How we made it: carbon removals fall short

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Hello from autumnal but warm London, and welcome back to the Climate Graphic: Explained after our summer break.

Capturing carbon underground has long been proferred as solution for the greenhouse gas emissions which will need to be dealt with to limit climate change.

However, the latest paper published by Nature journal, led by researchers from Imperial College London, warned that far less carbon dioxide can be safely stored underground than was previously thought. 

Limiting global warming to 1.5C above preindustrial levels as prescribed by the 2015 Paris agreement would mean sequestering 8.7 gigatonnes of CO₂ per year. But this amount would surpass the safe limit of storage described in the paper.

The report identified drawbacks and risks to the environment associated with relying on the technology, such as issues of cost and engineering and unintended pollution of groundwater.

One of the authors of the study Joeri Rogelj, who was involved in the Intergovernmental Panel on Climate Change special report on 1.5C of warming, said carbon storage should be seen as a “scarce resource” not “an unlimited solution to bring our climate back to a safe level”.

How we made it

To show the difference between the viable areas and the areas which were previously thought suitable for storing carbon around the world, we made a map.

To be specific, sedimentary basins, such as depleted hydrocarbon fields or deep saline aquifers within sedimentary basins, have the potential to be used as carbon capture storage areas. 

We used data provided by the authors of the report in shapefile format, to map the viable sedimentary deposits for carbon versus the total sedimentary deposits across the globe.

[Nerd note 1: a shapefile is geographically encoded data that can be read by GIS, graphical information system software, such as Qgis and Arcgis, which is open source.]

We also mapped the exclusive economic zones, a useful addition as they illustrate areas where territorial disputes could occur as a result of the basins stretching across those zones involving multiple countries. 

To distinguish between sedimentary basin types we used blues for offshore and browns for onshore. We used darker shades to draw the eye to viable areas.

[Nerd note 2: processing the data involved multiple platforms. The level of detail in the data meant it was a very large file, so we ran it through mapshaper.org, an online tool for simplifying map data. This made processing and manipulating the data easier and quicker. Changing colours, mapping economic zones and altering globe projections were all done in Qgis.]

Segmenting all the gathered information into various displays made it easier to digest. Rather than a two-dimensional map of the world, we chose to display the data as three globes, representing three hemispheres. This way the reader can focus on each of the regions rather than being overwhelmed by a single, extremely detailed global map.

The other benefit is that it allows more flexibility with the layout, especially on mobile devices where the vertical format is more appropriate.

Finally, stylistic edits, such as minor colour tweaks, adding a gradient and “outer glow” on the globes, were made in Adobe Illustrator. 

One interesting insight from the mapping exercise is that it shows countries with a history of low emissions often have higher carbon sequestration potential, while those that have historically emitted more tend to have less. This can be seen clearly in this scatter plot.

For the final chart, we wanted to show how the study had arrived at its final figure for the amount of underground storage potential after applying each subsequent exclusion layer.

A chart provided in the report was improved by changing it to a “waterfall” chart.

Representing the data this way made it easier to see the effect of each exclusion on the total underground carbon storage available. Error bars are also included for the total to show the uncertainty in the data.

Experts have strong views about the prospects for carbon storage. Ben Caldecott, director of the Oxford Sustainable Finance Group, said even the downgraded study estimates for storage capacity were still too optimistic because they did not factor in economic constraints. “Only a fraction of that estimated will be accessible at a price society will be willing to pay,” he said.

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