CRYSTAL BAY, Nev. (AP) – Climate change is changing the rules scientists have followed for decades as they try to identify the causes of Lake Tahoe’s loss of clarity and chart a course to restore the ecological health of the mountain lake. Until recently, the climatological cycles affecting Tahoe’s clarity had remained fairly constant since 1968. That’s when experts started dropping a white disk into the water to measure how far down it was visible – more than 102 feet (31 meters) back then compared to an average of about 69 feet (21 meters) today. Last year, winter clarity improved to 73 feet, but summer clarity declined to 64 feet – something scientists didn’t anticipate. They say it’s part of a change that sees the top of the water warming sooner than it used to, preventing the normal mixing of shallow and deeper water.