Just in case the previous post does not seem to help, some more information:

I have myself modelled scenarios with negative emissions for many years in TIMES, and I have not seen any problems with that, as long as I define the total net CO2 and GHG variables (VAR_COMNET) free, i.e. with negative bounds. However, if you are also referring to the VAR_COMPRD variables of the emissions in your model, then you may need to define those variables free as well (or having negative bounds).

Defining cumulative constraints on the emissions should not make any difference in that respect. Even if you use UC_CUMCOM, which creates new variables for cumulative amounts, TIMES automatically sets the lower bound of the cumulative variables to negative infinity, whenever the corresponding period-wise variables have been set with negative lower bounds. But if the problem persists, please provide some more info about the way you are defining the constraints and bounds.

I have myself modelled scenarios with negative emissions for many years in TIMES, and I have not seen any problems with that, as long as I define the total net CO2 and GHG variables (VAR_COMNET) free, i.e. with negative bounds. However, if you are also referring to the VAR_COMPRD variables of the emissions in your model, then you may need to define those variables free as well (or having negative bounds).

Defining cumulative constraints on the emissions should not make any difference in that respect. Even if you use UC_CUMCOM, which creates new variables for cumulative amounts, TIMES automatically sets the lower bound of the cumulative variables to negative infinity, whenever the corresponding period-wise variables have been set with negative lower bounds. But if the problem persists, please provide some more info about the way you are defining the constraints and bounds.