Morgan Stanley’s Stephen Roach hasn’t been purveying sweetness and light for a long time now. But this week’s commentary is particularly grim:

I don’t know where oil prices are going. But I do feel strongly that an important macro threshold has now been breached — one that adds unmistakable tension to the world economy’s greatest imbalances. At the current level of oil prices, I suspect one of two things will happen — either the over-extended American consumer will finally cave or the long-awaited US current account adjustment will finally unfold. Courtesy of a full-blown energy shock, the venting of global imbalances can no longer be deferred indefinitely. If consumers remain unflinching in the face of sky-high oil prices, a plunging saving rate will push an already outsize current account deficit to the flash point.

As always, duration matters. If oil prices fall back quickly and sharply, all will be forgotten and the consequences will be minimal. Unfortunately, that’s a bet the financial market consensus has been making for far too long. All this points to what could be the biggest macro call that any of us will have to make for a long time — the capitulation of the unflinching American consumer. Needless to say, this would have profound implications for the rest of the global economy — largely a US-centric world that is utterly lacking in support from autonomous domestic consumption.

Over the years, I’ve learned to be wary of betting against the American consumer. But the history of energy shocks argues to the contrary. Moreover, today’s saving-short, asset-dependent, overly-indebted consumer is far more vulnerable than in the past. After years of such warnings, investors, of course, have all but given up on that possibility. That’s precisely the time to worry the most.