A tar-sands facility: oil doesn't get any dirtier than this.At ForestEthics, persuading the world's largest corporations to treat the Earth ethically is our bread and butter. And it often starts with a letter. Last week, we mailed letters to more than 100 Fortune 500 companies, warning that their continued consumption of fuels from Canada’s tar sands—the world’s dirtiest oil—puts their brands at risk. As ForestEthics Executive Director Todd Paglia documented in a vivid slideshow for Grist last year, the tar sands manage to combine multiple local and global environmental hazards into a single industrial project—in fact, the largest industrial project …