The biggest icebergs breaking off Antarctica unexpectedly help to slow global warming as they melt away into the chill Southern Ocean, scientists said Monday. The rare Manhattan-sized icebergs, which may become more frequent in coming decades because of climate change, release a vast trail of iron and other nutrients that act as fertilizers for algae and other tiny plant-like organisms in the ocean. These extract carbon dioxide from the atmosphere as they grow, a natural ally for human efforts to limit the pace of climate change blamed on manmade greenhouse gas emissions. Ocean blooms in the wake of giant icebergs...