A team from the University of California, Berkeley concludes that reducing demand for light-duty vehicle (LDV) travel will likely be essential to meeting the international greenhouse gas emission and climate targets for the year 2050. Their open access paper is published in the IOP journal Environmental Research Letters.
Average per capita light-duty vehicle (LDV) transport CO2 emissions (kg CO2 person-1 yr-1) for a global sample of countries (2007) with a wide range of per capita incomes. Per capita transport CO2 emissions are decomposed into the product of two terms: per capita LDV use (horizontal axis, veh-km person-1 yr-1) and propulsion carbon intensity (vertical axis, g CO2 veh-km-1). Full explanation at Sager et al.
The quest for reduced LDV emissions is often posed as a technology issue of lower-carbon fuels or more efficient vehicles, Jalel Sager, Joshua Apte, Derek Lemoine and Daniel Kammen note in their paper. However, in their study they decomposed transport sector emissions into technological and behavioral drivers, and showed that even significant technological advances will be insufficient to meet climate goals, unless the growth in LDV use slows or reverses.
While policy options aimed to reduce the need for LDV travel typically receive far less attention than do technological measures, we find such demand avoidance options are likely essential to meeting mid-century GHG reduction goals.
...We find that innovation in a single area such as fuel economy does not offer a realistic, affordable, or resilient pathway to the LDV emission reductions necessary by mid-century. Instead, as social, technical, and infrastructural drivers of LDV GHG emissions interact multiplicatively, the responsibility should be spread over a portfolio of achievable improvements across the transport system. —Sager et al.
Sager et al. identify five potential elements for improvement in LDV GHG emissions:
- Fuel carbon intensity and vehicle energy efficiency (together representing LDV propulsion GHG intensity); and
- Increasing the vehicle occupancy rate; decreasing the mean per-trip distance; and reducing the per capita trip rate (together representing per capita LDV transport use).
In surveying 2007 LDV usage and fuel economy in an economically diverse set of countries, the team found that the large differences in per capita LDV GHG emissions—which range from ~100–4,000 kg CO2-eq yr-1—are principally explained by differing national per capita LDV use (range: 300–13,000 VKT (vehicle kilometers travelled) yr-1), rather than by fleet average fuel efficiency and carbon intensity factors, which reflect broadly similar car technology worldwide.
They found that in upper-income countries, intensive LDV use results in present-day emissions that exceed the 2050 target per capita range of 50–100 kg CO2-eq yr-1 by a factor of 10–80.
With global per capita LDV use of 10 000 km yr.1, GHG propulsion intensity would need to decline from current levels of around 300 g CO2-eq km-1 to around 5–10 g CO2-eq km-1 on a well-to-wheel basis. Such a performance level would require universal deployment of one or more of: electric vehicles (EVs) running on nearly zero-carbon electricity; cellulosic-biofuel-powered vehicles achieving 300 miles per gallon (0.78 L/100 km); or gasoline-fueled vehicles achieving in excess of 1,000 mpg (0.24 L/100 km).
Sager et al. suggest that meeting the 2050 climate targets will be feasible only with global implementation of robust policies to slow the growth rate of LDV VKT in low-income countries, and to reduce VKT in high-income countries. As an example, they note that by converging global 2050 per capita LDV VKT at levels currently typical of middle-income countries such as Mexico (~3000 km yr-1), a per capita LDV GHG target of 100 kg CO2-eq yr-1 could be met with medium-term technologies, such as 100 mpg (2.35 L/100 km) plug-in hybrid electric cars fueled on a mix of cellulosic biofuels and low-carbon grid electricity.
That particular strategy would require reducing cumulative growth in kilometers travelled to 60%, against the baseline projections of a 100% increase; and would require load factor convergence to around 1.65 persons per vehicle, typical of European averages in 2000.
“ While likely challenging, such efforts would not imply privation.” — Sager et al.
Achieving such a reduction in LDV VKT could be achieved by coordinated policies, the authors suggest—e.g., reducing average trip length and frequency by ~33% each while increasing the average load factor from ~1.5 to ~2 people per vehicle would be sufficient for the goal.
We have quantified the need for complementary policies required to achieve global climate targets in the light-duty vehicle sector. A truly unified framework would also account for freight interactions, technological options for mass transit, electricity sector emissions, and life-cycle assessments of LDVs and infrastructure. It would also explore in greater detail the substantial co-benefits of lower LDV usage by assessing, for instance, reductions in urban air pollution.
It is crucial to develop the institutional capacity for this effort; to share tailored policy implications with national, regional, and local governments across the world; and to establish standardized data collection mechanisms necessary for global policy evaluation. — Sager et al.