Commodity prices are well down, but actually some prices had been declining for quite a while. Iron ore, for example, peaked as long ago as February 2011. Moreover, in many cases declining commodity prices reflect additional supply, which usually carries a different – positive – implication for global growth, as opposed to weaker demand.
While global growth forecasts are being lowered, at this stage they see higher growth rates than in 2001, which was a relatively mild slowdown episode. Of course that is just a forecast – but so far there are not any actual data that invalidate that view.