John Holdren, President Barack Obama’s top science and technology advisor, said at a White House summit on clean energy on Tuesday that a “global low-carbon economy” by the year 2050 means that “cars, trucks and planes” will be powered by “electricity, biofuels or hydrogen.”

“A global low-carbon economy in 2050 is going to have to meet the energy needs of 9 or 10 billion people using technologies that will have to be more advanced than the technologies that are in place today,” he said. “Buildings, agriculture, machines must be dramatically more energy-efficient.”