A new study has found that using drones to deliver packages could help cut emissions - but the researchers say that is only the case if the drones are deployed 'sensibly'.
A number of companies and organisations are already investigating the prospect of using drones for deliveries, such as Amazon's 'Prime Air' project.
The new research has found that the environmental benefits of drone delivery are 'mixed', and the benefits are most prominent when light packages are involved.
The findings, published in Nature Communications, saw researchers conduct test campaigns with two drones, while also considering how the design and energy efficiency of drones is likely to improve in the coming years.
The authors write: "In most cases examined, the impacts of package delivery by small drone are lower than ground-based delivery. Results suggest that, if carefully deployed, drone-based delivery could reduce greenhouse gas emissions and energy use in the freight sector."
However, it also points out that the environmental impact of a drone depends on the type of electricity used to charge it.
The limited travel range of drones - currently around 4 kilometres - also means a drone delivery system would also require a new network of urban warehouses or waystations, which would come with their own energy demands.
Since many large urban areas are currently only served by one large regional distribution centre, a new system would require "dozens of new local warehouses to achieve the on-demand delivery model proposed for drones".
Lead author Joshua Stolaroff, of the Lawrence Livermore National Laboratory in California, explained: "A light package - say, a pair of sunglasses - flown by a small drone over a few miles, saves a lot of energy and greenhouse gas emissions compared to a delivery truck. But a larger package - say, a computer monitor - flown by a drone large enough to carry it, probably does worse than a delivery truck.
"Charging drones only with renewable and low-carbon electricity would be the easiest way [to improve efficiency]. They also might find creative ways to deliver goods from existing retail stores rather than building additional warehouses."
The researchers are calling on companies to focus on small packages "to realise the environmental benefits of drone delivery".
