Comma required to avoid syntactical (but not semantic) ambiguity?

Question

Consider this sentence:

You may worry about the Fed raising interest rates, or a market meltdown, but these risks should not change your investment plans.

Could the comma before "or" be omitted? I could argue no, because then the sentence would imply the reader may worry about the Fed "raising a market meltdown," which is clearly not the intent (A market meltdown is a separate risk). On the other hand, the phrase "raising a market meltdown" is clearly nonsensical, hence a comma may not be needed to clarify the writer's intent (the Fed could "cause" a market meltdown, but not "raise" one).

In other words, is it acceptable to omit a comma when its absence creates a syntactical ambiguity, but not a semantic ambiguity?

EDIT

Rephrasing is usually the best answer to these kinds of cases. It's a made-up example. Assume the sentence as phrased is my best option, and my only decision is comma or no comma.

My understanding is this:

• If a comma is optional, take it out.
• However, if there is any possibility of ambiguity (without the comma), leave it in.

Ideally, I'm looking for an answer that cites an authoritative source (e.g. Chicago Manual of Style).

Answer 1

Written English is governed by the principle "Anything which can be misunderstood will be".

There is thus no practical difference between a syntactical ambiguity and a semantic one: even if the ambiguity is resolvable with only a little effort, some readers will fail to make the effort and will either misunderstand your meaning or dismiss you as an idiot.

My own feeling is that it is your duty as a writer to make it as easy as possible for your reader to understand you, and as hard as possible for your reader to misunderstand you. If you can make your text more intelligible with a comma, put the comma in. If you don't like the comma, rewrite so it's not necessary:

You may worry about a market meltdown or the Fed raising interest rates, but ...

But as Edwin Ashworth points out, that has its own ambiguity: market meltdown may be taken as subject of raising. So:

You may worry about the Fed raising interest rates or the market melting down, but ...

Answer 2

Can you see any hikers, using binoculars?

Can you see any hikers using binoculars?

Obviously, the comma is not 'necessary': both are grammatical. But the first sentence would usually be taken to mean

Using binoculars, can you see any hikers?

Whereas the second can either mean that, or

Can you see any hikers who are using binoculars?

Disambiguation is important (in speech here, emphasis/context would disambiguate).

With

Can you see any hikers with the unaided eye?

There is no scope for confusion. The comma is not needed to aid disambiguation. It might be used to signal a pause in speech:

"I don't need binoculars. I can see the distant Cascade Range with the unaided eye!"

"Really. But I'm unconvinced. Can you see any hikers, with the unaided eye? I can count about 20."

But otherwise, there is no reason to include it.

With

You may worry about the Fed raising interest rates, or a market meltdown, but these risks should not change your investment plans.

There is no cause for lasting confusion amongst people used to this terminology, as they would discount as illogical the alternative meaning. But other people not sure of financial jargon (does 'raise' have a subject-specific sense in finance, as it does with verbs?) might be confused, and even those used to the jargon might stray momentarily into the common 'doing A or B' reading. So not compulsory to include the comma here, but daft not to.