A majority of photsynthesis on the earth occurs in the oceans, and a majority of that is due to the action of phytoplankton, which inhabit the to few meters of the ocean's water columns.
Phytoplankton blooms, situations where the quantity of phytoplankton increase dramatically, could be engineered
as one way to reduce greenhouse gasses, and possibly forestall other human effects on the environment.
Sadly, the quantity of nutirents needed to feed phytoplankton in these top few meters of the ocean decreases dramatically as one goes away from the shoreline, and out to sea.
Out to sea, the top few meters of the ocean therefore tend to be sparsely populated, and poorly inhabited... until recently... Currently, these top few meters of the ocean are becoming increasingly populated with human waste. Bits of plastic, which slowly break down into bite sized pieces, thus choking whales, sea birds, and other fish. Not to mention being a general eyesore.
I propose that the plastic products of our modern society be impregnated with nitrogen, minerals, and other nutrients needed to promote phytoplankton growth. As these bits of plastic drift out to sea, they will slowly break down, and release their nutrients, allowing phytoplankton to grow near them. As these plastics wash out to sea, encrusting species such as barnacles will begin to grow on them, and as there is an abundance of phytoplankton to feed on, these encrusting organisms will thrive. Their shells will then weigh down the bits of plastic, which will sink to the ocean's floor. Phytoplankton growth will increase, cutting down on greenhouse gasses in the atmosphere, and encrusting organisms will leave their shells on the human waste, therby weighing down the plastics so that they sink to the bottom, and are sequestered from further interaction with the environment until a much later date.
Now our trash can be used to sequester more carbon.