Ken Burns' latest PBS documentary, "The Dust Bowl," comes on the heels of the first presidential-election debates in 28 years not to address climate change. If the candidates won't talk about it, leave it to Burns and frequent collaborator Dayton Duncan to draw historic parallels between present-day environmental concerns and the disaster of the 1930s, which was caused by a combination of drought and environmentally destructive farming methods."It's a cautionary tale about who we are as human beings as much as anything else," Duncan said of "The Dust Bowl" (7 p.m. today and Monday on KUED Channel 7). "Our film was about nature, and it's also about human nature. We're not unique as Americans, but we might be a little more susceptible to it, that we believe that we can ignore the limits of the environment and of nature if it suits our purposes, and that if things are going on a roll, they will continue to go on a roll.