I seriously doubt that your motorcycle has much of an effect on the worldwide carbon content.  You have bought modern, efficient motorcycles with catalytic converters, fuel injection and oxygen sensors that monitor your fuel mixture many times a second - you are doing your part.

Have fun on them - don't get overly fussy about the mpg.  I once rode my little Ninja 250 very carefully around TN/NC the day before the others arrived - and I got 77mpg on it.  It was cool - but that is the last time I paid any attention.  I knew it ran well and got good mpg - so I didn't worry about it after that.  The fuel tank held enough that I could go 400 miles before I needed fuel - it lasted far longer than my bladder could!