The S&P 500 stock index is on track to celebrate the longest bull market in its history next summer, but Bank of America Merrill Lynch's chief investment strategist believes the era of low volatility and sky-high returns could end with a flash crash to rival that of 1987.

In a research note giving his overview of global investment strategy for next year, Michael Hartnett wrote: "The air in risk assets is getting thinner and thinner. Asset returns will likely peak in early 2018, but optimism fueled by recent stunning returns and historic low volatility could be followed by a sobering flash crash a la 1987, 1994 and 1998 as central banks, the major sedative of volatility, start to withdraw liquidity."