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. 

But batteries and fuel cells are still too heavy and expensive for long-distance hauling and shipping, as well as the vast majority of air travel. For these, the authors conclude, liquid fuels are likely to remain the preferred energy source, given the amount of energy that can be packed into a given weight and volume.

The researchers survey a range of solutions, including hydrogen or ammonia fuels, advanced biofuels, synthetic fuels, and solar fuels produced using what are known as artificial leaves (see “The race to invent the artificial leaf”). But none of these can be generated anywhere near as inexpensively as a standard gallon of gasoline or diesel. 

The researchers note that this area could demand special priority in research and development efforts. Such technologies could also provide a form of long-term energy storage for renewable sources like solar and wind, providing a crucial missing piece in the electricity sector as well. 

Cement and steel

Producing two of the world’s most common building materials, steel and cement, also generates considerable carbon dioxide. Carbon is used to reduce iron ore in the process of creating steel, while producing cement involves decomposing calcium carbonate using high heat, which generates emissions as well.

In both cases, this pollution could be reduced by adding systems that capture carbon before it’s released from the plants, as well as by shifting to different practices entirely. Cleaning up cement, for instance, may require switching to new materials that can actually capture and sequester carbon dioxide (see “What happened to green concrete?”).

But here again, the question is cost. Much of the global production of steel and cement occurs in poor parts of the world, where it’s especially hard to make the case for expensive upgrades or overhauls of legacy plants that can ordinarily operate for decades.

“Load-following” electricity

Renewable energy sources like wind and solar are providing a growing proportion of electricity, but the critical challenge has been that power generation plummets when the sun isn’t shining and the wind isn’t blowing. Operating a dependable grid thus requires leaning on other power plants that can quickly ramp production up and down to meet demand, a “load-following” job that’s increasingly fallen to carbon-spewing natural-gas plants.

The authors conclude that addressing this challenge without adding greenhouse emissions will require better carbon capture systems, more flexible nuclear plants, cheaper forms of large-scale energy storage, and various tools and incentives that can encourage residents and businesses to shift their electricity usage to times of peak production.

Notably, the paper surveyed only the parts of the energy and industrial systems where we lack affordable tools to eliminate greenhouse-gas pollution today. Among other challenges, the world also needs to figure out ways to significantly reduce the emissions produced from agriculture and land-use changes like deforestation. And, of course, we need to begin scaling up the technologies we do have available to clean up the electricity sector at a much faster rate (see “At this rate, it’s going to take nearly 400 years to transform the energy system”).