The ghost of William Jevons has haunted energy researchers in recent months, provoking debates on whether our best efforts to use less energy will merely lead us farther down the road of consumption. Jevons, a British economist, suggested in 1865 that increasing energy efficiency could backfire because it would allow further resource exploitation. He was thinking about coal at the time. And, indeed, as people got better at converting this black rock's energy into useful work, the work itself expanded.

The 'Jevons paradox' has persisted ever since. Today, the notion that there could be some modest 'rebound' effect that negates gains in energy efficiency is well known. For example, drivers of fuel-efficient vehicles might be inclined to drive more often, simply because they can afford to do so. It takes energy to create and install energy-efficient equipment; and money saved on energy could be spent elsewhere, so ultimately contributing to economic activity, which drives up energy consumption and greenhouse-gas emissions. (Indeed, one reason for the strong support for energy-efficiency measures is that they give people more money to spend, not necessarily to save.) According to another rebound effect, if fuel-efficiency regulations for vehicles in the United States and Europe curb petrol consumption, that should suppress the price of oil and encourage its use in other sectors or countries.

The world cannot solve all of its energy and climate woes with energy efficiency alone.

Last week, the Breakthrough Institute, based in Oakland, California, released a strongly phrased analysis that argues that these rebound effects are so large they could overwhelm many or all of the gains from energy-efficiency measures (see http://go.nature.com/m5ky3g). The advocacy group calls for massive investment in the research and development of low-carbon energy to counter what it says are over-optimistic assumptions about what can be accomplished through greater efficiency.

This is controversial stuff, and also frustrating. These are fundamentally interesting questions, but part of a debate that circles on itself. Was it really fair for Jevons to blame coal's expansion and technological diffusion on efficiency? Do people actually save money when their energy bills go down, or do they spend more money? Is the spread of refrigerators around the world purely because they have become more energy efficient, or because they are useful devices that keep food fresh for days on end? Is it the energy bill or the sticker price that people worry about when buying an appliance? And isn't technological diffusion and energy access throughout the world a good thing?

The rebound effects need to be considered, but they do not have to be viewed as paradoxical: they amount to economic expansion. Indeed, some researchers think that energy efficiency itself is a fundamental driver of economic growth, freeing up resources that can be used for other things, the deployment of low-carbon energy among them.

Despite its concerns about the rebound effect, the Breakthrough Institute argues that energy efficiency should nonetheless be pursued for exactly these reasons. Encouragingly, the discussion prompted by its report has led to plans from academics and industry experts on all sides of the debate to meet to wade through these issues.

The debate indicates that there must be deeper study of what energy efficiency could do if systematically deployed across an entire economy. The world cannot solve all of its energy and climate woes with energy efficiency alone; low-carbon energy technologies must be developed as well. But there seems to be no fundamental physical or economic reason that countries can't decrease their overall energy consumption while maintaining growth, and thus put the ghost of Jevons to rest.