Climate discussions typically center on the need to replace fossil-fuel power plants with technologies like wind turbines and solar panels.

But a new paper in Science offers a stark reminder that there are still huge parts of the global energy system where we simply don’t have affordable ways of halting greenhouse-gas emissions.

Air travel, long-distance transportation and shipping, steel and cement manufacturing, and remaining parts of the power sector account for 27 percent of global emissions from the energy and industrial sectors. And the authors say we need much more research, innovation, and strategic coordination to clean up these sources.

“If we’re really ambitious about meeting our climate targets, we need to be tackling these hard sectors now,” says the paper’s lead author, Steven Davis, an earth system scientist at the University of California, Irvine. (The more than 30 prominent coauthors include Sally Benson at Stanford, Ken Caldeira at the Carnegie Institution, Nathan Lewis of the California Institute of Technology, and Jessika Trancik and Yet-Ming Chiang at MIT.)

Aviation, trucking and shipping

The declining cost and improving performance of lithium-ion batteries and hydrogen fuel cells has made it possible to begin cleaning up big portions of the transportation industry, including cars, light-duty trucks, and short-haul semis.