Oh Aunty! That's a bit negative. Even a small positive is reducing the negative. True, as individuals we cannot have much effect but the more who try, the better. Every little helps!
Tying down exact figures is very tricky but common sense says something grown at home has to save on transport, packaging, refrigeration etc. Plus much of our food involves agricultural systems that liberate rather than sequester carbon.
If you accept that 18% of global greenhouse emissions come from agriculture then clearly a reduction is significant.
Every cardboard box we use as part of a mulch or in compost is sequestering carbon. Tiny amounts but positive.
Locally produce in a sustainable manner is good but most of us will buy from a big supermarket. A lettuce or cucumber shipped in from Almeria, garlic from China, beans from Peru - all things we can easily grow at home.
Plus growing your own makes you appreciate the value of food beyond the money paid for it. Since 25% plus of food produced is wasted, that's an easy win on reducing the 18%
Question is "is there a will?" because if we don't we're in bigger trouble.