Greenhouse gas emissions are most commonly reported at the national level, which tends to make us compare nations to other nations. This makes some sense, as national policy can significantly influence emissions trends. But it's easy to forget that borders are just lines on a map, and some lines have considerably more people inside them than others. The citizens of Luxembourg don't ensure their country's low carbon emissions because they're lightyears ahead of the people of China in terms of efficiency—there are just a whole lot fewer of them.

In order to make more meaningful comparisons, you obviously have to calculate emissions per person. And when you do that, the United States really sticks out. (As does Luxembourg, by the way.) It's not surprising that per capita emissions in the United States are much greater than in India, where millions of people still lack electricity. But why are they also much greater than in the wealthier Western nations in Europe?

To answer that question, we need to do more than divide a national total by population. We need to break down the contributions to a person's carbon footprint—the emissions behind the things we buy and do. Doing that in a detailed way is a challenge, and researchers haven't been at it that long. "A lot of the research that's been done has been done quite quickly [with] available data and resources," UC Berkeley's Chris Jones told Ars, "And there really is a lot of work to do."

Some of that research breaks national or (regional) economies into sectors to look at what comes in and goes out in each category. That also helps account for things like moving your manufacturing to another country. Other studies try to work from the bottom up, getting people to fill out surveys or even log diaries of their purchasing habits—an interesting but labor-intensive (and expensive) source of data.

A third approach combines more types of data into an econometric model, applying demographic information to the big economy-scale stuff. "And then once you've done that, kind of the cool thing is you can now scale emissions to any location within countries," Jones said. With data on things like home size, power plants, climate, and vehicle ownership, a more recognizable carbon footprint can be calculated for the community around you.

But any way you estimate it, the average American carbon footprint is in the ballpark of double that of the average European. (For the purposes of this article, "Europe" will refer to the pre-Brexit EU group of nations.) This huge difference is sometimes brushed off as a simple consequence of Europe's greater population density. It's not that simple, though—even the efficient high-rise cores of the biggest US cities can barely dip into the range of European national averages. So what exactly is it about American homes, communities, and behavior that adds up to explain this gap?

Excuse me?

In 2014, Chris Jones and his colleague Daniel Kammen published a map of US household carbon footprints by ZIP code based on that econometric method—the most detailed estimate out there. The US is pretty varied in a number of ways, which produces some regional patterns.

Emissions from electricity use are higher in areas that still had coal-dominated grids at the time, while home heating is a much larger factor in Northern states with colder winters. But most interesting is the pattern within and around major cities.

The average American household footprint is a little below 50 tons of CO 2 per year (actually "tons of CO 2 -equivalent" to include other greenhouse gases), but that number can drop to around 30 in city centers—closer to the average of a country like Germany. American cities are surrounded by sprawling suburbs, though, which swing very much in the opposite direction, going as high as 80 tons.

There are a number of reasons for that, including much larger homes and lengthy commutes. But even if European cities tend to trade suburban sprawl for a more concentrated area of medium density, it's not like Germany is a continuous megacity. Even US ZIP codes with considerably higher average population densities than European nations have higher household carbon footprints. What's going on here?

Is it the weather? Maybe Americans have to contend with colder winters and hotter summers? Not so. Heating and cooling demand is calculated in terms of "heating degree days" and "cooling degree days"—the number of days your thermostat kicks on combined with the difference in temperature inside and outside. On average, the United States does have more cooling degree days in the summer than Europe's average. But Europe has a larger lead in heating degree days. So if anything, it's European homes that have the larger heating/cooling demand, on balance.

(As an interesting sidenote, heating degree days are decreasing and cooling degree days are increasing as Earth's climate warms—and by more than you might think. Europe's average heating degree days have dropped about 15 percent since 1980. California's data, as an example, looks similar. And because home cooling runs on cleaner energy than heating in California, the net effect of climate change has reduced the average footprint a bit.)

What about industry? If the EU outsourced more of its manufacturing and materials industry, that might make it look cleaner than it really is. In reality, the US and Europe are relatively similar in this regard. If you go back to the national emissions chart above, the dashed lines show the effect of adjusting for imports. That boosts US and EU per capita emissions by about the same amount (while decreasing China's). And carbon-footprint estimates largely capture this, anyway, by assigning you the emissions associated with your consumption of goods and services.

Perhaps it's the energy mix? After all, a cleaner grid would shrink your footprint even with no change in behavior. But that doesn't explain much here, either—at least when comparing the averages for these two regions.

There are exceptions at smaller scales, like France's nuclear-dominated grid or Switzerland's grid, which is powered almost entirely by hydroelectric dams and nuclear power. And in the United Kingdom, the rapid replacement of coal with renewables over the last decade has reduced per capita emissions considerably. UK households used to have an above-average carbon footprint for the EU; that's no longer true.