diff --git a/R/MSSS.R b/R/MSSS.R
index c37e66faef6e16a68d0e11fb1209ced344579dea..ca854f30f8b18b3e8db3e82a1cb6d4487d9783c5 100644
--- a/R/MSSS.R
+++ b/R/MSSS.R
@@ -135,6 +135,10 @@ MSSS <- function(exp, obs, ref = NULL, time_dim = 'sdate', dat_dim = NULL,
     }
   } else {
     ref <- 0
+    .warning("If a reference dataset is not provided (ref = NULL), the default ", 
+            "value for the climatology is 0 and MSSS results will only be ",
+            "correct if 'exp' and 'obs' are anomalies. Provide a non-null ",
+            "'ref' for full-field data.")
   }
   if (!is.array(ref)) { # 0 or 1
     ref <- array(data = ref, dim = dim(exp))
diff --git a/R/RMSSS.R b/R/RMSSS.R
index 0326bfaa2c12239f746f086e3d760732de129968..3d1de43df26095c4195694c443000281be7fc036 100644
--- a/R/RMSSS.R
+++ b/R/RMSSS.R
@@ -145,6 +145,10 @@ RMSSS <- function(exp, obs, ref = NULL, time_dim = 'sdate', dat_dim = NULL,
     }
   } else {
     ref <- 0
+    .warning("If a reference dataset is not provided (ref = NULL), the default ", 
+            "value for the climatology is 0 and RMSSS results will only be ",
+            "correct if 'exp' and 'obs' are anomalies. Provide a non-null ",
+            "'ref' for full-field data.")
   }
   if (!is.array(ref)) { # 0 or 1
     ref <- array(data = ref, dim = dim(exp))