diff --git a/src/erfadatextra.c b/src/erfadatextra.c
index f70400c..c58f7ed 100644
--- a/src/erfadatextra.c
+++ b/src/erfadatextra.c
@@ -17,9 +17,10 @@
 */
 #include "erfa.h"
 #include "erfaextra.h"
+#include "erfadatextra.h"
 
-static eraLEAPSECOND *changes;
-static int NDAT = -1;
+static ERFA_THREAD_LOCAL eraLEAPSECOND *changes;
+static ERFA_THREAD_LOCAL int NDAT = -1;
 
 
 int eraGetLeapSeconds(eraLEAPSECOND **leapseconds)
diff --git a/src/erfadatextra.h b/src/erfadatextra.h
index afea18b..7bf4626 100644
--- a/src/erfadatextra.h
+++ b/src/erfadatextra.h
@@ -23,4 +23,30 @@
 int eraDatini(const eraLEAPSECOND *builtin, int n_builtin,
               eraLEAPSECOND **leapseconds);
 
+/*
+** For thread safety we want a way to define thread-local variables.
+** If no TLS is available, fall back to ordinary storage and disable
+** thread safety for those variables. This mostly follows the numpy
+** specification (as of 8203da6e4cdad722664fa740c7a9514104483f07)
+**
+** The order is:
+**   C++ >= 11 and GCC C >= 23 uses thread_local
+**   GCC: 11 <= C < 23 uses _Thread_local
+**   Clang uses __thread
+**   MSVC uses _declspec(thread)
+*/
+#if defined(__cplusplus) && __cplusplus >= 201103L
+#  define ERFA_THREAD_LOCAL thread_local
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#  define ERFA_THREAD_LOCAL thread_local
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#  define ERFA_THREAD_LOCAL _Thread_local
+#elif defined(__GNUC__) || defined(__clang__)
+#  define ERFA_THREAD_LOCAL __thread
+#elif defined(_MSC_VER)
+#  define ERFA_THREAD_LOCAL __declspec(thread)
+#else
+#  define ERFA_THREAD_LOCAL
+#endif
+
 #endif