Simple climate models (SCMs) are useful tools for exploring the global climate and carbon cycle (Calvin and Bond-Lamberty 2018). They have reduced computational complexity compared to larger Earth System Models (ESMs), trading a lower temporal and spatial resolution for a significantly faster runtime and reduced computing power. SCMs make it possible to quickly and efficiently conduct large-ensemble sensitivity studies, model coupling experiments, and policy analyses. SCMs use data collected from the field and parameters determined from larger model runs in order to investigate the effect of various biogeophysical forcings on the global climate and carbon cycle.
One SCM is the open-source model Hector (Hartin et al. 2015). Hector has active surface ocean chemistry, multiple terrestrial biomes, and is capable of running a variety of scenarios, including any of the representative concentration pathways (RCP), which show a number of different severity climate change scenarios (Moss et al. 2010). In a given model run, Hector computes the state of each carbon pool (e.g. atmosphere, soil) based on the flow of carbon between pools as modified by anthropogenic climate change (Figure 1).
However, Hector does not currently have the capability to track the movement of carbon within the model, meaning that, for example, users cannot reconstruct the original source pools of the carbon in each pool. Instead, it only reports the total carbon stored in each pool at any given time step (Figure 2).
Here we describe the addition of a novel tracking capability to Hector's code base. This allows Hector to track the origin of carbon within each of the model's carbon pools throughout the run, after a specified starting date. Thus, at the end of the run, each pool's contents can be broken down by the original source pool (defined based on when tracking was enabled), allowing deeper exploration of carbon movement and transfer, and more robust benchmarking. Below we describe how the code additions work technically, provide example tracking outputs for four major carbon pools, and discuss next steps in development and research.
We modified the model's code to record the origin of the carbon within each pool, in addition to keeping track of the total amount of carbon. When a user enables tracking within a model run, all of the carbon within each pool is marked as originating from that specific pool-e.g., all of the carbon within the atmosphere pool at that moment in time is tagged as originating from the atmosphere. These tags do not change over time as they are associated with that particular mass of carbon, and follow the carbon as fluxes move it throughout the model.
Fluxes are created from an existing pool and have the same origin proportions as the pools they originate from-i.e., if we create a 25 petagram carbon flux from the atmosphere, we assume that the atmosphere pool is well-mixed and remove carbon in equal proportions from each of the origin sub-pools within the atmosphere. The resulting 25 Pg C flux thus has the exact same origin fractions as the atmosphere at that point in time. The addition of a flux to a pool is what changes the origin proportions of that pool. For example, if we add a 25 petagram flux of carbon from the atmosphere pool to the vegetation pool, 25 petagrams of carbon will be added to the total value of the vegetation pool, and the origin proportions of that flux carbon (which as noted above matches that of the atmosphere) are added to the vegetation pool's list of origins for its already existing carbon. Subtraction is simpler as we continue our assumption that the pools are well mixed: we remove carbon from the pool in equal proportions to the carbon within the pool, simply decreasing the total value of carbon. In this way, each pool keeps track of where its carbon originated from and we are able to watch the pools' composition change over time through these pool-to-pool transfers.
Hector tracks carbon in the atmosphere, vegetation, soil, detritus, and ocean and can run multiple terrestrial biomes. We can also add additional pools, like fossil fuels, whose carbon we want to track. Each of these pools is initialized with a starting amount of carbon at the beginning of a run and tracking is turned on at a time specified by the user. For each time step within a run, we calculate all expected fluxes into and out of each pool (e.g. net primary production and land use change) and add/subtract these fluxes from our pools. When tracking is on, the origin of the carbon in each pool is updated as described above. If tracking is not on, the total amount of carbon within the pool is changed but the origin proportions stay the same. This process, with a few additional adjustments, repeats every timestep.
We also want to note that all of the changes to Hector's code are backwards compatible with previous versions of Hector and running Hector with tracking is completely optional. When tracking is not enabled, adding and subtracting fluxes from a pool functions as standard arithmetic operations that only keep track of the pool's overall value and not the carbons' origin fractions.
Currently tracking is implemented for atmosphere, vegetation, soil, detritus, and the ocean. Below we show our tracking results from each of these pools, excluding detritus due to its small size. All of the plots below were created by running Hector using RCP 8.5, a high emissions pathway (and therefore the most interesting carbon tracking results). Also note that we tracked from 1850 to 2300. However, Hector is only calibrated to 2100, so the findings after 2100 should be taken as provisional. We include the results between 2100 to 2300 to demonstrate interesting behavior within the soil pool. Note that at the beginning of each run, every pool of carbon is composed entirely of carbon from that pool, and thus the initial fluxes of carbon start off with high proportions of carbon from a single pool before they equilibrate. Thus in the first several years of the run, we can see a rapid rebalancing effect until the pools come into more of a steady state with respect to the distribution of source carbon in each pool. At the same time, an increasing amount of carbon from fossil fuels is being added to the system, which we can see especially in atmosphere, soil, and vegetation pools in Hector relative to their size.
Code for making the plots and animations is available at the author’s GitHub page here.
As far as we are aware, there are no other climate models-either simple or complex Earth System Models-with this type of tracking functionality. The addition of tracking to Hector thus creates a novel opportunity to explore more detailed carbon movement within the model and carry out robust benchmarking. For example, this new capability allows computation of the model's airborne fraction (Knorr 2009), a crucial carbon-cycle diagnostic that reflects how much of anthropogenic emissions remain in the atmosphere. Hector's carbon tracking also provides the foundational infrastructure needed for a future implementation of radiocarbon (He 2016). Additionally, tracking allows careful examination of the path and location of carbon from anthropogenic carbon sources, or natural system changes such as permafrost thaw (Woodard 2021), over time. Finally, we also envision this functionality being used in integrated assessments models (IAM) to examine how new economic policies may affect the carbon cycle.
We are hopeful that this tracking functionality will become a standard part of Hector when Version 3 is released. Additionally, we are working towards a manuscript in the second half of 2021.
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