The hot thing among “green” cities these days is pledging to get to 100 percent renewable energy — specifically, to get there by some future date, usually 2050. (Here’s a list of North American cities that have made the pledge.)

The trend has been gathering steam for a while. But it was bound to draw some criticism.

A new op-ed in the Wall Street Journal, by Charles McConnell of Rice University’s Energy and Environment Initiative, proposes to expose such pledges as misleading and silly. Not surprisingly, given its provenance, it badly misses the mark.

There are complexities and complications aplenty for cities that have taken on this ambitious goal. It begins with clearly distinguishing between renewable electricity and renewable energy. (The former is a small subset; the latter also includes transportation and heat energy from liquid fossil fuels.) Another is establishing a system of tracking energy that does not “double count” electrons, so that two different entities can’t claim to be consuming the same clean energy. Another is whether to involve utilities in these city efforts, or whether cities should try to bypass them.

But McConnell touches on none of those. Instead, he argues as though environmentalists are children, unaware of the most basic facts about electricity.

McConnell says there are no clean electrons. That’s irrelevant.

McConnell’s first beef with “100 percent renewable” claims is that they are, on the level of physics, not true.

Electrons are electrons — there are no clean and dirty ones. Once they are generated and fed into the grid, “coal electrons” and “solar electrons” cannot be distinguished or tracked. They all become part of one undifferentiated mix.

And that mix is, in practice, always partially dirty. No major grid in the world yet runs on 100 percent renewables. Every grid still relies on fossil fuels (most for a majority of their power).

Ergo, physically speaking, no city is consuming 100 percent renewables. No city (or business or individual homeowner) can, until the grid it’s connected to is powered by 100 percent renewables. All cities are, physically speaking, consuming part-dirty part-clean electricity.

But ... so what? This is one of those debating points that renewable energy critics find very clever, but at root it’s just semantics.

No one denies physics. No one involved in generating clean energy or procuring it for cities believes there is an actual market in clean electrons. There may be homeowners who do not read their utility literature carefully and believe they are literally buying clean power, but no one told them that, because no one believes it.

The way the market works is (longer description here): When an individual or city contributes to the generation of renewable energy, it gets credit for that renewable energy. If it buys enough renewable energy to cover its consumption, it can claim to be “consuming 100 percent renewable energy.”

Think of it as a shared pool of drinking water. Water comes in from several sources, some dirty, some clean. If I pay someone to procure $10 worth of clean water and add it to the pool, and I drink $10 worth of water, I can claim that my share of the water is clean. Of course I’m not literally drinking only clean water — I’m drinking from the same shared pool. But no one will be confused about that, will they? I’m responsible for an amount of clean water equal to my consumption; that’s what the claim means.

Buying clean energy credits through one’s utility is like paying someone to dump clean water into the pool.

It is not the most elegant solution, of course. In an actual market economy, consumers would choose their own electricity sources directly; that would make tracking easy. But the utility sector is not a market economy; it’s a quasi-socialist, semi-monopolistic Rube Goldberg contraption.

Clean energy credits are a kind of hack — the best consumers can currently do to channel their money to renewable energy. And the renewable energy those credits support is real.

In short, when a city is “run on 100 percent renewable energy,” that’s an accounting reality, not a physical reality.

But, again: So what?

McConnell says grids must always rely on fossil fuels. That is theology, not science.

McConnell then argues (well, says) that the small amounts of renewable energy supported by these city pledges only works because they sit on top of a grid built by fossil fuels.

This is his case:

There is no denying that wind and solar power are important to a balanced energy portfolio. But coal is the bedrock of affordable electricity, and it will remain so, no matter how much wishful thinking by environmental activists. Coal is abundant and reliable. Unlike wind and solar, coal generation can be dialed up and down in response to market conditions and to satisfy demand.

Fossil fuels, he says, are “the foundation upon which renewable power must stand.” Not do stand. Must stand.

Buried in these declarations of faith is a familiar argument: Solar and wind are variable (“You don’t know exactly when the sun is going to shine or when the wind is going to blow,” says the mayor of Denton, Texas); thus, the grid needs coal.

It is telling that McConnell offers virtually no evidence for that argument, because there is no evidence to be had.

First, coal is not the “bedrock of affordable electricity” — it is being undercut on the market every day by natural gas and renewable energy. That’s why it’s an industry in decline.

But more importantly, the counterbalance to variability is not coal — that’s a category error. The answer to variability is flexibility (longer explanation here). Insofar as wind and solar need backup, they need nimble, flexible backup that can ramp up and down as quickly as they do.

Coal is the opposite of that. Big coal plants (you have to make them huge to make them economic) are slow and expensive to ramp. They are designed to run all the time; that’s how they make money.

But having a big, dirty source running all the time impedes renewable energy growth; it doesn’t sustain it.

Flexibility will come, in the near term, from natural gas. In the longer term, it will come from dispatchable renewable sources (geothermal, small-run hydro, possibly some biomass), storage, load shifting, efficiency, and conservation. To get to 100 percent carbon-free electricity, we might need some nuclear power (especially the small modular kind). We might need carbon capture and sequestration, most likely attached to biomass and maybe natural gas.

But no grid operator will turn to coal for flexibility. Big, always-on “baseload” sources simply aren’t what a renewables-based grid needs.

So McConnell is correct that today’s grids rely on fossil fuels. But his contention that they must always do so — specifically, that they must always rely on coal — is ludicrous. It is being falsified even as we speak.

Cities are not deceiving anyone

When a city says it is getting 100 percent of its electricity from renewables, what it means is that it is buying a quantity of renewable energy equal to its energy consumption. Physically, it could not mean anything else.

Whether you find that deceptive is a matter of taste, I suppose. I don’t. If every US city paid for enough renewable energy to cover its consumption (and there was no double counting, which is no small detail), there would be an amount of renewable energy on the grid equal to the energy consumption of all US cities. It’s math.

What is deceptive is pretending that fossil fuels, much less specific fossil fuels, are necessary for reliability, and thus that 100 percent clean power will always be impossible.

McConnell is right that solar and wind electrons have no special properties; they are not “clean.” But neither do electrons from coal; they are not “reliable.” All the grid cares about for reliable service is that enough electrons get fed in at the right time. It is agnostic toward the source of those electrons. If we can feed in enough electrons without using coal, which is filthy and increasingly uncompetitive, why wouldn’t we?

Cities are not deceiving anyone. They are buying renewable energy and claiming credit for what they buy — in response to both public opinion and moral imperatives. It’s a perfectly reasonable way for ordinary citizens to express their support for clean energy.

What disturbs the old guard — people like McConnell, who used to be assistant secretary for fossil fuels at the Department of Energy and now works at Rice to “forge new public and private partnerships and to expand Rice’s existing relationships with companies like Shell, Chevron, ExxonMobil, and BP” — is that cities are endorsing a future in which coal and natural gas are obsolete.

The doomsaying from fossil fuel defenders will only get more shrill as that inevitable fate gets closer.