It is ironic that the United States seems set to become one of the world's cleanest and most energy-efficient counties, thanks in large part to fossil fuel.

The revolution in natural gas, pioneered and developed in the United States through the technology of hydraulic fracturing, means that energy consumption per head is set to decline in the country 0.5 percent each year for the next quarter-century.

It also means, in the words of the U.S. Department of Energy's latest Annual Energy Outlook, that: "The energy intensity of the U.S. economy , measured by primary energy use per dollar of gross domestic product, declines by 42 percent from 2010 to 2035."

That is an extraordinary rise in the efficiency of energy use, which is a fancy way of saying the United States will use less fuel to create more wealth.

And the new technology of natural gas is only a part of the explanation. By 2025, renewable fuels will be providing 16 percent of the country's energy. And by 2035, a much richer and more populous United States will still be emitting fewer carbon dioxide emissions than it did in 2005.

Last year, the United States invested almost $50 billion in clean energy, more than any other country in the world (probably helped by the imminent prospect of cuts in subsidies.) China was next, with investments of $45.5 billion, a Pew Charitable Trust report, "Who is Winning the Clean Energy Race?" stated. Germany was third, with $30.6 billion and Europe as a whole invested just more than $80 billion.

Overall, the world invested a record $263 billion in clean energy (not including nuclear power) last year, an increase of 6.5 percent over the previous year. That money bought an extra 83.5 gigawatts, 30GW of solar energy and 43GW of wind power. A good-sized nuclear or coal-fired power station generates about 1 gigawatt, sometimes a bit more.

As of today, the world has 539GW of generating power from clean energy. Wind is the biggest contributor, with 239GW, followed by hydro with 184GW, solar with 73GW and biomass with 57GW.

The potential to generate power, however, doesn't mean actual power; wind and sunshine can be fickle. But then the dramatic fall in the price of the photovoltaics that produce energy from sunshine is making solar power more and more competitive.

Australia, for example, last year celebrated "crossover day," when the price of solar power dropped to less than that of coal-fired electricity.

One of the main reasons why the next 25 years of U.S. energy look to be cleaner and more efficient is the way that coal-fired power stations, which are responsible for about a third of carbon dioxide emissions in the United States are being replaced with gas-fueled plants, which produce less than half the CO2 of the traditional coal plants. (If the technology of carbon capture and storage can be made economically viable, coal may yet make a comeback.)

By the end of next year, no major U.S. city will be hosting a coal-fueled power plant. The last two, each inside Chicago's city limits, close over the next 18 months. More coal-fueled plants are set to follow, under the Obama administration's announcement of much tougher carbon emission standards. To the horror of the coal industry, the new standards are almost impossible for coal-fired stations to reach under existing technology.

Steve Miller, chief executive officer and president of the American Coalition for Clean Coal Electricity, a group of coal-burning electricity producers, said the new regulations "will make it impossible to build any new coal-fueled power plants and could cause the premature closure of many more coal-fueled power plants operating today."

That may be an exaggeration. The current rules, which have yet to come into effect, won't apply to existing coal-fired plants or those built within the next year. And since half of U.S. electricity comes from coal, phasing it out will be a slow process. The Energy Department's study reckons that by 2035 coal will still be providing 37 percent of American electricity.

The changing nature of the primary energy that produces the power is only one aspect of growing energy efficiency. There is a healthy international competition in the attempts to squeeze more electricity from power stations. Turbine design is going through something of a revolution, as computer-aided designs tweak the shape of the blades to squeeze maximum output from the steam.

Back in 1949, the United States reckoned that it turned about 24 percent of coal's energy into usable power. Today, it converts on average just more than 30 percent. But in China's latest power stations, they claim to be using 37 percent of coal's energy and by the end of next year their latest GreenGen plant will be converting 42 percent. In a joint venture with Germany's Siemens group, Shanghai Electric is building turbines that generate a kilowatt-hour of electricity from 10 ounces of coal. Ten years ago, they needed 14 ounces.

One doesn't have to be a passionate environmentalist to welcome such improvements in energy efficiency and declines on carbon emissions. The real answer to that question in the title of the Pew report, "Who is Winning the Clean Energy Race?" is that the real winner should be the human race.