An arti­cle in last Wednesday’s NYT included a pretty remark­able sta­tis­tic. Steven R. Weis­man cited Diana Far­rell, direc­tor of the McK­in­sey Global Insti­tute, who esti­mates that petrodol­lars have held Amer­i­can inter­est rates three-​quarters of a per­cent lower than they would have been otherwise.

The macro­eco­nom­ics behind the rate supres­sion are simple—more avail­able dol­lars = more avail­able credit = lower inter­est rates—but con­sider for a moment what it means in real terms. Wednesday’s NYT quoted the 30-year fixed mort­gage rate at 5.80%, which means that a home­owner with a $300,000 mort­gage would pay $1760/month. Now let’s assume that Far­rell is right about the 0.75% sup­pres­sion. With­out the lower credit made pos­si­ble by petrodol­lars, the same home­owner would be look­ing at a 6.55% inter­est rate, which trans­lates into a $1906 monthly pay­ment. The dif­fer­ence, then, turns out to be $146 per month, or $1,752 per year. Which means that the mort­gage of our fic­tional Amer­i­can home­owner is being effec­tively sub­si­dized by oil money to the tune of 8.3% over­all. And that’s not saying any­thing about all of the other places the interest-​rate sup­pres­sion affects our lives (credit cards, car loans, etc.).

The trou­bling thing about this oil subsidy—for Amer­i­cans, anyway—is not that it’s hap­pen­ing but that it could dis­ap­pear some­day very soon. For the rate sup­pres­sion to ben­e­fit U.S. con­sumers, oil pro­duc­ers must want to keep their oil rev­enues in dollar-​denominated invest­ments. (The oil mar­kets trade in dol­lars.) For decades this has made sense, as the U.S. dollar was the reserve cur­rency of choice among for­eign gov­ern­ments. But the value of the dollar has been slip­ping by the day—according to the Econ­o­mist, it’s fallen 6% against a col­lec­tion of other cur­ren­cies since August—making dollar-​denominated invest­ments less attractive.

Indeed, Weisman’s arti­cle points to a grow­ing reluc­tance among oil-​producing coun­tries to keep their sky­rock­et­ing rev­enues in dollar-​based assets:

C. Fred Berg­sten, direc­tor of the Peter­son Insti­tute of Inter­na­tional Eco­nom­ics, said that while some coun­tries in the gulf were trying to diver­sify their invest­ments away from the dollar and into euros and pounds ster­ling, the Saudis were trying to quell that trend out of fear that the dollar will decline fur­ther and dimin­ish­ing the value of their assets.

What I’ve called an “oil subsidy” could equally be a “gold subsidy,” a “heroin subsidy,” or a “banana subsidy,” should any of those other com­modi­ties gen­er­ate oil-​sized rev­enues. The impor­tant fact here is that Amer­ica is an attrac­tive market for for­eign invest­ment. To be clear: the interest-​rate sup­pres­sion is only con­tin­gently depen­dent on the high price of oil. The cru­cial point is the abil­ity of dollar-​denominated invest­ments to attract investors, and that depends on their abil­ity to hold and increase value.

In a sense, then, and look­ing at this from a very wide angle, the interest-​rate sup­pres­sion is a kind of “superpower subsidy.” It will last as long as Amer­ica has the eco­nomic, polit­i­cal, and mil­i­tary strength to guar­an­tee that the dollar is a good store of value. How long that hap­pens is another ques­tion entirely.