diff --git a/CHANGELOG.md b/CHANGELOG.md
index 16c19d589..56a0ea89d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,13 +15,14 @@ Keep it human-readable, your future self will thank you!
 - Fix metadata serialization handling of numpy.integer (#140)
 - Fix negative variance for constant variables (#148)
 - Fix cutout slicing of grid dimension (#145)
-- update acumulation (#158)
+- Use cKDTree instead of KDTree
+- Implement 'complement' feature
 - Add ability to patch xarrays (#160)
 
 ### Added
 
 - Call filters from anemoi-transform
-- make test optional when adls is not installed Pull request #110
+- Make test optional when adls is not installed Pull request #110
 - Add wz_to_w, orog_to_z, and sum filters (#149)
 
 ## [0.5.8](https://github.com/ecmwf/anemoi-datasets/compare/0.5.7...0.5.8) - 2024-10-26
diff --git a/docs/using/code/complement1_.py b/docs/using/code/complement1_.py
new file mode 100644
index 000000000..ad77234d4
--- /dev/null
+++ b/docs/using/code/complement1_.py
@@ -0,0 +1,6 @@
+open_dataset(
+    complement=dataset1,
+    source=dataset2,
+    what="variables",
+    interpolate="nearest",
+)
diff --git a/docs/using/code/complement2_.py b/docs/using/code/complement2_.py
new file mode 100644
index 000000000..22ad78044
--- /dev/null
+++ b/docs/using/code/complement2_.py
@@ -0,0 +1,12 @@
+open_dataset(
+    cutout=[
+        {
+            "complement": lam_dataset,
+            "source": global_dataset,
+            "interpolate": "nearest",
+        },
+        {
+            "dataset": global_dataset,
+        },
+    ]
+)
diff --git a/docs/using/code/complement3_.py b/docs/using/code/complement3_.py
new file mode 100644
index 000000000..e20b30a39
--- /dev/null
+++ b/docs/using/code/complement3_.py
@@ -0,0 +1,4 @@
+open_dataset(
+    complement=dataset1,
+    source=dataset2,
+)
diff --git a/docs/using/combining.rst b/docs/using/combining.rst
index 86f1082fa..1d6acd1d2 100644
--- a/docs/using/combining.rst
+++ b/docs/using/combining.rst
@@ -182,3 +182,32 @@ The difference can be seen at the boundary between the two grids:
 To debug the combination, you can pass `plot=True` to the `cutout`
 function (when running from a Notebook), of use `plot="prefix"` to save
 the plots to series of PNG files in the current directory.
+
+.. _complement:
+
+************
+ complement
+************
+
+That feature will interpolate the variables of `dataset2` that are not
+in `dataset1` to the grid of `dataset1` , add them to the list of
+variable of `dataset1` and return the result.
+
+.. literalinclude:: code/complement1_.py
+
+Currently ``what`` can only be ``variables`` and can be omitted.
+
+The value for ``interpolate`` can be one of ``none`` (default) or
+``nearest``. In the case of ``none``, the grids of the two datasets must
+match.
+
+This feature was originally designed to be used in conjunction with
+``cutout``, where `dataset1` is the lam, and `dataset2` is the global
+dataset.
+
+.. literalinclude:: code/complement2_.py
+
+Another use case is to simply bring all non-overlapping variables of a
+dataset into an other:
+
+.. literalinclude:: code/complement3_.py
diff --git a/src/anemoi/datasets/create/functions/sources/xarray/field.py b/src/anemoi/datasets/create/functions/sources/xarray/field.py
index 257e2932b..3f4c2a5e7 100644
--- a/src/anemoi/datasets/create/functions/sources/xarray/field.py
+++ b/src/anemoi/datasets/create/functions/sources/xarray/field.py
@@ -120,6 +120,6 @@ def forecast_reference_time(self):
     def __repr__(self):
         return repr(self._metadata)
 
-    def _values(self):
+    def _values(self, dtype=None):
         # we don't use .values as this will download the data
         return self.selection
diff --git a/src/anemoi/datasets/data/complement.py b/src/anemoi/datasets/data/complement.py
new file mode 100644
index 000000000..ee324dd07
--- /dev/null
+++ b/src/anemoi/datasets/data/complement.py
@@ -0,0 +1,164 @@
+# (C) Copyright 2024 Anemoi contributors.
+#
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation
+# nor does it submit to any jurisdiction.
+
+
+import logging
+from functools import cached_property
+
+from ..grids import nearest_grid_points
+from .debug import Node
+from .forwards import Combined
+from .indexing import apply_index_to_slices_changes
+from .indexing import index_to_slices
+from .indexing import update_tuple
+from .misc import _auto_adjust
+from .misc import _open
+
+LOG = logging.getLogger(__name__)
+
+
+class Complement(Combined):
+
+    def __init__(self, target, source, what="variables", interpolation="nearest"):
+        super().__init__([target, source])
+
+        # We had the variables of dataset[1] to dataset[0]
+        # interpoated on the grid of dataset[0]
+
+        self.target = target
+        self.source = source
+
+        self._variables = []
+
+        # Keep the same order as the original dataset
+        for v in self.source.variables:
+            if v not in self.target.variables:
+                self._variables.append(v)
+
+        if not self._variables:
+            raise ValueError("Augment: no missing variables")
+
+    @property
+    def variables(self):
+        return self._variables
+
+    @property
+    def name_to_index(self):
+        return {v: i for i, v in enumerate(self.variables)}
+
+    @property
+    def shape(self):
+        shape = self.target.shape
+        return (shape[0], len(self._variables)) + shape[2:]
+
+    @property
+    def variables_metadata(self):
+        return {k: v for k, v in self.source.variables_metadata.items() if k in self._variables}
+
+    def check_same_variables(self, d1, d2):
+        pass
+
+    @cached_property
+    def missing(self):
+        missing = self.source.missing.copy()
+        missing = missing | self.target.missing
+        return set(missing)
+
+    def tree(self):
+        """Generates a hierarchical tree structure for the `Cutout` instance and
+        its associated datasets.
+
+        Returns:
+            Node: A `Node` object representing the `Cutout` instance as the root
+            node, with each dataset in `self.datasets` represented as a child
+            node.
+        """
+        return Node(self, [d.tree() for d in (self.target, self.source)])
+
+    def __getitem__(self, index):
+        if isinstance(index, (int, slice)):
+            index = (index, slice(None), slice(None), slice(None))
+        return self._get_tuple(index)
+
+
+class ComplementNone(Complement):
+
+    def __init__(self, target, source):
+        super().__init__(target, source)
+
+    def _get_tuple(self, index):
+        index, changes = index_to_slices(index, self.shape)
+        result = self.source[index]
+        return apply_index_to_slices_changes(result, changes)
+
+
+class ComplementNearest(Complement):
+
+    def __init__(self, target, source):
+        super().__init__(target, source)
+
+        self._nearest_grid_points = nearest_grid_points(
+            self.source.latitudes,
+            self.source.longitudes,
+            self.target.latitudes,
+            self.target.longitudes,
+        )
+
+    def check_compatibility(self, d1, d2):
+        pass
+
+    def _get_tuple(self, index):
+        variable_index = 1
+        index, changes = index_to_slices(index, self.shape)
+        index, previous = update_tuple(index, variable_index, slice(None))
+        source_index = [self.source.name_to_index[x] for x in self.variables[previous]]
+        source_data = self.source[index[0], source_index, index[2], ...]
+        target_data = source_data[..., self._nearest_grid_points]
+
+        result = target_data[..., index[3]]
+
+        return apply_index_to_slices_changes(result, changes)
+
+
+def complement_factory(args, kwargs):
+    from .select import Select
+
+    assert len(args) == 0, args
+
+    source = kwargs.pop("source")
+    target = kwargs.pop("complement")
+    what = kwargs.pop("what", "variables")
+    interpolation = kwargs.pop("interpolation", "none")
+
+    if what != "variables":
+        raise NotImplementedError(f"Complement what={what} not implemented")
+
+    if interpolation not in ("none", "nearest"):
+        raise NotImplementedError(f"Complement method={interpolation} not implemented")
+
+    source = _open(source)
+    target = _open(target)
+    # `select` is the same as `variables`
+    (source, target), kwargs = _auto_adjust([source, target], kwargs, exclude=["select"])
+
+    Class = {
+        None: ComplementNone,
+        "none": ComplementNone,
+        "nearest": ComplementNearest,
+    }[interpolation]
+
+    complement = Class(target=target, source=source)._subset(**kwargs)
+
+    # Will join the datasets along the variables axis
+    reorder = source.variables
+    complemented = _open([target, complement])
+    ordered = (
+        Select(complemented, complemented._reorder_to_columns(reorder), {"reoder": reorder})._subset(**kwargs).mutate()
+    )
+    return ordered
diff --git a/src/anemoi/datasets/data/join.py b/src/anemoi/datasets/data/join.py
index 6b7de3e64..30d1f1379 100644
--- a/src/anemoi/datasets/data/join.py
+++ b/src/anemoi/datasets/data/join.py
@@ -118,6 +118,7 @@ def variables(self):
     def variables_metadata(self):
         result = {}
         variables = [v for v in self.variables if not (v.startswith("(") and v.endswith(")"))]
+
         for d in self.datasets:
             md = d.variables_metadata
             for v in variables:
diff --git a/src/anemoi/datasets/data/misc.py b/src/anemoi/datasets/data/misc.py
index aad751f07..2d2493a3a 100644
--- a/src/anemoi/datasets/data/misc.py
+++ b/src/anemoi/datasets/data/misc.py
@@ -194,7 +194,7 @@ def _open(a):
     raise NotImplementedError(f"Unsupported argument: {type(a)}")
 
 
-def _auto_adjust(datasets, kwargs):
+def _auto_adjust(datasets, kwargs, exclude=None):
 
     if "adjust" not in kwargs:
         return datasets, kwargs
@@ -214,6 +214,9 @@ def _auto_adjust(datasets, kwargs):
     for a in adjust_list:
         adjust_set.update(ALIASES.get(a, [a]))
 
+    if exclude is not None:
+        adjust_set -= set(exclude)
+
     extra = set(adjust_set) - set(ALIASES["all"])
     if extra:
         raise ValueError(f"Invalid adjust keys: {extra}")
@@ -335,6 +338,12 @@ def _open_dataset(*args, **kwargs):
         assert not sets, sets
         return cutout_factory(args, kwargs).mutate()
 
+    if "complement" in kwargs:
+        from .complement import complement_factory
+
+        assert not sets, sets
+        return complement_factory(args, kwargs).mutate()
+
     for name in ("datasets", "dataset"):
         if name in kwargs:
             datasets = kwargs.pop(name)
diff --git a/src/anemoi/datasets/grids.py b/src/anemoi/datasets/grids.py
index 9e3a10b6d..e5d58f0ce 100644
--- a/src/anemoi/datasets/grids.py
+++ b/src/anemoi/datasets/grids.py
@@ -152,7 +152,7 @@ def cutout_mask(
     plot=None,
 ):
     """Return a mask for the points in [global_lats, global_lons] that are inside of [lats, lons]"""
-    from scipy.spatial import KDTree
+    from scipy.spatial import cKDTree
 
     # TODO: transform min_distance from lat/lon to xyz
 
@@ -195,13 +195,13 @@ def cutout_mask(
         min_distance = min_distance_km / 6371.0
     else:
         points = {"lam": lam_points, "global": global_points, None: global_points}[min_distance_km]
-        distances, _ = KDTree(points).query(points, k=2)
+        distances, _ = cKDTree(points).query(points, k=2)
         min_distance = np.min(distances[:, 1])
 
         LOG.info(f"cutout_mask using min_distance = {min_distance * 6371.0} km")
 
-    # Use a KDTree to find the nearest points
-    distances, indices = KDTree(lam_points).query(global_points, k=neighbours)
+    # Use a cKDTree to find the nearest points
+    distances, indices = cKDTree(lam_points).query(global_points, k=neighbours)
 
     # Centre of the Earth
     zero = np.array([0.0, 0.0, 0.0])
@@ -255,7 +255,7 @@ def thinning_mask(
     cropping_distance=2.0,
 ):
     """Return the list of points in [lats, lons] closest to [global_lats, global_lons]"""
-    from scipy.spatial import KDTree
+    from scipy.spatial import cKDTree
 
     assert global_lats.ndim == 1
     assert global_lons.ndim == 1
@@ -291,20 +291,20 @@ def thinning_mask(
     xyx = latlon_to_xyz(lats, lons)
     points = np.array(xyx).transpose()
 
-    # Use a KDTree to find the nearest points
-    _, indices = KDTree(points).query(global_points, k=1)
+    # Use a cKDTree to find the nearest points
+    _, indices = cKDTree(points).query(global_points, k=1)
 
     return np.array([i for i in indices])
 
 
 def outline(lats, lons, neighbours=5):
-    from scipy.spatial import KDTree
+    from scipy.spatial import cKDTree
 
     xyx = latlon_to_xyz(lats, lons)
     grid_points = np.array(xyx).transpose()
 
-    # Use a KDTree to find the nearest points
-    _, indices = KDTree(grid_points).query(grid_points, k=neighbours)
+    # Use a cKDTree to find the nearest points
+    _, indices = cKDTree(grid_points).query(grid_points, k=neighbours)
 
     # Centre of the Earth
     zero = np.array([0.0, 0.0, 0.0])
@@ -379,6 +379,19 @@ def serialise_mask(mask):
     return result
 
 
+def nearest_grid_points(source_latitudes, source_longitudes, target_latitudes, target_longitudes):
+    from scipy.spatial import cKDTree
+
+    source_xyz = latlon_to_xyz(source_latitudes, source_longitudes)
+    source_points = np.array(source_xyz).transpose()
+
+    target_xyz = latlon_to_xyz(target_latitudes, target_longitudes)
+    target_points = np.array(target_xyz).transpose()
+
+    _, indices = cKDTree(source_points).query(target_points, k=1)
+    return indices
+
+
 if __name__ == "__main__":
     global_lats, global_lons = np.meshgrid(
         np.linspace(90, -90, 90),