Relaxing US fuel-efficiency requirements for cars and light trucks (pickups and SUVs) —even for just a few years — would result in decades of elevated carbon emissions, say David Keith and colleagues at Massachusetts Institute of Technology in the US. Modern vehicles have such long lifespans that a new car sold today has a good chance of being on the road 20 years from now.

In light of their models, the researchers argue for continued upwards ratcheting of fuel-economy standards for new vehicles, and for policies that encourage the replacement of the most inefficient vehicles.

Introduced in the US in the 1970s in answer to rapid rises in the price of oil, Corporate Average Fuel Economy (CAFE) standards led to a gradual improvement in the fuel efficiency of cars and light trucks over subsequent decades. Targets set by the Obama administration would have seen the trend continue beyond 2020, reaching 39 miles per gallon (MPG) in real-world terms in 2025.

This continuous improvement requires constant innovation on the part of automakers, however, and the sector has lobbied for standards to be relaxed. Responding to these calls, the US Environmental Protection Agency and the Department of Transportation under the Trump administration have proposed freezing standards at the 2020 level of 31 MPG.

Keith and colleagues modelled the effect that such a freeze would have on the cumulative emissions from light vehicles out to 2050. The researchers assumed that the pause would last until 2026, after which standards would continue to be tightened. The trajectories with and without the freeze would then converge at 2050 on an allowable greenhouse gas emissions figure reduced by 75% from the 2018 level.

On this timescale, a six-year hiatus in efficiency improvements might not seem so significant, especially if the same standard is ultimately achieved whatever the near-term policies. As the researchers’ model showed, however, the longevity of modern vehicles imparts a great deal of inertia to the makeup of the fleet, so decisions taken today will have an effect for years to come.

If, for example, electric vehicles instantly became so popular that they accounted for all new purchases from tomorrow, in 20 years’ time 10% of the cars on the roads would still be combustion-driven. This lag means that just six years’ worth of sales of slightly less efficient vehicles could, according to Keith and colleagues, result in the emission of an extra 2.5 gigatonnes of CO 2 over the modelled period.

The same effect implies that, even if standards are driven up relentlessly, keeping transport-related emissions to a minimum might require an acceleration of the slow natural vehicle replacement rate. To achieve this, the researchers propose a feebate system in which financial penalties are applied to the least efficient vehicles while more efficient choices are rewarded.

The level of inducement would have to be set carefully, so that the emissions savings from having a more efficient fleet are not cancelled out by the carbon cost of manufacturing new vehicles.

“Greenhouse gas emissions occur throughout the vehicle lifecycle, from manufacturing and operation to retirement,” says Keith. “Determining the optimal time at which an individual vehicle should be retired depends on the attributes of the vehicle it will be replaced with. Models such as the one we demonstrate in this paper can inform the design of fleet management policies to ensure that real reductions in emissions are achieved.”

If the freeze does go ahead, its effects could extend far in space as well as time.

“Several other countries set their fuel-economy standards based on US policy — including Canada, Mexico and Saudi Arabia — so a rollback of CAFE could have wide-reaching consequences,” says Keith.

Keith and colleagues presented their perspective in Environmental Research Letters (ERL).