IT'S an uncomfortable feeling, when you've written about a subject dozens of times over the years, to suddenly be brought up short by an ontological question. Yet that's what's happened to me over the past few days, trawling around the eco-smug cities of Portland, Oregon and Austin, Texas, talking to people about green jobs. The question is: what is a green job, exactly?

I've always thought about "green jobs" as the category covering work with explicit and intentional environmental benefits. Working on a wind turbine: yes. Making solar panels: of course. Organic farming: okay. Farming in general: not really. Teaching science in a public high school: an admirable endeavour, but not a green job. But there are lots of jobs that fall in a gray area. Tofu can be swapped for emissions-heavy beef; does growing soybeans count? What if you manufacture bicycles? The more you think about this, the more uncertain you become. Most jobs can be greened or at least greenwashed; in Portland, for example, there is a vegan strip club. On the other hand, you could make a Malthusian argument that no job is green because anything that creates value for people helps them and facilitates their attrition campaign against the earth.

How to define green jobs may not seem like a pressing concern, but it does matter, because the parameters of the population determine its observable characteristics. That in turn will impact our views about green jobs and our interest, or lack thereof, in trying to create them. So let's look around. The Bureau of Labour Statistics actually has two definitions, one based on output and one based on process. The output criterion includes establishments that actually create products with environmental benefits. At the wastewater facility, for example, all the jobs are green. The process category is broader and will count workers who complete environmentally helpful tasks at establishments that are not necessarily classed as green overall—so, for example, the guy at Dupont who tests the water has a green job. Having two definitions may seem confusing, but it it's good to acknowledge ambiguity where it is known to exist.

Other outfits, however, want to pin it down. But their estimates vary widely. In 2008, the U.S. Conference of Mayors put the number at about 750,000 (PDF). In 2009, the Pew Charitable Trusts came up with a similar number, about 770,000. Those two are using a relatively narrow definition. Some analyses take different tacks. This 2010 report from Oxfam America, "A fresh look at the green economy", focuses on jobs that help with “climate resilience"; its tally is about 2m, including, for example, the 590,000 Americans who work in disaster response and preparedness. Others simply have a more liberal interpretation. This July 2011 report from the Brookings Institution, “Sizing the Clean Economy”, puts the figure at 2.7m, or 2% of the workforce, which is roughly on par with what the BLS would find using its output definition. Some people want a broader approach. I called up Chris Busch, policy director for the Blue-Green Alliance, an advocacy outfit comprised of labour and environmental groups. “How about the steel that goes into wind turbines?” he asked.

One thing that emerges from looking at the various numbers is that the purpose of the analysis may seep into the definition. The BLS is trying to be descriptive, which is why they're willing to offer two definitions, and why they're declining to get involved in controversy. In their FAQ someone asks whether they consider wages or safety in determining whether a job is green. A demurral: "Making such determinations would be inappropriate for a statistical agency, which must refrain from policy advocacy to main its credibility among data users." The (non-partisan) Brookings Institution also wants to provide information to policymakers, although with Brookings you get the sense that they want the policymakers to act on this analysis. Pew is a little more explicit in advocating for clean energy investments as a way to create more jobs—"Like all other sectors, the clean energy economy has been hit by the recession, but investments in clean technology have fared far better in the past year than venture capital overall"—which might explain why it defines the sector more narrowly. Brookings, by contrast, freely admits that job growth in the "clean economy" has been slower than the national average between 2003 and 2010—an average annual growth rate of 3.4% for clean jobs compared to 4.2% for the entire economy—but, like Pew, it finds that the narrower category of renewables and so on created jobs at "a torrid pace".

This points to an interesting conclusion. Environmentalists sometimes exaggerate the size of the green economy. This is, presumably, because it makes the sector seem more important to the economy as a whole. But as we see with the growth in rewnewables, the numbers may be more compelling when you winnow it down to the categories of strength. And for advocates, the broader definitions may actually be counterproductive. People find obvious exaggerations unconvincing, even offensive. It may be unfair to the environmental crowd that they're being hammered by the spectacular failure of Solyndra. But it was unfair to the taxpayers that politicians were pushing that company so hard. People may have a greater tolerance for uncertainty than politicians think. If politicians want to create "green jobs"—and whether they should try to do so is another question—candour is better than hype. More sustainable that way.