If the production doubles every two years then the sum of production also doubles every two years (to a first approximation)
consider sequence 1, 2, 4, 8, 16, ... , 2^n where n is the production after n periods of 2 years
then sum( 1, 2, 4, 8, 16, ... , 2^n ) = 2^(n+1) -1
after two more years...