The Great Barrier Reef (GBR) off the eastern coast of Queensland, Australia is the world's largest reef. In fact, it's the largest living thing on Earth spanning more than 2500km (GBRMPA). As climate changes in response to human activities, the GBR suffers the indirect effects of increased atmospheric CO2 emissions.
The most pronounced and widespread effect of climate change on the GBR is coral bleaching. Coral bleaching is the visible change in coral appearance as a result of the death (or loss) of symbiotic organisms that give the coral its color. As sea temperatures rise in response to climate change, the marine algae that live within the coral (and on its surface) die off - when they disappear, they leave behind the white coral structure. In small patches, coral bleaching isn't a problem - it can even be a good thing for growing new algae. However, the widespread bleaching resulting from average sea temperature increases that may or may not be permanent in nature highlights real problems for the reef. Currently 97% of the great barrier reef has been bleached or partially bleached. Will it ever restore itself and heal from the impacts of climate change? That's a question scientists are currently working on, and the future doesn't look too bright (link).
Other effects of climate change on the reef include ocean acidification (which impacts calcium carbonate formation of the coral itself) and sea level rise (which changes the location of the reef relative to the coastline).
For more information, check out the links below.