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Sustainable energy, sometimes called green energy, is energy produced and used in such a way that it "meets the needs of the present without compromising the ability of future generations to meet their own needs."
The energy transition to meet the world's needs for electricity, heating, cooling, and power for transport in a sustainable way is widely considered to be one of the greatest challenges facing humanity in the 21st century. Worldwide, nearly a billion people lack access to electricity, and around 3 billion people rely on smoky fuels such as wood, charcoal or animal dung in order to cook. These and fossil fuels are a major contributor to air pollution, which causes an estimated 7 million deaths per year. Production and consumption of energy emits over 70% of human-caused greenhouse gas emissions.
Proposed pathways for limiting global warming to 1.5 °C describe rapid implementation of low-emission methods of producing electricity and a shift towards more use of electricity in sectors such as transport. The pathways also include measures to reduce energy consumption; and use of carbon-neutral fuels, such as hydrogen produced by renewable electricity or with carbon capture and storage. Achieving these goals will require government policies including carbon pricing, energy-specific policies, and phase-out of fossil fuel subsidies.
When referring to methods of producing energy, the term "sustainable energy" is often used interchangeably with the term "renewable energy". In general, renewable energy sources such as solar, wind, and hydroelectric energy are widely considered to be sustainable. However, particular renewable energy projects, such as the clearing of forests for the production of biofuels, can lead to similar or even worse environmental damage when compared to using fossil fuel energy. Nuclear power is a safe zero-emission source, but its sustainability is debated because of nuclear waste production and finite uranium resources.
Moderate amounts of wind and solar energy, which are intermittent energy sources, can be integrated into the electrical grid without additional infrastructure such as grid energy storage. These sources generated 8.5% of worldwide electricity in 2019, a share that has grown rapidly. As of 2019, costs of wind, solar, and batteries are projected to continue falling.
The concept of sustainable development was described by the World Commission on Environment and Development in its 1987 book Our Common Future. Its definition of "sustainability", now used widely, was, "Sustainable development should meet the needs of the present without compromising the ability of future generations to meet their own needs." In its book, the Commission described four key elements of sustainability with respect to energy: the ability to increase the supply of energy to meet growing human needs, energy efficiency and conservation, public health and safety, and "protection of the biosphere and prevention of more localized forms of pollution."
Various definitions of sustainable energy have been offered since then which are also based on the three pillars of sustainable development, namely environment, economy, and society.
- Environmental criteria include greenhouse gas emissions, impact on biodiversity, and the production of hazardous waste and toxic emissions.
- Economic criteria include the cost of energy, whether energy is delivered to users with high reliability, and effects on jobs associated with energy production.
- Socio-cultural criteria include the prevention of wars over the energy supply (energy security) and long-term availability of energy.
Providing sustainable energy is widely viewed as one of the greatest challenges facing humanity in the 21st century, both in terms of meeting the needs of the present and in terms of effects on future generations. Bill Gates said in 2011:
If you gave me the choice between picking the next 10 presidents or ensuring that energy is environmentally friendly and a quarter as costly, I'd pick the energy thing.
Worldwide, 940 million (13% of the world) people do not have access to electricity, and 3 billion people rely on dirty fuels for cooking. Air pollution, caused largely by the burning of fuel, kills an estimated 7 million people each year. United Nations Sustainable Development Goal 7 calls for "access to affordable, reliable, sustainable and modern energy for all" by 2030.
Energy production and consumption are major contributors to climate change, being responsible for 72% of annual human-caused greenhouse gas emissions as of 2014. Generation of electricity and heat contributes 31% of human-caused greenhouse gas emissions, use of energy in transportation contributes 15%, and use of energy in manufacturing and construction contributes 12%. An additional 5% is released through processes associated with fossil fuel production, and 8% through various other forms of fuel combustion. As of 2015, 80% of the world's primary energy is produced from fossil fuels.
In developing countries, over 2.5 billion people rely on traditional cookstoves and open fires to burn biomass or coal for heating and cooking. This practice causes harmful local air pollution and increases the danger from fires, resulting in an estimated 4.3 million deaths annually. Additionally, serious local environmental damage, including desertification, can be caused by excessive harvesting of wood and other combustible material. Promoting usage of cleaner fuels and more efficient technologies for cooking is, therefore, one of the top priorities of the United Nations Sustainable Energy for All initiative. As of 2015[update], efforts to design clean cookstoves that are inexpensive, powered by sustainable energy sources, and acceptable to users have been mostly disappointing.
In 2020, the International Energy Agency warned that the economic turmoil caused by the coronavirus outbreak may prevent or delay companies from investing in green energy. The outbreak could potentially spell a slowdown in the world's clean energy transition if no action is undertaken.
Proposed pathways for climate change mitigation
Cost–benefit analysis work has been done by a disparate array of specialists and agencies to determine the best path to decarbonizing the energy supply of the world. The IPCC's 2018 Special Report on Global Warming of 1.5 °C says that for limiting warming to 1.5 °C and avoiding the worst effects of climate change, "global net human-caused emissions of CO
2 would need to fall by about 45% from 2010 levels by 2030, reaching net zero around 2050." As part of this report, the IPCC's working group on climate change mitigation reviewed a variety of previously-published papers that describe pathways (i.e. scenarios and portfolios of mitigation options) to stabilize the climate system through changes in energy, land use, agriculture, and other areas.
The pathways that are consistent with limiting warning to approximately 1.5 °C describe a rapid transition towards producing electricity through lower-emission methods, and increasing use of electricity instead of other fuels in sectors such as transportation. These pathways have the following characteristics (unless otherwise stated, the following values are the median across all pathways):
- Renewable energy: The proportion of primary energy supplied by renewables increases from 15% in 2020 to 60% in 2050. The proportion of primary energy supplied by biomass increases from 10% to 27%, with effective controls on whether land use is changed in the growing of biomass. The proportion from wind and solar increases from 1.8% to 21%.
- Nuclear energy: The proportion of primary energy supplied by nuclear power increases from 2.1% in 2020 to 4% in 2050. Most pathways describe an increase in use of nuclear power, but some describe a decrease. The reason for the wide range of possibilities is that deployment of nuclear energy "can be constrained by societal preferences."
- Coal and oil: Between 2020 and 2050, the proportion of primary energy from coal declines from 26% to 5%, and the proportion from oil declines from 35% to 13%.
- Natural gas: In most pathways, the proportion of primary energy supplied by natural gas decreases, but in some pathways, it increases. Using the median values across all pathways, the proportion of primary energy from natural gas declines from 23% in 2020 to 13% in 2050. 
- Carbon capture and storage: Pathways describe more use of carbon capture and storage for bioenergy and fossil fuel energy.
- Electrification: In 2020, around 20% of final energy use is provided by electricity. By 2050, this proportion more than doubles in most pathways.
- Energy conservation: Pathways describe methods to increase energy efficiency and reduce energy demand in all sectors (industry, buildings, and transport). With these measures, pathways show energy usage to remain around the same between 2010 and 2030, and increase slightly by 2050.
Moving towards energy sustainability will require changes not only in the way energy is supplied, but in the way it is used, and many say that reducing the amount of energy required to deliver various goods or services is essential. Others agree that energy efficiency is desirable, but say that electrification and its automatic efficiency gains (at least in the US as of 2020) would be sufficient to meet the Paris Agreement goals if pursued vigorously enough. Opportunities for improvement on the demand side of the energy equation are as rich and diverse as those on the supply side, and often offer significant economic benefits.
Efficiency slows down energy demand growth so that rising clean energy supplies can make deep cuts in fossil fuel use. A 2011 historical analysis demonstrated that the rate of energy efficiency improvements was generally outpaced by the rate of growth in energy demand, due to continuing economic and population growth. Because carbon emissions over the period studied were coupled with total energy use, despite energy efficiency gains total carbon emissions continued to increase. Thus, given the thermodynamic and practical limits of energy efficiency improvements, slowing the growth in energy demand has been said to be essential. However, unless clean energy supplies come online rapidly, slowing demand growth will only begin to reduce total emissions; reducing the carbon content of energy sources is also needed.[clarification needed] Any serious vision of a sustainable energy economy thus requires commitments to both renewables and efficiency.[unreliable source?]
Renewable energy sources
When referring to sources of energy, the terms "sustainable energy" and "renewable energy" are often used interchangeably, however particular renewable energy projects sometimes raise significant sustainability concerns. Renewable energy technologies are essential contributors to sustainable energy as they generally contribute to world energy security, and reduce dependence on fossil fuel resources thus mitigating greenhouse gas emissions.
In 2019, solar power provided around 3% of global electricity. Solar electricity production uses photovoltaic (PV) cells to convert light into electrical current. Photovoltaic modules can be integrated into buildings or used in photovoltaic power stations connected to the electrical grid. They are especially useful for providing electricity to remote areas. Although generally warranted for 25 years it is claimed that an average solar panel will last 40 years and almost all of it can be recycled.
Currently, photovoltaic (PV) panels only have the ability to convert around 24% of the sunlight that hits them into electricity. At this rate, solar energy still holds many challenges for widespread implementation, but steady progress has been made in reducing manufacturing cost and increasing photovoltaic efficiency. In 2008, researchers at Massachusetts Institute of Technology (MIT) developed a method to store solar energy by using it to produce hydrogen fuel from water. Such research is targeted at addressing the obstacle that solar development faces of storing energy for use during nighttime hours when the sun is not shining.
Research is ongoing in space-based solar power, a concept in which solar panels are launched into outer space and the energy they capture is transmitted back to Earth as microwaves. A test facility for the technology is being built in China.
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Solar water heating (SWH) is the conversion of sunlight into heat for water heating using a solar thermal collector. A variety of configurations is available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.
A sun-facing collector heats a working fluid that passes into a storage system for later use. SWH are active (pumped) and passive (convection-driven). They use water only, or both water and a working fluid. They are heated directly or via light-concentrating mirrors. They operate independently or as hybrids with electric or gas heaters. In large-scale installations, mirrors may concentrate sunlight into a smaller collector.
As of 2017, global solar hot water (SHW) thermal capacity is 472 GW and the market is dominated by China, the United States and Turkey. Barbados, Austria, Cyprus, Israel and Greece are the leading countries by capacity per person.
Wind power uses wind to provide the mechanical power through wind turbines to turn electric generators. In 2019, wind power provided approximately 6% of the global electricity supply. Wind power is a sustainable and renewable energy, and has a much smaller impact on the environment compared to burning fossil fuels. Wind farms consist of many individual wind turbines, which are connected to the electric power transmission network. Onshore wind is an inexpensive source of electric power, competitive with or in many places cheaper than coal or gas plants.
Onshore wind farms also have an impact on the landscape, as typically they need to be spread over more land than other power stations and need to be built in wild and rural areas, which can lead to "industrialization of the countryside" and habitat loss. Offshore wind is steadier and stronger than on land and offshore farms have less visual impact, but construction and maintenance costs are higher. After about 20 years wind turbine blades need replacing with larger blades, and research continues on how best to recycle them and how to manufacture blades which are easier to recycle.
Among sources of renewable energy, hydroelectric plants have the advantages of being long-lived—many existing plants have operated for more than 100 years. Also, hydroelectric plants are clean, have few emissions and can compensate for variations in wind and solar power. Criticisms directed at large-scale hydroelectric plants include: dislocation of people living where the reservoirs are planned, and release of greenhouse gases during construction and flooding of the reservoir.
However, it has been found that high emissions are associated only with shallow reservoirs in warm (tropical) locales, and recent innovations in hydropower turbine technology are enabling efficient development of low-impact run-of-the-river hydroelectricity projects. Generally speaking, hydroelectric plants produce much lower life-cycle emissions than other types of generation.
In 2015, hydropower supplied 16% of the world's electricity, down from a high of nearly 20% in the mid-to late 20th century. It produced 60% of electricity in Canada and nearly 80% in Brazil. As of 2017, new hydropower construction has stopped or slowed down since 1980 in most countries except China.
Biomass is biological material derived from living, or recently living organisms. As an energy source, biomass can either be burned to produce heat and to generate electricity or converted to modern biofuels such as biodiesel and ethanol.
Biomass is extremely versatile and one of the most-used sources of renewable energy. It is available in many countries, which makes it attractive for reducing dependence on imported fossil fuels. If the production of biomass is well-managed, carbon emissions can be significantly offset by the absorption of carbon dioxide by the plants during their lifespans. However this "carbon debt" may be paid back too late, or (especially in the United States) not properly accounted for. If the biomass source is agricultural or municipal waste, burning it or converting it into biogas also provides a way to dispose of this waste. Bioenergy production can be combined with carbon capture and storage to create a zero-carbon or negative-carbon system, but it is doubtful this can be scaled up quickly enough.
If biomass is harvested from crops, such as tree plantations, the cultivation of these crops can displace natural ecosystems, degrade soils, and consume water resources and synthetic fertilizers. In some cases, these impacts can actually result in higher overall carbon emissions compared to using petroleum-based fuels.
Biofuels are fuels, such as ethanol, manufactured from various types of biomass, such as corn or sugar beet. Biofuels are usually liquid and used to power transport, often blended with liquid fossil fuels such as gasoline, diesel or kerosene. As of 2020[update] which are sustainable biofuel is being debated.
Cellulosic ethanol has many benefits over traditional corn based-ethanol. It does not take away or directly conflict with the food supply because it is produced from wood, grasses, or non-edible parts of plants. Moreover, some studies have shown cellulosic ethanol to be potentially more cost effective and economically sustainable than corn-based ethanol. As of 2018, efforts to commercialize production of cellulosic ethanol have been mostly disappointing, but new commercial efforts are continuing.
Use of farmland for growing fuel can result in less land being available for growing food. Since photosynthesis is inherently inefficient, and crops also require significant amounts of energy to harvest, dry, and transport, the amount of energy produced per unit of land area is very small, in the range of 0.25 W/m2 to 1.2 W/m2. In the United States, corn-based ethanol has replaced less than 10% of motor gasoline use since 2011, but has consumed around 40% of the annual corn harvest in the country. In Malaysia and Indonesia, the clearing of 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. In 2015, annual global production of liquid biofuels was equivalent to 1.8% of the energy extracted from crude oil. It has been suggested that, due to the limited quantities that can be sustainably produced, it should all be aviation biofuel: because unlike other forms of transport long-distance aviation cannot be powered by batteries, hydrogen, ammonia or fuel cells.
Geothermal energy is produced by tapping into the thermal energy created and stored within the earth. It arises from the radioactive decay of an isotope of potassium and other elements found in the Earth's crust. Geothermal energy can be obtained by drilling into the ground, very similar to oil exploration, and then it is carried by a heat-transfer fluid (e.g. water, brine or steam). Within these liquid-dominated systems, there are possible concerns of subsidence and contamination of ground-water resources. Therefore, protection of ground-water resources is necessary in these systems. This means that careful reservoir production and engineering is necessary in liquid-dominated geothermal reservoir systems. Geothermal energy is considered sustainable because that thermal energy is constantly replenished.
Geothermal energy can be harnessed to for electricity generation and for heating. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. As of 2010, geothermal electricity generation is used in 24 countries, while geothermal heating is in use in 70 countries.[needs update] International markets grew at an average annual rate of 5 percent over the three years to 2015.[needs update]
Geothermal power is considered to be a sustainable, renewable source of energy because the heat extraction is small compared with the Earth's heat content. The greenhouse gas emissions of geothermal electric stations are on average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5 percent of that of conventional coal-fired plants.
Marine energy is mainly tidal power and wave power. As of 2020[update], a few small tidal power plants are operating in France and China, and engineers continue to try and make wave power equipment more robust against storms.
Non-renewable energy sources
Nuclear power plants have been used since the 1950s to produce a zero emission, steady supply of electricity, without creating local air pollution. In 2012, nuclear power plants in 30 countries generated 11% of global electricity. The IPCC considers nuclear power to be a low-carbon energy source, with lifecycle greenhouse gas emissions (including the mining and processing of uranium), similar to the emissions from renewable energy sources. As of 2020 nuclear power provides 50% of European Union low-carbon electricity and 26% of total energy production in Europe.
There is considerable controversy over whether nuclear power can be considered sustainable, with debates revolving around the risk of nuclear accidents, the cost and construction time needed to build new plants, the generation of radioactive nuclear waste, and the potential for nuclear energy to contribute to nuclear proliferation. These concerns have spurred the anti-nuclear movement and led to a decrease in the contribution of nuclear energy to the global electricity supply since 1993. At a global level, opposition to nuclear energy stood at 62 percent in 2011. Public support for nuclear energy is often low as a result of safety concerns, however for each unit of energy produced, nuclear energy is far safer than fossil fuel energy. The uranium ore used to fuel nuclear fission plants is a non-renewable resource, but sufficient quantities exist to provide a supply for hundreds of years.
Traditional environmental groups such as Greenpeace and the Sierra Club are opposed to all use of nuclear power. Individuals who have described nuclear power as a green energy source include philanthropist Bill Gates and environmentalist James Lovelock.
Safety assessment of already operating nuclear power plants to extend their lifetimes, perhaps up to 80 years, continues. Regardless of past accidents, nuclear power remains the safest energy source available per unit of energy as compared to other sources.
Some newer nuclear reactor designs are capable of extracting energy from nuclear waste until it is no longer (or significantly less) dangerous, and have design features that greatly minimize the possibility of a nuclear accident. These designs (for instance the molten salt reactor) have yet to be commercialized. Thorium is a fissionable material used in thorium-based nuclear power. The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation and reduced plutonium production. Therefore, it is sometimes referred as sustainable.
A prospective energy source is nuclear fusion (as opposed to nuclear fission used today). It is the reaction that exists in stars, including the Sun. Fusion reactors currently in construction (ITER) are expected to be inherently safe due to lack of chain reaction and do not produce long-lived nuclear waste. The fuel for nuclear fusion reactors are very widely available deuterium, lithium and tritium.
Sustainable energy system
There are multiple ways to make transport more sustainable. Public transport usually requires less energy per passenger than personal vehicles such as cars. In cities, transport can be made cleaner by stimulating nonmotorised transport such as cycling. Energy efficiency of cars has increased significantly, often due to regulation-driven innovation. Electric vehicles use less energy per kilometre, and as electricity is more easily produced sustainably than fuel, also contribute to making transport more sustainable. Hydrogen vehicles may propose an alternative to electric vehicles, but have seen smaller uptake.
Carbon capture and storage
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In theory, the greenhouse gas emissions of fossil fuel and biomass power plants can be significantly reduced through carbon capture and storage, although this process is expensive. According to the Intergovernmental Panel on Climate Change, the lowest cost path to meeting the 2 °C target includes massive deployment of one specific type of negative emissions technology called bioenergy with carbon capture and sequestration, or BECCS.[page needed] However, achieving this target through BECCS requires more resources than are currently available worldwide. For example, to capture 10 billion tons of CO2 per year (GtCO2/y) would require biomass from 40 percent of the world's current cropland.
Managing intermittent energy sources
Solar and wind are variable renewable energy (VRE) sources that supply electricity intermittently depending on the weather and the time of day. Overall intermittency can be reduced by combining these sources, and in some places batteries can also be combined to eliminate intermittency completely, so that the whole installation produces dispatchable power.
Most electric grids were constructed for non-intermittent energy sources such as coal-fired power plants. Half the world's electricity will need to be wind and solar by 2030, to limit the global rise in temperature to well below 2 °C by 2050. As larger amounts of solar and wind energy are integrated into the grid, it becomes necessary to make changes to the overall system to ensure that the supply of electricity is matched to demand.These changes can include the following:
- Using hydroelectricity, natural gas plants or nuclear plants to produce backup power
- Using grid energy storage to store excess solar and wind energy and release it as needed. The most commonly used storage method is pumped-storage hydroelectricity, which is feasible only at locations that are next to a large hill or a deep underground mine. Batteries are being deployed widely. Other storage technologies such as power-to-gas have been used in limited situations.
- Trading electricity with other locations in a regional grid or through long-distance transmission lines
- Reducing demand for electricity at certain times through energy demand management and use of smart grids.
- Energy market, or more specifically electricity market, changes so that power supply flexibility is better paid
As of 2019, the cost and logistics of energy storage for large population centers is a significant challenge, although the cost of battery systems has plunged dramatically. For instance, a 2019 study found that for solar and wind energy to replace all fossil fuel generation for a week of extreme cold in the eastern and midwest United States, energy storage capacity would have to increase from the 11 GW in place at that time to between 230 GW and 280 GW, depending on how much nuclear power is retired.
Some technologies provide short-term energy storage, while others can endure for much longer. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped. Grid energy storage is a collection of methods used for energy storage on a large scale within an electrical power grid.
Common examples of energy storage are the rechargeable battery, which stores electricity as chemical energy readily reconvertible to electricity to return to the grid, the hydroelectric dam, which stores energy in a reservoir as gravitational potential energy, and ice storage tanks, which store ice frozen by cheaper energy at night to meet peak daytime demand for cooling.
Hydrogen is a zero-emission fuel that can be produced by using electrolysis to split water molecules into hydrogen and oxygen. Hydrogen can play a role for storing energy in a sustainable energy system if the electricity used to produce it is generated from sustainable sources, such as wind or solar. 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. Hydrogen can be distributed by ship or through pipelines. Up to 20% can be mixed into natural gas pipelines without changing pipelines or appliances, but as hydrogen is less energy-dense this would only save 7% of emissions. As of 2020[update] trials are underway on how to convert a natural gas grid to 100% hydrogen, in order to reduce or eliminate emissions from residential and industrial natural gas heating. It can be used to power vehicles that have hydrogen fuel cells. As it has a low energy to volume content, it is easier to use in hydrogen-powered ships or heavy road vehicles than in cars and airplanes.
As of 2018, very little of the world's supply of hydrogen is created from sustainable sources. Nearly all hydrogen is created by steam methane reforming (SMR), which results in high greenhouse gas emissions but is currently cheaper than creating hydrogen via electrolysis. Although some carbon from SMR could be captured the process can be made more sustainable by using autothermal reforming with carbon capture and storage technologies to remove most of the carbon dioxide that is emitted.
Fuel switching from coal or wood to natural gas or LPG
On average for a given unit of energy produced, the greenhouse gas emissions of natural gas are around half the emissions of coal when used to generate electricity, and around two-thirds the emissions of coal when used to produce heat: however reducing methane leaks is imperative. Natural gas also produces significantly less air pollution than coal. Building gas-fired power plants and gas pipelines is therefore promoted as a way to phase out coal and wood burning pollution (and increase energy supply in some African countries with fast growing populations or economies), however this practice is controversial. Opponents argue that developing natural gas infrastructure will create decades of carbon lock-in and stranded assets, and that renewables create far less emissions at comparable costs. The life-cycle greenhouse-gas emissions of natural gas are around 40 times the emissions of wind and nuclear energy.
Electrification is a key part of using energy sustainably, as many mainstream sustainable energy technologies are electricity-based. As of 2018[update] 860 million people are estimated to have no electricity, of whom 600 million are in Sub-Saharan Africa. According to a 2019 report by the IEA, in sub-saharan Africa "current and planned efforts to provide access to modern energy services barely outpace population growth" and would still leave over half a billion people without electricity and over a billion without clean cooking by 2030. But according to the report this can be vastly improved, in part by accelerating electrification.
Government energy policies
Energy-specific programs and regulations have historically been the mainstay of efforts to reduce fossil fuel emissions. Successful cases include the building of nuclear reactors in France in the 1970s and 1980s, and fuel efficiency standards in the United States which conserved billions of barrels of oil. Other examples of energy-specific policies include energy-efficiency requirements in building codes, banning new coal-fired electricity plants, performance standards for electrical appliances, and support for electric vehicle use. Nevertheless, fossil fuel subsidies remain a key barrier to a transition to a clean energy system.
Carbon taxes are an effective way to encourage movement towards a low-carbon economy, while providing a source of revenue that can be used to lower other taxes or to help lower-income households afford higher energy costs. Carbon taxes have encountered strong political pushback in some jurisdictions, whereas energy-specific policies tend to be politically safer. According to the OECD climate change cannot be curbed without carbon taxes on energy, but 70% of energy-related CO2 emissions were not taxed at all in 2018. Some studies estimate that combining a carbon tax with energy-specific policies would be more cost-effective than a carbon tax alone.
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- Fossil fuel divestment
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