Stock markets worldwide have tumbled with investors unsettled by the continued slide in oil prices and fears about the impact on global growth, reports BBC.On Wall Street the Dow Jones and S&P 500 were 2% lower in early trading, while the Nasdaq fell 2.3%.The FTSE 100, Germany`s Dax and the Cac 40 in Paris are all trading about 2.5% lower - resuming the year`s sell-off.Many markets are now in so called "bear market" territory - a fall of 20% or more from their most recent peak.The falls in Europe and the US came after Asian stocks closed sharply lower.Markets in Dubai closed at a 28-month low, while in Japan shares fell to their lowest level since October 2014.Top emerging market shares and currencies were also caught up in the turmoil, with the Russian rouble hitting a new record low of 80.295 against the dollar.