If your car got better mileage, would you drive more?

Probably, but not enough to offset the fuel economy gain. A paper released yesterday by the American Council for an Energy-Efficient Economy (ACEEE) reports that increases in energy use stemming from improved resource efficiency are a real phenomenon, but their effects are modest.

Making vehicles, appliances and homes more efficient saves money and cuts energy consumption, which could lead people to consume more. This so-called rebound effect has led some to question whether pushing for better efficiency standards is even worth it if it will only serve to drive up consumption.

These fears are largely unfounded, according to the report's author, Steven Nadel, executive director of ACEEE. Analyzing research, economic modeling and published literature in the field, he concluded that direct and indirect rebound effects add up to 20 percent of the savings from efficiency improvements, far lower than some previous calculations from think tanks.

Nadel explained that direct effects include things like weatherizing a home to save energy but then running the heater higher in the winter. Indirect effects are further removed and more abstract, dealing with how people use their saved resources.

A household that saves money by weatherizing may spend its extra money buying a bigger television, for example. This affects not only electricity bills but also the resources used to build and deliver a television, thus affecting the wider economy.

However, efficiency rebounds are not set in stone and can vary depending on certain factors. "For passenger vehicles, rebound effects are actually declining," said Nadel. He noted that this resulted in part from the economic downturn as well as higher gasoline prices and increasing traffic congestion in many parts of the country. These rebounds can be further reduced through consumer education and emissions taxes.

Areas where 'rebound' doesn't apply

Other efficiency applications have an even smaller rebound. "Are you going to open the door of your refrigerator more because it's more efficient? Of course not," said Jonathan Koomey, a consulting professor at Stanford University. "There's a whole class of devices where the rebound effect doesn't apply."

However, some critics have argued that econometric modeling shows that the rebound can wipe out savings from efficiency upgrades entirely.

Nadel asserted that these models are flawed and optimize the present-case scenario so that any deviation from them results in a worse outcome. "Given all the rhetoric on indirect rebound, I thought I would find better-quality studies to support the indirect claims," he said.

But as the planet gets warmer, simply buying a hybrid car won't stop the flow of greenhouse gases. "You need to do more than just efficiency. You need to reduce overall emissions," said Koomey. For this to happen, the entire energy infrastructure, from generators to appliances, has to become less carbon-intensive.

With the world's population surging and billions around the world seeking the air conditioning, televisions and cars that are so ubiquitous in the United States, efficiency is a necessary but insufficient mechanism for reducing our environmental impact.

"The lesson is that there is moderate rebound that should be factored in, but it's a good thing that helps the economy grow," Nadel said.