1. Introduction
In 2015, the global population reached 7.3 billion with an increase of about 1 billion in the last twelve years alone [
1]. According to the United Nations [
2], the proportion of the population living in urban areas is expected to increase from 54% to 66% by 2050. This proportion of urban inhabitants is even greater in Latin America and the Caribbean, where approximately 80% of the population already resides in urban areas [
2].
Policies and activities to address climate change in tropical Latin American countries generally focus on mitigating carbon emissions from the industrial and transportation sectors [
3]. In Brazilian and Colombian cities, innovative transportation policies and programs have been reported as effective policies to mitigate local-scale anthropogenic carbon dioxide (CO
2) emissions [
3]. In Medellin, Colombia, the use of cable cars that reach less accessible, low income neighborhoods have been touted as measures that both promote social inclusion and provide other co-benefits such as offsets of carbon emissions [
4,
5]. Similarly, the role of deforestation and forest degradation in global-scale anthropogenic CO
2 emissions has been well established in the environmental literature, particularly for tropical forests [
6]. However, less is known about the influence of existing urban forests, deforestation in peri-urban areas, and commonly touted urban tree planting programs and their role in local-level CO
2 regulation in cities such as those from Neotropical Latin America [
7]. Indeed, urban areas worldwide play a major role, accounting for 71–76% of energy related CO
2 emissions [
8].
While many different strategies at the national and local-level are needed to address increasing greenhouse gases emissions, local-level nature-based solutions can play an important role in mitigating local and regional urban environmental quality issues, not only in cities in high income countries, but also in low-middle income countries such as Colombia [
9,
10,
11]. These nature-based solutions have recently been proposed in Europe as not only an approach for addressing climate change, but also for creating employment opportunities and a green economy [
12,
13]. Unfortunately, to our knowledge the concept has been little discussed outside the European Union and the United States [
13]. However, such approaches are arguably more important in cities in regions such as Latin America, rather than in Europe, due to the marked socio-economic disparities and high proportion of vulnerable populations. The use of urban and peri-urban forests and agriculture, for example, are solutions that could also be used for improving urban air quality and food security, reducing temperatures, conserving biodiversity, maintaining water quality, and increasing employment through tree maintenance and green space management [
12,
13,
14,
15].
Urban trees directly remove CO
2 from the atmosphere through sequestration, and can reduce building cooling demands through shading and evapotranspiration, thus indirectly reducing CO
2 emissions from the use of fossil fuels to produce energy for building cooling [
16]. Additionally, herbaceous vegetation and soils contribute to carbon sequestration and storage, and provide numerous other ecosystem services such as atmospheric pollution removal, biodiversity, and storm water regulation [
17,
18,
19]. However, in addition to these benefits, urban trees can generate costs or ecosystem disservices, by incurring maintenance and management costs, production of allergenic pollen, and can lead to CO
2 emissions through decomposition, soil respiration, and maintenance-related fossil fuel burning activities [
17,
20]; as such, these types of costs and emissions must also be accounted for.
Understanding the CO
2 offset potential and effectiveness of urban tree plantings can help determine and promote their viability as a nature-based solution. A characterization and framework for nature-based solutions that includes an assessment of multiple co-benefits for the urban Neotropics is warranted. Such assessment would require the accurate estimates of benefits, one of which is carbon sequestration [
21].
Improving carbon estimates would also improve the incorporation of urban tree plantings into carbon markets and climate policy. In December 2015, the United Nations Framework Conference on Climate Change (UNFCCC) produced the 2015 Paris Agreement. As a country party to the convention, Colombia submitted their new emissions target, committing to reductions of 20% below business as usual emissions by 2030, or 30% below business as usual with sufficient international support [
22]. One mechanism for high income countries to provide this support to Colombia is through the Clean Development Mechanism (CDM) [
23]. This mechanism allows developed countries to invest in offset projects in developing countries in exchange for certified emissions reduction credits [
23]. The CDM accounting methodologies for receiving credits for a project must either be developed or adopted from a past project and approved.
From 2008 to 2012, a methodology for accounting for CO
2 sequestration from urban afforestation/reforestation, AR-AMS0002, was in effect, but is since inactive [
23]. Currently, AR-AMS0002 does not account for avoided emissions from cooling or emissions from tree maintenance or plantings, only net sequestration [
24]. However, a carbon offset registry with associated protocols has recently been developed for urban forest projects [
25].
Previous studies in temperate high income regions of the world have looked at the potential of urban trees to offset CO
2 at multiple scales. In the US, national-level carbon storage and sequestration by urban trees was evaluated [
26], as well as state level urban forest carbon sequestration [
18]. Other authors have reported city-scale carbon sequestration by urban forests and public trees in several US and Chinese cities [
16,
19,
27,
28,
29]. More recently, C storage and sequestration by urban trees in European cities such as Barcelona, Bolzano, Rome, and elsewhere are increasingly being studied [
17,
30,
31,
32]. A study much closer to Medellin estimated average carbon stocks per tree for Bogota, Colombia’s public urban forests [
9].
Some studies have also investigated management, maintenance, and socio-ecological dynamics and the interaction between urban tree carbon sequestration and emissions [
20,
27,
33]. Another body of research is exploring if the use of trees is indeed an effective policy and activity to mitigate local CO
2 emissions [
14,
16,
17,
28,
34,
35]. In terms of policy effectiveness, studies in the United States have documented that local level reforestation of degraded peri-urban areas can cost effectively be used to improve air quality while providing other co-benefits, such as CO
2 sequestration and wildlife habitat [
36].
Another study found that urban tree plantings “may be cost-effective” in certain locations in the US [
15]. However, a different study found that urban forest management was “moderately” effective as a mitigation strategy in two cities in Florida, USA, relative to other reduction strategies [
14]. Similarly, studies in Europe raise awareness to the fact that different urban tree and vegetation carbon calculation methods, such as the choice of allometric equations, carbon estimation models, resolution of geospatial data, and reporting of different carbon pools (i.e., above ground versus below ground carbon pools) will yield very different C sequestration results [
17,
32].
While the above studies have mostly analyzed cities in the US, Europe, and China, few have explored the use of trees as a nature-based solution for climate change mitigation in Neotropical cites and low-middle income countries. Assessments and studies have, however, looked at the use of innovative transportation strategies and policies to offset CO
2 emission in Latin American countries, such as in Colombia [
4,
5]. The need for other innovative measures (i.e., nature-based solutions) is necessary as a response to Colombia’s projected rates of urbanization and population vulnerable to adverse climate change effects [
5].
This study aims to develop an approach for assessing the potential use of public trees in Neotropical cities to offset CO2 emissions where there is often a lack of data and information. Specifically, the objectives of this study are to: (1) estimate carbon storage and sequestration by public trees in the urban area of the MAAV (Metropolitan Area of the Aburrá Valley); (2) estimate the relative CO2 emissions offset potential by the MAAV’s public trees; and (3) discuss the effect that proposed large scale tree planting can have on #1 and #2.
5. Conclusions
The use of a nature-based solutions approach for addressing climate change and other environmental and socio-economic problems has only recently been proposed in the scientific literature in the form of perspective pieces and in the European Union context. Noticeably missing from this discourse, are: (1) Evidence based examples of such strategies and solutions; (2) Data, information, tools, and models for documenting, monitoring, modeling, and assessing such solutions; and (3) examples from the developing world where such approaches can have a likely greater impact in providing more effective, sustainable, and equitable solutions. To these ends, although the urban ecosystem service and green infrastructure approach is used in Latin America, this study provides one of the first insights into the role of greening of neotropical cities, via tree planting programs, as a viable nature-based solution strategy.
Although urban forests, as a nature-based solution, provide many co-benefits; developing more accurate and region-specific methods and models for estimating carbon offsets by urban forests can help improve access to international funding for these types of nature based solution projects. Urban forests may also be incorporated into carbon markets on national or regional scales. The Climate Action Reserve, the carbon offset registry for the North American carbon market, has approved two protocols for urban forest projects: Urban Tree Plantings and Urban Tree Management. Both protocols only account for standing carbon; avoided emissions from cooling and maintenance emissions are not counted.
The consideration of nature-based air quality, socio-economics, biodiversity, and in particular CO2 benefits from public trees can help guide management of these natural resources and the use of public space. As a potential carbon offset strategy, public tree plantings were competitive but less effective than other strategies in the region. Accounting for additional ecosystem services and social equity benefits may increase the favorability of tree plantings as a nature-based solution to the challenges of urbanization. Future studies could take a more holistic approach, assessing the economic feasibility and social acceptability of public tree plantings for the MAAV. More regional data and biomass equations based on urban trees as opposed to forest trees can improve the measurement of costs and benefits. Additionally, revising methods of selecting reference cities to better reflect the closest match could help to minimize the error associated with regional differences. Further studies on urban forests in low-middle income countries, especially in regions with rapid growth rates, are needed for improved urban planning and potential international investment in nature-based solutions.