It took a major financial crisis to slightly slow down the global emissions of greenhouse gases. According to the International Energy Agency, emissions dipped in 2009 due to the recession and then climbed back to a record level just a year later. A new study from the Global Carbon Project confirms this trend and says 2010 saw the largest absolute jump in emissions in any year since the Industrial Revolution.

I couldn't help but contrast these grim numbers with the frenzy of the holiday shopping season. By now we are all well aware that we can make or break the economy through our purchasing habits.

I was feeling downright unpatriotic, having done less than my share of shopping this whole year, until I came across the clothing company Patagonia's full-page ad in the New York Times on Black Friday. Patagonia took a courageous stand and asked consumers to buy less. The ad went on to say, "Don't buy what you don't need. Think twice before you buy anything."

This raises an uncomfortable issue that's been literally swept under the rug for many years. After more than two decades of talking about sustainable consumption -- and in spite of advances in resource efficiencies and renewable energy -- we are not all that much closer to consuming sustainably. The fact is, it is enormously difficult to do this without actually reducing what we consume.

Take clean energy, for example. Supplies are currently limited, costs can be higher, and breakthroughs are still needed -- specifically in battery technologies, solar energy and biofuels -- before renewables can replace fossil energy on a large scale. In time, with the right investments and policies, the breakthroughs can happen but we are not there yet.

Moreover, the IEA estimates that 80 percent of emissions from the power sector in 2020 are already locked in because of existing power plants and new construction currently in the pipeline -- which would make it very challenging for emissions to peak in this decade and then decline per IPCC's stabilization scenario [PDF] to keep the mean temperature increase under 3 degrees C.

Can we rely primarily on energy efficiencies in the meantime? Up to a point, yes; but efficiencies are not a panacea for all our energy problems. Energy efficiencies are known to cause rebounds, which can reduce potential energy savings by stimulating additional energy use. Rebounds can occur economy-wide as well as at the level of individual consumers and firms.