Four people were killed in food riots in Haiti. From Bolivia to Uzbekistan there have been violent protests against the doubling of food prices. In Italy, mothers are marching against the price of pasta. The World Food Programme has seized up and the World Bank on 13 April forecast that 100 million people face starvation. It should not have come as a surprise.

Conventional explanations for the food crisis range from climate change to dietary change in China, from global overpopulation to the switch of agricultural production to biofuels. These long-term factors are important but they are not the real reasons why food prices have doubled or why India is rationing rice or why British farmers are killing pigs for which they can't afford feedstocks. It's the credit crisis.

This latest food emergency has developed in an incredibly short space of time - essentially over the past 18 months. The reason for food "shortages" is speculation in commodity futures following the collapse of the financial derivatives markets. Desperate for quick returns, dealers are taking trillions of dollars out of equities and mortgage bonds and ploughing them into food and raw materials. It's called the "commodities super-cycle" on Wall Street, and it is likely to cause starvation on an epic scale.

The rocketing price of wheat, soybeans, sugar, coffee - you name it - is a direct result of debt defaults that have caused financial panic in the west and encouraged investors to seek "stores of value". These range from gold and oil at one end to corn, cocoa and cattle at the other; speculators are even placing bets on water prices.

Just like the boom in house prices, commodity price inflation feeds on itself. The more prices rise, and big profits are made, the more others invest, hoping for big returns. Look at the financial websites: everyone and their mother is piling into commodities. It is the great bull market of the Noughties. The trouble is that if you are one of the 2.8 billion people, almost half the world's population, who live on less than $2 a day, you may pay for these profits with your life.

This speculation doesn't happen on its own, however. Commodities such as gold and oil are favourite "hedges" against falling currencies. But this time all manner of other commodities, such as wheat and rice, have been swept along in the inflationary slipstream.

Investment houses, pension funds, private equity groups and banks are driven by profit not morality, and they invest wherever they can see the biggest return. It is not a conspiracy, but it is a conscious strategy, backed by the central bankers of the west as they try to help Wall Street back on its feet. Put another way, the banks are exporting our debts to the developing world. The collapse of the dollar means that most international commodities are more expensive for poor people to buy. The dollar's decline is a direct result of the low interest rate policy of the US Federal Reserve and the Bank of England, which shockingly cut interest rates on 10 April even as inflation spiralled.

When interest rates are below the rate of inflation, investors have to keep moving their funds from sector to sector in search of higher returns. In the 1990s they piled into internet stocks. When that bubble burst in the 2000 stock-market crash, they shifted into property and complex collateralised debt dealing based on US "sub-prime" mortgages. Now, with the collapse of the property bubble - not just in the US but across the world - investors are on the move again, and the only place left is commodities. It's the third bubble and it's hitting the developing world hard.

There are other reasons for food shortages: the diversion to biofuels because of the depletion of oil reserves, the increasing population, changing eating habits in south-east Asia - all these are putting long-term pressure on agricultural resources. But the efforts of institutions such as the US Federal Reserve to revive the economy on the back of a commodities boom have dramatically speeded up global inflation.

Will it work? Will the new "asset bubble" restore the profits of the banks and revive the US economy? In the short term, possibly yes - but at terrible human cost. In the end, the US may be cutting its own throat. Once speculative prices get out of control, there is no knowing when they will stop. Oil is now more than $100 a barrel. Resource-rich countries such as Russia are suddenly world powers again. Hungry people are desperate people. This might be the bubble to end all bubbles.