Lots of explanations have been advanced as to why world trade fell so rapidly during 2008-9 — far more rapidly than at the start of the Great Depression. Problems associated with trade finance, and the vertical disintegration of modern manufacturing production, are the two that come up most frequently.
I’d like to offer another, more banal explanation: the composition of world trade is very different today than 80 years ago. In 1929, just 44 per cent of world merchandise trade involved manufactured goods. That proportion increased to 70 per cent in 2007. The reason this matters is that manufacturing is more volatile than the rest of the economy, and it was the output of and trade in manufactures, rather than primary products, which collapsed during the Great Depression.
Between 1929 and 1930, the volume of world trade in manufactures fell almost 15%, while trade in non-manufactures actually increased by 1% (I have to say I wonder about that, but this what what my source says). Weighting these two indices by the shares of manufactures and non-manufactures in total world trade, you get an implied fall in total world trade of 6 per cent in 1930 versus the 7.5 per cent actually experienced. Repeating the exercise, but this time using 2007 weights rather than 1929 weights, yields a counterfactual decline in world trade of 10 per cent in 1930 — equal to the decline the WTO is predicting for 2009. The changing composition of world trade can thus explain a lot, it seems.
An implication is that whenever the world economy recovers, world trade will recover with it, unless a surge of protectionism occurs in the meantime.