Pollution levels in Iowa’s water have been abnormally high this winter. That’s forcing the state’s largest city, Des Moines, to run an expensive nitrate removal system that they usually only use in the summer. Scientists say it’s a sign of a problem that is only growing. As Earth warms due to human-caused climate change, the ground isn’t staying frozen as consistently in many places. That means snow is melting or instead falling as rain, and intense rainfall events that can hit that thawed ground are becoming more frequent. That all adds up to more winter days when nitrates are likely to reach unhealthy levels.