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Usage and Grammar
Q. Would you use “less” or “fewer” with “CO₂ emissions”? To me, “emissions” seems like a measurable, uncountable substance, so I would say “less.” However, a quick search on Google Ngrams shows “fewer emissions” is more common. Which is correct? Or should it be “lower” instead? And if so, why?
A. Your preference for “less” makes a lot of sense. In formal writing, however, “fewer” has long been preferred with a plural noun, no matter how difficult it might be to count. According to that preference, if it takes a plural verb, you would use “fewer”: so, fewer CO₂ emissions, but less CO₂ (carbon dioxide is a mass—or noncount—noun: CO₂ is; CO₂ emissions are). It may be helpful to consider the case of “data.” Even though “data” doesn’t look like a plural, you would write “less data” or “fewer data” depending on whether you consider “data” as a mass noun (as in common usage) or as a plural (as in the sciences).
We had a lot less data to support our hypothesis than we wanted. [data as mass noun]
The second group of researchers returned fewer data than the first. [data as a plural noun]
The latter usage—including the use of “fewer”—is supported by the AMA Manual of Style (10th ed., sec. 7.8.2). In other words, the data support(s) “fewer emissions.”
The alternative “lower emissions” can also work. Unlike temperatures or costs, which can be higher or lower in the singular or the plural, we don’t normally talk of a higher or lower emission singular. But we do talk of higher or lower emissions plural—where “level(s)” is understood. And an Ngram comparison of “fewer,” “less,” and “lower” as adjectives modifying “emissions” (adding “_ADJ” to a term filters out other parts of speech) shows that “lower emissions” is more common than the other two phrases combined.
In sum, “fewer” is considered to be correct, but “lower” is a good alternative—and well established. Meanwhile, though “less emissions” seems like a rational choice (it’s unambiguous, and it has the advantage of being the shortest option), avoid it if your goal is to satisfy any sticklers.