Say 85% or so. Not with plastic, though!
Is 15% light penetration a killer for life in the seas? What would happen to winds and rainfall? Can scientists design such a life form - white algae perheps - that we could use for building material and as a carbon sink?
It has been done before.
One catch is that the oceans are already fairly reflective. You’d get more benefit from whitewashing the land.
I don’t think that’s true. Wikipedia’s figures put open-ocean albedo at around 0.06, while land albedos range from 0.08 to 0.40. They also have a handy graph (below) showing the typical ranges.
Of course, in terms of “bang for your buck” it’s probably easier to whitewash the land than the oceans, so it may be true that if we were to seriously consider this, we’d want to start with the land.
I recall reading that someone was already working on a lighter variety of corn plant (maize for our European friends). Those acres and acres of corn in the Midwest…
And there have been studies on replacing black tar on flat roofs with white elastomers. But that was about air-conditioning costs.