Would you believe a scientist who can't do simple maths?

Apparently (it's always apparently, may or could or might) the Earth is running out of plants. What's the Earth using the plants for, I hear you ask (well I asked it anyway). It's not the Earth using the plants at all, of course, but us. "Scientist claims Earth may run out of plants" wails CBS News uncritically, repeating an article "Will Earth Run Out of Plants?" in LiveScience, which begins

Humans may be very close to extracting all of the Earth's available plant resources, says a University of Montana researcher.

In fact, said Steven Running, a professor in the university's College of Forestry and Conservation, humanity may realistically have only 10 percent or so of our planet's annual plant resources in reserve, with little ability to boost yearly growth. The calculations don't suggest that humanity is on the verge of starvation, Running said, but they do indicate there are limits to our species' growth.

Note that he's in the College of Forestry and Conservation - I'll return to that factoid in a while.

Thanks to satellite measurements, researchers can now calculate how much vegetation the Earth produces each year. Over 30 years of observation, Running said, the number has stayed remarkably stable at 53.6 petagrams (one petagram is one trillion kilograms, or about 2.2 trillion pounds).

That's a lot of greenery. But humans use about 40 percent of it annually, Running said. The number would seem to offer plenty of wiggle room for humanity, but in fact, only about 10 percent of the remaining vegetation is up for grabs, he said.

That 10% of the remaining vegetation (60%) must be 6% of the whole, right? Nope - this is where the prof's arithmetic goes astray (or has already gone astray?).

"If humans are appropriating about 40 percent of annual production, if another 50 percent we can't harvest and appropriate, then that only leaves about 10 percent," Running said. "Well, that starts to sound a lot closer to a planetary boundary."

That sounds a lot closer to a mishtake, shurely? Is it 6% or is it 10% prof? In fact it isn't either. If, as the prof states, 10% is left, then we have 10% out of the 50% left to use - I make that 10/50 - 20% left from what we can use, or put another way, we could possibly increase our use by 10/40 or 25%. Doesn't seem quite so bad when you put it that way, does it? Why didn't the prof put it that way?

"What we've found is that the vast majority of that other 60 percent isn't available at all," he said. It's either locked up in root systems and unharvestable, conserved in national parks or wilderness areas crucial for biodiversity, or simply in far Siberia or the middle of the Amazon, where there are no roads and no way to harvest it.

No roads, eh? No hope for mankind then? "Far Siberia" where there are no roads, like the Trans-Siberian Highway and the Trans-Siberian Railway? I've heard of the Amazon - it's a big forest in South America, or something. Conservationists (remember College of Forestry and Conservation?) have been wailing for decades about forest clearance and the road building necessary to move in the equipment and transport to do the dirty deed. Doesn't deforestation decrease the vegetation he's talking about? If that's caused a decrease, as it must have, doesn't that mean that there has to have been an increase in other vegetation to keep the total "remarkably stable"? Bear with me, I'm just a simple chap, and obviously can't grasp the complex science involved.

There are arguments against this imminent vegetal limit, Running said: It's arguably possible that humanity could increase plant production with fertilizer or irrigation (though both of those are also in limited supply and have downsides such as pollution), or that we could construct more roads into the Amazon to avail ourselves of more natural resources.

Just a minute - we are increasing plant production with fertilizer and irrigation - crop yields are increasing worldwide. So if "we" construct more roads we could avail ourselves of more of the 50% of natural resources that he said "isn't available at all"?

"And again, you would question how far that would go and whether that's a planet we want, where every single acre from wall to wall has been completely harvested and is in some sort of controlled annual plant production cycle," he said.

"Controlled annual plant production cycle" - it's called agriculture, prof.