Fog is a defining element of summer in Santa Cruz, obscuring the view of day trippers descending the hills to the coast and prompting kids to bundle up to hop on their bikes for summer adventures. Its fingerprints are visible in the vast coastal forests, even when it isn’t hanging in the air. The redwood trees towering in a clear blue sky soak up moisture from the fog on gray days. It is often their only source of water for months at a time. … But many scientists believe that fog is declining, another casualty of global warming.