Global shares have fallen sharply as concerns about weak global economic growth knock investor confidence.In the US, the Dow Jones opened down almost 300 points or 1.8 per cent at 16,025, before recovering slightly.European markets were also sharply lower. the main Cac 40 index in France fell 2.4 per cent while Germany's Dax index and the UK's the FTSE 100 were down 1.8 percent.Oil prices also continued to slide with Brent crude falling below 85 dollar a barrel and US light crude dipping to 81.60 dollar.Brent crude has fallen by 20% since the summer on concerns of oversupply.