The rapidly growing trade in derivatives poses a mega-catastrophic risk" for the economy and most shares are still "too expensive", ... investor Warren Buffett has warned. The derivatives market has exploded in recent years, with investment banks selling billions of dollars worth of these investments to clients as a way to off-load or manage market risk. But Mr Buffett argues that such highly complex financial instruments are time bombs and "financial weapons of mass destruction" that could harm not only their buyers and sellers, but the whole economic system. Derivatives are financial instruments that allow investors to speculate on the future price of, for example, commodities or shares - without buying the underlying investment. Outstanding derivatives contracts - excluding those traded on exchanges such as the International Petroleum Exchange - are worth close to $85 trillion, according to the International Swaps and Derivatives Association. Some derivatives contracts, Mr Buffett says, appear to have been devised by "madmen". He warns that derivatives can push companies onto a "spiral that can lead to a corporate meltdown", like the demise of the notorious hedge fund Long-Term Capital Management in 1998.