Mapping Coral Bleaching



Over half of the world’s coral has been lost to bleaching, and decline is only accelerating. The pigment of corals is created by microscopic algae called zooxanthellae. Zooxanthellae live inside the coral in a symbiotic relationship, and is crucial to the survival of coral. But they are also very sensitive to changes in the environment. An increase of temperature in the seas, pollution, and an increase in acidity can all cause zooxanthellae to leave the coral, causing bleaching. When the algae leaves, the coral dies. 

Coral reefs form the backbone of vibrant ocean ecosystems containing a multitude of fauna and flora. With reefs dying at breakneck speed, populations of marine life are dwindling too. 

I wanted to map out the bleaching of the world’s corals to visualize just how much of our reefs we have already destroyed. 


I gathered multiple shapefiles containing data on different marine ecoregions of the world, on coral reefs, and I found a csv file documenting global coral bleaching. 

Methodology & Visualization

To construct the map, I utilized Carto, a location visualization tool that allow users to create maps to analyze spatial data. 

The first dataset I imported contained all the Marine Ecoregions Of the World (MEOW) but did not specify areas that were coral reefs from other ecoregions so it was not helpful for this project.

I then imported a dataset containing coral reefs which also specified how concentrated each reef was.

I layered a CSV dataset of coral bleaching information on top of the coral reef displaying each data point as a point on the map. This dataset contains information on the severity of coral bleaching, ranging from none to high, but also includes “unknown”.

In order to visualize the data meaningfully, I used the data analytics tool filter column option to filter out data points with “no” coral bleaching and with “unknown” coral bleaching.

I felt like although the map presented a lot of information, it was not digestible. I therefore decided not to different coral reefs by density of coral and recolour the bleaching so that high bleaching would be white and low bleaching would be closer to the coral colour. I also used a blue base map to be more in-keeping with the ocean theme of the project.

While this was an improvement on the previous map, I felt like it didn’t map well onto the real world. Coral bleaching is the erosion of coral, not the addition of whiteness on existing coral. So by changing the base map to a whiter colour palette I was able to communicate this better. I also added a legend for easier understanding of the visualization.


Using Carto to map the bleaching of our reefs was an engaging way to show the extent and seriousness of this problem. The map shows how widespread the problem is and how severe it is in many marine ecoregions. 

However, the majority of data points on the coral bleaching dataset specified that the data for coral bleaching was “unknown” so it is unclear if this visualization is factually accurate. I also did not have as much control over the design of the map as I would have liked because the shapefiles were predetermined by their creators.