If cement producers were a country, they'd be the third-largest carbon emitter in the world, behind only China and the US. The manufacturing of cement used to make concrete produces an estimated 8% of global carbon emissions, which is nearly 3B tons of carbon dioxide per year.

In the epicenter of SoCal's cement industry, San Bernardino County's high desert in the Inland Empire—a region rich in limestone, the key ingredient of cement—two huge cement plants produce more combined carbon emissions than the Golden State's aviation industry.

The plants, CalPortland's Cemex facility in Oro Grande and a Mitsubishi Cement plant in Lucerne Valley, along with four other cement manufacturing facilities in CA have made significant gains in energy efficiency over the past two decades. According to the state Air Resources Board, the plants collectively have reduced emissions by about 17% compared to levels in 2000.