As of 2018, over one hundred municipalities in the United States have committed to using 100% renewable energy in the coming decades. Yet, complete decarbonization paths have yet to be trodden. Recent collaborative research out of Cal Tech and Carnegie Mellon published in Science magazine addresses the challenges of energy sectors that are difficult-to-decarbonize—meaning the technology to do so doesn’t exist at scale, or that the method of electrification is not as intuitive as in other sectors—and argues that addressing these sectors is an essential step toward a carbon neutral future. Because technical development of new technologies can be burdened by long lead times and because infrastructure generally has a long life span, the decarbonization of difficult-to-decarbonize energy systems is a necessary and urgent intent.

The research discusses a carbon neutral (or net-zero carbon) energy future. This is slightly different than a 100% renewable energy outlook. While 100% renewable energy focuses exclusively on power generation, and implies that all power will come directly from non-emitting renewable sources like solar and wind energy, carbon neutrality minimizes the net-result of all summed carbon emissions, proposing an energy mix that could include low-carbon sources, carbon capture, or carbon recycling.

There are some sectors that could enjoy a straightforward path to decarbonization because the necessary zero-carbon technology already exists, such as light-duty electric transportation and electrification of household amenities such as heating, cooling and lighting. In other industries, complete decarbonization paths are not straightforward. Among these are aviation, long-distance transport and shipping, the production of carbon-intensive materials such as steel and cement, and a reliable electricity supply able to meet varying demand.

Decarbonization paths are far from settled, and the differences in projections of a 100% renewable future have flamed emotions so far as to even spur lawsuits. But, as the recent IPCC report reminds us, we are losing time, and the enduring infrastructure of the future is built today. Still, difficult-to-eliminate emissions are remarkably underrepresented in climate change assessment models, even though they are responsible for over one quarter of global CO2 emissions. Further, the demand for difficult-to-decarbonize services continues to grow.

The challenges of transitioning some sectors to net-zero carbon systems must not be understated. While converting light-duty vehicles to run on electricity and hydrogen fuel is relatively straightforward, requiring only small shifts in range and refueling infrastructure, other transportation subsectors—heavy-duty vehicles, airplanes and ships—require energy sources with higher energy densities. This means that energy-dense liquid fuels such as hydrogen, ammonia and biofuels might still be needed for some transportation sectors like long-distance trucking. Battery and hydrogen-power may be used for short-distance freight transport, but for long-distance, heavy-duty trucks carried by renewable sources can often carry about 40% less goods by weight than their diesel-fueled equivalents per mile.

In steel and cement manufacturing, chemical processes with very high heat requirements are necessary. If demand for these resources stays the same, net-zero solutions may depend on carbon capture and storage, at least in the short-term. Given that the demand for steel and cement is rising, more research and development of carbon management is necessary to ensure that this sort of technology can quickly be used in practice when necessary.

To meet international climate targets and to keep global warming from surpassing the 2°C (3.6°F) threshold rise in global temperature, implementing net-zero energy systems for difficult-to-decarbonize services will be essential. When proposing possible pathways towards decarbonization, this work argues that combining currently separate energy sectors and industrial processes will be vital for a transition to a net-zero framework that is cost-effective. Instead of looking at difficult-to-decarbonize sectors separately, an integrated approach across industries should be taken when analyzing feasible solutions.

Can municipalities meet their 100% renewable energy targets? The answer will likely depend on multiple variables. Demand reductions will be important, as will prioritizing sustainability and social equity when considering new infrastructure. And, implementing infrastructure and technology that decarbonizes sectors with difficult-to-eliminate emissions will be necessary. If energy consumption continues along current trajectories, a net-zero carbon approach may be more apt than one depending solely on solar, wind and battery storage. Regardless, the transition towards a cleaner future will require significant shifts in infrastructure and in inter-industrial relationships. The time to begin those shifts is now.