Question: how is carbon dioxide affecting the great barrier reef

Good question skyelee it is complicated but here is a start. Much of the excess CO2 is absorbed by the oceans – in fact science is indicating that oceans have absorbed ~50% of the excess CO2 produced by humans in last 200 years. The absorbed CO2 then results in chemical changes to the oceans, especially surface waters (<10m deep). The CO2 bonds with sea water to form carbonic acid and this chemical change has resulted in decreases in pH by ~0.1 while this is generally called ocean acidification the oceans are still alkaline. A few other chemical reaction take place and what happens is that fewer carbonate ions are available for use by animals who need them to make shells or calcium bodies. Scientists predict that the GBR waters will reduce from a pH of ~8.2 to 7.8 over the next 100 years which has large implication for corals as they need carbonate for their skeletons. Hope that made sense.