As the case for corn-based ethanol unravels, a lot of pundits and green-minded investors have settled on a new panacea: ethanol from sugar cane, which thrives in the tropics.

Thomas Friedman has been blustering about it for years now; Richard Branson recently hinted he might start investing in it.

Sugarcane is a deeply ironic crop on which to hang a “sustainable energy revolution.” Historically, the spread of sugarcane in Caribbean islands and South America involved vast clear-cutting of coastal forests.

Socially, its legacy may be worse. To run the bustling cane plantations of the Americas during the colonial period, European powers relied on ruthlessly exploited African slaves.

Still a highly labor-intensive crop, cane evidently remains under the shadow of that atrocious past. Even today, Brazil’s much-heralded ethanol miracle is built on the backs of “forced” cane workers. From a Reuters story:

Amnesty International criticized poor working conditions and forced labor in Brazil’s fast-growing sugar cane sector on Wednesday, as the government tries to promote the cane-based ethanol industry as a way to reduce poverty.

Sometimes the circumstances involved not cane fields, but ethanol refineries:

Amnesty said that in March 2007, 288 workers were rescued from forced labor at six cane plantations in Sao Paulo state, and 409 workers from an ethanol distillery in Mato Grosso do Sul state.

It goes on:

In November 2007, inspection teams found 831 indigenous cane cutters working in poor conditions, also in Mato Grosso do Sul, while over 1,000 people “in conditions analogous to slavery” were released in June from a sugar plantation in Para state.

Amnesty claims that Brazil’s official response to the problem has often been “symbolic” at best. “Clearly resources are limited, areas are huge, and the number of companies in this sector is expanding in an alarming way,” an Amnesty researcher told Reuters.

It’s important to note that these aren’t isolated incidents. Brazil’s cane-ethanol industry thrives in global markets in no small part due to the “comparative advantage” of a large and unprotected workforce. From a 2006 Land Research Action Network report:

After Australia, Brazil has the lowest cost of production of sugar in the world because it exploits workers. In the state of São Paulo, the cost of production is $165 dollars per ton. In the European Union the cost is $700 dollars per ton.

Now highly mechanized, the cane industry still requires seasonal bursts of hand labor — and offers little or no work in down times. And like U.S. vegetable farms, can plantations rely on great influxes of migrant labor. Land Research Action Network again:

These workers often begin their activities in debt. One of the frequent debts encountered before beginning work is with transportation (usually clandestine, called “excursions”) that costs on average R$200.00 per worker migrating from the Northeast to São Paulo. The migrant workers are seduced by “cats” or “coyotes” who are usually the owners of the buses which make the journey.

In the sugarcane regions, so-called “dormitory cities” have increased, where migrant workers live in tenement houses, or overcrowded barracks, without ventilation or minimal hygienic conditions. Despite their precarious situation, the cost of housing and food for sugarcane workers is much higher than the average paid by the local population.

I wish Thomas Friedman — next time he’s in Brazil chatting up cab drivers, development gurus, and ethanol execs — would go spend the night in one of these dreadful barracks, and then turn in a shift in a cane field (where he’d earn about a buck for every ton of cane he harvested, Land Research Action reports). The experience may add a bit of pique to his next gurglings about the genius of cane ethanol.