Human activities can affect the carbon cycle in numerous ways: deforestation, fossil fuels, rice paddies etc etc.
But that's beside the point.
I'm in Year 13. I chose to do something just as obscure for my coursework (Urban Heat Island Transect). Yesterday I got into a spat with my teacher because I took her too literally on the 4000 word limit and I did not write enough, among other things. I also got told off for changing my hypotheses after submitting my CRF form, because measuring air quality is pretty darn difficult without a £500 piece of equipment or bits of sticky tape stuck to every lamp post. I'm really struggling to complete my NEA simply because of how difficult it is to make good geographical/specification references to my UHI subject while also giving good data analysis, from data taken from my local area alone.
My advice: go to the beach like everyone else and measure pebbles. Don't do something overly complicated like this because you're going to struggle with picking up marks later on.