diff --git a/autonerves/site/ral.yaml b/autonerves/site/ral.yaml
index 200e493..75c6903 100644
--- a/autonerves/site/ral.yaml
+++ b/autonerves/site/ral.yaml
@@ -28,5 +28,15 @@ partitions:
     note: 8 A100s on two nodes (4 per node, unverified).
 pue: {low: 1.14, central: 1.31, high: 1.45, unverified: true}  # R89 measured 1.28-1.45 (mean 1.31); new centre ~1.14; hosting building unverified.
 grid_region: GB
-ci_basis: {value: location_annual_average, unverified: true}
+ci_basis: {value: regional, unverified: true}   # Ruling F6 (2026-10-28): regional central, national and DESNZ as bounds.
+# Grid carbon intensity, gCO2e per kWh (hpc-campaign report §1.4, survey carbon.md §4):
+# - regional: NESO Carbon Intensity API, region 12 "South England" (Harwell OX11), 2025 forecast series;
+#   central = annual mean, low/high = half-hourly p5/p95.
+# - national: NESO GB national 2025 actual; central = mean, low/high = half-hourly p5/p95.
+# - desnz: DESNZ 2025 UK company-reporting factor (0.177 kg/kWh; lags the grid by ~2 years).
+# Unverified until a human confirms the hosting building's region and the year to use.
+carbon_intensity:
+  regional: {low: 81, central: 188, high: 308, unverified: true}
+  national: {low: 51, central: 129, high: 231, unverified: true}
+  desnz: {value: 177, unverified: true}
 energy_accounting: {value: false, unverified: true}  # Slurm AcctGatherEnergyType not confirmed.
diff --git a/autonerves/site_config.py b/autonerves/site_config.py
index dd8c79a..78e4d98 100644
--- a/autonerves/site_config.py
+++ b/autonerves/site_config.py
@@ -23,7 +23,11 @@
         gpu_tdp_w: {low: 250, high: 400, unverified: true}
     pue: {low: 1.14, central: 1.31, high: 1.45, unverified: true}
     grid_region: GB
-    ci_basis: location_annual_average
+    ci_basis: regional
+    carbon_intensity:             # gCO2e per kWh, one entry per basis
+      regional: {low: 81, central: 188, high: 308, unverified: true}
+      national: {low: 51, central: 129, high: 231, unverified: true}
+      desnz: {value: 177, unverified: true}
     energy_accounting: false
 
 Any value may be written plainly (``gpu_only: true``), as ``{value: ...}`` or as a
@@ -32,6 +36,13 @@
 used as a range and reported as unverified, never silently trusted. ``pue`` is always
 a range.
 
+``carbon_intensity`` is optional: the grid's carbon intensity in gCO2e/kWh under one or
+more named bases (e.g. ``regional``, ``national``, ``desnz``), each a value or a range.
+When it is given, ``ci_basis`` names the basis an estimate uses centrally and must be
+one of its keys; the other bases show how far the choice of basis moves the answer. A
+site without the table gives no carbon figure (energy only); nothing falls back to a
+world-average default.
+
 A partition with ``gpu_only: true`` takes GPU jobs only: a planner must never place a
 CPU-only job there (``Site.cpu_partitions`` leaves it out, ``Site.check_cpu_job``
 raises).
@@ -68,6 +79,7 @@
     "pue": "range",
     "grid_region": "str",
     "ci_basis": "str",
+    "carbon_intensity": "carbon_intensity",
     "energy_accounting": "bool",
     "note": "str",
 }
@@ -191,6 +203,10 @@ def __init__(self, raw: Dict, source: Optional[Path] = None):
             for key in ("pue", "grid_region", "ci_basis", "energy_accounting")
             if key in raw
         }
+        self.carbon_intensity = {
+            basis: SiteValue.from_yaml(value)
+            for basis, value in (raw.get("carbon_intensity") or {}).items()
+        }
 
     def partition(self, name: str) -> Partition:
         try:
@@ -236,6 +252,14 @@ def ci_basis(self):
         value = self.values.get("ci_basis")
         return None if value is None else value.central
 
+    def ci(self, basis: Optional[str] = None) -> Optional[SiteValue]:
+        """
+        The grid carbon intensity (gCO2e/kWh) under ``basis`` (default ``ci_basis``),
+        or ``None`` when the site gives no ``carbon_intensity`` for it.
+        """
+        basis = basis or self.ci_basis
+        return self.carbon_intensity.get(basis) if basis else None
+
     @property
     def energy_accounting(self) -> bool:
         value = self.values.get("energy_accounting")
@@ -246,6 +270,11 @@ def unverified(self) -> List[str]:
         The dotted names of every value marked ``unverified``.
         """
         names = [key for key, value in self.values.items() if value.unverified]
+        names += [
+            f"carbon_intensity.{basis}"
+            for basis, value in self.carbon_intensity.items()
+            if value.unverified
+        ]
         for partition in self.partitions.values():
             names += [
                 f"partitions.{partition.name}.{key}"
@@ -350,6 +379,32 @@ def validate(raw) -> List[str]:
         pue = raw["pue"] if isinstance(raw["pue"], dict) else {}
         if isinstance(pue.get("low"), (int, float)) and pue["low"] < 1:
             errors.append("pue: a PUE is at least 1")
+    if "carbon_intensity" in raw:
+        table = raw["carbon_intensity"]
+        if not isinstance(table, dict) or not table:
+            errors.append("carbon_intensity must map at least one basis to a value")
+        else:
+            for basis, value in table.items():
+                where = f"carbon_intensity.{basis}"
+                _check_value(where, "number", value, errors)
+                values = value if isinstance(value, dict) else {"value": value}
+                for key in ("value", "low", "high", "central"):
+                    number = values.get(key)
+                    if isinstance(number, (int, float)) and number < 0:
+                        errors.append(f"{where}.{key} cannot be negative")
+            basis = SiteValue.from_yaml(raw.get("ci_basis")).central
+            if basis is None:
+                errors.append("ci_basis: name the carbon_intensity basis to use")
+            elif basis not in table:
+                errors.append(
+                    f"ci_basis {basis!r} is not a carbon_intensity basis "
+                    f"({sorted(table)})"
+                )
+            elif SiteValue.from_yaml(table[basis]).central is None:
+                errors.append(
+                    f"carbon_intensity.{basis}: the ci_basis basis needs a value "
+                    f"(or a low/high range); it is empty"
+                )
 
     return errors
 
diff --git a/test_autonerves/test_site_config.py b/test_autonerves/test_site_config.py
index d98d802..546c061 100644
--- a/test_autonerves/test_site_config.py
+++ b/test_autonerves/test_site_config.py
@@ -146,3 +146,69 @@ def test_nerves_imports_no_autofit():
         "assert not [m for m in sys.modules if m.split('.')[0] == 'autofit'], 'autofit'"
     )
     subprocess.run([sys.executable, "-c", code], check=True, cwd=Path(conf.__file__).parents[1])
+
+
+def test_ral_carbon_intensity_bases(config):
+    site = config.push_site("ral")
+
+    assert site.ci_basis == "regional"
+    assert site.ci().central == 188
+    assert site.ci().range == (81, 308)
+    assert site.ci("national").central == 129
+    assert site.ci("desnz").central == 177
+    assert site.ci("absent") is None
+    assert {"carbon_intensity.regional", "carbon_intensity.desnz"} <= set(
+        site.unverified()
+    )
+
+
+def test_carbon_intensity_is_optional():
+    assert validate(VALID) == []
+    site = load_site_from(VALID)
+    assert site.carbon_intensity == {}
+    assert site.ci() is None
+
+
+def test_carbon_intensity_validation():
+    def errors(table, basis="regional"):
+        return validate({**VALID, "ci_basis": basis, "carbon_intensity": table})
+
+    assert errors({"regional": {"low": 80, "central": 190, "high": 310}}) == []
+    assert errors({"regional": 190, "national": {"value": 130}}) == []
+    assert any("not a carbon_intensity basis" in e for e in errors({"national": 130}))
+    assert any("negative" in e for e in errors({"regional": -5}))
+    assert any("must be a number" in e for e in errors({"regional": "high"}))
+    assert any("low 300 is above high" in e for e in errors({"regional": {"low": 300, "high": 100}}))
+    assert any("at least one basis" in e for e in errors({}))
+    assert any(
+        "name the carbon_intensity basis" in e
+        for e in validate({k: v for k, v in VALID.items() if k != "ci_basis"} | {"carbon_intensity": {"x": 1}})
+    )
+
+
+def load_site_from(raw):
+    from autonerves.site_config import Site
+
+    return Site(dict(raw))
+
+
+def test_ci_basis_basis_must_have_a_value():
+    errors = validate(
+        {
+            **VALID,
+            "ci_basis": "regional",
+            "carbon_intensity": {"regional": {"value": None, "unverified": True}, "national": 129},
+        }
+    )
+    assert any("carbon_intensity.regional: the ci_basis basis needs a value" in e for e in errors)
+    # Another basis may be empty: only the one used centrally must have a value.
+    assert (
+        validate(
+            {
+                **VALID,
+                "ci_basis": "national",
+                "carbon_intensity": {"regional": {"value": None, "unverified": True}, "national": 129},
+            }
+        )
+        == []
+    )