Everyone’s favorite McKinsey study suggests that America can shed a huge chunk of its emissions through costless measures, primarily in the realm of energy efficiency. The fly in this delicious low-carbon ointment is that the freebie cuts haven’t so far happened by themselves, and it’s never entirely clear how well an analyst’s report is going to translate into reality. How nice, then, that New York City is gearing up to provide the proof point we’ve all been waiting for.

Mayor Bloomberg’s office recently released a plan to drop the carbon emissions of the municipal government 30 percent from 2006 levels by 2017. The plan will cost about $2.3 billion, but the city expects to recoup these costs by 2015 — an average payback of less than eight years across a large portfolio of projects.

A full 57 percent of the savings come from projects related to building retrofits and improved maintenance — utterly boring stuff like boiler replacements, upgraded heating and cooling systems, better steam trap maintenance, etc.

A further 17 percent comes from wastewater treatment. Wastewater presently is a huge source of methane. Capturing that methane not only creates a direct reduction in greenhouse-gas emissions, but also provides a source of renewable energy.

Another 11 percent will come from a new rail and barge network to replace long-haul garbage trucking.

The remaining 15 percent is divided up among a large number of piecemeal initiatives. The sexy, cutting-edge stuff — solar photovoltaic, daylight harvesting, green walls, etc. — make up a tiny sliver of the planned reductions. No breakthroughs required.

The city’s plan calculates the cost-per-metric ton of carbon dioxide reduced across all project types. This metric provides a handy benchmark for any future climate legislation, allowing a direct comparison of financial incentives to costs, as well as a gauge of the economic impact of any carbon reduction effort.

Except there almost aren’t any costs in the city’s plan. The average abatement cost across all project types is negative $52 per metric ton, meaning that every ton reduced offers $52 in energy savings. Even when costs are analyzed by project type, almost all projects pay for themselves. Improved street lighting, for example, yields an amazing $149 of savings per ton (almost enough to balance out solar PV’s fairly painful price of $157 per ton).

The plan offers further benefits beyond direct emissions reductions. In particular, by shaving 220 megawatts off peak power demand, the city will significantly reduce strain on the grid and pressure for new power plants.

The only real rub is the aggressive pace required to meet the plan’s goals. Implementation of roughly 2,200 projects over the next decade will require a roughly eight-fold increase in the present pace of efficiency improvements.

Another consideration, of course, is the fact that the municipal government only accounts for 6.5 percent of total emissions in New York. This is a big chunk, to be sure, but it hints at the huge potential savings if the private sector follows suit.