Hello, I have a question concerning the interpretation of the InVEST Carbon model output. The very simple version, provides as outcomes a map of the sum of all four carbon pools. I saw multiple references that treat this as carbon supply and compare it with an estimated demand (e.g. carbon per person multiplied by population, etc.). Is this the correct interpretation of the provided result? Can the tot_c_cur.tif, a rasters showing the amount of carbon stored in each pixel be used as a supply indicator?
I am asking this because computing this map and taking the sum of all pixels in a region and comparing to an estimated demand (carbon/pers * pop) gives really large values for all regions (unexpected, as some are cities while other rural areas). On the other hand, taking the mean of the carbon pool map per region, and compare this to the demand gives an opposite situation of only largely negative values for all regions (also rather unexpected).
Should I take the sum of all pixels in a region or rather the mean? And why are the result so unexpected? More generally, how do I interpret/use the outcome of InVEST Carbon?
I’m curious about what “supply” and “demand” mean in this case. Do you want to compare carbon storage to something like the total carbon emissions from the local population? What would this be used for?
We usually just use the model outputs to show where carbon is (or is not) stored on the landscape, as an additional service to show synergies or tradeoffs with other services. If we have scenarios, then we can tell how that changes over time, and maybe link that change to policies that aim to reduce carbon emissions for a location.
Personally, I have never linked population numbers to carbon storage, especially since carbon storage and sequestration affect the whole globe. When I have calculated a “service” metric using population (as we often do with sediment or nutrient, since beneficiaries are more local and easier to quantify and locate), we just multiply (total population in that pixel or watershed or whatever the unit is x total supply in the pixel or watershed or whatever). And we consider the result to be a unitless, relative ranking of where there the most sediment retention (or whatever) is provided to the most people.
I have seen many studies treating the InVEST Carbon as “supply of ecosystem service” while the demand is computed as pop*emission/capita. I am quite sure about the “demand” side, but I would like to link the carbon emission to the potential sequestration. I thought that InVEST Carbon would compute this map (i.e. based on LULC how much carbon sequestration is happening), but if I understand correctly InVEST only computes sequestration as a difference between storage at two timesteps. So if a pixel keeps constant LULC the sequestration stay 0, correct?
I would be happy about any suggestion on what to use to show “supply” (sequestration capacity), in order to compare to demand (emission). Thank you.
but if I understand correctly InVEST only computes sequestration as a difference between storage at two timesteps. So if a pixel keeps constant LULC the sequestration stay 0, correct?
That’s correct, the model is simply taking the storage difference of the future scenario from the current to get the delta_cur_fut.tif output. Positive values indicate sequestered carbon, negative values indicate carbon that was lost.
I wonder if @rmgriffin would have any experience or recommendations when trying to apply this model how you’re describing.
I think the terminology is not appropriate then. Or I am not sure I understand the model. Sequestration in this model has nothing to do with the capacity of an environment to uptake carbon, but rather only shows difference in stocks before and after. I would argue this cannot be called sequestration, or it might lead to very problematic use of the outcomes of the model.
But back to my question, I saw different studies using the simply carbon stock map as “supply” and compare this to the “demand” which they calculate as carbon emission. In my view this use of the model is also wrong, as carbon stocks do not give information about the supply. However, since many studies followed this procedure, I am probably misinterpreting some information. Any help please?
It would be helpful to see an example of the studies you are mentioning.
To your other point, the model calculates changes in the stock of carbon in carbon pools, which is the IPCC definition of carbon sequestration/emission.
It is a simple “bookkeeping” model though and may not capture all of the details you’d want for your research questions.
Here two examples where carbon storage is used as supply, and emission as demand. I do not need a detailed model, simple bookkeeping suffice, but I want to understand how can a map of carbon storage (not sequestration) can be compared against carbon emission. The two are not linked. But the study above shows that this is common practice. What am I missing?
And to the other point, the InVEST model calculates sequestration as a difference between pools. Thus, a LULC “forest” that stays “forest” over time, results as having a sequestration of 0. I think we can agree that this is not the case. And it would be problematic as well to use this map and compare it against carbon emission, as e.g. forests would appear to not have any impact when this is clearly not the case. Or again, what am I missing here?
@matteoriva, one thing that we can do to show change within a single landcover type over time with this model is to create different classes that reflect the change. For example, instead of a single “Forest” class, you can make “Forest - 1-20 years old”, “Forest - 20-40 years” etc, provide different carbon pool values, and assign them as appropriate in the current and future land cover maps.
Otherwise, this is simply the limitation of such a simple tool. If you need to dynamically grow trees and calculate the full carbon cycle, there are more complex models out there for that purpose.
Thank you the suggestion. What bothers me is not the model simplicity. I just don’t understand how the model is used in the studies I mentioned above. Is this a correct use of the carbon storage map?
I’m having issues accessing the second study. The first study appears to be described as sequestration, but the maps in the results appear to be storage, as they are given for fixed years based on the LULC in those years. The supplement doesn’t address this directly, so without any more detail from the authors I’m inclined to agree with your assessment that they are treating carbon stock maps as “supply,” which is inconsistent with sequestration being the actual ecosystem service people benefit from.