In order to determine how much carbon is captured in different coffee systems and what that translates into when different types of coffee are traded, we need information about the carbon sequestered per hectare as well as the average yields associated with the different systems.
Studies of carbon sequestration in agricultural systems, especially different management regimes in coffee, are few. However, from those that have been done in recent years, we find that the coffee agroforestry system can capture amounts of carbon that approach or even surpass those fixed by certain forest systems.
The amounts are quite variable in the different coffee studies, but, on average, a shaded system of coffee (and this includes both "rustic" coffee, with lots of tree diversity provided by trees formerly forming a natural forest, as well as "polyculture" coffee, where the diversity is due to farmers' having planted the shade trees) sequesters about 90 metric tons of carbon per hectare in the above ground biomass (no studies have looked at the amount of carbon in the soil or below ground biomass).
By contrast, a sun coffee plantation captures 12 metric tons of carbon per hectare.
As for yields, the global average for coffee is about 750 kilograms of green coffee per hectare. Shaded systems produce less; sun systems produce more. Taking conservative estimates for yields of the 2 systems, it is safe to assume a yield of 400 kilograms/hectare as an average for shade coffee systems, and 1000 kilograms/hectare for sun coffee.
So, how does all this relate to the coffee trade and the kind of coffee people end up drinking? There is a difference in the amount of carbon being fixed at the farm level, but what about beyond the farm gate? We can determine this by taking a container of coffee as the representational unit. Assume there are 15 metric tons of green coffee in a container.
For a sun system, where 1000 kilograms/hectare is the yield, that means that 15 hectares are needed to fill that container. With 12 metric tons of carbon fixed per hectare in this sun system, that means that 180 metric tons of carbon are being sequestered on the land that produced this coffee (12 x 15 = 180).
For the shade system, where yields are lower, it takes 37.5 hectares to fill the container. Given that 90 metric tons of carbon are fixed per hectare in the shade system, the container represents 3375 metric tons of carbon being captured on the land that produced the coffee. That is nearly 19 times the amount of carbon fixed in the sun coffee system.
Note* This applies only to the above ground biomass of these systems. It does not account for transport, processing or roasting, nor does it figure in the below ground carbon or the "leakage" in the form of the coffee beans themselves that get harvested.