snow albedo to near pristine values, while having multiple other benefits for climate, human health, agricultural productivity, and environmental aesthetics. Already soot emissions from coal are decreasing in many regions with transition from small users to power plants with scrubbers." Jacobson suggests converting "[U.S.] vehicles from fossil fuel to electric, plug-in-hybrid, or hydrogen fuel cell vehicles, where the electricity or hydrogen is produced by a renewable energy source, such as wind, solar, geothermal, hydroelectric, wave, or tidal power. Such a conversion would eliminate 160 Gg/yr (24%) of U.S. (or 1.5% of world) fossil-fuel soot and about 26% of U.S. (or 5.5% of world) carbon dioxide." According to Jacobson's estimates, this proposal would reduce soot and CO2 emissions by 1.63 GtCO2–eq. per year. He notes, however, "that the elimination of hydrocarbons and nitrogen oxides would also eliminate some cooling particles, reducing the net benefit by at most, half, but improving human health," a substantial reduction for one policy in one country. For diesel vehicles in particular there are several effective technologies available. Newer, more efficient diesel particulate filters (DPFs), or traps, can eliminate over 90% of black carbon emissions, but these devices require ultra-low sulfur diesel fuel (ULSD). To ensure compliance with new particulate rules for new on-road and non-road vehicles in the U.S., the EPA first required a nationwide shift to ULSD, which allowed DPFs to be used in diesel vehicles in order to meet the standards. Because of recent EPA regulations, black carbon emissions from diesel vehicles are expected to decline about 70 percent from 2001 to 2020." Overall, "BC emissions in the United States are projected to decline by 42 percent from 2001 to 2020. By the time the full fleet is subject to