Remember when Japan was going to become the world\u2019s biggest economy? Don\u2019t laugh. Herman Kahn, the Rand Corp. futurist who partly inspired the character of Dr. Strangelove, predicted as far back as 1970 that Japan\u2019s gross domestic product would overtake the U.S. around the year 2000. Decades later, his prediction still seemed on track. Thanks in part to the strength of the yen, Japan was just months away from matching the U.S. in GDP terms, one 1995 Los Angeles Times article predicted. Halfway through that country\u2019s lost decade, it\u2019s hard to believe that this wasn\u2019t a fringe view \u2013 but it was nonetheless the subject of an essay in the august Foreign Policy magazine and a well-received accompanying book. If you extrapolated early-1990s growth rates on a chart, indeed, the forecast seemed almost irrefutable \u2013 but that\u2019s ultimately a lesson about the hazards of extrapolating lines on charts. As my colleague Daniel Moss has argued, we should heed that mistake every time we predict that China is on the brink of a similar coup. China\u2019s official figures for the size of its economy are about 16 percent larger than they should be, and measures of real GDP growth were overstated by about 2 percentage points between 2008 and 2016, according to a study published by the Brookings Institution Thursday. Combined with the country\u2019s own downgrade of its GDP growth for this year to a range of between 6 percent and 6.5 percent earlier in the week, that represents a sort of profit warning for the hchineconomy. It\u2019s worth reflecting that this particular stock has already had its share of analyst downgrades. Back in 2010, Standard Chartered Plc was predicting that China\u2019s economy would overtake the U.S. by 2020, and would be nearly twice the size by 2030. Four years later, IHS Markit Economics forecast the tipping point would come the same 10 years into the future, in 2024. Those forecasts seem fanciful in the light of the way China\u2019s economy has slowed \u2013 and America\u2019s has accelerated \u2013 during the era of President Xi Jinping. At market exchange rates and current prices, U.S. GDP was $20.54 trillion in 2018, about 60 percent larger than China\u2019s $13.09 trillion. That just shows how misleading extrapolations can be. If you had inflated China\u2019s 2011 GDP at the 8 percent rate then considered the minimum politically tolerable, and grew America\u2019s at its then-10 year average of 1.8 percent, you\u2019d have put the tipping point around 2023. Do the same forecast now with China\u2019s growth at the 6-percent end of the official forecast band, and the U.S. would still be seen falling behind in 2028, roughly in line with Goldman Sachs Group Inc.\u2019s Jim O\u2019Neill\u2019s longtime forecast. What happens, though, if we see the U.S. growing from here at its 20-year average rate of 2.2 percent, and apply Brookings\u2019 adjustments to China\u2019s current GDP and growth-rate forecasts? In that eventuality, China\u2019s GDP continues to trail the U.S. all the way out to 2050. By that time, demographic factors will be weighing heavily against catch-up growth. China\u2019s workforce, nearly five times the size of America\u2019s at present, will only be a bit more than three times as large, meaning total factor productivity will have to be growing still faster to close the gap \u2013 and all the evidence is that, to the contrary, it\u2019s been contracting. That\u2019s not a reason to count China out. For one thing, as my colleague Noah Smith has written, it\u2019s already the larger economy according to purchasing-power parity (a way of adjusting for the fact that the same income buys a higher standard of living in China). For another, such measures are heavily influenced by exchange rates, one reason Japan seemed so mighty when the yen was nearing 80 cents in the mid-1990s. On top of that, as Bloomberg Intelligence economist Tom Orlik argued Friday, China\u2019s official GDP numbers may be more robust these days than Brookings is giving them credit for. Most importantly, though, it\u2019s a lesson that numbers like these aren\u2019t really a solid-enough foundation for building such grand strategic narratives, however tempting it is to do so. Even at its current scale, China is a scant 15 percent of the global economy, and it\u2019s unlikely to crack more than 20 percent at market exchange rates in the foreseeable future. Fretting over adjustments to long-run economic forecasts is really just a proxy for our deeper anxieties about the fracturing of democracy in Western countries and the growing confidence of authoritarian governments from Beijing and Brasilia to Moscow and Ankara. If we want to ensure the international liberal order survives another century, we\u2019d do much better to focus on those real problems in the present, rather than fixate on the impending doom of an imagined future.