What’s a percent or two? Or three? Not much, sometimes. But a lot when we’re talking about carbon dioxide emissions that are throwing earth’s climate out of whack. And quite a lot when effort is going into ranking emission sources to help prioritize our responses to the climate crisis.

These thoughts are occasioned by a scoop in the Guardian (U.K.) reporting that world shipping — essentially, freighters and tankers moving goods and raw materials — accounts for “up to 5% of the global total” of carbon emissions. “CO2 output from shipping [is] twice as much as airlines,” shouts the Guardian headline, in light of the 2-3 percent share of emissions associated with air travel.

Goodness, a dozen eco-blogs seemed to mutter in unison, have we been barking up the wrong tree? Were we wrong to hammer globe-girdling celebs and fret over cheap air fares, instead of targeting ships carrying shirts from Bangkok to Berlin and plasma screens from Seoul to San Francisco?

Well, not so fast.

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I ran the numbers, and it looks to me like CO2 emissions from world shipping are in the same range as those from aviation, and perhaps even a bit less. Which suggests that climate campaigners need to keep targeting air travel along with world trade (the activity filling up all those ships) — and every other human activity leading to greenhouse gas emissions. It also suggests two less obvious lessons, which I’ll come to in a moment.

First, the numbers. Global emissions of carbon dioxide from fossil-fuel burning totaled nearly 27 billion tonnes in 2003. (Those are metric tons, each a thousand kilos, or around 10 percent more than a U.S. ton.) According to BP, the source relied on in the Guardian article, the global fleet of 70,000 ships uses approximately 200 million tonnes of heavy oil annually. Most of the atoms in those hydrocarbon molecules are carbon. Assume for simplicity that they’re all carbon. Burning all that fuel would convert the 200 million tonnes of carbon to carbon dioxide, each molecule of which weighs 3.67 times as much as each carbon atom. Voilà, the carbon dioxide from shipping would weigh 734 million tonnes, which is 2.7 percent of 27 billion (the world’s CO2 from all fossil fuels, in tonnes).

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Actually, though, not all of fuel oil is carbon. Some is hydrogen, which combusts to water vapor, some is sulfur, which produces the notorious pollutant sulfur dioxide, etc. A more thorough calculation, shown in this spreadsheet, yields 2.4 percent for shipping’s share of world CO2. And that’s comparing current shipping to a 2003 world total. Using a uniform base year would probably drop shipping’s share of CO2 to just a tad over 2 percent, or half of the 4-5 percent advertised by the Guardian.

What are my two lessons?

First, check startling claims. They may be right, but sometimes they’re startling only because an underlying number is off. And pay attention to the source. The Guardian correspondent, John Vidal, has written terrific stories on the climate crisis, but even he can trip on the numbers. In late 2005, he wrote that “emissions of global warming gases from the United States have nearly doubled in 14 years,” an obvious exaggeration, which he didn’t acknowledge when I questioned it. I’ve kept a wary eye ever since.

Second, tracking carbon emissions can be problematic. Some sectors, like power generation, are perfectly transparent. Others, like shipping, aviation, and, of course, the U.S. and other militaries, are much less so (Vidal makes this point, to his credit). Will a carbon cap-and-trade system, which depends on allocating or auctioning emission allowances, carve out exceptions for such sectors? In contrast, a carbon tax, levied “upstream” in the fossil fuel chain, neatly sidesteps this difficulty. (Yes, the tax should be global, but in the interim we can adopt the suggestion from France to levy carbon-equivalent fees on imports from non-taxing countries.)

Still, it’s good to be reminded of world trade’s part in the climate crisis. Along with year-round, all-weather bicycle commuting, and stomping on every incandescent bulb I can unscrew, I’m doing my best to buy locally. Of course, it would be a lot simpler with a carbon tax that added “trade-miles” to the price of my next pair of winter gloves.