The words blasted to cellphones around Oregon’s capital city were ominous: “Civil emergency . prepare for action.” Within half an hour, a second official alert clarified the subject wasn’t impending violence but toxins from an algae bloom detected in Salem’s water supply. Across the U.S., reservoirs that supply drinking water and lakes used for recreation are experiencing similar events with growing frequency. The trend represents another impact of global warming and raises looming questions about the effects on human health, researchers say.