The global switch to renewable energy has passed a “positive tipping point” where solar and wind power will become even cheaper and more widespread, according to two United Nations reports released Tuesday, describing a bright spot amid otherwise gloomy progress to curb climate change. Last year, 74% of the growth in electricity generated worldwide was from wind, solar and other green sources, according to the U.N.’s multiagency report, called Seizing the Moment of Opportunity.