The easiest to implement is energy conservation. Allwood et al. (2011) found that applying the best available practices already available would cut global energy consumption by 73%. Among the practices they identified were installing triple-glazing windows and wall insulation in homes, using tankless water heaters, and reducing the weight of cars to 300 kg (661 lbs). Even using saucepan lids when cooking to increase the efficiency of the cooking process (as demonstrated by Bill Nye) and reduce the energy used in food preparation plays its part. The main message here is that even small changes by individual people and companies can add up to make big differences in total energy consumption (see also this article by Napp et al. 2012), even if widespread adoption of 300 kg cars is more pipe dream than reality. Better energy conservation would make the next steps far easier.
Second, we need to replace coal-fired power plants. After all, those produce ~41% of CO2 emissions in the USA. There are two main non-fossil fuel options for most areas (solar and wind) and several options that are more local solutions (hydropower, geothermal, wave power). While somewhat controversial in the US, renewables—mainly wind and solar—produced 25% of Germany's energy in the first half of 2012. The main reason renewables are succeeding in Germany while floundering in the US isn't that Germany gets more sunlight than the US—they actually get far less than every part of the continental US due to their more northerly latitude.
|Available solar energy between the US, Germany, and Spain. Source: http://www.ilsr.org/germany-solar-power-wins/|
Third, we need to go back to the future on transportation, especially for commuting, by re-installing trolley lines. Prior to 1945, the majority of Americans, especially those living in cities, did not have cars. They didn't need them. Just about every American city—regardless of size—had an extensive electric trolley system that made car ownership unnecessary, even Los Angeles. In 1936, National City Lines, with funding from General Motors, Standard Oil of California, Philips Petroleum, and Firestone Tire, began buying streetcar lines around the nation with the express purpose of destroying them and replacing them with General Motors buses, as documented in United States vs. National City Lines, Snell 1974, and the 1996 documentary film Taken for a Ride. The express purpose was simple: Make public transit so inconvenient, uncomfortable, and unreliable that people would buy cars instead. The companies also calculated that once the trolley rails were torn out, the cost of re-installing them would be prohibitive. And to ensure that trolleys could not be rebuilt, the major trolley lines through cities were paved over as interstate highways, which in hindsight could have been expected—the Commerce Secretary under President Eisenhower who decided where to place the interstate highways was also the Chairman of General Motors. Their scheme worked. In the years since, we've just become even more reliant on the car to the point where for most Americans owning a car is an absolute necessity as shown by walkability scores.
Fourth, for short trips (< 5 miles) and errands, we need to encourage bicycling. Not only is cycling the most efficient form of transportation, it's also good for the economy and for health. The best ways to encourage cycling? Build off-road trails and bike lanes.
In summary, we need to encourage energy conservation in homes and businesses (which many utilities are now doing, i.e. Vectren Energy), revise our government policies on renewable energy to stimulate demand instead of supply, and rethink our transportation system in order to wean ourselves from fossil fuels.