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In this update to the 4^th edition of Energy Science, we discuss some of the main developments that have occurred in the last two years. Climate change is increasingly noticeable worldwide, and we summarize the IPCC 6^th assessment and the international meetings at COP27 and COP27. Renewable energy production has been accelerating, particularly from solar PV and offshore wind, but the percentage of energy from fossil-fuels is not decreasing fast enough. Investment in the transition must increase significantly to meet climate targets, and fossil fuel combustion decrease far more rapidly.

We highlight the importance of storage in making wind and solar power more dispatchable, and describe research on promising new long duration battery technologies. Short sections follow on developments in nuclear power technology, the rapid growth of EVs, hydrogen, and heat pumps. Solar power is the fastest growing renewable and its progress and integration on farmland with food production, called agrivoltaics, are described. How emissions from agriculture could be reduced and the effect of the short-lived GHG methane on global warming are explained. Geoengineering the Arctic by refreezing parts of it could give more time for mitigation and carbon removal looks increasing necessary to meet climate targets. Finally, the progress made on the transition away from fossil fuels to clean energy and the actions needed are summarized.

Climate change

The effects of climate change are becoming increasing apparent. In 2022, many regions experienced extreme heat waves and flooding. Notably, in Bangladesh, India, and Pakistan, with the monsoon in Pakistan leaving a third of the country under water. Europe saw heatwaves causing wildfires in Spain, Portugal, and France, and widespread devastation. Water shortages on rivers in France threatened nuclear power plants, and low river levels on the Rhine in Germany reduced barge traffic. The USA experienced Hurricane Ida, which caused 114 deaths and a total loss of $65 billion, and heatwaves, wildfires, and water shortages in the West, and floods in the East. In China, drought caused sections of the Yangtze River to dry up in places, followed later by floods due to excessive rainfall. Africa was also affected, with wildfires in Algeria and catastrophic flooding in South Africa.

A high angle view of a flooded area Description automatically generated with low confidence

Flooding in Pakistan 2022 (Getty Images)

These weather extremes reflect the increased global warming, which is higher over land than over sea (1.6 ^oC c.f. 0.9 ^oC in mid 2010s), notably in the Arctic where it is around three times greater than the world average of about 1.2 ^oC in 2022. The Northern Hemisphere jet stream meanders more, because of the lowered Pole-to-Equator temperature difference, causing more weather patterns to stall and exacerbate heat waves and flooding. As more water vapour is held in the hotter atmosphere, the global warming not only heats the land but causes it to dry out making wildfires more likely.

The cost of natural disasters over the period 2000-2021 has shown an increasing trend with a 44% increase in the total for the decade 2011-2020 compared with that in 2001-2010. Analysis of many extreme weather events over the last few decades (attribution studies) has established beyond reasonable doubt that their severity and increasing frequency can be attributed to human-induced climate change, including: 71% of the 504 extreme weather events, 93% of the 152 extreme heat events, and 56% of the 126 excess rainfall and flooding events (CarbonBrief 2022).

Global CO₂ emissions and COP 26 & 27

Global CO₂ emissions have remained relatively flat since 2015 but will exhaust the carbon budget for 1.5 ^oC in around 10 years if they remain constant. This stresses the importance of improving the mitigation pledges (NDCs) of countries, which following the COP21 meeting in Paris 2015 would only limit warming to 3.2 ^oC. Countries’ NDCs were due to be revised in 2020 at COP26, but COP26 took place in Glasgow in November 2021, a year later than planned because of the Covid-19 pandemic.

While fossil fuels were explicitly addressed for the first time at a COP, it was only decided to phase down, rather than out, the use of coal. Curbing methane emissions and setting up effective carbon markets were also agreed, as were ending subsidies for fossil fuels. Aligning the finance sector with net-zero and proposals to curtail deforestation and increasing finance for adaption of natural habitats and of communities were accepted, as was accelerating the transition to EVs, the decarbonising of shipping and the built environment, and investment in renewables. However, progress on mitigation was insufficient, with pledges only limiting warming to around 2.8 ^oC.

COP27 in November 2022 saw the developed nations at long last agreeing to set up a Loss and Damage fund for developing countries to address the consequences of climate change that exceed adaptation limits. Any national liability that such a fund might signify, which has stymied progress in the past, was explicitly excluded. The urgency and importance of this fund has been heightened by recent climatic disasters such as the floods in Pakistan. However, financial details still need to be worked out and there was pressure on the World Bank to give more support in this area. The loss and damage fund will be in addition to the climate finance of $100 billion a year for mitigation and adaption promised to developing nations in 2009 but still not fully delivered. Part of the finance is needed to deal with migration from the worst affected regions, which up to now has been mostly to within the countries’ borders.

While no further significant progress was made on restricting fossil fuels over the COP26 decision to phase down coal, it was noted at COP27 that more and more businesses and financial institutions are committing to net-zero. But plans must be more robust and address concerns over greenwashing. In addition, the need for more resilient and sustainable agriculture was recognised for the first time. Also, a master plan for accelerating the decarbonization of the power, road transport, steel, hydrogen, and agricultural sectors was announced.

While some further national determined contributions (NDC) to mitigate emissions had been announced before COP27, they would only put the world on track for 2.4-2.6 ^oC warming by 2100, even if fully enacted. Emissions are not falling nearly fast enough to keep warming below 1.5 ^oC. To accelerate their own mitigation efforts, Indonesia (population 275 million) announced at the G20 meeting, held alongside COP27, a Just Energy Transition Partnership (JETP) which will include finance to reduce coal use and boost renewables. This follows on from the setting up by South Africa of the first JETP at COP26. Further such initiatives are needed globally. But above all, actions not just pledges are required urgently.

IPCC 6^th assessment report on climate change

The IPCC 6^th assessment report (IPCC 2021-23) concluded that it was unequivocal that human influence has warmed the atmosphere, ocean, and land, with each of the last four decades successively warmer. During this time the Arctic and Greenland ice cover and the extent of glaciers worldwide have markedly decreased, while in the last 30 years the rate of sea level rise has increased by about 50% to 3.9 mm per year (NASA 2022). Climate change is worldwide, and as global warming increases, heatwaves and heavy precipitation are more frequent and extreme, as are concurrent heatwaves, droughts, fires, and flooding.

The more intense and frequent extreme weather events have caused widespread damage to both nature and people and have led to increased migration and some irreversible impacts. Coastal cities are particularly at risk. Over 40% of the world’s population are highly vulnerable to climate change, with the most disadvantaged disproportionately affected. Near-term actions that limit global warming to close to 1.5 ^oC by cutting emissions would substantially reduce losses and damage. Governments, civil society, and the private sector must prioritise risk reduction, equity, and justice, and commit adequate resources and finance. To maintain biodiversity and healthy ecosystems, a third to half the Earth’s surface must be protected to ensure future food and fresh water supplies. Current global financial flows for adaptation are insufficient, especially in developing countries.

Mitigation options are now available to halve GHG emissions by 2030, through cutting fossil fuel use, widespread electrification powered by renewables, reducing demand, and utilising alternative fuels such as hydrogen. Significant potential lies in shifting to EVs and more sustainable healthy diets, in reusing and recycling materials, and in integrating renewable energy into buildings. Land can remove carbon dioxide at scale but cannot compensate for delayed mitigation in other sectors.

While many countries have now set targets and net zero is to the fore in industry and finance, the transition away from fossil fuels to renewables is happening far too slowly, even though the economic implications point to an overall benefit. NDCs announced by October 2021 make it likely that warming will exceed 1.5 ^oC and harder to limit warming to less than 2 ^oC. Developing countries still lag in deployment of low-carbon technologies and will require finance.

Policies should ensure a just and fast transition with immediate deep GHG emission reductions in all sectors and accelerated adaptation measures; these are critical for global sustainable development. The aim should be net-zero as close to the 2050 as possible to limit warming to below 2 ^oC and close to 1.5 ^oC. The decade 2021-2030 is critical and will involve high upfront investments and potentially disruptive changes. Sustained net negative global CO₂ emissions could correct some overshoot but would entail adverse impacts.

Renewable energy capacity

*Hydropower (GW)*	2019	2020	2021	2022
*World*	1312	1334	1363	1393
*Solar PV (GWp)*	2019	2020	2021	2022
Asia and Oceania	353	436	518	637
Americas	81	102	132	163
Middle East and Africa	15	19	22	26
Europe	142	163	190	228
*World*	592	720	861	1053
*Wind onshore (GW)*	2019	2020	2021	2022
Asia and Oceania	266	342	380	418
Americas	146	165	186	198
Middle East and Africa	7	8	8	9
Europe	174	183	196	210
*Onshore*	593	697	770	836
*Wind offshore*	2019	2020	2021	2022
Asia	6	9	28	32
Europe	22	25	26	31
*Offshore*	28	34	54	63
*World*	621	732	824	899
*Bioenergy (GW)*	2019	2020	2021	2022
*World*	124	133	141	148
*Geothermal (GW)*	2019	2020	2021	2022
*World*	14	14	15	15
*Concentrated Solar (GW)*	2019	2020	2021	2022
*World*	6	7	6	7
*Marine (GW)*	2019	2020	2021	2022
Asia	0.3	0.3	0.3	0.3
Europe	0.2	0.2	0.2	0.2
*World*	0.5	0.5	0.5	0.5

(IRENA 2023)

The period 2010 to 2021 has seen an enormous improvement in the competitiveness of the variable renewables. The global weighted average LCOE of newly commissioned utility-scale solar PV projects declined by 88% between 2010 and 2021 to USD 0.048/kWh, whilst that of onshore wind fell by 68% to USD 0.033/kWh, and offshore wind by 60% to USD 0.075/kWh (IRENA 2022).

In 2022 renewable energy capacity increased by 9.6%, the largest ever, which accounted for 83% of global power additions, with solar PV capacity passing one TWp and offshore wind capacity growing from a small base by almost 50% since 2019 (IRENA 2023). Offshore is primarily off Asia and Europe but is starting to take off in the US.

However, the pace needs to increase significantly, and fossil fuel use quickly decrease, for warming to be kept close to 1.5 C. The percentage of energy supplied by fossil fuels only dropped from 85% to 82% in the five-year period 2016-2021 (BP Statistical Review 2022).

Off-grid renewable energy

Home and mini-grid systems have seen tremendous growth as costs have dropped sharply in the last decade, with the population provided with lighting and other electrical services by these systems doubling in the period 2013 to 2016 to 130 million. The total capacity grew from about 4 GW to 6 GW with the main growth in solar PV. Since then, the capacity has doubled to 12.4 GW by 2022 with solar photovoltaic increasing from about 2 to 5 GW. Most of the growth has been in Asia and Africa and is enabling many people to have access to electricity. Off-grid renewables can play a very valuable role in enabling universal access, crucial for a good standard of living.

Of the world’s total off-grid renewable energy capacity in 2022, solar PV accounted for 5.1 GW, hydropower 1.8 GW, and other (mostly biomass) 5.6 GW. Most capacity was in Asia 9.8 GW (solar PV 3.5 GW and hydropower 1.3 GW) and Africa 1.4 GW (solar PV 1.0 GW and hydropower 0.3 GW) (IRENA Off-grid 2022).

Global electricity generation by source

Coal	10323	Renewables	8349	Gas	6500
Nuclear	2684	Other	784
Total	28640	TWh

(CarbonBrief/IEA 2023)

The power sector is decarbonizing relatively quickly with the percentage from renewables and nuclear 39% in 2022 c.f. 31% in 2018. The increase in 2022 of solar (24%) and wind (17%) was the largest ever, meeting 80% of the global demand growth, and their share in electricity generation reached 12%. The think tank Ember forecast that fossil fuel generation will fall by 0.4% in 2023, with greater falls in future years as wind and solar capacity increases further (Ember 2023). However, the transition must greatly accelerate, particularly in the industry, buildings, and transport sectors.

Cost of electricity from solar PV with storage

Increasingly, renewable energy farms are being built with storage attached to enable supply to be better matched to demand. For example, in the Mojave Desert in California the Eland PV plus storage project, due to be commissioned in 2023, will consist of 400 MW of PV and 300 MW/1200 MWh of storage, which will enable electricity to be supplied throughout the day and into the evening after the sun has set. The power purchase agreement (PPA) price is US$40 per MWh, with the cost of electricity from the PV at US$20 per MWh. This PPA is very competitive with fossil fuel generation.

The reason storage only adds $20 per MWh is because about three quarters of the electricity generated in a day is from the PV farm and only a quarter from storage, with all the electricity receiving $40 per MWh. In a typical day 3500 MWh is from PV and 1200 MWh is discharged from storage. The 3500 MWh from PV receives 3500 × 40 = $140,000 which, after covering the PV costs of $20 per MWh, gives $70,000 towards the cost of storage. The 1200 MWh from storage receives 1200 × 40 = $48,000, giving a total of $118,000 for 1200 MWh from storage or $98.3 per MWh. This value, as shown below, is close to the LCOS of a utility lithium-ion battery used once a day with a capital cost of $200 per kWh, which is about the expected price in 2023.

Levelized Cost of Electricity from Storage (LCOS)

The cost of electricity from a battery depends on the costs of charging, discharging, and O&M. The discharge cost (Discharge) depends on the capital cost per kWh (Capex) and the battery cycle life (cif ). For cyc discharge cycles per year, the lifetime of the battery (N years) is given by N = cif/cyc. Because of degradation over its lifetime, the effective capacity is a factor f (about 90%) of the initial, and the amount of this used is the depth of discharge DOD, typically 80%. So, neglecting discounting,

Discharge = Capex/(cyc × N × f × DOD) $ kWh^-1

The round-trip efficiency (η ) of each charge-discharge cycle is typically 85-90%, so the cost of charging per kWh discharged is P/η, where the cost of the electricity used to charge the battery is $P kWh^-1. The annual O&M charge per kWh is a percentage OM (roughly 2%) of Capex. The cost of electricity from storage (COS) in $ kWh^-1would then be:

COS = P/η + Capex/(cyc × N × f × DOD) + OM × Capex/(cyc × f × DOD)

To allow for future cash flows having a lower present value, the discharged electricity must be discounted, changing N in the above equation to Dc = [1 - (1 + r)^(-N)]/r, where r is the discount rate and N the number of years, with the result that the levelized cost of electricity from storage (LCOS) is given in $ kWh^-1by:

LCOS = P/η + Capex/(cyc × Dc × f × DOD) + OM × Capex/(cyc × f × DOD)

For example, if Capex = $200 kWh^-1, P = $0.02 kWh^-1, cif = 7000, cyc = 350, f = 0.9, N = 20, η = 0.85, DOD = 0.8, OM = 0.02, and r = 0.05 then

COS = $0.079 kWh^-1 or $79 per MWh, and LCOS = $0.103 kWh^-1 or $103 per MWh,

Decreasing the number of cycles per year but keeping the same lifetime N for discounting increases the LCOS. For instance, a battery cycled only once a week would need to be some seven times cheaper to give the same LCOS. When comparing the costs of storage, it is important to ensure that the uses are the same.

In practice, other important factors should be considered: for example, the cost of the electricity for charging can change significantly, e.g., it can be very low when renewable generation is high; the need for additional generation or transmission lines may be avoided; and for a wind or solar farm, the addition of storage can enable them to compete for back-up (balancing) generation.

Electricity market reform

In competitive electricity markets, the wholesale price is generally determined by the ‘merit order’ of the power plants bidding to supply electricity for a certain period. The plants are included in increasing order of their bids until the demand is reached, and the wholesale price is set by that of the last included generator. The increased profit of the earlier suppliers enables investment in these plants, which should tend to push the more expensive plants out of the market and decrease the wholesale price.

This was the case until the fraction of renewable generators, primarily wind and solar farms, became significant. These have essentially zero marginal costs, and so are first in the merit order, but being variable (intermittent) are not always dispatchable and therefore cannot guarantee meeting demand. Fossil fuel powered plants are therefore still needed and these determine the wholesale price. Increasing the output from renewable generators hurts the profitability of fossil fuel plants, as they run less often, but they are still needed as back-up to meet occasional demand.

Fixing the price by the merit order can therefore discourage investment in plants with finite marginal costs and, when these marginal costs are high, it doesn’t reflect the lower cost of generation by variable renewables and retail prices can be very expensive. Capacity markets that provide support for dispatchable plants have been introduced but these don’t address the low cost of renewables, which is called the marginal price challenge.

Separating out renewable from dispatchable generators has been suggested as the way forward. The electricity price from dispatchable plants could be based on their merit order, while that for renewables on their contract-for-difference prices. This would provide support for dispatchable plants, while giving the consumer the benefit of lower cost electricity from renewables which would promote low-carbon electrification.

Storage requirements for electrical grids

It is expected (McKinsey 2022) that the global electricity supply will triple to about 75,000 TWh by 2050 to meet the increased demand from electrification and rising living standards. A large fraction of generation will be from wind and solar farms but, as these are variable sources, flexible supplies will be required to meet demand when their output is low. The demand can be altered somewhat through demand-side management, but this can only mainly help with diurnal changes, e.g., by charging batteries or heating well-insulated water tanks in off-peak periods. Inter-seasonal variations can generally be met by a suitable mix of wind and solar generation, but periods of low wind and weak sunshine (dark doldrums) and interannual differences increase the need for balancing supplies of not only enough capacity (TWh) but also of enough power (TW).

Flexibility can be provided by overcapacity of the variable renewable generators and interconnectors allowing trade of electricity with neighbouring regions, together with firm low-carbon generators (such as from biogas, hydrogen turbines, gas + CCS, nuclear, or geothermal plants) and electricity storage. The need for overcapacity and storage is lessened if some 10-20% of electricity is from firm low-carbon generators, and the overall cost of decarbonizing the power supply and the amount of firm capacity required can be reduced if long duration storage is available at a low enough price.

The likely option for very long duration (>150 hr) is green hydrogen stored underground. The demand for green H is growing rapidly (e.g., for steel production and shipping as the hydrogen derivative fuels methanol or ammonia), which will drive down its cost. Several options are possible for long duration energy storage (LDES) of ~6-150 hr, but two iron-based batteries may be already cheap enough (~20$ per kWh): one is a flow battery by EES, the other an iron-air battery by Form Energy (see Electricity Storage section). As many units will be required, there is considerable scope for cost reductions through the learning effect.

An estimate (McKinsey 2021) of the global amount of LDES required to decarbonize the global power supply (if the fall in costs and predicted performances are realised) is a capacity of 85-140 TWh with a power 1.5-2.5 TW. This LDES capacity would be together with about a similar amount, roughly split equally, of Li-ion for up to ~6 hr and green H storage. Widespread deployment of LDES is predicted to occur when the penetration of renewables reaches 60-70%.

Besides providing a route to decarbonization, LDES can help alleviate the curtailment (which can waste significant amounts of energy) that occurs when supply is larger than demand or when transmission lines become congested (which can require more expensive natural gas-fired generators to ramp up to meet demand downstream). Such storage could be considerably cheaper and faster to install than upgrading or constructing transmission lines and will be an initial market for their use.

Electricity storage

Lithium-ion batteries

For lithium-ion batteries, a cathode that is increasingly being considered in place of nickel-manganese-cobalt (NMC) or nickel-cobalt-aluminium (NCA), is lithium iron phosphate (LFP) because of its safety, long cycle life, cobalt-free nature, and lower cost: around US$ 150 per kWh in 2022, compared with US$ 175 per kWh for a typical NMC or NCA battery pack (Fastmarkets 2022). Their share of the global battery market is expected by the company UBS to rise from 17% in 2020 to 40% in 2030. In a LPF battery, lithium ions move between an iron phosphate cathode and a graphite anode. Separating the electrodes is a porous membrane that is permeable to lithium ions but prevents electrical contact.

If we assume that when fully charged the lithium intercalated in the graphite has the form LiC₁₂, which is half the theoretical maximum amount of lithium, and the cathode has the form FePO₄, then the molecular weight of the electrodes per mole of exchanged lithium is M = 0.30 kg. The charge capacity (neglecting other components) is given by:

Q_c = nF/(3600 × M) Ah kg^-1

where F is Faraday’s constant, and n is the number of electrons exchanged. As n = 1, Q_c = 90.3 Ah kg^-1. The nominal potential is 3.2 V, giving a theoretical maximum specific energy of ~290 Wh kg^-1. In practice, values of ~160 Wh kg^-1 have been achieved (2022).

While this specific energy is less than that of NMC and NCA cells, which have reached greater than 250 Wh kg^-1, recent progress in improving the cell-to-pack ratio have made LFP batteries more competitive. Their longer cycle life, greater safety, as well as their good energy density, make LFP batteries very suitable for EVs and for grid storage applications, such as with wind and solar PV farms. When used on the grid, lithium-ion batteries typically discharge over a period of up to 4 hours and provide intra-day balancing.

Sodium batteries

The much higher availability and lower cost of sodium compared to lithium has promoted interest in sodium-ion batteries. These work in a similar way to lithium-ion ones with the sodium ions intercalating in the electrodes. The larger size of Na compared to Li ions makes repeatable intercalation harder, but progress has been made in the last decade. Prototype Na-ion batteries are safe, easily recycled, have good performance at low temperatures, and have an estimated cost of $40 per kWh (Na battery 2022). These batteries can store about two thirds the energy of an equivalently sized Li-ion one, making them suitable for light transport, such as tuk-tuks in India, and for stationary applications, such as with solar farms.

A large market exists for such uses, and globally several companies are near large-scale manufacturing capability. In India, Reliance Industries, who acquired the UK Na-ion battery developer Faradion in 2022, in California Natron Energy, in Sweden Altris Energy, and in China CATL, which is the largest Li-ion battery producer in the world.

Another sodium-based battery is the sodium-sulfur battery. Existing Na-S batteries employ molten electrodes at temperature of around 300 ^oC and have been used for stationary energy storage, but their relatively limited cycle life and safety issues have restricted their deployment. Attention has more recently focused on room temperature (RT) designs, and in December 2022 a Chinese-Australian collaboration reported a RT Na-S battery with an energy density four times that of lithium-ion and potentially far cheaper (Na-S battery 2022). It maintained 50% of its initial capacity after 1000 cycles, a far higher stability than previously obtained. What performance and cost can be realised at a commercial scale will be the determining factors for its success.

Iron flow battery

In the last decade the American company ESS has developed a competitive iron flow battery. The electrolytes are solutions of iron chloride which are pumped past the anode and cathode when the battery is in operation. On discharging, the anode loses electrons from iron (Fe⁰⁺) plated out on the anode going into solution as Fe²⁺ ions, while the cathode gains electrons that convert Fe³⁺ ions to Fe²⁺ ions in solution. On charging, these processes are reversed. The reactions on discharging are:

Anode:	Fe + 2Cl^- -> FeCl₂ + 2e^-	V ^o = - 0.44 V
Cathode:	2FeCl₃ + 2e^- -> 2FeCl₂ + 2Cl^-	V ^o = + 0.77 V
Overall:	Fe + 2FeCl₃ -> 3FeCl₂	V ^o_cell = 1.21 V

The solutions are separated by a semi-permeable membrane that allows Cl^-ions to pass through to maintain charge neutrality. A small percentage of H⁺ ions in solution gain electrons and become H₂ molecules. These are changed back to H⁺ ions in a small fuel cell, and this ‘proton pump’ maintains the correct composition of the electrolytes.

ESS currently offers a 400-600 kWh (50-90 kW) behind-the-meter unit and a larger 3 MW utility-scale battery with a 74 MWh/6 MW per acre footprint. The batteries have a design life of 25 years, a round-trip efficiency of 70-75%, and are low-cost (estimated ~20 $ per kWh), safe, and sustainable (Iron-flow 2022). ESS claims that its batteries are ideal for 4-12 hr flexible energy storage applications, such as remote solar installations plus storage microgrids and load shifting. As well as other orders, it will supply Sacramento Municipal Utility District SMUD with flow batteries with a total capacity of 200 MW/2 GWh beginning in 2023.

Metal Air Batteries

Metal-air batteries have a metal anode, an electrolyte, and an air-breathing cathode. As the oxygen is not stored in the battery and the anode is (approximately) a pure element, these batteries have potentially high energy densities. On discharging the maximum specific energy decreases, as the anode becomes heavier as it oxidises; for example, for iron-air from a maximum of about 1200 down to 750 Wh kg^-1 and for lithium-air from about 11,000 down to 3500 Wh kg^-1. These values are based on the active components; the inclusion of the mass of inactive components, such as that of the electrolyte and housing, can make the practical energy density much less, but still significantly higher than current lithium-ion batteries (250 Wh kg^-1).

Metal-air batteries can be designed as primary (non-rechargeable) or secondary (rechargeable) batteries, and primary zinc-air batteries have been used since the early 1930s for high-capacity applications; these are discarded when the zinc charge is exhausted. Designing secondary metal-air batteries has been much more challenging.

Iron-air

An alkali iron-air battery’s overall half-cell reactions can be simplified as follows:

Air Cathode	½ O₂ + H₂O + 2e^- ↔ 2OH^-	V⁰ = + 0.40 V
Fe Anode	Fe + 2OH^- ↔ Fe(OH)₂ + 2e^-	V⁰ = - 0.88 V
Overall,	Fe + ½O₂ + H₂O ↔ Fe(OH)₂	Cell voltage = + 1.28 V

where the standard reduction potentials V⁰ are given. As the anode is oxidized on discharge, the cell voltage is given by [0.4 - (-0.88)] V. Secondary metal air batteries require the regeneration of the oxidised metal; they therefore require a bi-functional catalyst on the conductive air-breathing cathode membrane to catalyse both the oxygen reduction and evolution reactions. The membrane must be impermeable to the passage of the alkaline electrolyte, which is between the metal anode and air cathode, but permeable to air.

At the anode, the oxidation and reduction of the metal must be reversible. This is helped in iron-air batteries by the low solubility of iron hydroxide, which inhibits the formation of dendrites (tree-like growths) and gives potentially a long cycle life. The iron-air battery was first developed in the 1970s, but the formation of an iron hydroxide layer was found to limit the discharge rate because of its poor conductivity. The charging efficiency was also reduced by the competing side reaction of hydrogen evolution at the Fe anode, as the water to hydrogen (2H₂O + 2e^- → H₂ + 2OH^-) reduction potential is more positive than that of iron hydroxide to iron (-0.83 V cf. -0.88 V).

But in recent years it has been discovered that using high purity iron with the addition of small amounts of other compounds, such as bismuth oxide and iron sulfide (which is electrically conductive), can significantly reduce the evolution of hydrogen, inhibit the corrosion of iron, and greatly increase the discharge rate of the battery. As the iron hydroxide layer is thin, the capacity of the battery is proportional to the surface area of the iron, which can be increased by using nanocomposites of iron and conductive materials.

The development of suitable high surface area iron anodes and catalysed membranes for the air cathodes enabled Form Energy (USA) to announce in 2021 the outline design for a high capacity and low-cost iron-air battery for long duration storage. The basic battery container is about the size of a washing machine and contains around 50 cells, each consisting of an anode of small iron pellets and an air breathing cathode immersed in a water-based non-flammable alkaline electrolyte (Form Energy 2023). The design is inherently safe, environmentally benign, and operates at ambient temperatures. Iron is also very abundant and cheap.

Form Energy estimate that their batteries will store energy at about a tenth of the cost of lithium ion at around $20 per kWh (see Levelized cost of energy storage section). They are building a demonstration store with a capacity of around 1000 MWh capable of 1 MW output for 4 days duration- its size will be about an acre (0.4 ha). The batteries can be sited anywhere and, if their predicted performance and low cost per kWh are realised, the duration of cheap back-up power for 4 days will help very considerably in handling the variability (intermittency) of wind and solar power generation.

Long cycle life and low cost rather than high energy density and rapid power handling abilities are key for grid storage, and storage installations would include Li-ion batteries to handle spikes in demand with the iron-air batteries meeting slower, longer duration requirements. At the end of 2022, Form Energy announced that its first US manufacturing plant will be in Weirton in West Virginia. Results are expected in 2024.

If its potential is realised, then the production of direct reduced iron (DRI) from one U.S. plant alone, which is around 2 million tonnes per year, could provide annually an estimated 500 GWh of storage in Fe-air batteries; this could buffer 5 GW for 4 days.

Lithium-air

Another metal-air battery for which a significant claim was made in 2021 is for the lithium air battery. The Japanese National Institute for Materials Science (NIMS) and SoftBank Corp. announced in December 2021 (Li-air 2021) that they had developed a lithium-air battery with an energy density over 500 Wh kg^-1, significantly higher than current Li-ion batteries. It can be charged and recharged at room temperature.

It has been estimated (Li-air 2021) that batteries with over 500 Wh kg^-1 will be needed for electrifying long haul and high-capacity airplanes, so if this performance can be achieved at scale and the cycle life is good then this battery will be a major advance in decarbonizing air transport and other areas requiring very high energy densities.

Aluminium-air

Developments have also been seen in the last few years on primary aluminium-air batteries. Their maximum potential energy density at 8.1 kWh kg^-1is second only to lithium-air, and the batteries typically consist of an aluminium anode, air cathode, and electrolyte such as KOH. In the process of discharging the aluminium anode is converted to aluminium hydroxide. Aluminium is the most abundant metal on Earth and is relatively cheap at 2$ kg^-1; the hydroxide can also be easily converted back to aluminium.

The Israeli company Phinergy has already made large primary Al-air batteries to provide energy-backup for telecom towers and are developing primary Al-air batteries for EVs (Phinergy 2022). Currently their Al-air batteries have three times the energy density of Li-ion and can be swapped in and out of a car in a few minutes. There is no need for a rapid charging infrastructure and the battery can give a long driving range. They could be valuable in India, where many do not have easy access to electric charging points, and in 2022 Phinergy announced a collaboration with the Indian company Hindalco. Phinergy already have several prototype cars driving in Israel and India. [These batteries could also be used as a range extender for EVs powered by Li-ion.]

In America, the Wright Electric aviation company is planning a 100-seat electric short hop aircraft that will be powered either by a hydrogen fuel-cell or an aluminium-air battery (Electric Aviation 2021). Presently, aluminium-air looks like giving a shorter range but higher payload than hydrogen fuel cells, as it has a potentially higher volumetric but lower specific (gravimetric) energy density. Furthermore, Al-air could be cheaper and easier to operate with. However, there is much development to be done before commercial flights are seen with either energy source.

Zinc-air

A Canadian company Zinc8 has been developing a zinc-air battery and plans to pilot its technology in New York State in 2023 (Zinc8 2023). When discharging the battery, zinc particles are oxidised to zincate particles, Zn(OH)₄, which are decomposed to zinc, water, and oxygen when charging. The cost is predicted to decrease significantly as the battery capacity increases and to be some five times cheaper than Li-ion for 100 hours storage. Zinc is cheap and relatively plentiful, so this battery, if successful, could be very useful for grid storage.

Sand battery

The Finnish company Polar Night Energy (Sand battery 2022) have developed a sand battery that can store excess renewable electricity as heat for use weeks or even months later. An industrial version of a standard resistive heater is used to generate hot air to heat the sand up to temperatures around 500-600 ^oC. When heat is required, air is blown through the sand via heat exchange pipes and the exiting hot air funnelled into a heating system for homes or industry. The prototype battery is 4 m in diameter and 7 m tall, surrounded by a thick outer layer of insulation, and contains 100 tonnes of sand, sufficient to store 8 MWh, and capable of discharging with 100 kW of thermal power for 80 hours. Larger batteries are being planned, and the long duration cheap storage system could help provide clean heat in winter, at times when renewable generation is low.

Liquid CO₂ storage

The Italian company Energy Dome unveiled a prototype 2.5 MW/4 MWh storage unit in June 2022 (Energy Dome 2022). The unit stores energy by liquifying CO₂ gas, and when electricity is required, the liquid CO₂ is expanded through a turbine that is coupled to a generator. The CO₂ gas is stored in a flexible plastic dome at ambient temperature and pressure and is liquified by first compressing the gas to 60 bar, which raises its temperature to 300 ^oC. The CO₂ is passed over steel shot and quartz ‘bricks’, storing the heat of compression for later use, and cooling and condensing the CO₂. The liquid CO₂ is then stored at ambient temperature under pressure in a tank. To generate electricity, the liquid CO₂ is passed over the heated ‘bricks’, which produces hot pressurised gas. The gas is then expanded through a turbine that drives an electricity generator, with the exhaust CO₂ stored in the flexible dome.

The company is currently building a 20 MW/200 MWh facility due for completion in late 2023 or early 2024. This ten-hour duration system uses off-the-shelf equipment, and the projected round-trip efficiency is 75-80%. The estimated cost of storage is below $100 per MWh and the company is aiming for $50-60 per MWh in the next few years. That would be about twice as cheap as the cost of storage with lithium-ion batteries.

High density pumped hydro

The start-up company RheEnergise has developed a liquid (High-Density Fluid R-19) with a density 2.5 times that of water (High-density Hydro 2022). The fluid is water with a high solid content that is milled to a fine power and treated so that it does not coagulate. The company predicts that a significant number of pumped hydro sites will be possible, and these could provide long-term energy storage for renewables.

The power P of a pumped fluid site is dependent on the volume flow rate of the fluid Q , its density ρ, and the difference in height h (the head) of the upper and lower reservoirs, and given by the formula P = ρQgh (see Chapter 6.2). The energy stored U is proportional to the volume of fluid V in the upper reservoir and is given by U = ρVgh. When using the fluid R-19, the system would be closed with the fluid recirculated.

When the head available is h_f, the expressions for P and U show that the same power and stored energy can be achieved with the fluid R-19 as with water by using a reservoir of volume
V_f = (ρ_w h/ρ_f h_f) V_w and a flow Q_f = (ρ_w h/ρ_f h_f) Q_w. So, for example with the same head, the volume and flow would be 2.5 times smaller, and the size of the turbines and pipes also reduced, which would significantly decrease the capital cost of construction. Sites with lower elevations and smaller heads could also be suitable.

Overall costs are predicted to be >40% lower than those for a corresponding lithium-ion battery store and round-trip efficiency is estimated as ~80%. The company is testing for wear of components from continuous operation with R-19 and is planning to build a demonstration
1 MW/4 MWh plant in 2023.

Geomechanical pumped storage

The company Quidnet is exploring the potential of storing electricity by using it to pump water underground, where it expands cavities and is stored under high pressure (Geomechanical Pumped 2022). When energy is needed, the high-pressure forces water up through a turbine to generate electricity. Quidnet is exploring the reversibility of the process and how widespread are suitable locations. If successful, this technology might provide cheap ~10 hr duration storage.

Nuclear power

Nuclear power is maintaining its capacity worldwide (~400 GW) with lifetime extensions and plant upgrading. Its share of global electricity supply in 2022 was 9%. Most new orders or plans are in the Asian region, and nearly all reactors under construction have capacities of order 1 GW. The IEA envisage a twofold increase in global capacity for net-zero by around 2050 (IEA 2022) The economics alone make any larger share unrealistic and nuclear power is no substitute for massive solar and wind capacity. However, a 5-10% share in electricity generation may help in balancing the variability of renewables, and in the West smaller reactors (SMRs) are being considered as a more viable design.

Small Modular Reactors (SMRs)

For net-zero, a large fraction of electricity generation must be from wind and solar generators but, as these are variable sources, flexible supplies will be required to meet demand when their output is low. Small modular nuclear reactors are being considered for part of this role. These can give firm and more flexible power than conventional nuclear reactors. They would be manufactured in factories and assembled on site, which could avoid the increasing costs and long delays in construction that have bedevilled the nuclear power industry in the West, resulting in most of the reactors under construction since 2017 being of Chinese or Russian designs (Nuclear 2022). Safety concerns for SMRs are like those for larger reactors and with passive design features should be acceptable. Safe waste disposal will still be required. The smaller size means financing will be easier and may improve social acceptance and attract private investment. SMRs will give energy security and Canada, France, the United Kingdom, and the United States have shown increased interest and support.

The larger numbers of smaller units could also give some economies of scale if significant units are made; however, many different designs for these SMRs are under development so the likely market size for any one design is small. Moreover SMRs, which have an output less than 300 MW, lose the economies of size that more conventional nuclear reactors of around a 1000 MW output have. It is by no means certain that SMRs will be the most economical way of providing ~ 5 - 10% of firm clean power to help balance supply and demand. The alternatives of green hydrogen powered gas turbines, BECCS, or long duration batteries may well prove cheaper.

Fusion power

Several compact fusion reactor designs are under development and a major advance was made in 2021 by MIT and the startup Commonwealth Fusion Systems when a large bore high temperature superconducting magnetic produced a field of 20 Tesla (Compact Fusion 2021). Such a high field capability will enable a tokamak of roughly a tenth the volume of ITER achieve the same power output; this improvement comes about from the fusion power density depending on the fourth power of the magnetic field. Such a smaller size tokamak will reduce the construction time and investment required, as well as speed up the innovation cycle.

In December 2021, the JET laboratory in the UK more than doubled its own world record for the energy output of a fusion reaction – 59 megajoules over 5 seconds (JET 2021). It marks a boost for the ITER fusion reactor under construction at Cadarache, due to be completed by 2035. But commercial fusion reactors based on this design are not expected until the second half of the 21^st century.

In a separate development, in December 2022, scientists at the Lawrence Livermore National Laboratory achieved a net gain in energy using inertial confinement fusion (Inertial Fusion 2022). A commercial reactor would need to reproduce the result many times a second and overcome other energy losses, but it at least shows that the technology is capable of operating in the right ballpark.

Electric vehicles (EVs)

The sale of EVs is rising across the world with 1 in 7 new cars in 2022 electric (1 in 4 in China) compared with 1 in 70 in 2017, and fossil-fuelled cars have seen a drop in sales of 25% (World Economic Forum 2023). However, the pace is not fast enough to meet climate targets. The IEA estimated the emissions in 2022 from SUVs, the most popular car, as almost 1 GtCO₂ (IEA SUVs 2023). Bans on buying ICE cars have been announced by several countries with the EU target by 2035, but the cost, range, and lack of charging points is slowly the transition. EVs are still more expensive than the equivalent sized ICE car and while home charging can take advantage of off-peak rates, fast public chargers can be much more expensive. EVs’ range has doubled in the last decade and is now over 300 km, but it is still about half that of a typical ICE car in the US. The growth in EVs will significantly help boost, by almost a factor of seven (McKinsey Battery 2023), the annual demand for lithium-ion batteries to 4.7 TWh by 2030, which will drive down their cost and improve their performance. However stronger policies worldwide will be needed to accelerate the transition.

Hydrogen and heat pumps

Interest in clean hydrogen (green or blue) has increased significantly in the last few years, with over 680 megawatt projects announced, about half in Europe, a seventh in North America and a tenth in China (McKinsey Hydrogen 2022). It can play a major role in decarbonising steel manufacture, which accounts for around 8% of global emissions, heavy transport, ammonia for fertiliser production, petrochemicals, and flexible power generation among other applications. More than 50 steelmaking projects have been announced worldwide, mainly in Europe. By 2050, clean hydrogen use could abate 7 GtCO₂annually, about 20% of current annual global energy-related CO₂ emissions.

Burning hydrogen can provide high temperature heat, which is useful in industry, but for home heating, heat pumps will generally be more economic. The losses in producing hydrogen combined with the COP of heat pumps of around three means that heat pumps are a much more efficient way of heating homes that typically more than offsets the cost of converting the heating system. Globally, heat pumps provided 10% of space heating in 2021 and there has been a considerable growth in sales- in Europe sales increased by almost 40% and globally by 11% (IEA Heat Pumps 2023). Particularly strong uptake was seen in France and the US where for the first time more heat pumps were sold that gas boilers. Financial incentives are available in over 30 countries covering more than 70% of the heating demand; however, more support and technical innovation will be needed, together with a quadrupling of global manufacturing, to meet climate targets.

For heavy transport, clean hydrogen can be used for making fuels such as methanol and ammonia for shipping and kerosene for aviation. It is used in fuel-cells for powering trains, trucks, and buses, though for the latter two, battery powered vehicles are a much more efficient use of electricity from renewables and may prove cheaper. For flexible generation, hydrogen can be stored for a long time in salt caverns and used in gas turbines when required.

Solar PV

Solar PV showed the largest growth in power generation capacity in 2022. The huge increase in cumulative production in the last decade has seen the global average solar PV’s LCOE fall to USD 0.048/kWh in 2021 (IRENA LOCE 2022), making it, together with onshore wind, cheaper in nearly half the world than existing coal or gas-fired generation (Bloomberg 2021). Over a third of new solar PV is added to rooftops with most of these attached to a grid, and huge rooftop potential is available globally, with 1815 TWh/y at a cost of $66/MWh in India and 4375 TWh/y at a cost of $68/MWh in China (Rooftop Solar PV 2021); in both these countries rooftop PV could help significantly with decarbonizing their electricity supply.

Tandem solar cells made from silicon with a thin film of perovskite on top have now reached an efficiency in the laboratory of 32.5% (Tandem 2022), and near 30% cells are expected commercially in 2023. Their smaller footprint, 75% of single-junction silicon cells with a typical efficiency of 22.5% (Silicon Panels 2023), will be valuable for rooftop installations, and they promise significantly cheaper electricity than from single-junction cells.

Solar organic PV can be produced on flexible sheet, and are light and can be transparent, which means that they can be simply mounted on walls, in place of windows, and have even been applied to the cylindrical surface of an 80-meter-high wind turbine tower (Organic Solar 2022). Their efficiency has improved in the laboratory to close to 20%, and around 10% efficient commercial systems now available. These could provide the cheapest electricity with the lowest carbon footprint and have an important niche market.

Solar PV panels are also being mounted on floating platforms, and a large 60 MW project on a reservoir opened in Singapore in 2021 (Floating Solar 2021). Besides saving land, the PV panels are kept cool, which improves their performance.

Solar farms and wind farms are subject to nimbyism, and community involvement and incentives like cheaper electricity for those affected can help with obtaining planning approval. That land must be reserved for farming is generally not the case (see Agrivoltaics- solar farms on farmland section). Nor is the argument that their variability means that a large fraction of electricity must come from nuclear power, which would be very uneconomic, and unnecessary as their variability can be handled using only a small amount (10-20%) of flexible generation (see Storage requirements for electrical grids).

The best regions for solar PV are often not close to where electricity is needed, and a very ambitious project under development plans to transmit 3.6 GW of power from Morocco to the UK via four underwater HVDC cables (Solar Link 2022). The electricity will be generated by solar and wind farms that benefit from the good sun and steady trade winds in Morocco, and a 20 GWh, 5 GW battery system will enable power to be supplied 20+ hours a day. This firm but flexible source once complete will provide 8% of the UK electricity and help balance UK supply and demand.

Agrivoltaics- solar farms on farmland

Solar farms can often be located on land unsuitable for agriculture or on buildings, but with the need for far more solar capacity good quality farmland is sometimes considered. The impact on food production will depend on the fraction of farmland proposed. In the UK, for example, solar farms only occupy 0.1% of the land area, and for net-zero 0.3% is envisaged which is equivalent to 0.5% of the land used for farming. Climate change poses the greatest threat to food production, with the risk in the UK that the proportion of best land drops from 38% of farmland down to 11% by 2050. Already crop failures are occurring from extreme heat and drought.

But it is not either electricity or food, the land used for the solar farm can also grow crops and graze livestock, often with little reduction in yields, as many crops don’t require the full amount of sunlight. Mixing native vegetation, tomatoes, watermelons, peppers, and grazing sheep in with the solar panels, for instance, can provide food, electricity, a pollinator habitat, and manure for the crops. Some crops are clearly not suitable such as tall fruit or nut trees, but leafy greens or berries can benefit from the extra shade. The potential of agrivoltaics depends on the climate. Very promising results have been seen, for example, in the western US with its high sunshine and lack of water but in the north-east some crop yields have been depressed.

Interest has been heightened by the effects of climate change making yields of crops erratic- solar panels can provide a stable source of income and also a cheap source of electricity. Japan is a leader in agrivoltaics, and many projects are underway around the world; in Germany alone, the Fraunhofer Institute estimate the potential as 1700 GWp. In 2021, the agrivoltaics total capacity exceeded 14 GWp and has been growing fast, and could be an important source of mitigation (CarbonBrief Agrivoltaics 2022).

Agricultural emissions and impact

Agriculture was responsible for emissions of 9.3 GtCO₂eq per year within the farm gate, 19% of the global total in 2018. About 8% is from land use, mainly deforestation to create land for grazing ruminants (mainly cattle) and growing animal feed to meet the increasing global demand for meat. Direct emissions are 11% from methane (CH₄), mainly from ruminant enteric fermentation (4%), and nitrous oxide (N₂O) from fertilizer application (FAO 2020). [About 1% of global CO₂ emissions come from ammonia production for fertilisers, and this could be cut by using green hydrogen in the Haber-Bosch process.]

Intensive farming of cropland involves significant use of fertilizers, pesticides, tilling, and high yielding monocultures that have fed the growing global population (2.5 billion 1950, 8 billion 2023). However, tilling degrades the soil through loss of soil organic matter and excess fertiliser and pesticide application have caused serious pollution and loss of biodiversity (Fertilisers 2023). Crop-related emissions account for about a third of those from agriculture, while animal-related emissions give rise to the other two-thirds.

More regenerative or integrated farming practices that typically involve cover crops, crop rotations, and less tilling, together with more timely and reduced application of additives can improve soil quality and reduce crop-related emissions and damage. Using little-to-no additives (as in organic farming) and perennial varieties give further improvements but generally at the expense of less yield per hectare. And expanding arable land through deforestation to offset this yield gap can negate the benefits (Organic Farming 2019).

In urban areas, vertical indoor farming, designed to produce very high yields per hectare with low water requirements, can also relieve the pressure on the environment (Vertical Farming 2021). However, even with the sharp drop in the cost of LEDs, the capital and energy costs can be high. These farms, together with greenhouses, may best complement outdoor farming through providing fresh leafy vegetables and fruit.

Animal-related emissions account for about 11% of global emissions (FAO 2022), of which cattle are responsible for approximately 65%, with other ruminants around 15%, and pigs and chicken each about 10%. Reducing ruminant numbers would therefore significantly cut emissions, release pasturelands and some arable land, and reduce deforestation. Moreover, as methane is short-lived with a lifetime in the atmosphere of about 12 years (compared with several hundred years in the case of carbon dioxide emissions), it is the emission rate that determines its contribution to global warming. A cut in methane emissions of 4%, for example, would give a one-off reduction in global warming of about 0.1 °C (see Global warming potential (GWP) of short lived GHGs).

Less consumption of ruminant meat in developed countries, where the current amount eaten is often higher than recommended on health grounds, could be helped by taxes on meat that differed by type (Meat Tax 2022). The reduction could allow a higher fraction of free-range organic production of livestock, which has low CO₂ (if no land-use change) and low N₂O emissions and is better for animal welfare as it avoids factory farms, but the yield of meat is lower than with intensive livestock farming.

With less livestock, vegetable protein can provide a substitute for meat and demand is rising fast in developed countries. Fermentation processes that can produce artificial meat are also being developed, but high costs are currently a barrier (Fermentation 2021). Currently meat substitutes are still only a small percentage of meat sales.

For many in low-income countries, ruminant meat provides essential micronutrients, and livestock rearing is very important culturally and economically. Livestock will therefore remain important globally, but with reduced numbers and less intensive farming practices their contribution to global warming can be significantly reduced. The increased arable land available for food production together with avoiding competition from biofuels and a reduction in the wastage of food, 25% of which currently perishes post-harvest, could feed the estimated ten billion in 2050 without further land clearances and significantly reduce agricultural emissions (WRI 2018). The grazing land released could be used for increasing biodiversity, and for carbon sequestration in vegetation, soils, and in trees through afforestation.

Global warming potential (GWP) of short-lived GHGs

The global warming potential GWP_H of a gas is the ratio of the amount of energy radiated by the Earth that is absorbed by the gas over a period of H years following the emission of a pulse of one tonne, relative to that absorbed by a pulse of one tonne of CO₂ over the same period. For CO₂, with its very long lifetime in the atmosphere, the absorption is essentially constant in time over periods of hundreds of years and causes the Earth’s temperature to rise, until the increase in the amount radiated by the Earth equals the energy absorbed and the equilibrium between the Earth’s incoming and outgoing radiation is restored. For example, methane has a GWP₂₀ of 82 but a GWP₁₀₀ of 28, reflecting the short lifetime of about 12 years for a pulse of methane emitted into the Earth’s atmosphere.

If the emission rate of a short lived GHG, with a lifetime T, is constant at one tonne per year, then after a time H, when H >> T, the amount in the atmosphere will be constant and the global warming will be equal to that from an additional GWP_H × H tonne of CO₂ (neglecting indirect effects).

For non-CO₂ GHGs, their contribution is typically given in terms of CO₂eq by R × GWP₁₀₀ where R is the emission rate in tonne per year. If the total annual CO₂eq rate is E tonne per year, then the percentage p of a non-CO₂ GHG is given by:

p = R × GWP₁₀₀ × 100/E

If the GHG is short-lived, then the global warming from this percentage is equal (Short-lived GHG 2016), from above, to the addition of:

R × GWP₁₀₀ × 100 = p × E tonne of CO₂

The number of tonnes of CO₂ for 0.3 ^oC of warming has been estimated as 500 Gt, though with considerable uncertainty (Carbon Budget 2021). So, neglecting any effect in this estimate from changes in non-CO2 GHGs, if the percentage of methane emissions could be reduced from 15% (its current value) to 11%, then as E is approximately 50 GtCO₂eq per year the drop in global warming would be about 0.12 ^oC, which would give the world a little more time to reach net zero.

Geoengineering

Refreezing the Arctic

Global warming has caused the Arctic ice cover to shrink in size significantly- the area has decreased by almost 13% a decade since 1979. The effect is a decrease in albedo, as ice is much more reflective than water, with the result that the Arctic region has been warming up more than twice as fast as the global average. This has reduced the temperature difference between the Arctic and the Equator. This decrease can cause weather patterns to move more slowly and increase the chance of their staying in one place which can create an extreme event, such as severe flooding or a heat wave (CarbonBrief 2019).

The British company Real Ice proposes to demonstrate in 2023 the practicality of refreezing parts of the Arctic by flooding areas of ice with water in winter temperatures of minus 50 ^oC (Arctic 2022). The water would freeze and thicken the ice sheet sufficiently that it retains its integrity throughout the summer when the surface of the ice is melting. This would increase the albedo and lessen warming. Real Ice plan to use green hydrogen fuelled water pumps.

A particular attraction of this idea is that it mimics natural processes and would give more time for mitigation. However, making it happen at the scale required is a massive and expensive task, but might prove to be a valuable tool for mitigating global warming.

Carbon dioxide removal (CDR)

Modelling by the IPCC suggests that limiting warming to 1.5^oC – 2^oC will require carbon dioxide removal as well as extensive mitigation (Carbon removal 2020). A substantial quantity can be through reforestation, afforestation, reducing deforestation, and using bioenergy with carbon capture and storage (BECCS). BECCS has a few GtCO₂ potential, but taking up too large a land area risks sustainability, and it is best used for carbon dioxide removal and balancing power. The carbon removal from forestation can offset a significant fraction of the hard-to-abate emissions from agriculture, industry, and heavy transport, which may amount to about 20% of current emissions, i.e., ~10 GtCO₂ per year. (Though every effort should be made through electrification and the use of clean fuels to reduce this quantity.) However, the amount of carbon removal require for climate targets will very likely be more than can be sustainably provided by nature-based solutions (NBS), or hybrid methods such as BECCS, as these would then have adverse impacts on water availability, food security, biodiversity, and land quality (CarbonBrief NBS 2021). Considerable forestation programmes are underway, but management is needed for removal to be permanent, and losses through wildfires can occur.

So, several GtCO₂ of high-tech strategies such as direct air capture (DAC) or enhanced mineralization will be required. Currently DAC is very expensive and energy intensive, and carbon prices are too low to promote a market. At present, 99.9% of CDR is through forestation projects, estimated at ~2 GtCO₂ per year and only 0.1% by other means (Oxford CDR 2023), with only about 0.01 MtCO₂ by DAC. The scale of investment in carbon removal was only $400 million in the 4 years 2018-2021 but ramped up to $2 billion in the first 4 months of 2022 (CDR 2022).

Despite numerous disappointments over the last two decades, there is growing optimism that carbon capture technologies will be successful. The IEA has identified three main developments (IEA CCUS 2022):

New business models, that focus on industrial hubs that share the cost of transportation and storage of carbon dioxide, rather than large stand-alone projects.

More investment support for carbon capture, utilization, and sequestration, notably the 45Q tax credit in the USA, which incentivizes the use of carbon capture and storage in industries such as steel, cement, refineries, and chemicals, which account for over a third of emissions in the USA.

International commitments towards net zero by around the middle of this century.

Progress on the transition away from fossil fuels

The global demand for energy dropped by about 6% in 2020 following the outbreak of the Covid-19 pandemic and the imposition of lockdowns. As restrictions relaxed, demand had returned to about pre-pandemic levels by 2022, but funds diverted to tackle the disease and keep food affordable slowed progress on energy access globally. The Russian-Ukraine war, which started in February 2022, has caused tragic losses and large energy price rises following restrictions on Russian fossil fuel exports.

The price rises have escalated concerns over energy security and the importance of energy efficiency to reduce demand, which have boosted investment in local wind and solar farms and consequently efficiency, through electrification of transport and heating. It has seen growth in biogas in Europe as a substitute for natural gas, but also some renewed investment in fossil fuels. Nuclear power, including some SMRs, is also experiencing some resurgence and demand for clean hydrogen is growing rapidly.

Under existing policies, global fossil fuel demand is now expected by the IEA to peak by the mid-2020s, but globally the amount of finance planned for clean power needs to triple by 2030 to be on track to displace fossil fuels and bring the combustion of fossil fuels close to zero, and hold warming close to 1.5 ^oC (IEA Outlook 2022). While wind and solar reached a 10% share of global power generation in 2021, equal to that of nuclear, the percentage of energy supplied by fossil fuels only dropped from 85% to 82% in the five-year period 2016-2021 (BP Statistical Review 2022). To be on track to meet the Paris climate targets, the IPCC estimate that global emissions must drop by 45% by 2030 from 2010 levels and reach net-zero by 2050 (Net-zero 2022).

Programmes that are accelerating the transition include the Inflation Reduction Act in the USA, the RepowerEU plan in Europe, Japan’s Green Transformation (GX) program, and the Green Economy initiative in South Korea. China is also seeing a massive build-up of clean energy, and India aims for net zero by 2070 with half of its electricity from renewables by 2030. In emerging markets and developing economies, notably where the cost of capital has been high for renewable projects, continued investment in fossil fuels is aided by vested interests and existing infrastructure. The favourable economics for clean electricity generation alone is insufficient to guarantee a fast transition.

Significant finance will be needed to manage the retirement of existing fossil fuels as well as expanding renewables and developing new jobs to give a just transition. For instance, critical mineral mining and carbon removal projects could absorb some of the employment lost, and the large subsidies for fossil fuels should be redirected towards the transition to renewables. But too fast a transition has the danger of leaving some in energy poverty. Investment by countries and businesses in low-carbon technology is growing and equalled that in fossil fuels for the first time in 2022 (BloombergNEF 2023). It needs to be in developing economies to ensure clean growth, as well as in developed countries where the largest emissions are (Investments 2022). Constraints on access to finance is already challenging coal generation and the falling cost of battery storage combined with utility-scale solar is also affecting the competitiveness of gas and coal generation in several parts of the world.

Expanded and modernised grids, allowing for distributed as well as centralized generation, will be required for faster progress. Demand for and supplies of fossil fuels need to decline rapidly and can be targeted through restricting end uses, such as ICE cars and gas appliances in homes, exploration, and exports, and through carbon pricing and incentivizing low-carbon alternatives. Nationalizing fossil fuel companies, as argued by Holly Buck in Ending Fossil Fuels (Ending Fossil Fuels 2021), could hasten near-zero production. Tax schemes need to be aligned with net-zero and alternative revenues for governments devised to replace the large amounts from fossil fuel taxes, such as the duty on petrol and diesel.

Carbon pricing, either through taxes (including border adjustment taxes) or emission trading schemes (ETS) using permits, is expanding but only covers about 30% of global emissions (IMF 2022). Moreover, although the highest price is around $90 per tonne CO₂, the average is $6, and this must increase quickly for carbon pricing to be effective at making fossil fuel combustion uneconomic. An estimate (the social cost of carbon) based on the cost of the damage that the emission of 1 tonne of CO₂ causes is very hard to determine and several countries including the UK determine the carbon price as that required to provide a certain limit to global warming, i.e., to the cumulative emissions. This target consistent approach is in line with that of the IPCC, and it has been estimated that the price should be $75 per tonne by 2030 for warming to be limited to close to 1.5 ^oC (IMF 2022).

Allowing combustion of fossil fuels without mandatory capture risks greenwashing, continued pollution, and insufficient global carbon removal for net zero, given the current lack of scale of carbon capture projects. Carbon offsets often only avoid or reduce emissions and have often been abused, so carbon removal projects are favoured. Nature-based ones such as tree planting and enhancing areas of kelp, mangroves, salt marshes have considerable potential estimated at several GtCO₂ per year. But high-tech strategies such as enhanced mineralization and direct air capture will be needed. Net-zero alone risks a licence to emit with incomplete capture which argues (Ending Fossil Fuels 2021) for planning for near-zero fossil fuel production together with net-zero emissions.

References

Arctic 2022: https://www.thetimes.co.uk/article/the-plan-to-refreeze-the-arctic-qqzt6xdk2

Bloomberg 2021: https://www.bloomberg.com/news/articles/2021-06-23/building-new-renewables-cheaper-than-running-fossil-fuel-plants

BloombergNEF 2023: https://about.bnef.com/blog/global-low-carbon-energy-technology-investment-surges-past-1-trillion-for-the-first-time/

BP Statistical Review 2022: https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html

Carbon Budget 2021: https://climateanalytics.org/briefings/is-the-15c-limit-still-in-reach-faqs/

Carbon Removal 2020: https://www.iea.org/commentaries/going-carbon-negative-what-are-the-technology-options

Carbon Brief 2019: https://www.carbonbrief.org/qa-how-is-arctic-warming-linked-to-polar-vortext-other-extreme-weather/

CarbonBrief 2022: https://www.carbonbrief.org/mapped-how-climate-change-affects-extreme-weather-around-the-world/

CarbonBrief Agrivoltaics 2022: Factcheck: Is solar power a ‘threat’ to UK farmland? - Carbon Brief

CarbonBrief/IEA 2023: https://www.weforum.org/agenda/2023/02/renewables-world-top-electricity-source-data/

CarbonBrief NBS 2021: https://www.carbonbrief.org/qa-can-nature-based-solutions-help-address-climate-change