diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_Bio_interpolate.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_Bio_interpolate.py
index bd3a213..2609eda 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_Bio_interpolate.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_Bio_interpolate.py
@@ -8,16 +8,13 @@
     load_cmip7_aerosol_biomass_list,
     save_cmip7_aerosol_biomass,
 )
-from aerosol.cmip7_HI_aerosol import (
-    CMIP7_HI_AEROSOL_BEG_YEAR,
-    CMIP7_HI_AEROSOL_END_YEAR,
-    esm_hi_aerosol_save_dirpath,
-)
+from aerosol.cmip7_HI_aerosol import esm_hi_aerosol_save_dirpath
 from cmip7_ancil_argparse import (
     common_parser,
     percent_parser,
 )
 from cmip7_ancil_common import cmip7_date_constraint_from_years
+from cmip7_HI import CMIP7_HI_BEG_YEAR, CMIP7_HI_END_YEAR
 
 
 def parse_args():
@@ -42,8 +39,8 @@ def load_cmip7_hi_aerosol_biomass(args, species):
         species,
         args.dataset_date_range_list,
         cmip7_date_constraint_from_years(
-            CMIP7_HI_AEROSOL_BEG_YEAR,
-            CMIP7_HI_AEROSOL_END_YEAR,
+            CMIP7_HI_BEG_YEAR,
+            CMIP7_HI_END_YEAR,
         ),
     )
 
@@ -54,8 +51,8 @@ def load_cmip7_hi_aerosol_biomass_percentage(args, species):
         species,
         args.percent_date_range,
         cmip7_date_constraint_from_years(
-            CMIP7_HI_AEROSOL_BEG_YEAR,
-            CMIP7_HI_AEROSOL_END_YEAR,
+            CMIP7_HI_BEG_YEAR,
+            CMIP7_HI_END_YEAR,
         ),
     )
 
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_SO2_interpolate.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_SO2_interpolate.py
index e3aea5d..05b9aed 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_SO2_interpolate.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_SO2_interpolate.py
@@ -4,8 +4,6 @@
 from ast import literal_eval
 
 from aerosol.cmip7_HI_aerosol import (
-    CMIP7_HI_AEROSOL_BEG_YEAR,
-    CMIP7_HI_AEROSOL_END_YEAR,
     esm_hi_aerosol_ancil_dirpath,
     esm_hi_aerosol_save_dirpath,
 )
@@ -18,7 +16,11 @@
     common_parser,
     dms_filename_parser,
 )
-from cmip7_HI import fix_esm15_hi_ancil_date
+from cmip7_HI import (
+    CMIP7_HI_BEG_YEAR,
+    CMIP7_HI_END_YEAR,
+    fix_esm15_hi_ancil_date,
+)
 
 
 def parse_args():
@@ -41,8 +43,8 @@ def load_cmip7_hi_so2_aerosol_anthro(args, species):
     return load_cmip7_hi_aerosol_anthro(
         args,
         species,
-        beg_year=CMIP7_HI_AEROSOL_BEG_YEAR,
-        end_year=CMIP7_HI_AEROSOL_END_YEAR,
+        beg_year=CMIP7_HI_BEG_YEAR,
+        end_year=CMIP7_HI_END_YEAR,
     )
 
 
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol.py
index 8cfb60d..7bd9c1f 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol.py
@@ -1,10 +1,6 @@
 from pathlib import Path
 
 from cmip7_ancil_constants import ANCIL_TODAY
-from cmip7_HI import CMIP7_HI_BEG_YEAR, CMIP7_HI_END_YEAR
-
-CMIP7_HI_AEROSOL_BEG_YEAR = CMIP7_HI_BEG_YEAR - 1
-CMIP7_HI_AEROSOL_END_YEAR = CMIP7_HI_END_YEAR
 
 
 def esm_hi_aerosol_ancil_dirpath(ancil_root_dirname):
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol_anthro.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol_anthro.py
index 3bccd6a..734a167 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol_anthro.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_HI_aerosol_anthro.py
@@ -5,13 +5,10 @@
     cmip7_aerosol_anthro_interpolate,
     load_cmip7_aerosol_anthro_list,
 )
-from aerosol.cmip7_HI_aerosol import (
-    CMIP7_HI_AEROSOL_BEG_YEAR,
-    CMIP7_HI_AEROSOL_END_YEAR,
-    esm_hi_aerosol_save_dirpath,
-)
+from aerosol.cmip7_HI_aerosol import esm_hi_aerosol_save_dirpath
 from cmip7_ancil_argparse import common_parser
 from cmip7_ancil_common import cmip7_date_constraint_from_years
+from cmip7_HI import CMIP7_HI_BEG_YEAR, CMIP7_HI_END_YEAR
 
 
 def parse_args(species):
@@ -30,8 +27,8 @@ def parse_args(species):
 def load_cmip7_hi_aerosol_anthro(
     args,
     species,
-    beg_year=CMIP7_HI_AEROSOL_BEG_YEAR,
-    end_year=CMIP7_HI_AEROSOL_END_YEAR,
+    beg_year=CMIP7_HI_BEG_YEAR,
+    end_year=CMIP7_HI_END_YEAR,
 ):
     return load_cmip7_aerosol_anthro_list(
         args,
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_SO2_interpolate.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_SO2_interpolate.py
index b9f594f..38ee7e8 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_SO2_interpolate.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_SO2_interpolate.py
@@ -12,6 +12,7 @@
 from cmip7_ancil_common import (
     INTERPOLATION_SCHEME,
     esm_grid_mask_cube,
+    extend_years,
     save_ancil,
 )
 
@@ -97,6 +98,12 @@ def save_cmip7_so2_aerosol_anthro(
     # Need to remove the sector coordinate before saving
     # because high doesn't have it
     so2_low.remove_coord("sector")
+
+    # Extend the historical time series, if any,
+    # by duplicating the first and last years
+    so2_low = extend_years(so2_low)
+    so2_high = extend_years(so2_high)
+
     # Use the CMIP6 DMS
     cmip6_dms = dms_load_fn(args)
 
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_anthro.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_anthro.py
index 8189bc8..4f35578 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_anthro.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_anthro.py
@@ -9,6 +9,7 @@
 from cmip7_ancil_common import (
     INTERPOLATION_SCHEME,
     esm_grid_mask_cube,
+    extend_years,
     fix_coords,
     save_ancil,
 )
@@ -74,4 +75,7 @@ def cmip7_aerosol_anthro_interpolate(
     esm_cube.attributes["STASH"] = iris.fileformats.pp.STASH(
         model=1, section=0, item=stash_item
     )
+    # Extend the historical time series, if any,
+    # by duplicating the first and last years
+    esm_cube = extend_years(esm_cube)
     save_ancil(esm_cube, save_dirpath, args.save_filename)
diff --git a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_biomass.py b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_biomass.py
index 1793a22..b35dcef 100644
--- a/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_biomass.py
+++ b/CMIP7/esm1p6/atmosphere/aerosol/cmip7_aerosol_biomass.py
@@ -11,6 +11,7 @@
 from cmip7_ancil_common import (
     INTERPOLATION_SCHEME,
     esm_grid_mask_cube,
+    extend_years,
     save_ancil,
     set_coord_system,
 )
@@ -140,6 +141,11 @@ def save_cmip7_aerosol_biomass(args, load_pc_fn, load_fn, save_dirpath):
         model=1, section=0, item=131
     )
 
+    # Extend the historical time series, if any,
+    # by duplicating the first and last years
+    low_esm = extend_years(low_esm)
+    high_esm = extend_years(high_esm)
+
     save_ancil([low_esm, high_esm], save_dirpath, args.save_filename)
 
     now = datetime.now()
diff --git a/CMIP7/esm1p6/atmosphere/cmip7_HI.py b/CMIP7/esm1p6/atmosphere/cmip7_HI.py
index 9e04159..b8d103c 100644
--- a/CMIP7/esm1p6/atmosphere/cmip7_HI.py
+++ b/CMIP7/esm1p6/atmosphere/cmip7_HI.py
@@ -1,11 +1,9 @@
 import calendar
 from pathlib import Path
 
-import iris
 import mule
 from cmip7_ancil_constants import (
     ANCIL_TODAY,
-    MONTHS_IN_A_YEAR,
     UM_VERSION,
 )
 from cmip7_PI import CMIP7_PI_YEAR
@@ -46,39 +44,3 @@ def fix_esm15_hi_ancil_date(ifile, ofile):
         field.lbdatd = calendar.monthrange(field.lbyr, field.lbmon)[1]
 
     ff.to_file(ofile)
-
-
-def extend_hi_years(cube):
-    """
-    Extend a cube representing a monthly time series by duplicating
-    and adjusting the first and last years.
-    Based on Crown copyright code from ozone_cmip6_ancillary_for_suite.py
-    by Steven Hardiman of the UK Met Office.
-    """
-    time_coord = cube.coord("time")
-    time_points = time_coord.points
-    if (months := len(time_points)) <= MONTHS_IN_A_YEAR:
-        raise ValueError(
-            f"Cannot extend a cube containing {months} months of data. "
-            f"More than {MONTHS_IN_A_YEAR} months are required."
-        )
-
-    # Duplicate the first year.
-    length_one_year = time_points[MONTHS_IN_A_YEAR] - time_points[0]
-    beg_year = cube[:MONTHS_IN_A_YEAR].copy()
-    beg_year_tc = beg_year.coord("time")
-    beg_year_tc.points = beg_year_tc.points - length_one_year
-    if time_coord.has_bounds():
-        beg_year_tc.bounds = beg_year_tc.bounds - length_one_year
-
-    # Duplicate the last year.
-    length_one_year = time_points[-1] - time_points[-1 - MONTHS_IN_A_YEAR]
-    end_year = cube[-MONTHS_IN_A_YEAR:].copy()
-    end_year_tc = end_year.coord("time")
-    end_year_tc.points = end_year_tc.points + length_one_year
-    if time_coord.has_bounds():
-        end_year_tc.bounds = end_year_tc.bounds + length_one_year
-
-    # Return a cube with extended years.
-    cubelist = iris.cube.CubeList((beg_year, cube, end_year))
-    return cubelist.concatenate_cube()
diff --git a/CMIP7/esm1p6/atmosphere/cmip7_ancil_common.py b/CMIP7/esm1p6/atmosphere/cmip7_ancil_common.py
index 2829aba..f4cf260 100644
--- a/CMIP7/esm1p6/atmosphere/cmip7_ancil_common.py
+++ b/CMIP7/esm1p6/atmosphere/cmip7_ancil_common.py
@@ -10,7 +10,10 @@
 import iris
 import mule
 import numpy as np
-from cmip7_ancil_constants import UM_VERSION
+from cmip7_ancil_constants import (
+    MONTHS_IN_A_YEAR,
+    UM_VERSION,
+)
 
 INTERPOLATION_SCHEME = iris.analysis.AreaWeighted(mdtol=0.5)
 
@@ -72,6 +75,40 @@ def set_gregorian(var):
     time.units = newunits
 
 
+def extend_years(cube):
+    """
+    Extend a cube representing a monthly time series by duplicating
+    and adjusting the first and last years.
+    Based on Crown copyright code from ozone_cmip6_ancillary_for_suite.py
+    by Steven Hardiman of the UK Met Office.
+    """
+    time_coord = cube.coord("time")
+    time_points = time_coord.points
+    # Do not extend a cube containing less than two years of data.
+    if len(time_points) < MONTHS_IN_A_YEAR * 2:
+        return cube
+
+    # Duplicate the first year.
+    length_one_year = time_points[MONTHS_IN_A_YEAR] - time_points[0]
+    beg_year = cube[:MONTHS_IN_A_YEAR].copy()
+    beg_year_tc = beg_year.coord("time")
+    beg_year_tc.points = beg_year_tc.points - length_one_year
+    if time_coord.has_bounds():
+        beg_year_tc.bounds = beg_year_tc.bounds - length_one_year
+
+    # Duplicate the last year.
+    length_one_year = time_points[-1] - time_points[-1 - MONTHS_IN_A_YEAR]
+    end_year = cube[-MONTHS_IN_A_YEAR:].copy()
+    end_year_tc = end_year.coord("time")
+    end_year_tc.points = end_year_tc.points + length_one_year
+    if time_coord.has_bounds():
+        end_year_tc.bounds = end_year_tc.bounds + length_one_year
+
+    # Return a cube with extended years.
+    cubelist = iris.cube.CubeList((beg_year, cube, end_year))
+    return cubelist.concatenate_cube()
+
+
 def set_coord_system(cube):
     coord_system = iris.coord_systems.GeogCS(6371229.0)
     cube.coord("latitude").coord_system = coord_system
diff --git a/CMIP7/esm1p6/atmosphere/nitrogen/cmip7_HI_nitrogen_generate.py b/CMIP7/esm1p6/atmosphere/nitrogen/cmip7_HI_nitrogen_generate.py
index 2cedc97..d4bcf92 100644
--- a/CMIP7/esm1p6/atmosphere/nitrogen/cmip7_HI_nitrogen_generate.py
+++ b/CMIP7/esm1p6/atmosphere/nitrogen/cmip7_HI_nitrogen_generate.py
@@ -2,8 +2,8 @@
 from pathlib import Path
 
 from cmip7_ancil_argparse import common_parser
+from cmip7_ancil_common import extend_years
 from cmip7_ancil_constants import ANCIL_TODAY
-from cmip7_HI import extend_hi_years
 from nitrogen.cmip7_nitrogen import (
     cmip7_nitrogen_dirpath,
     load_cmip7_nitrogen,
@@ -54,6 +54,6 @@ def esm_hi_nitrogen_save_dirpath(args):
     # Load the CMIP7 datasets
     nitrogen_cube = load_cmip7_nitrogen(args, cmip7_hi_nitrogen_filepath)
     # Regrid to match the ESM1.5 mask and extend the time series
-    esm_cube = extend_hi_years(regrid_cmip7_nitrogen(args, nitrogen_cube))
+    esm_cube = extend_years(regrid_cmip7_nitrogen(args, nitrogen_cube))
     # Save the ancillary
     save_cmip7_nitrogen(args, esm_cube, esm_hi_nitrogen_save_dirpath)