San Francisco Chronicle: When leaders from the Group of Eight nations agreed last week to cut carbon emissions in half by 2050, all the commentary debated their failure to set shorter-term goals. But all of that chatter ignores the iceberg in the room: How on earth will those nations or any others meet even that relatively modest goal? The scientific community argues that by 2050, carbon emissions must be reduced 80 percent below the level in the year 2000, if serious global warming is to be averted. I would …