Valuation Levels

Taking last Friday's US large cap market close and using current and projected next twelve month's earnings, the justification for a well above average P/E ratio rests on no negative impacts to the key inputs of earnings, growth of earnings, and an appropriate discount rate. If, however, an exogenous event were to occur with the global economy limping along at just above stall speed, what are the odds that such an impact were to occur - specifically to earnings and its growth rate? And, therefore, shouldn't such a risk give one pause to the justification of an above average valuation level for equities?