1. That has led to a reduction of CO2 emissions by approximately 425 tonnes per year
  2. That has led to a reduction in CO2 emissions by approximately 425 tonnes per year.
  3. That has led to a reduction of approximately 425 tonnes per year in CO2 emissions.

I remember reduction/increase is always followed by "in" to mean something is reduced or increased. But here I see reduction of, which is a puzzle to me. I tried my best to offer 2) and 3). I am wondering which is the right one.

  • I think you have the right idea in 2 and 3. – user3169 Oct 12 at 5:04

As far as I'm aware, there is no hard & fast grammatical rule dictating whether in or of should follow the word 'reduction' in sentences similar to your examples, but rather this depends on the object being described and the type of quantity which is being reduced.

Usually a reduction in describes a result, where the subject is the quantity being reduced, for example:

This has led to a reduction in CO2 emissions.

Whereas a reduction of tends to place the focus on the reduction itself, i.e. the action which produced the result, for example:

A reduction of violent video games has led to a reduction in violent crime.

But then the above could also be written:

A reduction in the number of violent video games has led to a reduction in violent crime.

A reduction of could also be used in phrases where the reduction itself is the subject of the phrase, and not the quantities being reduced, for example:

Prepare the sauce from a reduction of red wine & vinegar.

  • As for my specific example of talking about quantifiedly reduced amount of carbon emmissions, which one should sound better? – Mike Philip Nov 24 at 15:48
  • @MikePhilip Number 2 or 3 – Lee Mac Nov 24 at 15:59
  • @MikePhilip I would choose number 3, or a hybrid of that and number 2: "[...] a reduction in CO₂ emissions of [...]" (replacing the "by"). We normally talk about "a reduction of" a quantity, or "reducing by", but rarely "a reduction by". – Darael Nov 24 at 19:24

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