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Cheng et al. Carbon Footprints 2024;3:10 https://dx.doi.org/10.20517/cf.2023.53 Page 11 of 13

However, the climate performance of greenhouses deserves further investigation if agricultural robots or
agricultural machinery were deployed in large-scale intelligent greenhouses.

CONCLUSIONS
Non-CO greenhouse gas emissions on dynamic carbon footprints complicate the comparison of the
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climate performance of two vegetable production systems: Traditional sunlight greenhouses and plant
factories. In this study, we used lettuce production as an example and established the LCA models of
sunlight greenhouses and plant factories to quantitively compare their climate advantages under different
grid decarbonization scenarios, with conclusions as follows.

First, artificial light plant factories have no climate advantage in lettuce production relative to traditional
sunlight greenhouses at the baseline scenario. Second, nitrogen fertilizer and polyethylene film consumption
in the traditional greenhouse production system are the main sources of greenhouse gas emissions, whereas
electricity consumption is a huge contributor to the climate load of the plant factory. Third, scenario
analysis shows that power grid decarbonization with upstream methane control added the climate
advantage of plant factories, whose climate performance gradually increased over time relative to the
greenhouse. Fourth, although nitrous oxide emissions reduce the climate advantage of the traditional
sunlight greenhouse, plant factory cultivation still fails to exhibit better climate performance unless it is fully
powered by hydroelectricity. Our analysis contributes to the low-carbon development of vegetable
production systems in the context of China’s power grid decarbonization.

Follow-up investigations can be conducted to further compare the climate performance of greenhouses and
plant factories. Firstly, besides polyethylene film, glass is usually used as coverings for sunlight greenhouses.
Glass covering materials have a long lifespan but produce high carbon emissions in glass production, so
there is a trade-off when selecting the materials of greenhouse coverings for climate performance analysis.
Secondly, plant factories or vertical farming facilities can be developed at significantly varied scales, such as
urban areas, communities, or even households. Thus, the power sources also vary accordingly, and the top-
down and bottom-up technological pathway is needed for local policymakers to choose global optimum
[30]
vegetable production technologies to avoid troublesome foci . Thirdly, different crops (e.g., lettuce and
strawberry) have different demands on temperature, light, nutrients, etc. These varied materials input leads
to different carbon emissions, which deserves in-depth discussion under varied decarbonization pathways.
Fourthly, for crop transportation distance, urban plant factories generally belong to the category of
“zero km”, thus emitting fewer GHGs, causing less food loss, and keeping high quality by reducing the
transport distance compared with vegetables transported from rural areas, which is an interesting topic for
further investigation.

Notably, plant factories, as supplements to vegetable production facilities, can not completely replace
sunlight greenhouses in China's context. For example, in remote areas with extremely harsh climate
conditions (like frontier sentry), plant factories show the potential to provide a sufficient supply of
vegetables. This reduces the challenges of long-distance transportation and preservation tasks. Compared to
rural greenhouses, urban plant factories also provide cultural and educational services for metropolitan
areas. Additionally, building plant factories becomes possible in densely populated urban areas, which
[31]
enhances the food security of large cities, such as during pandemics . Therefore, climate performance and
sustainable production are not the only criteria for judging the feasibility of plant factories in urban areas.
The construction of plant factories is also necessary to improve future urban food security. Thus, comparing
the climate performance of urban plant factories and rural traditional sunlight greenhouses is an interesting
but challenging topic, except for considering the temporal dimensions of the short-lived and powerful