Step change in PCT_CROP between 2023 and 2024 in ctsm5.3 SSP landuse.timeseries

[xueyao]

Hi all,

I'm running CTSM transient SSP simulations and noticed a step change in global cropland area between 2023 and 2024 in the landuse.timeseries files. I'd like to know whether it is expected.

What version of the code are you using?
ctsm5.3

Have you made any changes to files in the source tree?
no

Files used (1.9x2.5):

• SSP2-4.5: /glade/campaign/cesm/cesmdata/inputdata/lnd/clm2/surfdata_esmf/ctsm5.3.0/landuse.timeseries_1.9x2.5_SSP2-4.5_1850-2100_78pfts_c240908.nc
• SSP1-2.6, SSP3-7.0, SSP5-8.5: generated with ctsm5.3.080/tools/mksurfdata_esmf, e.g.
  ./gen_mksurfdata_namelist --res 1.9x2.5 --ssp-rcp SSP5-8.5 --start-year 1850 --end-year 2100 --nosurfdata --namelist ssp585.namelist

What I see:
Global cropland area, computed as AREA × LANDFRAC_PFT × PCT_CROP/100, jumps between 2023 and 2024 in SSP126/245/370/585 scenarios (figure attached):

In every scenario, this is the largest year-to-year change after 2015. The original LUH2 trajectories (Hurtt et al., 2020, GMD, Fig. 4a) show no such discontinuity (figure for your reference):


What I think is happening:The file metadata shows Vegetation_type_raw_data_filename = mksrf_landuse_ctsm53_histTRENDY2024_1850.c240728.nc. In rawdata/CTSM53RawData/:

• globalctsm53histTRENDY2024Deg025_240728/ covers 1700–2023
• globalctsm53TRSSP{119,126,245,370,460,585}Deg025_240728/ cover 2015–2100

So the landuse.timeseries seems to use TRENDY2024 historical data through 2023 and the SSP raw data from 2024 onward. If the SSP raw data are still harmonized to the original LUH2 v2h at 2015, rather than to TRENDY2024, that would explain the step.

Questions:

1. Is this discontinuity known or expected?
2. Were the TRSSP* raw files harmonized to TRENDY2024 at any point? If not, is a harmonized version planned (e.g., for CTSM5.4 / CMIP7)?
3. For SSP studies, would you recommend the CMIP6 files (c190214, LUH2 v2h + SSP throughout) instead, or some other way to handle the transition?

Thanks for any guidance!

 

[slevis]

1. Your explanation is correct, so the discontinuity is expected.
2. No new harmonization was done.
3. The next harmonization can be expected with CMIP7.