A recent University of Washington study found that when the same values were used with 10 different online calculators, the results varied greatly. In one category, the bottom line for a typical American homeowner varied by more than 32,800 pounds of carbon produced per year.Given how complex it is to calculate a footprint and all the assumptions involved, I am not surprised that there is so much difference. I think the calculators do need to be more transparent and let you know exactly how they calculate their value and what assumptions were made.
One reason for the wide range is that emissions from air travel, the couple's largest carbon producer, are often calculated very differently, said Clark Williams-Derry, research director at Sightline Institute, a nonprofit research center that studies carbon calculators.
Another part of the variation stems from the fact that different calculators include different behaviors in their calculations. For example, some ask for the amount of garbage a household produces each year, while others use the national average (1,606 pounds), and others don't include garbage at all.
Another reason for the differences is that most calculators use different internal numbers as conversion factors and standards to come up with their results, Steinemann said.
"There are so many different ways to calculate, using different variables, different standards and different assumptions," she said. "There's no one absolute right best number, so each calculator seems to use something different."
Web sites offering the calculators also often don't let the user see what those numbers are.
"The other problem is that individual calculators don't tell you the assumptions behind their calculations," Steinemann said. "Even if there isn't one standard calculator, they should at least be able to be transparent, so people know what's being included, what isn't being included, and what's not being calculated."
Links to 10 carbon calculators in the article.
via Seattle PI