A skeptical question about financial speculation and food prices

Tom Philpott at Mother Jones blames food price increases since 2007 on financial speculators.  He is quite harsh on the fools like me, who thought rising food prices reflected resource scarcity.

Here is my question in his comments section:

I don't understand how speculation could raise food prices for more than a few months. 

The financial speculators are placing bets on the futures market.  If they expect prices to go up in January, they will buy a contract today that guarantees them a certain amount of commodity in January.  When January comes around, they sell their paper commodities on the ordinary spot market.  If they are correct that prices rose, they make a profit, because their January sale is more than the price they originally paid. 

An essential feature of this game is that the speculators do not want to take possession of a ton of corn or soybeans.  For speculators, the game is not "buying and more buying," it is "buying and then selling."  The buying drives up prices, and the selling drives down prices.  Commodity futures are different from "dot com" stocks, which can be held for a long time.

Basically, I find it much more plausible -- as an economist and as an environmentalist -- to see rising food prices as a symptom of real resource scarcity.

But I'll keep an open mind.  Can somebody explain more clearly: beyond the first few months of a speculative boom, how can financial speculators keep driving the price higher and higher?