IN WONKISH circles, energy efficiency used to be known as “the fifth fuel”. Now, most blueprints for tackling climate change assume that saving energy has a big part to play. A recent report by the McKinsey Global Institute suggests that energy efficiency alone could get the world halfway to a greenhouse-gas concentration goal recommended by many scientists. Even though overall energy use is rising as the global economy grows, the world is using less energy to generate output; intensity is falling by 1.5% a year. Price is a big factor. In countries with higher fuel prices, there is more incentive for manufacturers and consumers alike to be more energy efficient. America's energy intensity was falling by a 0. ... Read full article