The use of energy is considered sustainable if it meets the needs of the present without compromising the needs of future generations. Definitions of sustainable energy typically include environmental aspects such as greenhouse gas emissions, and social and economic aspects such as energy poverty. Renewable energy sources such as wind, hydroelectric power, solar, and geothermal energy are generally far more sustainable than fossil fuel sources. However, some renewable energy projects, such as the clearing of forests to produce biofuels, can cause severe environmental damage. The role of non-renewable energy sources has been controversial. Nuclear power is a low-carbon source and has a safety record comparable to wind and solar, but its sustainability has been debated because of concerns about nuclear proliferation, radioactive waste and accidents. Switching from coal to natural gas has environmental benefits but may lead to a delay in switching to more sustainable options. Carbon capture and storage technology can be built into power plants to remove their carbon dioxide emissions, but is expensive and has seldom been implemented.
The global energy system, which is 85% based on fossil fuels, is responsible for 76% of the greenhouse gas emissions that cause climate change. Around 790 million people in developing countries lack access to electricity and 2.6 billion rely on polluting fuels such as wood or charcoal to cook. Reducing greenhouse gas emissions to levels consistent with the Paris Agreement will require a system-wide transformation of the way energy is produced, distributed, stored, and consumed. As the burning of fossil fuels and biomass is a major contributor to air pollution, which causes an estimated 7 million deaths each year, the transition to a low-carbon energy system would have strong co-benefits for human health. Pathways exist to provide universal access to electricity and clean cooking technologies in ways that are compatible with climate goals, while bringing major health and economic benefits to developing countries.
Climate change mitigation scenarios describe pathways in which the world would rapidly shift to low-emission methods of generating electricity, rely less on burning fuels for energy, and rely more on electricity instead. For some energy-intensive technologies and processes that are difficult to electrify, many scenarios describe a growing role for hydrogen fuel produced from low-emission energy sources. To accommodate larger shares of variable renewable energy, electrical grids require flexibility through infrastructure such as energy storage. To make deep cuts in emissions, infrastructure and technologies that use energy, such as buildings and transport systems, would need to be modified to use clean forms of energy and also to conserve energy. Some critical technologies for eliminating energy-related greenhouse gas emissions are not yet mature.
Wind and solar energy generated 8.5% of worldwide electricity in 2019, a share that has grown rapidly, while costs have fallen and are projected to continue falling. The IPCC estimates that 2.5% of world GDP would need to be invested in the energy system each year between 2016 and 2035 to limit global warming to 1.5 °C (2.7 °F). Well-designed government policies that promote energy system transformation can lower greenhouse gas emissions and improve air quality simultaneously, and in many cases can also increase energy security. Policy approaches include carbon-pricing, renewable portfolio standards, phase-outs of fossil fuel subsidies, the development of infrastructure to support electrification and sustainable transport, and funding research, development, and demonstration of new clean energy technologies.
"Energy is the golden thread that connects economic growth, increased social equity, and an environment that allows the world to thrive. Development is not possible without energy, and sustainable development is not possible without sustainable energy."
The United Nations Brundtland Commission in its 1987 report, Our Common Future, described the concept of sustainable development, for which energy is a key component. It defined sustainable development as development that "meets the needs of the present without compromising the ability of future generations to meet their own needs". This description of sustainable development has since been referenced in many definitions and explanations of sustainable energy.
No single interpretation of how the concept of sustainability applies to energy has gained worldwide acceptance. Working definitions of sustainable energy encompass multiple dimensions of sustainability, such as environmental, economic, and social dimensions. Historically, the concept of sustainable energy development has focused on emissions and on energy security. Since the early 1990s, the concept has broadened to encompass wider social and economic issues.
The environmental dimension includes greenhouse gas emissions, impacts on biodiversity and ecosystems, hazardous waste and toxic emissions, water consumption, and depletion of non-renewable resources. Energy sources with low environmental impact are sometimes called 'green energy' or 'clean energy'. The economic dimension covers economic development, efficient use of energy, and energy security to ensure that each country has constant access to sufficient energy. Social issues include access to affordable and reliable energy for all people, workers' rights and land rights.
The current energy system contributes to many environmental problems, including climate change, air pollution, biodiversity loss, the release of toxins into the environment, and water scarcity. As of 2019, 85% of the world's energy needs are met by burning fossil fuels. Energy production and consumption are responsible for 76% of annual human-caused greenhouse gas emissions as of 2018. The international Paris Agreement on climate change aims to limit global warming to well below 2 °C (3.6 °F) and preferably to 1.5 °C (2.7 °F); achieving this goal will require that emissions be reduced as soon as possible and reach net-zero by mid-century.
The burning of fossil fuels and biomass is a major source of air pollutants. The combustion of coal releases gases which form into ground-level ozone and acid rain, especially if the coal is not cleaned before combustion. Air pollution is the second-leading cause of death from non-infectious disease. Around 91% of the world's population lives with levels of air pollution that exceed the World Health Organization's (WHO) recommended limits. The WHO estimates that outdoor air pollution causes 4.2 million deaths per year.
Cooking with polluting fuels such as wood, animal dung, coal, or kerosene is responsible for nearly all indoor air pollution, which causes in an estimated 1.6 to 3.8 million deaths annually, and also contributes significantly to outdoor air pollution. Health effects are concentrated among women, who are likely to be responsible for cooking, and young children.
Environmental impacts extend beyond the by-products of combustion. Oil spills at sea harm marine life and may cause fires which release toxic emissions. Around 10% of global water use goes to energy production, mainly for cooling in thermal energy plants. In dry regions, this contributes to water scarcity. Bioenergy production, coal mining and processing, and oil extraction also require large amounts of water. Excessive harvesting of wood and other combustible material for burning can cause serious local environmental damage, including desertification.
Meeting existing and future energy demand in a sustainable way is a critical challenge for the global ambition to reduce the impact of climate change while maintaining economic growth and enabling living standards to rise. Reliable and affordable energy, particularly electricity, is essential for health care, education, and economic development. As of 2020, 790 million people in developing countries do not have access to electricity, and around 2.6 billion rely on burning polluting fuels for cooking.
Improving energy access in the least-developed countries and making energy cleaner are key to achieving most of the United Nations 2030 Sustainable Development Goals, which cover issues ranging from climate action to gender equality. Sustainable Development Goal 7 calls for "access to affordable, reliable, sustainable and modern energy for all", including universal access to electricity and to clean cooking facilities by 2030.
Energy efficiency--using less energy to deliver the same goods or services--is a cornerstone of many sustainable energy strategies. The International Energy Agency (IEA) has estimated that increasing energy efficiency could achieve 40% of greenhouse gas emission reductions needed to fulfil the Paris Agreement's goals.
Energy can be conserved by increasing the technical efficiency of appliances, vehicles, industrial processes and buildings. Another approach is to use less materials that require a lot of energy for production, for example through better building design and recycling. Behavioural changes such as using videoconferencing rather than business flights, or making urban trips by cycling, walking or public transport rather than by car, are another way to conserve energy. Government policies to improve efficiency can include building codes, performance standards, carbon pricing, and the development of energy-efficient infrastructure to encourage changes in transport modes.
The energy intensity of the global economy (the amount of energy needed per unit of GDP) is a rough indicator of the energy efficiency of economic production. United Nations targets for 2030 include a doubling of the rate of improvement in energy efficiency. Energy intensity has been gradually decreasing for decades, however improvements have slowed in recent years, and a faster rate of efficiency improvement would be necessary to meet global targets for 2030. Efficiency improvements often lead to a rebound effect in which consumers use the money they save to buy more energy-intensive goods and services. Recent technical efficiency improvements in transport and buildings have been largely offset by trends in consumer behaviour, such as purchasing larger vehicles and homes.
Renewable energy technologies are essential contributors to sustainable energy, as they generally contribute to global energy security and reduce dependence on fossil fuel resources, thus mitigating greenhouse gas emissions. Renewable energy projects sometimes raise significant sustainability concerns, such as risks to biodiversity when areas of high ecological value are converted to bioenergy production, wind or solar farms.
Hydropower is the largest source of renewable electricity while solar and wind energy are growing rapidly. Photovoltaic solar and onshore wind are the cheapest forms of new power generation capacity in most countries. For more than half of the 770 million people who currently lack access to electricity, decentralised renewable energy solutions such as solar-powered mini-grids are likely to be the cheapest method of providing access by 2030. United Nations targets for 2030 include substantially increasing the proportion of renewable energy in the world's energy supply.
The Sun is Earth's primary source of energy, a clean and abundantly available resource in many regions. In 2019, solar power provided around 3% of global electricity, mostly through solar panels based on photovoltaic cells (PV). The panels are mounted on top of buildings, or installed in utility-scale solar parks. Costs of solar photovoltaic cells have dropped rapidly, driving a strong growth in worldwide capacity. The cost of electricity from new solar farms is competitive with, or in many places, cheaper than electricity from existing coal plants. Various projections of future energy use identify solar PV as one of the main sources of energy generation in a sustainable mix.
Most components of solar panels can be easily recycled, but it is not always done in the absence of regulation. Panels typically contain heavy metals, so they pose environmental risks if put in landfill. Solar panels require energy for their production, equivalent to under two years of their own generation, but less if materials are recycled rather than mined.
In concentrated solar power, solar rays are concentrated by a field of mirrors, heating a fluid. Electricity is produced from the resulting steam with a heat engine. Concentrated solar power can support dispatchable power generation, as some of the heat is typically stored to enable electricity to be generated when needed. In addition to electricity production, solar energy is used more directly; solar thermal heating systems are used for hot water production, heating buildings, drying and desalination.
Wind has been an important driver of development over millennia, providing mechanical energy for industrial processes, water pumps, and sailing ships. Modern wind turbines are used to generate electricity, and provided approximately 6% of global electricity in 2019. Electricity from onshore wind farms is often cheaper than existing coal plants, and competitive with natural gas and nuclear. Wind turbines can also be placed offshore where winds are steadier and stronger than on land, but construction and maintenance costs are higher.
Onshore wind farms, often built in wild or rural areas, have a visual impact on the landscape. While collisions with wind turbines kill both bats and to a lesser extent birds, these impacts are fewer than from other infrastructure such as windows and transmission lines. The noise and flickering light created by the turbines can be annoying, and constrain construction near densely populated areas. Wind power, in contrast to nuclear and fossil fuel plants does not consume water to produce power. Little energy is needed for wind turbine construction compared to the energy produced by the wind power plant itself. Turbine blades are not fully recyclable and research into methods of manufacturing easier-to-recycle blades is ongoing.
Hydroelectric plants convert the energy of moving water into electricity. In 2020, hydropower supplied 17% of the world's electricity, down from a high of nearly 20% in the mid-to-late 20th century.
In conventional hydropower, a reservoir is created behind a dam. Conventional hydropower plants provide a highly flexible, dispatchable electricity supply and can be combined with wind and solar power to meet peaks in demand and to compensate when wind and sun are less available.
On average, hydropower ranks among the energy sources with the lowest levels of greenhouse gas emissions per unit of energy produced, but levels of emissions vary enormously between projects, with the highest emissions tending to be in tropical regions. These emissions are produced when the biological matter that becomes submerged in the reservoir's flooding decomposes and releases carbon dioxide and methane. Deforestation and climate change can reduce energy generation from hydroelectric dams. Depending on location, large dams can displace residents and cause significant local environmental damage; potential dam failure could place the surrounding population at risk.
Compared to reservoir-based facilities, run-of-the-river hydroelectricity generally has less environmental impact. However, its ability to generate power depends on river flow, which can vary with daily and seasonal weather. Reservoirs provide water quantity controls that are used for flood control and flexible electricity output while also providing security during drought for drinking water supply and irrigation.
Geothermal energy is produced by tapping into deep underground heat, harnessing it to generate electricity, and using it to heat water and buildings. The use of geothermal energy is concentrated in regions where heat extraction is economical: a combination is needed of high temperatures, heat flow and permeability (the ability of the rock to allow fluids to pass through). Power is produced via wells drilled into reservoirs.[clarification needed] Fluids heat up underground, and can be captured as steam to drive a heat turbine. Together with solar thermal, geothermal energy met 2.2% of worldwide demand for heating in buildings in 2019.
Geothermal energy is a renewable resource because thermal energy is constantly replenished from neighbouring hotter regions and the radioactive decay of naturally occurring isotopes. Per unit of electricity produced, the median life-cycle greenhouse gas emissions of geothermal electric stations are less than 5% of the emissions of coal. Geothermal energy carries a risk of inducing earthquakes, needs effective protection to avoid water pollution, and releases toxic emissions, which can be captured.
Biomass is renewable organic material that comes from plants and animals. It can either be burned to produce heat and to generate electricity or converted to modern biofuels such as biodiesel and ethanol, which can be used to power vehicles.
The climate impact of bioenergy varies considerably depending on where biomass feedstocks come from and how they are grown. For example, burning wood for energy releases carbon dioxide; those emissions can be significantly offset if the trees that were harvested are replaced by new trees in a well-managed forest, as the new trees will absorb carbon dioxide from the air as they grow. However, the establishment and cultivation of bioenergy crops can displace natural ecosystems, degrade soils and consume water resources and synthetic fertilisers. Approximately one-third of all wood used for fuel is harvested unsustainably. Bioenergy feedstocks typically require significant amounts of energy to harvest, dry, and transport, and the energy usage for these processes may emit greenhouse gases. In some cases, the impacts of land-use change, cultivation, and processing can result in higher overall carbon emissions for bioenergy compared to using fossil fuels.
Use of farmland for growing biomass can result in less land being available for growing food. In the United States, corn-based ethanol has replaced around 10% of motor gasoline, which requires a significant proportion of the yearly corn harvest. In Malaysia and Indonesia, clearing forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for endangered species. Since photosynthesis captures only a small fraction of the energy in sunlight, producing a given amount of bioenergy requires a large amount of land compared to other renewable energy sources.
Second-generation biofuels which are produced from non-food plants reduce competition with food production, but may have other negative effects including trade-offs with conservation areas and local air pollution. Relatively sustainable sources of biomass include algae, waste, and crops grown on soil unsuitable for food production. If the biomass source is agricultural or municipal waste, burning it or converting it into biogas provides a way to dispose of this waste.
Carbon capture and storage technology can be used to capture emissions from bioenergy power plants. This process is known as bioenergy with carbon capture and storage (BECCS) and can result in net carbon dioxide removal from the atmosphere. However, BECCS can also result in net positive emissions depending on how the biomass material is grown, harvested, and transported. Deployment of BECCS at scales described in some climate change mitigation pathways would require converting large amounts of cropland.
Marine energy has the smallest share of the energy market. It encompasses tidal power, which is approaching maturity and wave power, which is earlier in its development. Two tidal barrage systems, in France and in South Korea, make up 90% of total production. While single marine energy devices pose little risk to the environment, the impacts of larger devices are less well known.
Switching from coal to natural gas has advantages in terms of sustainability. For a given unit of energy produced, the life-cycle greenhouse-gas emissions of natural gas are around 40 times the emissions of wind or nuclear energy, but are much less than coal. Natural gas produces around half the emissions of coal when used to generate electricity and around two-thirds the emissions of coal when used to produce heat. Reducing methane leaks in the process of extracting and transporting natural gas could further decrease its climate impact. Natural gas produces less air pollution than coal.
Building gas-fired power plants and gas pipelines is promoted as a way to phase out coal and wood burning pollution and increase energy supply in some African countries with fast growing populations and economies, but this practice is controversial. Developing natural gas infrastructure risks carbon lock-in and stranded assets, where new fossil infrastructure either commits to decades of carbon emissions, or has to be written off before it makes a profit.
The greenhouse gas emissions of fossil fuel and biomass power plants can be significantly reduced through carbon capture and storage (CCS), however deployment of this technology is still very limited, with only 21 large-scale CCS plants in operation worldwide as of 2020. The CCS process is expensive, with costs depending considerably on the location's proximity to suitable geology for carbon dioxide storage. Most studies use a working assumption that CCS can capture 85-90% of the emissions from a power plant. If 90% of emitted is captured from a coal-fired power plant, its uncaptured emissions would still be many times greater than the emissions of nuclear, solar or wind energy per unit of electricity produced. Since coal plants using CCS would be less efficient, they would require more coal and thus increase the pollution associated with mining and transporting coal.
Nuclear power plants have been used since the 1950s as a low-carbon source of baseload electricity. Nuclear power plants in over 30 countries generate about 10% of global electricity, and as of 2019, over a quarter of all low-carbon energy, the second largest source after hydropower.
Nuclear power's lifecycle greenhouse gas emissions--including the mining and processing of uranium--are similar to the emissions from renewable energy sources. Nuclear power uses little land per unit of energy produced, compared to the major renewables, and does not create local air pollution. The uranium ore used to fuel nuclear fission plants is a non-renewable resource, but sufficient quantities exist to provide a supply for hundreds to thousands of years. Climate change mitigation pathways consistent with ambitious goals typically see an increase in power supply from nuclear.
There is controversy over whether nuclear power is sustainable. Radioactive nuclear waste must be managed over multi-generation timescales and nuclear power plants can create fissile material that could be used for nuclear weapon proliferation. The perceived risk of nuclear accidents has a major influence on public opinion of nuclear energy, although for each unit of energy produced, nuclear energy is far safer than fossil fuel energy and comparable to renewable sources. Public opposition often makes nuclear plants politically difficult to implement. Experts from the Joint Research Centre (JRC), the scientific expert arm of the EU, stated in April 2021 that nuclear power is "sustainable". Two other groups of experts--SCHEER (Scientific Committee on Health, Environmental and Emerging Risks) and "Article 31"--largely confirmed JRC findings in July 2021.
Reducing the time and the cost of building new nuclear plants have been goals for decades, but progress has been limited. Various new forms of nuclear energy are in development, hoping to address the drawbacks of conventional plants. Fast breeder reactors are capable of recycling nuclear waste and therefore can significantly reduce the amount of waste that requires geological disposal but have not yet been deployed on a large-scale commercial basis. Nuclear power based on thorium, rather than uranium, may be able to provide higher energy security for countries that do not have a large supply of uranium. Small modular reactors may have several advantages over current large reactors: it should be possible to build them faster, and their modularization would allow for cost reductions via learning-by-doing. Several countries are attempting to develop nuclear fusion reactors, which would generate small amounts of waste and no risk of explosions.
The emissions reductions necessary to keep global warming below 2°C will require a system-wide transformation of the way energy is produced, distributed, stored, and consumed. For a society to replace one form of energy with another, multiple technologies and behaviours in the energy system must change. For example, transitioning from oil to solar power as the energy source for automobiles requires the generation of solar electricity, modifications to the electrical grid to accommodate fluctuations in solar panel output and higher overall demand, adoption of electric cars, and networks of electric vehicle charging facilities and repair shops.
Many climate change mitigation scenarios envision three main aspects of a low-carbon energy system:
Some energy-intensive technologies and processes are difficult to electrify, including aviation, shipping, and steelmaking. There are several options for reducing the emissions from these sectors: biofuels and synthetic carbon-neutral fuels can power many vehicles that are designed to burn fossil fuels, however biofuels cannot be sustainably produced in the quantities needed and synthetic fuels are currently very expensive. For some applications, the most prominent alternative to electrification is to develop a system based on sustainably-produced hydrogen fuel.
Full decarbonization of the global energy system is expected to take several decades and can mostly be achieved by deploying existing technology. The International Energy Agency states that further innovation in the energy sector, such as in battery technologies and carbon-neutral fuels, is needed to reach net-zero emissions by 2050. Development of new technologies requires research and development, demonstration and cost reductions via deployment. The transition to a zero-carbon energy system would bring strong co-benefits for human health: The WHO estimates that efforts to limit global warming to 1.5 °C could save millions of lives each year from air pollution alone. With responsible planning and management, pathways exist to provide universal access to electricity and clean cooking by 2030 in ways that are consistent with climate goals. Historically, several countries have made rapid economic gains through coal usage, particularly in Asia. However, there remains a window of opportunity for many poor countries and regions to "leapfrog" fossil fuel dependency by developing their energy systems based on renewables, given adequate international investment and knowledge transfer.
To deliver reliable electricity from variable renewable energy sources such as wind and solar, electrical power systems require flexibility. Most electrical grids were constructed for non-intermittent energy sources such as coal-fired power plants. As larger amounts of solar and wind energy are integrated into the grid, changes have to be made to the energy system to ensure that the supply of electricity is matched to demand. In 2019, these sources generated 8.5% of worldwide electricity, a share that has grown rapidly.
There are various ways to make the electricity system more flexible. In many places, wind and solar generation are complementary on a daily and a seasonal scale: there is more wind during the night and in winter when solar energy production is low. Linking different geographical regions through long-distance transmission lines allows for further cancelling out of variability. Energy demand can be shifted in time through energy demand management and the use of smart grids, matching the times when variable energy production is highest. With grid energy storage, energy produced in excess can be released when needed. Further flexibility could be provided from sector coupling, that is coupling the electricity sector to the heat and mobility sector via power-to-heat-systems and electric vehicles.
Building overcapacity for wind and solar generation can help to ensure that enough electricity is produced even during poor weather - during optimal weather energy generation may have to be curtailed, if excess electricity cannot be used or stored. The final demand-supply mismatch may be covered by using dispatchable energy sources such as hydropower, bioenergy, or natural gas.
Energy storage helps overcome barriers to intermittent renewable energy and is an important aspect of a sustainable energy system. The most commonly used storage method is pumped-storage hydroelectricity, which requires locations with large differences in height and access to water. Batteries, and specifically lithium-ion batteries whose costs have been coming down rapidly, are also deployed widely. Batteries typically store electricity for short periods; research is ongoing into technology with sufficient capacity to last through seasons. Pumped hydro storage and power-to-gas (converting electricity to gas, and back) with capacity for multi-month usage has been implemented in some locations.
As of 2019, 37% of global electricity is produced from low-carbon sources, that is renewables and nuclear energy. Fossil fuels, primarily coal, produce the rest of the electricity supply. One of the easiest and fastest ways to reduce greenhouse gas emissions is to phase out coal-fired power plants and increase renewable electricity generation. As of 2018, about a quarter of all electricity generation came from renewable sources other than biomass.
In addition to decarbonizing electricity generation, climate change mitigation scenarios envision extensive electrification -- the use of electricity as a substitute for the direct burning of fossil fuels. Many options exist to produce electricity sustainably, but producing fuels or heat on a large scale sustainably is relatively difficult. Ambitious climate policy would see a doubling of energy consumed as electricity by 2050, from 20% in 2020.
One of the challenges in providing universal access to electricity is distributing power to rural areas. Off-grid and mini-grid systems based on renewable energy, such as small solar PV installations that generate and store enough electricity for a village, are important solutions. Wider access to reliable electricity would lead to less use of kerosene lighting and diesel generators, which are currently common in the developing world.
Infrastructure for generating and storing renewable electricity requires minerals and metals, such as cobalt and lithium for batteries and copper for solar panels. Recycling can meet some of this demand if product lifecycles are well-designed, however achieving net zero emissions would still require major increases in mining for 17 types of metals and minerals. A small group of countries or companies sometimes dominate the markets for these commodities, raising geopolitical concerns. Cobalt, for instance, is mined in Congo, a politically unstable region where mining is often associated with human rights risks. More diverse geographical sourcing may ensure the stability of the supply chain.
Hydrogen is a gas that can be burned to produce heat or can power fuel cells to generate electricity, with zero emissions at the point of usage. The overall lifecycle emissions of hydrogen depend on how it is produced. Nearly all of the world's current supply of hydrogen is created from fossil fuels. The predominant method is steam methane reforming in which hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6-9.3 tonnes of carbon dioxide. While carbon capture can remove a large fraction of these emissions, the overall carbon footprint of hydrogen from natural gas is difficult to assess as of 2021 , in part because of emissions created in the production of the natural gas itself.
Electricity can be used to split water molecules, producing sustainable hydrogen provided the electricity was generated sustainably. But this electrolysis is currently more expensive than creating hydrogen from methane, and the efficiency of energy conversion is inherently low. Hydrogen can be produced when there is a surplus of intermittent renewable electricity, then stored and used to generate heat or to re-generate electricity. It can be further transformed into synthetic fuels such as ammonia and methanol.
Innovation in hydrogen electrolysers could make large-scale production of hydrogen from electricity more cost-competitive. There is potential for hydrogen to play a significant role in decarbonising energy systems because in certain sectors, replacing fossil fuels with direct use of electricity would be very difficult. Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals. For steelmaking, hydrogen can function as a clean energy carrier and simultaneously as a low-carbon catalyst replacing coal-derived coke. Disadvantages of hydrogen as an energy carrier include high costs of storage and distribution due to hydrogen's explosivity, its large bulk compared to other fuels, and its tendency to make pipes brittle.
There are multiple ways to make transport more sustainable. Public transport frequently emits fewer greenhouse gases per passenger than personal vehicles, especially with high occupancy. High-speed rail journeys, which use much less fuel, can replace short-distance flights. Stimulating non-motorised transport such as walking and cycling, particularly in cities, can make transport cleaner and healthier.
The energy efficiency of cars has increased because of technological progress, but shifting to electric vehicles is an important further step towards decarbonising transport and reducing air pollution. A large proportion of traffic-related air pollution consists of particulate matter from road dust and the wearing-down of tyres and brake pads. Substantially reducing pollution from these sources cannot be achieved by electrification; it requires measures such as making vehicles lighter and driving them less.
Making freight transport sustainable is challenging. Hydrogen vehicles may be an option for larger vehicles such as lorries which have not yet been widely electrified because of the weight of batteries needed for long-distance travel. Many of the techniques needed to lower emissions from shipping and aviation are still early in their development, with ammonia (produced from hydrogen) a promising candidate for shipping fuel. Aviation biofuel may be one of the better uses of bioenergy, providing some carbon is captured and stored during manufacture of the fuel.
To heat buildings, alternatives to burning fossil fuels and biomass include electrification through heat pumps or electric heaters, geothermal energy, central solar heating, waste heat, and seasonal thermal energy storage. Heat pumps provide both winter heat and summer air conditioning through a single appliance. The IEA estimates heat pumps could provide over 90% of space and water heating requirements globally.
A highly efficient way to heat buildings is through district heating, in which heat is generated in a centralized location and then distributed to multiple buildings through insulated pipes. Traditionally, most district heating systems have used fossil fuels, but modern and cold district heating systems are designed to use high shares of renewable energy.
Cooling of buildings can be made more efficient through passive building design, planning that minimizes the urban heat island effect, and district cooling systems that cool multiple buildings with piped cold water. Air conditioning requires large amounts of electricity and is not always affordable by poorer households. Some air conditioning units use refrigerants that are greenhouse gases; the international Kigali Amendment requires that these be replaced with climate-friendly refrigerants.
In developing countries where populations suffer from energy poverty, polluting fuels such as wood or animal dung are often used for cooking. Cooking with these fuels is generally unsustainable because they release harmful smoke and because harvesting wood can lead to forest degradation. The universal adoption of clean cooking facilities, which are already ubiquitous in rich countries, would dramatically improve health and have minimal effects on climate. Clean cooking facilities typically use natural gas, LPG, or electricity as the energy source; biogas systems are a promising alternative in some contexts. Improved cookstoves that burn biomass more efficiently than traditional stoves are an interim solution where transitioning to clean cooking systems is difficult.
Over one-third of energy use is by industry. Most of that energy is deployed in thermal processes: generating steam, drying, and refrigeration. The share of renewable energy in industry was 14.5% in 2017--mostly low-temperature heat supplied by bioenergy and electricity. The more energy-intensive activities in industry have the lowest shares of renewable energy, as they face limitations in generating heat at temperatures over 200 °C (390 °F).
For some industrial processes, such as steel production, commercialisation of technologies that have not yet been built or operated at full scale will be needed to eliminate greenhouse gas emissions. The production of plastic, cement and fertilizers also requires significant amounts of energy, with limited possibilities available to decarbonise. A switch to a circular economy would make industry more sustainable, as it involves recycling more and thereby using less energy compared to extracting new raw materials.
"Bringing new energy technologies to market can often take several decades, but the imperative of reaching net-zero emissions globally by 2050 means that progress has to be much faster. Experience has shown that the role of government is crucial in shortening the time needed to bring new technology to market and to diffuse it widely."
Well-designed government policies that promote energy system transformation can lower greenhouse gas emissions and improve air quality simultaneously, and in many cases can also increase energy security.
Energy-specific programs and regulations have historically been the mainstays of efforts to reduce fossil fuel emissions. Some governments have committed to dates for phasing out coal-fired power plants, ending new fossil fuel exploration, requiring that new passenger vehicles produce zero emissions, and requiring new buildings to be heated by electricity instead of gas. Renewable portfolio standards have been enacted in several countries requiring utilities to increase the percentage of electricity they generate from renewable sources.
Governments can accelerate energy system transformation by leading the development of infrastructure such as long-distance electrical transmission lines, smart grids and hydrogen pipelines. In transport, appropriate infrastructure and incentives can make travel more efficient and less car-dependent. Urban planning to discourage sprawl can reduce energy use in local transport and buildings while enhancing quality of life. Government-sponsored research, and procurement and incentive policies, have historically been critical to the development and maturation of clean energy technologies such as solar PV and lithium batteries. In the IEA's scenario for a net zero-emission energy system by 2050, public funding is rapidly mobilised to bring a range of nascent technologies to the demonstration phase and to incentivize deployment.
Carbon pricing is an approach that gives industries and consumers an incentive to reduce emissions while giving them flexibility in how to do so, e.g. by shifting to low-emission energy sources, improving energy efficiency, or reducing their use of energy-intensive products and services. Carbon pricing has encountered strong political pushback in some jurisdictions, whereas energy-specific policies tend to be politically safer. Most studies indicate that to limit global warming to 1.5°C, carbon pricing would need to be complemented by stringent energy-specific policies. As of 2019, most carbon pricing mechanisms place a lower price on carbon than what would be needed to achieve the goals of the Paris Agreement. Carbon taxes provide a source of revenue that can be used to lower other taxes or to help lower-income households afford higher energy costs. Some governments are exploring the use of carbon border adjustments, which place tariffs on imports from countries with less stringent climate policies, to ensure that their industries remain competitive.
The scale and pace of policy reforms that have been initiated as of 2020 are far less than needed to fulfil the climate goals of the Paris Agreement. In addition to domestic policies, greater international cooperation will be required to accelerate innovation and to assist poorer countries in establishing a sustainable path to full energy access.
Countries may support renewables to create jobs. The International Labour Organization estimates that efforts to limit global warming to 2 °C would result in net job creation in most sectors of the economy. It predicts that 24 million new jobs would be created in areas such as renewable electricity generation, improving energy-efficiency in buildings and the transition to electric vehicles, while 6 million jobs in the fossil fuel industry would be lost. Governments can make the transition to sustainable energy more politically and socially feasible by ensuring a just transition for workers and regions that depend on the fossil fuel industry, to ensure they have alternative economic opportunities.
Mobilising sufficient finance for innovation and investment is a prerequisite for the energy transition. The IPCC estimates that to limit global warming to 1.5 °C, US$2.4 trillion would need to be invested in the energy system each year between 2016 and 2035. Most studies project that these costs, equivalent to 2.5 percent of world GDP, would be small compared to the economic and health benefits. Average annual investment in low-carbon energy technologies and energy efficiency would need to be six times more by 2050 compared to 2015. Underfunding is particularly acute in the least developed countries. The Paris Agreement includes a pledge of additional funds for poorer countries of $100 billion per year, but this goal has not been achieved. The mobilised money reaches poorer countries via more than a hundred channels, often controlled by rich countries.
The UNFCCC estimates that climate financing totalled $681 billion in 2016, with most of this being private-sector investment in renewable energy deployment, public-sector investment in sustainable transport and private-sector investment in energy efficiency.
Fossil fuel funding and subsidies are a significant barrier to the energy transition. Direct global fossil fuel subsidies were $319 billion in 2017, and $5.2 trillion when indirect costs such as air pollution are priced in. Ending these could lead to a 28% reduction in global carbon emissions and a 46% reduction in air pollution deaths. Funding for clean energy has been largely unaffected by the COVID-19 pandemic, and the required economic stimulus packages offer possibilities for a green recovery.
According to the Global Burden of Disease study 1.6 million people died prematurely in 2017 as a result of indoor air pollution ... But it's worth noting that the WHO publishes a substantially larger number of indoor air pollution deaths...
Our analysis points to end-uses that maximise sequestration (storage of carbon) as being optimal in 2050. These include wood in construction and the production of hydrogen, electricity, industrial products and potentially also aviation biofuels, all with carbon capture and storage. Many current uses of biomass are not in line with longterm best-use and these will need to change.