AAA-rated government bonds are used to back basically all futures contracts, to provide ubersafe assets to big banks and pension funds, to stabilize currency fluctuations, and to serve as totally-safe better-than-cash assets in a whole wide variety of situations. If a government loses its AAA rating, its bonds can’t be used like this anymore, because big institutional investors no longer regard them as 100% safe.
So AAA debt is a really valuable renewable resource to global financial markets. And a big orange wedge on that chart above is currently risking downgrade. If the US Treasury is downgraded, anything that’s backed with US Treasury bonds suddenly becomes a little more risky and a little less valuable, all at once. Through the magic of leveraging, that corresponds to trillions of dollars of outstanding transactions, which would stand to lose billions of dollars in value. And there’s no good substitute to use in the meantime, because there aren’t that many AAA-rated governments and many of them are small countries (like Denmark or Luxembourg) with relatively little outstanding debt.
Hopefully this explains why a potential US downgrade could cause some really serious issues for global markets.
