China sold $18 billion of U.S. Treasury debt in December

China dumped billions of America’s debt in December.

The largest owner of U.S. debt, China sold $18 billion of U.S. Treasury debt in December.

And it’s not alone. Japan sold even more: $22 billion. In the past year, Mexico, Turkey and Belgium have also lowered their holdings of U.S. debt, all of which have led to a record annual dump by central banks.

Many countries are suffering from the global economic slowdown, forcing central banks to pull out all the stops to help buttress their economies.

Central banks in Japan and Sweden have resorted to negative interest rates to spur banks to lend more; the European Central Bank is buying bonds issued by its member countries; the People’s Bank of China is injecting cash into its financial system.

For many central banks, selling U.S. Treasuries gives them the cash to prop up their collapsing currencies.

“These interventions are trying to add some air to the parachute,” says Win Thin, head of emerging market currency strategy at Brown Brothers Harriman.

In total, central banks sold off a net $225 billion in U.S. Treasury debt last year, the most since at least 1978, the last year of available data. In 2014, there was a net increase of $45 billion, according to CNNMoney’s analysis of Treasury data published Tuesday.

Foreign governments sold more U.S. Treasuries than they bought in 11 out of 12 months last year, according to Treasury data.

The U.S. debt dump is a sign of two things:

1. How aggressive central banks are acting to keep their economy afloat amid global weakness.

2. After years of building up savings — the so-called “global savings glut” — countries are starting to sell off their reserves.

Currency collapse: central banks to the rescue?

A weak currency often reflects a slowing or shrinking economy. For instance, Russia and Brazil are both in recession and their currencies have collapsed in value.

Both these countries along with many developing countries depend on commodities like oil, metals and food, to power their growth. Commodity prices — especially oil prices — tanked last year and have continued to do so in 2016.