Anthropogenic CO2 emissions have two negative effects on the World System: (1) CO2 pumped into the atmosphere increases the greenhouse effect that drives global warming and (2) CO2 absorbed by the oceans (between 30-40% of emissions) creates ocean acidification (lower pH).
Ocean pH is lowered through the creation of carbonic acid. As ocean pH is lowered, corals die and calcifying organisms who need carbonate to form their shells (carbonate is depleted by carbonic acid formation) start dissolving and dying. When corals and calcifying organisms start dying, food chains are disrupted. Since humanity is at the top of the food chain, humanity starts to suffer.
There are only two hopes here: (1) reduce CO2 emissions (substantially) and (2) hope that corals and calcifying organisms can evolve to flourish in low pH waters (evolution is slow). Since people who do not accept climate change science typically also do not accept evolution, there is really no hope for them except to deny that any of this is happening.
Nova recently aired a segment titled Lethal Seas (trailer above) that discussed the topic in depth. The entire show can been seen here.