PHOENIX (AP) — A dangerously hot summer is shaping up in the U.S. West, with heat suspected in dozens of recent deaths, including retirees in Oregon, a motorcyclist in Death Valley, California and a 10-year-old boy who collapsed while hiking with his family on a Phoenix trail.

Heat is the top cause of weather-related fatalities nationwide. But because investigations of suspected heat deaths can take months, and a is used by counties to count them, it is unknown exactly how many people died in the recent heat wave beginning July 1. There are indications it was an especially deadly two weeks.

“This is just a harbinger of things to come,” Joellen Russell, a climate scientist at the University of Arizona in Tucson, said Friday. “The floods, droughts, wildfires, heat waves, hurricanes, thunderstorms: We have activated all this extreme weather with the extra carbon dioxide we put into the atmosphere.” Here are some things to know: Where most deaths occurred Nineteen deaths are being investigated for possible heat-related causes in Northern California’s Santa Clara County, where a heat wave this month pushed temperatures into the low triple digits.

The medical examiner’s office reported that four people who died were homeless and nine were older than 65. At least 16 people are suspected to have died from record high temperatures in Oregon, mostly in the metro Portland area. There have been nine confirmed heat-related deaths this year in Clark County, Nevada, which enco.