I'm not sure why, but this doesn't feel like a good "as a whole" scenario to me. "Cumulative impact" sounds right ("accumulative " feels like slightly the wrong word). :) Even the original paragraph has a few inelegant aspects to it. I'd say this: "Each individual needs to do their share to influence the environmental big picture. Switching, perhaps, to a source of green power such as solar panels. If everyone made even modest contributions, the cumulative impact would be considerable." That only leaves the question of whether the statement is true. Surely a modest effect from everyone (modest relative to each individual's contribution) would result in a modest impact overall (proportionately modest relative to the overall problem)?

Hello Huy! What you have written is possible, but it sounds better as "The impact as a whole would be considerable." We normally use this expression to emphasize the idea 'cumulative' or 'complete.' Your example works, but because the expression modifies a noun, we usually see it immediately after the noun. Examples would be: "It´s always sad to fire employees, but it's necessary for the company as a whole to improve." "Prisons are often dangerous, but they make society as a whole a safer place." If you have any other questions just reply or send me a message! Best regards, Lee

When you use, "as a whole", it means you are considering all parts of something, even if some of the parts do not fit the general picture. The keyword here is "considering", so unfortunately it cannot replace the word "accumulative".

Is that your real name?