By Rob Cox
The author is a Reuters Breakingviews columnist. The opinions expressed are his own.

Stop worrying about the tech bubble – there may be an even bigger one inflating beyond the confines of Silicon Valley. The corporate urge to merge has gone into global hyper-drive this year. Deal activity has surged as investors egg companies on and bid up the shares of acquirers well beyond mathematical explication, or prudence. As new metrics from interested parties are trotted out to justify the irrational, it’s time to exercise caution.

So far this year companies have announced some $1.3 trillion worth of transactions around the world, according to Thomson Reuters data. That’s nearly double the level of activity a year ago. European corporations have fueled even greater increases. Much of this is pent-up demand and a delayed response to the past year’s remarkable runup in stock market values.

But after years of relatively restrained M&A movement, giddy shareholders appear to be over-titillating the animal spirits of executives and directors into taking on ever riskier transactions.

Consider the huge regulatory hurdles that Pfizer is attempting to jump in its efforts to seal a $100 billion-plus merger with AstraZeneca and shift its tax domicile to the UK, or the French political sensitivities that General Electric is taking on with its $13.5 billion bid for Alstom’s power assets. Equally, Comcast’s $45 billion takeover of Time Warner Cable raises severe antitrust questions. These may be overcome by divesting assets. The fear that Comcast may succeed, though, is compelling rival AT&T to mull a stab at DirecTV despite having been recently thwarted in trying to acquire T-Mobile US.