6.5 Improving Energy Sustainability

Is it feasible to eliminate GHG emissions – to reach net zero emissions?

Plans exist that define emissions reduction strategies to avoid global warming beyond 1.5°C above pre-industrial temperatures or to return quickly to temperatures less than 1.5°C above historic temperatures. However, the longer progress towards sustainable energy use is delayed, the greater the levels of global warming that will be reached. As a practical matter, we have seen that nations have neither pledged nor made sufficient emissions reductions to avoid increasing their GHG emissions and therefore increasing global warming. Following the 2024 presidential elections, the US has made and committed to making steps backwards away from a goal of net zero emissions.

The IPCC’s 6th Assessment Report provided extensive guidance, particularly in the Working Group III report on climate mitigation, for actions to reduce emissions and to remove CO₂ from the atmosphere. The International Energy Agency’s (IEA) Net Zero by 2050 plan^[1] provides a roadmap with many specific milestones for reaching net zero emissions. For the US, the National Renewable Energy Laboratory developed four separate scenarios for achieving 100% clean electricity by 2035^[2].

The IEA breaks the path to decarbonization into 7 processes, with changing importance over time (Fig 1).

• Energy efficiency – decreasing in importance over time as efficiency gains become permanent
• Behavioral change and avoided demand – increasing in importance as technology helps to reduce energy demand
• Electrification – increasing in importance to move energy demand to energy forms that can be supplied cleanly
• Renewables – supply approximately half of emissions reductions until 2030, and supply 90% of electricity by 2050.
• Hydrogen and hydrogen-based fuels – initially a transition fuel to avoid transmission and distribution needs, then a mainstay for flexibility in electricity and for transportation.
• Bioenergy – traditional biomass for cooking ceases by 2030, solid bioenergy provides flexible fuel, heat, CO₂ removal. Liquid biofuel becomes important for aviation.
• Carbon capture, utilization, and storage – offsets emissions during transition, especially in emerging and developing economies, removes CO₂ while permitting use of natural gas.

Figure 1. Decarbonization pathway for IEA Net Zero by 2050 plan. Emissions increase between 2020 and 2030, offset by decarbonization processes. Emissions increase from 2030 to 2050, offset by a different balance of decarbonization processes to reach zero emissions by 2050. International Energy Agency. CC BY.

Several of steps needed to reach zero emissions and beyond require technology and infrastructure not currently available. Carbon capture and storage, hydrogen technologies, and enhanced energy transmission systems will all need to be improved and extensively deployed to halt and eventually reverse global warming. It is not enough to create clean energy – it must be stored so that supply can meet demand, it must be transported from production sites to end users, and new energy types must be integrated into the existing power grid so that control of the power supply remains smooth.

Some processes that currently exist, particularly in carbon uptake by ecosystems and agricultural soils still need extensive study. As a result of ongoing developments and research needs, all scenarios to halt and reverse global warming have considerable uncertainty built into them. Some aspects may go faster than anticipated, but so far, where large uncertainties exist, deployment has been very modest.

Reaching net zero emissions will neither immediately halt nor reverse global warming

As noted in Chapter 2, research indicates that if all anthropogenic GHG emissions were to cease, global temperatures would level off in a few decades – not immediately, but perhaps within a lifetime. But a return to pre-industrial temperatures – a decrease in planetary temperature – will require many centuries. Partly this is due to the lifetime of greenhouse gases in the atmosphere, but CO₂ and heat have also been absorbed by the ocean, including the deep ocean, and all of that heat and CO₂ must be purged to return the planet to pre-industrial temperatures.

The IPCC scenarios that result in global warming that remains below 1.5°C above historic levels (or quickly returns to levels below 1.5°C following an overshoot) all combine zero emissions with removal of GHG from the atmosphere. Carbon reduction strategies are a suite of practices, techniques, and associated technologies that manage carbon either by reducing emissions of GHG or by removing GHG already present in the atmosphere. Carbon capture and storage is one technique that involves capturing carbon emissions at the source – at power plants and industrial sites – and redirecting them into storage in the deep subsurface, or removing CO₂ directly from the atmosphere through a variety of means and also storing them geologically. Some emissions may be used to create carbon-based products, in carbon capture, use, and storage (Fig 2).

Figure 2. A generic carbon capture, use, and storage system. International Energy Agency CC BY.

Although deployment of carbon capture and storage is increasing, it still falls short of levels considered necessary to fulfill its role as part of carbon management strategies needed to hold warming at or below 1.5°C (Fig 3). Technology for carbon capture at emissions sites is still improving; support for development and implementation can speed progress, but political and economic barriers may exist.

Figure 3. Carbon capture capacity compared to needed capacity for the International Energy Agency’s net zero emissions (NZE) scenario, 2020-2030. International Energy Agency. CC BY.

NZE = Net Zero Emissions by 2050 Scenario. Includes large-scale projects with a capture capacity over 100 000 t per year (1 000 t per year for DAC). Capture projects for CO₂ use are included as long as CO₂ is used in fuels, chemicals, polymers, building materials, or for yield boosting. Within planned carbon-capture-use-storage (CCUS) industrial hubs, only identified CO₂ capture projects are included (not the full potential capture capacity of industrial hubs for which capture sources are not specified).

Nature-based climate solutions are also available for carbon capture and storage. Plants take up CO₂ during photosynthesis and store the resulting carbohydrates in biomass. Soil can sequester carbon from organic material. Some organic material, such as woody tissue, is naturally resistant to decomposition. But organic material can also become sequestered within soil particles, and bound to the surface of soil particles, holding it in the soil without decomposition to CO₂.

Land management practices of ecosystems including halting deforestation, restoring naturally occurring forests (reforestation), planting forests where forests did not previously exist (afforestation), restoration of ecosystems with high plant biomass and high-carbon soils such as occur in wetlands, some tropical forests, and boreal forests can increase CO₂ removal from the atmosphere and retention in ecosystems. Regenerative agriculture that seeks to maintain and improve soil health is similarly useful. We will see more about these practices in later chapters.

Barriers to a transition to clean energy

Complex energy systems

Energy contributes to quality of life for individuals and drives economies of nations. Traditional energy systems are over a century old and worked well to advance society into the modern age through the use of high-density energy resources – fossil fuels. However, the greenhouse gases associated with that progress are an outstanding example of an externality that was never calculated into the overall expense of the energy systems, and the cost of addressing that externality has become extreme, and, unsurprisingly, unwelcome. Cost reductions have helped to ease the transition, but as energy demands continue to increase, the overall share of cleaner energy sources in the world energy portfolio has increased rather little.

Wholesale change in energy portfolios is therefore something to approach with care. Science can describe a path to clean energy, but implementing such large changes involves a host of other actors in policy, politics, communication and other social and economic areas.

National energy grids are typically complicated and highly regulated. Energy is subject to regulation at local, state, and federal levels, in the US, and even changes associated with conventional energy can be mired in delays.  Watch video click here for a discussion of the complexities associated with the US power grid and the increasing power demands associated with electric cars (if you like, stop at time code 11:13). Note that most of the relevant elements of the Inflation Reduction Act in the US, referenced in the video, have been halted by the subsequent US administration.

Social, political, and economic forces

From the earliest stages of global warming, scientific information has been met with distrust and disbelief. As global warming progresses and scientific certainty grows, along with loss of life and property, distrust and disbelief persist. Considerations of financial and political power have led to persistent misinformation and opposition^[3]. These, in turn, are sometimes offset by the economic and social advantages associated with reducing air and water pollution and climate change.

In the past, new technologies that were in the best interests of society often received support from governments in the form of direct subsidies, tax exemptions, protection of patents, etc. As technologies mature, industry begins to take over support in order to make profits, and prices begin to become competitive with existing technologies, so that market forces begin to encourage adoption of the new technology. The more consistent the support, and the more considered the transition from government support to private support to widespread adoption, the more straightforward the process. Development of both technology and policy to halt and reverse climate change has often lacked support and consistency. The changes envisioned are large, which creates the potential for economic and political winners and losers. The US, in particular has seen see-sawing support for climate-related actions and policies. Changes in financial incentives for climate-related actions have global repercussions; supply chains for climate-related technologies, including for critical minerals, have global reaches. The result has been slower progress, but not halted progress, in combating global warming.

For a discussion of the global processes related just to carbon capture and sections, watch this video that discusses the status, trends, and barriers to carbon capture and sequestration as a global mechanism to reduce and reverse carbon emissions. From the The 2nd High-Level Roundtable on Carbon Management Technologies, held in Riyadh, Saudi Arabia in February 2023. Note that most of the relevant elements of the Inflation Reduction Act in the US, referenced in the video, have been halted by the subsequent US administration, whose intransigence in matters of energy production contributes significantly to the barriers to decarbonization.

Geoengineering: an emergency intervention for climate change

Geoengineering involves major interventions into Earth’s energy processes in order to reduce or reverse climate change. Proponents suggest such interventions as emergency measures to avoid the worst impacts from climate change. However, safe testing procedures would require a test planet, and ours is the only one available. The balance between the documented evident harms from present and future climate change on one side and the potential harm of impacts from geoengineering is very hard to estimate, given the large number of unknowns on both sides.

Solar radiation modifications

The group of geoengineering interventions grouped together as solar radiation modifications seek to increase planetary reflectance or albedo to reduce the warming impacts of solar radiation.

• Injecting sulfur dioxide into the stratosphere to increase albedo. As we learned in chapter 2, atmospheric sulfates are associated with acid rain, which can cause severe air and water pollution and significant damage to terrestrial and aquatic ecosystems. Modeling work on sulfate geoengineering suggests that the approach is unlikely to cause catastrophic harm, but would change the location of acid rain from industrial regions to less disturbed parts of the planet where sensitive ecosystems may be at risk.^[4] Sulfate injections are achievable with current technology and would last 1-3 years in the atmosphere, requiring ongoing injections.
• Spraying aerosols of sea salt (basically, seawater) into low-elevation marine clouds to enhance cloud cover and associated reflectivity. This practice is achievable with current technology. Effects would be short-lived, so it would be easy to stop impacts. A recent review of research needed to understand this approach emphasizes that the effectiveness of the practice is still unclear, and that risks of changes to regional temperature and rainfall and resulting impacts to human and natural systems are also unclear.^[5]
• Orbiting mirrors or sunshades. This approach would carry large blocking, reflecting, or solar-panel-covered bodies into a point in space where they would be stable and would intercept solar radiation. This approach is currently only hypothetical as the fleet of spacecraft needed for the operation and the engineering technology needed for the sunshades do not exist at this time. Proponents point out that deploying sunshades avoids actions on Earth, can be undone at need (if you have the spacecraft to deploy them, theoretically, you have the spacecraft to move them out of line with the sun to return incoming radiation), and could, potentially, supply additional energy to Earth if the sunshades were designed to collect or focus solar energy to terrestrial energy facilities.

Carbon dioxide removal

A variety of geoengineering techniques are suggested that can remove carbon dioxide from the atmosphere using biological and geological approaches. Four of the most common suggestions follow.

• Direct-air capture removes carbon dioxide directly from the atmosphere and then injects it deep underground to sequester it there, permanently. In 2024,  the International Energy Agency reported that several nations had plans to implement carbon removal in the coming years, but the process is expensive and the volumes of C to be removed are still small relatively to the overall need.^[6]
• Nature-based climate solutions facilitate carbon uptake by plants and soil in natural ecosystems and also in agro-ecosystems. In addition, some solutions seek to maintain carbon in natural carbon sinks, particularly in organic soils in tundra, wetlands, and peatlands, including tropical rainforest peatlands. Our understanding of the details of carbon processes in natural ecosystems and agroecosystems is still incomplete^[7], but researchers estimate that land systems presently absorb perhaps 30% of current emissions.^[8] Reforestation and protection of existing forests, wetland restoration, and supporting large grazer populations on tundra are some of the recommended approaches for increasing carbon sequestration in natural systems. Chapter 7 describes some of these approaches for using agroecosystems for carbon sequestration. 
• Ocean fertilization uses iron additions to the ocean to increase algal biomass in less productive parts of the ocean. The algae take up CO₂ during their lifetimes, during photosynthesis and incorporate it into their biomass. When they die, some of the biomass sinks to the ocean floor, which is a global C sink. Ocean fertilization experiments have been performed, and they do increase uptake of C by algae. However, these experiments also have the potential to cause ecosystem-level changes that propagate through the ocean, reducing productivity by using up nutrients to produce algae that would otherwise be used elsewhere in the ocean. Models of fertilizing the Southern Ocean suggest significant impacts to tropical fisheries, demonstrating the potential for far-reaching ripple effects from the practice.^[9]
• Enhanced weathering of rock formations to absorb CO₂ during weathering – CO₂ from the atmosphere is chemically fixed into the minerals that form during weathering. In these approaches, ground rock would be applied to oceans or land. Powdering the rock (olivine is one suggested mineral to use) greatly increases its surface area, providing more surface for weathering reactions that transform CO₂ into rock. The chemistry is not always straightforward and rock with the wrong composition could actually increase GHG emissions. But the largest problem with enhanced weathering is the cost and energy use of mining, pulverizing and transporting rock. ^[10]

Just as the use of natural gas was put forward as a means of transitioning from coal to decarbonized energy sources – a way to buy time while technology was developing and society was adapting – geoengineering is suggested as a means of offsetting carbon emissions while society continues to make progress towards decarbonizing. One major criticism of geoengineering is the same as a major criticism leveled against use of natural gas as a transition fuel: those who profit from slowing progress towards decarbonization will use the breathing room gained from geoengineering to continue business as usual with fossil fuels, slowing progress and dulling public perception of the urgency of the climate crisis.

However, as mentioned above in this section, all modeled climate futures that avoid global warming above 1.5 or 2°C require some kind of removal of carbon dioxide or faster-than-decarbonization-only reduction in GHG emissions. To date, carbon capture and storage is the primary means at hand.

The United Nations Convention on Biodiversity created an embargo on large-scale geoengineering in 2010, pending strong scientific justification for its use and full understanding of potential impact. Some have noted that concerns about impacts from geoengineering have mostly successfully stymied work to determine feasibility of some approaches. ^[11] In the face of growing concerns about climate-change impacts and growing pressure to slow climate change, perceptions of nuclear energy have changed considerably. If the pace of decarbonization proves unequal to the task of significantly slowing climate change, perceptions of geoengineering may similarly change.

Energy sustainability is not only about global warming

Global warming is probably the most urgent aspect of energy sustainability at this time, because the resulting climate change affects everyone, everywhere, with increasing force. Climate change touches directly on Sustainable Development Goals 7 (affordable and clean energy) and 13 (climate action), but also on many others, including Goal 3 (good health and well-being), 12 (responsible consumption and production), 14 (life below water) and 15 (life on land).

Increasingly, energy sustainability is also associated with additional social goals, including 1 (no poverty), 2 (zero hunger), 4 (quality education), 5 (gender equality) and 11 (sustainable communities). While developed countries are learning to integrate new, advanced technologies into their energy grids in order to reduce GHG emissions, many areas of the world still have limited access to clean fuels for cooking (Fig 3). The burden of gathering fuel for cooking fires in developing countries falls largely on women and children (particularly girls)^[12], contributing to inequality and lack of education. Similar issues related to unequal impacts arise with mining activities associated with energy. Unequal impacts of energy generation include impacts from climate-change, air and water pollution, and land-use change^[13]. Some of these sustainability issues will be improved as efforts to reduce climate change continue. Legacy problems associated with prolonged, unequal environmental impacts and related social and economic impacts, will require more focused attention.

Figure 3. Proportion of the population with access to clean fuels for cooking, in 2021. OurWorldinData.org. CC BY.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.