A major report published Monday warns global warming has pushed the planet’s stores of ice to a widespread collapse that was “unthinkable just a decade ago,” with some level of Arctic sea ice certain to vanish in summer months before the year 2050, even as countries drastically reduce their fossil fuel emissions. Climate scientists say the only way to avoid further catastrophe is to take urgent steps now. If fossil fuel pollution is allowed to continue to grow, the Arctic could lose most of its sea ice by 2030.