SSP GHG Atmgas Generator

Overview

Create scenario-specific atmospheric concentration forcing for EcoSIM by splicing historical monthly CO2, CH4, and N2O data with RCMIP annual SSP concentration pathways. Prefer the bundled script for reproducibility and unit-safe NetCDF writing.

Data Sources

• Historical input: an EcoSIM-style NetCDF with time, fractional year, and CO2, CH4, N2O variables.
• Future pathway input: RCMIP concentrations annual means CSV, usually rcmip-concentrations-annual-means-v5-1-0.csv.
• Default RCMIP URL:

https://rcmip-protocols-au.s3-ap-southeast-2.amazonaws.com/v5.1.0/rcmip-concentrations-annual-means-v5-1-0.csv

Use the World region and variables:

• Atmospheric Concentrations|CO2 in ppm; write EcoSIM units as ppmv.
• Atmospheric Concentrations|CH4 in ppb; write EcoSIM units as ppbv.
• Atmospheric Concentrations|N2O in ppb; write EcoSIM units as ppbv.

RCMIP scenario model labels are scenario-specific. Preserve them in metadata instead of assuming a single IAM model across all SSPs.

Quick Start

Run from the repository root:

.venv-cmip6/bin/python .agents/skills/ssp-ghg-atmgas-generator/scripts/build_ssp_atmgas.py \
  --historical-netcdf /path/to/fatm_hist_GHGs_1750-2025.nc \
  --rcmip-csv /path/to/rcmip-concentrations-annual-means-v5-1-0.csv \
  --output-dir result/ssp_gas_concentrations \
  --scenarios ssp245 ssp585 \
  --end-year 2100

If the RCMIP CSV is not local, either let the script download it with --download-rcmip or download it first with an approved network command and pass --rcmip-csv.

Workflow

1. Inspect the historical NetCDF before writing outputs:
   • Confirm variables CO2, CH4, N2O, and year exist.
   • Confirm historical gas units are ppmv/ppbv-compatible.
   • Confirm the fractional-year convention. EcoSIM atmgas files usually encode month as calendar_year + month / 12, so December 2022 is 2023.0.
2. Extract RCMIP annual means for requested scenarios and gases from Region == "World".
3. Keep historical monthly values through the last complete historical calendar year unless the user asks for a different splice year.
4. Expand SSP annual values to monthly future values. The default script policy assigns each month of a calendar year that year's annual RCMIP mean; do not invent a future seasonal cycle unless explicitly requested.
5. Write one NetCDF per scenario with EcoSIM variable names and units:
   • year(time) as fractional AD year.
   • CO2(time) in ppmv.
   • CH4(time) and N2O(time) in ppbv.
6. Write compact CSV summaries with key years and annual scenario values for auditability.

Output Rules

• Put outputs under result/ unless the user provides another location.
• Name scenario files clearly, for example fatm_GHGs_ssp245_1750-2100.nc.
• Preserve enough metadata to reproduce the splice: historical source path, RCMIP source, scenario, scenario model label, splice calendar year, monthly expansion policy, and creation timestamp.
• Treat ppm as ppmv and ppb as ppbv for EcoSIM atmgas forcing, and note that this is a mixing-ratio naming convention.
• For a requested end calendar year of 2100, the final fractional year coordinate will be 2101.0 because it represents December 2100 in the existing EcoSIM convention.

Validation Checklist

• Print or inspect each output file's dimensions, first/last year, variable units, and 2100 CO2/CH4/N2O values.
• Check continuity at the splice: the first future month should use the selected scenario annual value for that calendar year.
• Ensure N2O is present in both historical and scenario outputs; do not deliver CO2/CH4-only products when the request mentions greenhouse gases or atmgas.
• Report source and unit assumptions in the final response.

Bundled Script

Use scripts/build_ssp_atmgas.py for the full workflow. It can read a local RCMIP CSV or download one, create NetCDF outputs, and write summary CSV files.