When it comes to carbon offsets, invest in clean energy.

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Today was a good, green day. I rode my bike to the post office, a dentist appointment, and a lunch meeting. In doing so, I eliminated 4.3 miles of car travel that, according to the NativeEnergy carbon calculator, would have spewed four pounds of CO2 into the environment. Oh, that everyone could be as climate-conscious as me.

Sadly, the sentiment was short-lived. Eager to see how my model lifestyle compared to that of lesser beings, I took the Earth Day Network’s online ecological footprint quiz. Turns out my footprint is 16 percent heavier than the average American’s (excessive air travel cancels out quite a few bike trips, apparently). The quiz’s stinging conclusion: “If everyone lived like you, we would need 6.2 planets.”

Humbled and mildly indignant, I did what any consumer would do: I looked to buy my way out of the problem. Enter carbon offsets, the handy mechanism that allows us to convert our units of personal carbon emissions into dollars for wind farms, reforestation projects, and other conscience-cleansing pursuits. The theory is that such investments cancel out the environmental harm we do.

Reforestation has long been popular, but the environmental payoff is relatively low.

Because air travel is my biggest vice, I started by plugging a year’s worth of flights into various online travel calculators. The cost of redemption for 34,000 air miles: $168, according to NativeEnergy; $160.89 per MyClimate; or $64.95 on TerraPass. When I added up my total carbon footprint (air travel, auto, and home energy), the price tags ranged from $180 to $408 per year. Why would anyone spend $408 when she could pay $180?

“The cheaper the program, the more likely it is that the quality is not very good,” says Wolfgang Strasdas, who recently completed a study on carbon offsets for the International Ecotourism Society. Strasdas judged carbon-offset companies on three main factors: how credible they are, how they spend your money, and how they calculate emissions. The latter accounts for the discrepancies in price, especially regarding air travel.

The UN’s Intergovernmental Panel on Climate Change (IPCC) says that the greenhouse effect of an airplane burning fuel at a high elevation is 2.7 times that of the actual emissions. Some scientists have placed that number as high as 5.8; others deem the issue too complex to factor. As a rule of thumb, Strasdas says, a program that calculates air-travel emissions according to the IPCC standard should sell offsets that add about 10 to 20 percent to the cost of an economy ticket. The other piece of the pricing picture is that companies charge different rates for each ton of carbon they offset. A recent Tufts Climate Initiative study of 13 carbon-offset companies found prices ranging from $5.50 to $27.40 per ton of carbon. It did not find a definitive link between low price and low quality.