Chicago Tribune: When President Barack Obama lands Thursday in Canada for his first foreign visit as the nation’s chief executive, his pledge to wean Americans off “dirty” oil will collide with the host country’s aggressive push to send even more crude to the U.S. Obama is promising to tackle the growing threat of climate change and promotes green energy as a solution. At the same time, refineries across the Midwest are poised to dramatically increase their emissions of heat-trapping pollution, a …