Schwarzenegger is a dual Austrian/United States citizen. He holds Austrian citizenship by birth and has held U.S. citizenship since becoming naturalized in 1983. Being Austrian and thus European, he was able to win the 2007 European Voice campaigner of the year award for taking action against climate change with the California Global Warming Solutions Act of 2006 and plans to introduce an emissions trading scheme with other US states and possibly with the EU.

What year did Schwarzenegger become a naturalized citizen of the United States?