Congress is about to confront the challenge of coal, and much of what we hope to do to reduce the threat of global warming hinges on these decisions. There's a useful test to use whenever the challenges of fossil fuel dependence and global warming come up: We must reduce the threat of global warming without worsening our dependence on foreign oil; and we must reduce the threat of oil dependence without worsening global warming. When it comes to coal, it's that second part of the equation that brings up some sticky issues. Coal has been a big part of our …