TL;DR

• A rapid and large-scale reduction in car use is necessary to achieve short-term emissions targets, meet stringent carbon budgets that limit temperature rise to 2°C at most and to avoid excessive demand for additional technology, material, and minerals.
• Relying on efficiency and electrification policies takes too long due to the turnover time of the fleet, even with the ban on the sale of fossil fuel vehicles moved forward and accelerated scrapping of ICEVs. This is because the vast majority of carbon emissions that fill up the remaining carbon budget originate from cars that already exist. 
• Policies that can reduce these emissions include retrofitting the existing fleet with electric engines and reducing the distance driven by cars. Policy efforts should put increasing emphasis on mitigating emissions from cars that exist today rather than solely focussing on the electrification of new cars.
• Without increased consensus on regional and sub-sectoral carbon budgets, it is difficult for regional policymakers in urban areas to apply policies that meet the Paris Agreement.

Cities are recognised as a key area for mitigation globally being responsible for 70% of global carbon emissions and consuming two-thirds of the world’s energy [1]. Yet, cities present us with a big decarbonisation challenge with millions of people relying on carbon and energy intensive services every day.  Nevertheless, in the case of transport, high population densities allow for short distances to be travelled between people and places and previously existing transport networks can be built on to provide low-carbon mobility options. However,  transportation accounts for 30% of global energy consumption [2] and even if the entire current UK car fleet were electrified, an additional electricity usage comparable to the entire nation’s current renewable generation capacity would be necessary [3]. 

Energy and emissions can be mitigated in three fundamental ways: efficiency improvements, technological substitutions, and demand side solutions [4]. Demand side solutions are often not prioritised in favour of electrification of existing technologies, for example in the UK, national policy ambition proposes to meet climate targets by achieving only 9.5% through reducing demand [5]. Yet, electrifying private car use encounters two limits to the mitigation of fleet-wide energy and emissions. Firstly, powering a private vehicle at low occupancy requires a fundamental amount of energy (and embedded resources) which is much higher than shared and active modes of transport. Secondly, given that just 1 in 50 new cars globally are electric and that cars have a lifetime of 15-20 years, it would take decades for the entire car fleet to transition away from fossil fuels [6, 7] and would require significant amounts of raw material. In contrast, demand-side solutions allow for immediate reductions without needing to wait for deploying or developing new technologies [8, 9]. There is consensus in transport literature that a mix of policies is necessary [10, 11, 12] , but it is imperative for policymakers to understand how the policy mix interacts as a whole [13] .

In a recent article published in Nature Communications, we develop the Urban Transport Policy Model (UTPM) to compare the effect of various energy efficiency improvement policies in London’s car fleet as well as a total reduction in car usage to explore what is necessary to meet regional carbon budgets compliant with the Paris Agreement, using London as an urban case study. This started out as my master's project supervised by Dr. Oytun Babacan, and later developed into a longer project incorporating Dr. Drew Pearce and Prof. Jenny Nelson's work in order to publish the results.

The carbon budgets specify the remaining carbon emissions that can be released to have a “reasonable” chance of remaining within a 1.5 °C or 2 °C temperature rise [14] and emphasis should be put on whether cumulative emissions released between today and the future remain within this budget. We include the CCC’s 1.5°C compatible pathway for surface transport [5] as well as a more stringent estimate of the carbon budget for London from the Tyndall Centre [15] due to its increased fairness approach.

How different policies affect cumulative emissions 

Figure 1 shows that the current system cannot reach stringent carbon budgets without adopting highly aggressive and disruptive policies. Electrification, including moving the phase out date forward, results in cumulative emissions 7 times greater than the Tyndall carbon budget for the “well below 2 °C and pursuing 1.5 °C” global temperature target. In general, policies that push for electrification encounter a trade-off between decreased use-phase emissions and increased embedded emissions. Nevertheless, our analysis shows that moving forward the ban on the sale of new internal combustion engine vehicles from 2030 to 2025 results in an overall reduction in emissions by 6%. If all electric vehicles were powered by renewable energy, this would reduce emissions of travel using battery electric vehicle (BEV) by 30% leading to an overall reduction in fleet-wide emissions by 2%.

Embedded emissions can be reduced by policies such as light-weighting, retrofitting conventional ICEVs with electric engines and implementing stricter manufacturing standards.

Reducing the mass and size of cars (light-weighting) means less energy is spent on the average 1.6 passengers that occupy a 5-seater car [17]. Recent analysis has found that most SUVs in the UK are bought by people in cities [18] and that increases in fuel consumption from larger cars have cancelled emission saving effects from EVs on a global scale [19]. Our analysis has shown that light-weighting all new cars by 20% leads to a reduction in fleet-wide emissions by 

Retrofitting also provides a reasonable emission reduction of 8% when one third of scrapped fossil fuel cars are converted to EVs instead. However, retrofitting a large proportion of the ICEV fleet is currently not seen as viable as the development cost per vehicle type and cost of conversion for most vehicles is higher than manufacturing a new electric car due to economies of scale and design incompatibilities. Yet, with the right incentives and government support to drive innovations, retrofits can become cheaper and more commonplace.

Embedded emissions of BEVs can be reduced by 42% through implementing stricter manufacturing standards. Policies should therefore incentivise the operation of EV battery recycling and manufacturing on a national level, where jurisdiction is possible over the energy sources and manufacturing practices used.

Applying modal shift policies result in the most substantial emission reductions due to less car travel activity overall. The reduction in emissions with reduction in distance driven is approximately linear with a 43% and 81% decrease in car distance driven by 2027 resulting in a 42.8% and 70.5% decrease in cumulative emissions respectively. Although the emissions released from alternative modes are increased, the resources needed for active travel and shared transport infrastructure are significantly less than private vehicle ownership [20, 21] . Active travel also holds many co-benefits, such as the health benefits gained from physical activity and the freeing up of urban spaces from traffic thus creating safer and less polluted cities [22]. 

Local transport policy plans fail to meet the Tyndall carbon budget, with the 2018 Mayor’s Transport Strategy (proposing a 12% reduction in distance travelled by car by 2041) [23] achieving a reduction in cumulative emissions of -20.1% and the Element Energy report published for the Greater London Authority (proposing a 27% reduction in vehicle kilometres by 2030) [24] achieving a reduction in cumulative emissions of -31.6% with tailpipe emissions exceeding the Tyndall carbon budget by more than double. Nevertheless, this scenario meets the 1.5°C pathway derived from the UK’s Committee on Climate Change (CCC) carbon budget [5] which is considered lenient among other carbon budgets [25, 26].

Cars currently on the road cause majority of emissions

Tailpipe emissions (black bars in Figure 1) represent the largest proportion of cumulative emissions in the combined policy cases and a key area for mitigation. In the baseline case, 78% of these tailpipe emissions are caused by cars in the existing fleet made pre-2020 rather than new cars introduced in the period of 2020-2050. In the combined policies cases, 99% of the tailpipe emissions stem from the existing fleet. Hence, it is the exhaust emissions from cars on the road today living out their life that are responsible for 68% of the total emissions in the combined case and that are using up more than 92% of the carbon budget, even if disruptive policies are implemented. Therefore, policies solely focusing on reducing the carbon footprint of new cars cannot be sufficient in achieving climate targets without introducing regulations that target the existing fleet. This can be done through measures that encourage less driving (e.g., modal shift), retrofits of the existing fleet, fuel-saving driver behaviour (e.g., optimal speed travel, which is not modelled in this study) and potentially low carbon synthetic fuels, although their impact on emissions is not established.

Designing a policy mix

In Figure 1, we demonstrate two additional scenarios that meet the Tyndall carbon budget. Both require BEVs to be powered by 100% renewable electricity, a 2025 fossil fuel phase-out, 33% retrofitting, 40% light-weighting, strict standards on EV manufacture as well as a massive and rapid reduction in car travel activity. The scale of reduction necessary is either a 72% reduction in distance driven by 2025 or an 84% reduction in distance driven by 2027. This translates to just 1 in every 12 and 1 in every 24 trips made by car, respectively. Thus, waiting longer to act on car travel demand requires much more stringent policy measures in the future.

Some policies are effective but too slow for generating meaningful impact

Figure 2 shows the use-phase emissions of the baseline case and several policy options. Only a reduction in distance driven results in short term emission reductions that meet 2030 targets. Although all policies reduce emissions by a factor of 4 or more by 2050, the rate of emission reductions vary greatly. Policies that rely on the turnover of the fleet such as electrification and light-weighting, are too slow considering emissions globally need to be almost halved by 2030 in order to meet 1.5°C carbon budgets [27]. However, to reach equal per capita emissions globally in 2030, the UK needs to decarbonise faster, reaching a 70% emissions reduction by 2030 [28].

Uncertainties in carbon budgets and mitigation efforts

The exact degree of mitigation effort required in terms of reducing car travel activity is very sensitive to the carbon budget. When considering multiple carbon budgets, derived under different methods and allocation principles from the global carbon budget for 1.5°C, the amount of reduction in car travel activity that is necessary is uncertain, as seen in Figure 3. For example, taking the UK carbon budgets from the CCC (for a 50% chance of meeting 1.5°C and considered lenient among other carbon budgets [21, 22]) allows, in a small set of cases, for car travel activity to increase as ‘business as usual’, although this is at the expense of total emissions including electricity generation and embedded emissions. Additionally, for the combined policy case (assuming the highest mitigation efforts in electrification, fuel efficiency and modal shift), an uncertainty in carbon budget of ± 4 MtCO2eq results in an uncertainty in car travel activity of ± 20%. Since there is no agreed consensus on regional and sub-sectoral carbon budgets, and uncertainties are large, it makes it difficult to ascertain exactly how far we need to go with reducing car travel activity. This may explain why there is currently little consensus on the level of demand-side mitigation required within climate policy. Our work makes it clear that urgent frameworks and agreements need to be developed on sub-national sectoral carbon budgets for regional policymakers in urban areas to direct serious and confident efforts at policies that meet the Paris Agreement.

Cover image

 James Mckay -Dreams of a Low Carbon Future

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