单项选择题

Discussions about reducing greenhouse gas emissions usually concentrate on cleaner ways of generating energy: that’s because they promise that we can lower emissions without having to change our energy-hungry ways. But whereas new generation techniques take years to come on stream, efficiency can be improved today, with existing technologies and know-how.
To calculate how much energy could be saved through such improvements, Julian Allwood and colleagues at the University of Cambridge analysed the buildings, vehicles and industry around us and applied "best practice" efficiency changes to them. They found that 73 percent of global energy use could be saved by introducing such changes.
Many people are unaware of the scale of opportunities for reducing energy demand, says Allwood. By showing how global energy demand can fall to a quarter of its current level without any decline in services, the team hope to redress the balance. "We think it’s pretty unlikely that we’ll find a good response to the threat of global warming on the supply side alone," Allwood says. "But if we can make a serious reduction in our demand for energy, then all the options for changing the energy supply look more realistic. " Not all of the changes might be suitable for immediate introduction, Allwood admits. But increasingly tough emissions standards for passenger cars, particularly in Europe, will drive down average vehicle weights, he says.
Nick Eyre, leader of the Lower Carbon Futures group at the University of Oxford, says some of the assumptions made by the team on how much energy could be saved by efficiency measures may even be too conservative. For example, it is possible to design buildings that are more efficient than the Passivhaus(低能耗房屋) standard they use as their practical limit. Buildings complying with this standard must consume less than 15 kilowatt-hours per square metre of energy in heating each year.
Even so, achieving the 73 percent cut in energy consumption will depend on how people use the more efficient technologies, Eyre says. "A Passivhaus building will not perform to its design standard if its occupants open windows when it’s cold outside." However, the team’s conclusions are "powerful", he says, and the suggestion that major investment should be going into buildings, vehicles and factories instead of the energy system has major political implications.
The aim of saving 73 percent of energy use is up to ______.