Urban reforestation in cities in the Global South could be a cost-effective path to offsetting some part of global city emissions, according to a new study. And the researchers are calling their findings a conservative estimate, calculated with due concern for land use conflicts and other fundamental constraints.

Aiming to correct the misperception that urban reforestation would have “insignificant” climate mitigation potential, a team from the National University of Singapore (NUS) modelled the global potential and limits of this natural climate solution (NCS). The researchers published their results in the journal Environmental Research Letters.

Choosing urban areas with a density of at least 1,500 inhabitants per square kilometre and a minimum population of 50,000, the team found that 17.6% of all city areas are suitable for reforestation. Maximizing that potential could offset 1.1% of annual global emissions from cities.

At the regional level, some areas could see significant effects. “Among the cities analyzed, 1,189 are potentially able to cost-effectively offset more than 25% of their city carbon emissions through reforestation,” write the authors. On that basis, they urge policy-makers to give urban NCS “a place on global and local agendas.” 

To date, urban reforestation has been largely dismissed as a potential global climate mitigation tool, largely because “urban areas consist mostly of impervious surfaces that occupy less than 1% to 3% of global land and constantly face a multitude of competing land uses,” the researchers note.

Alert to these constraints, the NUS team was careful to exclude not only impervious surfaces, but also forests and croplands. “Protected areas, airports, golf courses, sporting grounds, parks, and gardens” were likewise excluded, even if such areas could end up being suitable for micro-efforts at forest building. 

Noting that much (77.7%) of the climate mitigation potential of urban reforestation is located in the Global South, the NUS researchers urged both researchers and potential practitioners “to be particularly sensitive to the context and challenges faced by cities in the Global South, from governance capacities to socioeconomic needs.” They added that “cities in the Global South tend to have much higher population densities and are projected to continue growing rapidly, exerting additional pressures on urban land.” 

The study authors are also careful to note that their land use exclusions were made “to provide a better estimate of likely climate mitigation potential, and not intended to imply whether they should be reforested, a decision best made locally with full consideration of the benefits, opportunity costs, and socioeconomic justice issues.” 

In the interests of long-term stakeholder buy-in, the authors add that urban reforestation should be “well-planned in order to reduce potential ecosystem disservices from trees, such as allergenic pollen, release of toxic compounds, damage to people and properties, pests, and human-wildlife conflicts.”