## 2.21.9. Model Equilibration and its Acceleration[¶](#model-equilibration-and-its-acceleration "Permalink to this headline")
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For transient experiments, it is usually assumed that the carbon cycle is starting from a point of relatively close equilibrium, i.e. that productivity is balanced by ecosystem carbon losses through respiratory and disturbance pathways. In order to satisfy this assumption, the model is generally run until the productivity and loss terms find a stable long-term equilibrium; at this point the model is considered ‘spun up’.

Because of the coupling between the slowest SOM pools and productivity through N downregulation of photosynthesis, equilibration of the model for initialization purposes will take an extremely long time in the standard mode. This is particularly true for the CENTURY-based decomposition cascade, which includes a passive pool. In order to rapidly equilibrate the model, a modified version of the “accelerated decomposition” [(Thornton and Rosenbloon, 2005)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#thorntonrosenbloom2005) is used. The fundamental idea of this approach is to allow fluxes between the various pools (both turnover-defined and vertically-defined fluxes) adjust rapidly, while keeping the pool sizes themselves small so that they can fill quickly To do this, the base decomposition rate \\({k}\_{i}\\) for each pool _i_ is accelerated by a term \\({a}\_{i}\\) such that the slow pools are collapsed onto an approximately annual timescale [Koven et al. (2013)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#kovenetal2013). Accelerating the pools beyond this timescale distorts the seasonal and/or diurnal cycles of decomposition and N mineralization, thus leading to a substantially different ecosystem productivity than the full model. For the vertical model, the vertical transport terms are also accelerated by the same term \\({a}\_{i}\\), as is the radioactive decay when \\({}^{14}\\)C is enabled, following the same principle of keeping fluxes between pools (or fluxes lost to decay close to the full model while keeping the pools sizes small. When leaving the accelerated decomposition mode, the concentration of C and N in pools that had been accelerated are multiplied by the same term \\({a}\_{i}\\), to bring the model into approximate equilibrium Note that in CLM, the model can also transition into accelerated decomposition mode from the standard mode (by dividing the pools by \\({a}\_{i}\\)), and that the transitions into and out of accelerated decomposition mode are handled automatically by CLM upon loading from restart files (which preserve information about the mode of the model when restart files were written).

The base acceleration terms for the two decomposition cascades are shown in Tables 15.1 and 15.3. In addition to the base terms, CLM5 also includes a geographic term to the acceleration in order to apply larger values to high-latitude systems, where decomposition rates are particularly slow and thus equilibration can take significantly longer than in temperate or tropical climates. This geographic term takes the form of a logistic equation, where \\({a}\_{i}\\) is equal to the product of the base acceleration term and \\({a}\_{l}\\) below:

(2.21.64)[¶](#equation-21-65 "Permalink to this equation")\\\[ a\_l = 1 + 50 / \\left ( 1 + exp \\left (-0.1 \* (abs(latitude) - 60 ) \\right ) \\right )\\\]
