For as long as humanity has relied on fossil fuels, there's been a tight relationship between economic growth and the carbon-dioxide emissions that are heating the planet. When a country's economy expands, its energy use and carbon pollution go up, up, up. When a recession strikes, energy use drops and emissions sink back down.

But that relationship has never been perfectly symmetrical, according to a new study in Nature Climate Change by Richard York of the University of Oregon. The uptick in carbon pollution from a given amount of growth tends to be significantly bigger than the drop in carbon output from an equal-sized recession. Essentially, there's a ratcheting effect, as people get used to a higher-carbon lifestyle and maintain it even during a downturn.

York looked at World Bank data from 150 countries between 1960 and 2008. What he found was that carbon-dioxide emissions tend to rise 0.73 percent for every one percentage point increase in GDP per capita. By contrast, emissions only drop 0.43 percent for every point decline in GDP per capita.

In a lot of ways, that makes sense. York pointed out to LiveScience that after the collapse of the Soviet Union in 1991, many former states saw their economies plummet, with per capita GDP shriveling all the way down to sub-Saharan levels in a few countries. But while their carbon emissions dropped, they didn't plunge all the way down to sub-Saharan levels as well. That's because these former Soviet states had already built a lot of infrastructure, such as roads and factories, that didn't disappear entirely during the recession.

This ratcheting effect can help explain why, after the recent financial crisis, global emissions didn't drop quite as sharply as many researchers had expected. While the United States has managed to cut its carbon pollution by 7.7 percent since 2006 — thanks to a combination of weak growth, swapping out coal for natural gas, and increased oil efficiency — that hasn't been true of the world as a whole. Global greenhouse-gas emissions quickly rebounded in 2010 and hit a record high in 2011.

In any case, few environmentalists think we should tackle global warming by inducing a lasting recession or stopping economic growth altogether. (Though, granted, a few have raised the idea.) Instead, most tend to suggest that we try to "decarbonize" the economy — severing the link between growth and greenhouse gas pollution, so that we can have the former without the latter. But York's study offers evidence that recessions aren't likely to buy us very much time or space to do so.

Related: U.S. cut its carbon emissions in 2011 — but China erased the gains