Greg Mitchell, a researcher at the prestigious Scripps Institute of Oceanography, expects seaweed to become a multi-trillion dollar industry -- sometime after a ten year developmental period. This new industry should disrupt the use of fossil fuels, according to Mitchell.
According to the Biomass Handbook, cultivated seaweed can yield close to 130 tons per ha per year. Fast growing willow may yield just above 10 tonnes per ha per year. And miscanthus grass can yield 15 tonnes dry mass per ha per year. Giant King Grass (PDF) may produce 5 X or more yield than miscanthus, in tropical climates.
Those are rough figures which are subject to change as faster-growing strains are developed via several means. Besides rapid growth and 6 X a year harvesting, seaweed takes advantage of large areas of the Earth's surface which cannot be utilised to grow land trees or land grass for biomass. Seaweed essentially doubles available biomass growing area -- or more -- which throws conventional calculations for biomass potential out the window. Some problems must be worked out, but by the time humans truly need the massive quantities of biomass they can get from seaweed (and special grasses and trees), the problems should have been solved.