At the start of the 21st century, the United States was widely considered to be the world's sole superpower. This is beginning to change, with the rapid rise of China.
The chosen metrics are,
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Nominal GDP per the International Monetary Fund (or another reliable organization, chosen by Metaculus administrators).
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Military expenditures per the Stockholm International Peace Research Institute (or another reliable organization, chosen by Metaculus administrators).
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The Nature Index. If this index is discontinued, then International Science Ranking takes its place. If both are discontinued, then this question resolves ambiguously.
This question resolves January 1st on the year during which each published report listed demonstrates that the United States is behind China.