This analysis is concerned largely with the design of policies that will be most effective at achieving large emissions cuts before 2050. However, many of the policies discussed in this book (e.g., energy efficiency standards for buildings and vehicles, incentives or requirements for reduced industrial methane emissions, and even carbon pricing) cannot, by themselves, reduce emissions to zero or below.
In the scenario used throughout this book, which gives the world a 50% chance of staying below 2°C of warming, emissions become negative after about 2070. This result is common across climate models: To have at least a 50% chance of keeping the world below two degrees of warming, global emissions must steadily decline to zero and ultimately become negative (i.e., removing more CO2 from the air than is added) in the second half of the century. Even if the two-degree target is not achieved, to stabilize the climate at any temperature whatsoever (e.g., 3°, 4°) will probably require similarly dramatic emission cuts, albeit later in time. Therefore, there is no way around the need to reduce emissions to near-zero or below.
This section considers technologies that may be necessary in the long term (after 2050) to achieve the emission reductions required by a future with less than two degrees of warming and policies that may be necessary to adapt to climate change. The technologies discussed in this chapter may not be ready for widespread deployment today, but policies to accelerate their progress must begin now so that they will be sufficiently mature by the time they are needed.
One important note: The measures here must not detract from efforts to rapidly cut emissions using the more traditional tools discussed in the rest of this book. Progress on the techniques needed for post-2050 complete decarbonization should be completely additional to and simultaneous with efforts to achieve near-term impact. There is no mopping up the last 10% of carbon emissions if we don’t eliminate the first 90%!
When allocating limited resources, it is important to remember that many post-2050 solutions are still in research and development (R&D) stages and therefore require less money to make satisfactory progress in the near term than solutions that are ready to deploy at global scale (e.g., wind, solar, efficient industrial equipment and building components).
Policies to Support Post-2050 Technologies
This section discusses policies to help emerging technologies for achieving zero or negative emissions. Although support for further research in these technologies is broadly needed, three policies in particular can help accelerate their development.
- Government support for R&D is the core policy for ensuring that carbon-reducing technologies reach maturity. These technologies have large, positive social externalities (benefits whose economic value cannot be captured by the company deploying the technology), so without government support, companies may choose to direct their R&D efforts elsewhere.
- Strong carbon pricing is critical to accelerating these technologies. By putting a price on carbon, governments can help create additional economic value for carbon-reducing technologies and encourage private sector investment.
- Finally, some of these technologies will require large-scale demonstration plants or projects to achieve cost reductions through learning by doing. Therefore, governments will probably need to subsidize the construction and operation of a number of demonstration plants or large projects until the technologies discussed in this chapter are better understood.
Technologies for Further Reducing Emissions
Carbon Capture and Sequestration
It may be possible to fully eliminate carbon emissions from the electricity system and to electrify many end uses. For example, renewables and nuclear may be able to supply all electricity needs when combined with flexible demand, large balancing areas, energy storage, and overbuilding wind and solar while putting excess electricity to a useful, non–time-sensitive purpose such as the creation of hydrogen.
However, there are some sources of CO2 emissions that may be difficult to eliminate. For example, manufacturing the clinker in cement releases CO2 emissions and the share of clinker in cement likely cannot be reduced below a certain percentage without affecting the material’s structural properties. Another example is the manufacture of new iron and steel (rather than reforging scrap iron and steel in an electric arc furnace), which uses carbon not just as a source of energy but also as a chemical-reducing agent.
Innovations in material science may one day allow the replacement of cement or steel with novel materials with similar structural properties. However, it may not be possible to eliminate all industrial emissions, particularly if efforts to develop and commercialize novel materials encounter problems or cannot be scaled cost-effectively to satisfy the global demand for these materials.
Carbon capture and sequestration (CCS) provides a means whereby humans may continue to manufacture traditional materials without adding CO2 to the atmosphere. A CCS system extracts CO2 from a stream of waste gases, uses pressure to liquefy the CO2, transports it to a geologically suitable region, and pumps it underground for indefinite storage.
CCS is already used successfully in the oil and gas industry for enhanced oil recovery, and demonstration facilities using CCS for industrial process and power generation exist around the world. Some CCS power plants might use the Allam cycle, a combustion process that uses CO2 as the working fluid and produces a very pure stream of CO2 exhaust, which is easier to capture than CO2 diluted in air.
CCS may be used by power plants burning biomass (such as wood) rather than coal or natural gas. This is called bioenergy with CCS. Because the carbon in biomass was recently removed from the atmosphere by plants, storing it underground reduces atmospheric CO2 concentrations.
In addition to the challenges related to the CCS technology itself, bioenergy CCS faces additional hurdles. One issue is the amount of land needed to grow bioenergy crops, which may be very large. Care must be taken to ensure bioenergy CCS does not result in food insecurity or in deforestation to obtain additional cropland. There exist promising research directions that aim to address these challenges.
For example, more R&D is needed to develop multifunctional land uses (e.g., to allow the same land to produce food and bioenergy crops). Another route is to derive high-value alternative fuels from bioenergy crops (e.g., liquid transportation fuels) before the residue is burned for bioenergy CCS, thereby improving the economics of devoting land to bioenergy crops.
Atmospheric CO2 Removal
Achieving negative emissions necessarily involves removing CO2 from the atmosphere. Apart from bioenergy with CCS, various techniques have been proposed to accomplish this, although they are in early research stages.
Direct Air Capture
Although all techniques in this section capture CO2, direct air capture usually refers to the use of chemical processes to extract CO2 from the atmosphere, analogous to the way scrubbers capture CO2 from the air inside spacecraft. Unlike bioenergy with CCS, these systems do not use large amounts of land, so they would not pose food security or deforestation risks.
Direct air capture systems need a lot of energy. To achieve negative carbon emissions, direct air capture systems must be powered by emission-free energy, such as wind, solar, or nuclear power, and that energy must not be taken from other users who would then rely on fossil energy instead. (That is, the emission-free energy used by a direct air capture system must be strictly additional to other emission-free energy uses.)
The other challenge facing direct air capture systems is cost. The estimated cost of a system that captures 1 million tons of CO2 per year (roughly 0.02% of annual U.S. emissions) was $2.2 billion as of 2011. Over the plant’s lifetime, the all-in cost is $600 per ton of CO2, roughly eight times higher than the cost per ton to capture CO2 from the flue gas of a coal power plant. (Exhaust streams feature higher CO2 concentrations, which makes the CO2 easier to capture.)
Research can help improve the energy efficiency and lower the capital cost of direct air capture systems. As with other technologies to remove CO2 from the atmosphere, carbon pricing can provide an economic incentive and the possibility of financial returns.
In nature, when certain types of minerals (such as olivine) are exposed to air and water, they undergo chemical reactions that extract CO2 from the atmosphere and store it as a carbonate mineral. These minerals make their way to the ocean, where organisms use the minerals to form shells and skeletons. When the organisms die, the material sinks into the deep ocean and eventually may be converted to limestone.
Although this natural process is too slow to help reduce atmospheric CO2 concentrations on human timescales, it may be possible to accelerate the natural process. For example, if large quantities of olivine and similar minerals were mined, finely ground (to increase their surface area), and spread on beaches or other land exposed to water and the atmosphere, the rate of CO2 capture could be accelerated.
Unfortunately, given current scientific understanding, the amount of olivine we would need to use would be very large, and the mining, transport, grinding, and spreading of the olivine would have to be done in a manner that releases few if any carbon emissions in order to achieve net sequestration.
Additionally, for the sequestration to be sufficiently rapid, the olivine may have to be ground to particles with a mean diameter of less than 10 microns, a microscopic size that is easily aerosolized and could be inhaled (as PM10). More research would be needed to develop improved techniques before enhanced weathering could be considered a viable option for CO2 removal on human timescales.
Phytoplankton are photosynthetic organisms in the ocean that extract CO2 from seawater to build their bodies. When plankton die, they sink to the ocean floor, sequestering the CO2 in their bodies.
Like other organisms, phytoplankton need a variety of nutrients to survive. In many parts of the ocean, iron is the limiting nutrient that constrains phytoplankton growth. Therefore, it has been proposed that the ocean may be seeded with iron, encouraging phytoplankton growth, as a means of accelerating CO2 sequestration.
There are a number of challenges with this approach. Many phytoplankton produce toxins, so encouraging their growth could lead to an increase in harmful algal blooms that threaten the health of marine ecosystems (and can harm or kill humans who eat contaminated seafood). Also, when phytoplankton die, the bacteria that decompose them may deplete the oxygen in the water, leading to a “dead zone” that suffocates animal life. Finally, algal growth in one area can inhibit algal growth in another area, and nutrients other than iron may become limiting nutrients in some places, so the effectiveness of iron seeding at increasing overall phytoplankton numbers has been questioned.
More research could help determine whether ocean fertilization can be done safely and whether it offers significant CO2 removal potential.
Biofuels and Synthetic Fuels for Transport
It may be possible to electrify many forms of transport, such as light-duty on-road vehicles. However, it can be difficult to electrify certain transport options, such as commercial aircraft, because of the requirements for fuel of high energy density. One option for these vehicle types is a carbon-neutral biofuel or other synthetic fuel.
Ethanol is a biofuel that is already widely used for transport, but ethanol derived from corn offers only 20% less greenhouse gas emissions than petroleum gasoline on a lifecycle basis, and vehicles cannot run on 100% ethanol without special engine designs. To achieve zero emissions, a biofuel must be carbon neutral on a lifecycle basis, and it is more likely to be adopted if it is a drop-in replacement for gasoline (or diesel).
Ethanol made from cellulose, the inedible substance that forms the leaves and stalks of plants, can be made from agricultural residues rather than corn, lowering lifecycle greenhouse gas emissions. Various companies have experimented with obtaining biofuels from algae, although most of these businesses failed or pivoted to higher-value products, such as cosmetics or food additives, upon realizing the magnitude of the technical challenges involved.
Another approach involves creating fuels directly from sunlight using a chemical or biological process. This has the potential to avoid the inefficiency of using plants to convert sunlight into biomass, then converting that biomass to an energy-dense liquid fuel. These approaches are all in research stages and will need long-term, consistent government support to have a chance at commercialization.
It is also possible to use hydrogen as a chemical fuel. Hydrogen has several advantages over carbon-neutral biofuels. First, the technology is more mature. We are able to produce hydrogen today with little or no greenhouse gas impact by using electricity from renewables to split water into hydrogen and oxygen. Also, hydrogen does not emit any pollutants when used for energy; the only byproduct is water vapor. Even if carbon neutral, biofuels may emit particulates and other pollutants harmful to human health when burned.
An important downside of hydrogen is that, to achieve sufficient energy density for use in a vehicle, the chemical must be stored at very high pressure or very low temperature. This necessitates a bulky and heavy storage system. For example, the first ship to use hydrogen as a fuel (albeit not its primary fuel) was recently ordered. One of the technical challenges of this ship design is to store liquid hydrogen at –253°C.
As a result, hydrogen is more likely to be used in shipping or long-distance land-based transportation than in aircraft. The development of a large hydrogen distribution network and fueling stations would also be necessary.
Similarly, transporting hydrogen over large distances for use in equipment would require a new pipeline network. In some regions, such as the United States, it could be very difficult and expensive to construct the new pipeline infrastructure necessary to transport hydrogen for use across the economy.
The main thing governments can do to further the use of hydrogen is to promote the development of standards for hydrogen use in ships and other vehicles, including through international bodies, such as the International Maritime Organization. Government can also facilitate the buildout of hydrogen fueling and distribution infrastructure, if it is merited by sufficient demand.
Dietary and Behavioral Change
Some sources of greenhouse gas emissions, such as enteric fermentation in ruminants, may be difficult to tackle through technology this century. One option for lowering some types of emissions is for the government to use policies to shift human diet or behavior.
For example, ending subsidies to crops used for animal feed (such as corn) or taxing beef and lamb may reduce demand for these goods. If demand is sufficiently reduced, a ban may become feasible. Although this may sound politically unlikely, bans of this sort already exist and enjoy broad public support; for example, government restrictions make it impossible to slaughter horses for meat in the U.S.
Behavioral change is not limited to targeting emission sources with few technological options; it may be helpful in a broad variety of circumstances. For example, zoning may be used to encourage walking and biking rather than driving private cars.
Albedo (Reflectivity) Modification
Climate change is caused by the increase in heat-trapping gases in Earth’s atmosphere. Radiant heat is infrared radiation, which is emitted by the surface of Earth after it absorbs sunlight and increases in temperature. If sunlight is reflected rather than absorbed, it may reenter space, bypassing the heat-trapping gases in the atmosphere.
Therefore, one approach to tackle climate change is to increase the albedo (reflectivity) of Earth, so less sunlight is absorbed by the surface and turns into heat.
The main proposed mechanism of albedo modification is to inject tiny particles into the stratosphere, where they would act like a sunshade, scattering a fraction of the sunlight back into space. This effect can occur in nature after a large volcanic eruption, which can release large quantities of particle-forming sulfur dioxide. This work is still in the R&D stage and could be accelerated through government support.
Other mechanisms involve increasing the albedo of Earth’s surface, such as by using light colors for rooftops and pavement. Albedo ranges from 0.0 (perfectly absorbing) to 1.0 (perfectly reflective). In urban areas, each 0.01 increase in albedo over a square meter of surface area results in a cooling effect equivalent to avoiding 7 kg of CO2 emissions. High-albedo surfaces can also mitigate the urban heat effect and reduce the need for air conditioning in warm climates, saving energy and potentially reducing emissions. Government can promote the use of high-albedo materials through building codes and through direct procurement for public roadway and sidewalk materials.
Policies For Adapting To A Warmer World
When planning for a post-2050 future, it is wise to make investments in technologies and measures that will help humanity adapt to climate change. Adaptation can be categorized as structural or physical (e.g., building seawalls or creating new crop or animal varieties), social (e.g., evacuation planning or preparing for migration flows), or institutional (e.g., insurance ownership requirements or urban planning for climate change). Many of these measures require long lead times, and efforts to tackle them should begin now.
For example, it may be desirable to genetically engineer new varieties of crops that are resistant to drought, heat waves, or different sorts of pests yet are safe for the environment. Work to develop suitable crops may take many years. Governments should provide R&D support for adaptation technologies today.
Governments necessarily take the lead on resilient urban planning, disaster planning, construction of protective infrastructure (e.g., seawalls and early warning systems), and other institutional measures. National governments may require the participation of local governments, particularly for measures best undertaken at the local level, such as by stipulating that cities must take climate change into account in their urban plans. Insurance against natural hazards can be required by government, and premiums may be structured in a way to encourage people to reduce their exposure to damage from extreme weather events.
Governments also must plan for both internal and cross-border migration. The United Nations High Commissioner for Refugees has estimated that “up to 250 million people may be displaced by the middle of this century as a result of extreme weather conditions, dwindling water reserves . . . a degradation of agricultural land . . . [and] to escape fighting over meagre resources.”
This is roughly 50 times the number of refugees from the Syrian civil war, or 3.2% of today’s global population. (Lest this figure sound too high to be credible, consider that the United Nations already tracks 64 million refugees, asylum seekers, and internally displaced people, many of whom were forced to relocate by problems caused or exacerbated by climate change.)
Working to ensure that adequate infrastructure, housing, and job opportunities are available, and that migrants can be successfully integrated into new societies, will be critical to preserving political stability. Without proper preparations, the reaction to refugee flows could compound the physical damages caused by climate change.
Enacting strong policy to mitigate emissions as soon as possible offers the most cost-effective opportunities to limit damage to human societies. However, human needs in a post-2050 world will be different from human needs in 2020 or 2030, and the technology necessary to satisfy those needs does not exist at present. Therefore, even as we act to cut emissions today, it is crucial to begin work on strategies with long lead times to obtain the technologies we will need in the latter part of the century and beyond.
In particular, research into CCS technology, various ways to remove CO2 from the atmosphere, and certain high-importance adaptation measures (e.g., genetically engineered crops, to avoid famine) should be robustly supported by present-day government policy and investment. This will help to ensure we have a full portfolio of options with which to tackle future challenges as they arise.