Blazes started by humans accounted for 84 percent of all US wildfires over the past two decades, according to a team co-led by a UMass Amherst researcher. Such fires “tripled the length of the fire season and dominated an area seven times greater than that affected by lightning-caused fires,’’ the team found. The research was oversee by Bethany Bradley at UMass and Jennifer Balch at the University of Colorado Boulder. The findings are reported in Proceedings of the National Academy of Sciences.