Bug Afforestation

[flohump]

In case of high CO2 prices or plantations the model in some cases moves age classes beyond the time horizon (50yrs) to ac0, which results in lower CDR from afforestation than keeping those higher age-classes. The model does this because the incentive for afforestation (=increase of ac0) is higher than the costs.
The problem is twofold.

a) vm_btm_cell is a free variable for some sources and pollutants. v56_emis_cell = vm_btm_cell.
Therefore, in equation q56_emission_costs_cell_oneoff the emission costs caused by shifting age-classes from ac50 to ac0 can be offset by generating some negative emissions in vm_btm_cell, within the bounds of vm_btm_reg/vm_emissions_reg.

Solution:
***fix vm_btm_cell to zero for non-CO2 emissions from land-use change
vm_btm_cell.fx(j,emis_source_cell,pollutants)$(not sameas(pollutants,"co2_c")) = 0;
***fix vm_btm_cell to zero for CO2 emissions from ag. production (non land-use change)
vm_btm_cell.fx(j,emis_source_reg,"co2_c") = 0;
***fix vm_btm_cell to zero for CO2 emissions from beccs (not used)
vm_btm_cell.fx(j,"beccs",pollutants) = 0;

b) If there is regrowth in a cell, the model can maintain the carbon stock with less area. Therefore, the model can move area from ac55 to ac0 without any costs for vegc. There are costs for litc. But theses costs are lower than the incentive for planting new trees.
Cutting down tree which still sequester carbon to replant immediatly for the purpose of carbon sequestration is something which would never happen in the real world.

*ac0 can only increase if total afforested land increases.
Solution:
q32_aff_ac0(j2) ..
v32_land(j2,"aff","ac0") =l= sum(ac, v32_land(j2,"aff",ac)) - sum((ct,ac), p32_land(ct,j2,"aff",ac));