Presenting Numbers for Energy

There are a lot of numbers thrown around in the conversation about energy efficiency and climate change. I thought it would be worthwhile for us to break down and play with two sets of numbers that people are frequently confronted with -- temperature and miles per gallon. The math to tweak the presentation is simple and obvious, but the message / the understandability / the impact of these changes can be great.

Celsius vs. Fahrenheit

The scale chosen to present temperature data matters more than you would probably think. Most accepted sources on global warming put the potential worldwide temperature rise at 2 to 3 degrees Celsius. The problem is that for people who do not use the Celsius on a day-to-day basis (its not what your weather man reports to you) -- that number is almost meaningless. The danger is that it begins to psychologically seem like quite a small number. 2 to 3 degrees Celsius is 3.6 to 5.4 degrees Fahrenheit. Those numbers are significantly larger, and would have more of an emotional impact to us Americans.

To continue to humanize these numbers, let's imagine the 3.6 to 5.4 degrees Fahrenheit in other terms: instead of having an average July temperature of 75 degrees Fahrenheit, New York City would instead experience Atlanta's average July temperature of 80 degrees Fahrenheit. That would be like the entire city of New York moving 480 miles to the south. Or, like getting in a car and driving due south on a highway for 8 hours to feel the warmth.


In the US, we measure our automobile's efficiency in miles per gallon (MPG) -- that is, given one gallon of gasoline, how far can I drive this car? Our European friends, however, measure the efficiency of their cars in the number of liters it takes to drive 100 kilometers (in the units Americans use: how many gallons does it take to drive 100 miles) -- they're looking at gallons per mile (GPM).

Why is this inversion important? Doesn't it measure the same thing? Let's look at a few examples:

Class of car Avg. MPG Gallons per 100 miles Gallons per 10,000 miles
S.U.V. 15 6.6 666
Mid-sized car 25 4 400
Compact car 35 2.8 286
Hybrid 45 2.2 222

Take a close look at that table -- the MPG goes "up" by 10 miles per gallon for each type of car, but the amount of fuel used drops down a lot as we go from 15 MPG to 25 MPG, then by a smaller amount as we go from 25 MPG to 35 MPG, and again by a smaller amount from 35 MPG to 45 MPG.

What the MPG scale fails to show is that for every single mile per gallon improvement from the lower end of the scale means pretty large fuel savings when you actually have to drive that car. Just moving from a 15 MPG S.U.V. to a 25 MPG mid-sized car has a profound effect in the amount of fuel used over 10,000 miles -- 266 fewer gallons, in fact! (And, arguably where it matters the most as the biggest "bang for the buck" is getting S.U.V.s off the road)

For both of these -- looking at temperature numbers and fuel efficiency numbers -- the way that we choose to present them to people can make the impact feel incredibly real. Next time you hear people talk about how much hotter the planet will get, feel free to refer to the thermometers we put together. And, the next time you are deciding between two cars, think about comparing their fuel efficiency on GPM instead of MPG.

But, most importantly, when talking about energy and climate change, remember that numbers can be confusing, or may not be taken seriously. Take care when choosing exactly how to present it.

Update 5 January 2009 -- Rick Larrick and Jack Soll from Duke University got in touch with us! If you're interested in learning more about the implications of GPM vs MPG, then read their work which was also recognized by the New York Time's "Year in Ideas".