What type of investigation to plant height if the amount of available light is reduced due to global dimming

If the amount of available light is reduced, then plants will receive less sunlight to perform nearly as enough photosynthesis as they would normally. As a result of this, plants will not grow as quickly, and their height may likely decrease. 

Hope this helps a little. 

X/7 = 18/21
Lets Reduce:
21/7=3 so we know for every 3 in 18/21 we have one in x/7
This means 6/7=18/21
Check:
18/6=3
21/7=3