GCAM 8.5 / Transportation Units Changes

[pralitp]

Dear GCAM Modelers,

We have just merged Core Model Proposal 422 GCAM Bugfix Fall 2025 / GCAM v8.5
While this CMP consists of mostly minor fixes, performance tweaks, and helpful features it also includes one which will affect modelers who have custom XML inputs or process results for the Transportation sector: We have changed the service price and quantity units in GCAM v8.5!

The motivation was simplifying the C++ code, avoid hardwired unit conversions (such as BTUs) and keep numbers as close to 1 as possible. Consistent with the way most of the other sectors are treated in GCAM. The natural units of fuel inputs to the Transportation sector are EJ for quantities and 1975$/GJ for prices. We have flexibility on what the output units may be. Given this we have now changed (note: “service” are “person” for Passenger and “ton” for Freight):

• Price Units change from 1990$/service-km to 1975$/service-km
• Output units change from million-service-km to billion-service-km

Naturally the processing in gcamdata has been updated in this CMP to reflect this change. In particular, change the gdp_defalator to 1975 instead of 1990 for any non-energy cost data; and for any IO-coefficients replace the conversion of CONV_MJ_BTU with CONV_ONES_THOUS (given no longer assuming BTU in the C++ and wanting billion instead of million). Users will need to make similar changes in any processing or inputs they may have that is separate from the standard L254 gcamdata chunks.

GCAM 8.5 is now available in the latest version of "master". However, given this is not deemed a "major release" it has not yet been included as part of any release package. As always, users are free to choose when exactly to merge the latest master into their research branches and may want to factor this in their decision making.

Thanks,
CMP Team