Wednesday closed the books on an extraordinary quarter. A 40% rebound in Brent crude oil, record-high diesel prices, bond yields at 20-year highs and AI's threat to wipe out humanity gave investors no shortage of reasons to retrench. Yet faced with that formidable wall of worry, US and world stocks still scaled fresh record highs.Equities weren't exactly booming. The S&P 500 rose only 2%. But considering the dismal backdrop and the fact that the third quarter followed the strongest three-month period in six years, that’s not too shabby. Only last month, the S&P 500 and MSCI All Country World index were at their peaks, while the tech-heavy Nasdaq traded at a new high of 27,288 points just last week.Wall Street's Teflon-like qualities have been something to behold. That is largely thanks to corporate earnings growth in the second quarter, which came in at a stunning 53.7%.Of course, it's a little murkier under the surface. Around 40% of S&P 500 stocks are down for the year, and a quarter of the index's constituents have fallen 10% or more, according to Ben Carlson at Ritholtz Wealth Management.Strategists at Goldman Sachs reckon market breadth is the narrowest since 2000. A handful of tech giants are boosting an otherwise middling market, with AI increasingly the only game in town. More than half of the S&P 500's market capitalisation is now composed of AI or AI-adjacent companies.The global equity picture was more mixed than in the US, with benchmark indices in Japan, China and Europe losing ground.None gave up as much as South Korea's KOSPI, which tumbled around 20%. Context is key, though. The tech-heavy Korean index had soared nearly 70% in the previous quarter in a retail-driven AI frenzy that prompted regulators to issue a string of measures to cool the mania.As we begin the final quarter of 2026, the question is whether equities can continue to grind higher. Forecasts for AI- and tech-infused earnings growth remain lofty, but, as the third quarter showed, many of the obstacles in Wall Street's path are just getting bigger. BOND BLUESThe most glaring concern is the cost of capital, which is rising — rapidly. The 10-year US Treasury yield, still the world's benchmark borrowing cost against which trillions of dollars of loans are referenced globally, rose more than 85 basis points in the quarter. That's one of its biggest quarterly rises in the last 50 years. Yields across most of the US Treasury curve are at their highest levels since the mid-2000s, and it's a similar story in Europe and Japan.This reflects a major hawkish shift in global interest rate expectations on the back of rising price pressures. The Federal Reserve, under the stewardship of new Chair Kevin Warsh, raised rates in September for the first time in three years.Sovereign bonds are being hit from all sides. Energy supply shocks stemming from the US-Iran and Russia-Ukraine wars are stoking inflation fears. Then there’s the pressure on governments to ramp up spending on defense, energy security and AI infrastructure, despite their precarious fiscal positions.Uncle Sam's finances aren't encouraging, with total US debt crossing the $40tn barrier in August. That appeared to be a manna for crypto investors, who pushed bitcoin up 43% in the July-September period, making the leading cryptocurrency one of the biggest winners across the asset spectrum.Credit, meanwhile, came under heavy pressure. US investment-grade and high-yield corporate bonds racked up their biggest quarterly losses since 2022, according to ICE Bank of America aggregate index data. Much of that can be pinned on the glut of new AI-related borrowing, which is expected to remain strong in the final quarter of the year and rise next year. WHAT'S NEXT?Looking forward, the hawkish rate outlook in the US remains a major headwind. While current market pricing puts the likelihood of an October hike at roughly 50%, the bond market is indicating the Fed has more work to do. Will the Fed's next step be later this month, just days before the US midterm elections, or in December?Ultimately, it will be difficult for the fourth quarter to match the drama of the last three months, but investors should brace themselves. On top of more expected interest rate rises in the US and around the world, worries about AI's threats to financial stability, the long-term employment outlook and human existence itself are unlikely to fade quickly. The US-Iran war is still raging, too.Can equities shrug all that off again? Investors may get an answer as the third-quarter earnings season kicks into gear in the next few weeks.The opinions expressed here are those of the author, a columnist for Reuters.