India, Aug. 7 -- Italy placed all of its major cities under the highest heat alert Thursday as a blistering heat wave gripped Europe, setting a national temperature record in Austria, fueling wildfires in France and disrupting energy and transport systems across the continent. The extreme weather has pushed temperatures above 40 degrees Celsius (104 Fahrenheit) in several countries, prompting health warnings for millions of people. Across Europe, authorities are grappling with the consequences of extreme heat: health alerts for millions in Italy, record temperatures in Austria, drought-related disruptions to energy and shipping in Central Europe, and wildfires stretching from France to the Balkans.

Scientists and officials warn that such...