Is planting trees in cities useful against the climate crisis?

Since the consequences of the effects of global warming have passed from scientific news to news reports, a single solution seems to have conquered the scene: planting trees. As proof of the relevance of this “magic wand” for the climate crisis, on 21 September, on the occasion of National Tree Day, the Minister of the Environment and Energy Security Gilberto Pichetto Fratin said that “trees have a big and positive impact on the environment of our cities: they are a value to be defended and strengthened. The National Recovery and Resilience Plan provides that in the next three years are distributed 6.6 million plants in 14 metropolitan cities. The agreements with eleven administrations for the achievement of the annual objective of over 1.6 million trees planted in 2022”.

An important effort, five times greater than what was done recently given that always in line with the Pnrr, in the context of the so-called Climate Decree, the same ministry had launched in the two-year period 2020-2021 the planting of approx 300 thousand trees always in the areas of 14 metropolitan cities. Finally, a large public investment for environmental purposes that aims at a quantifiable result in hundreds of thousands of new plants that should mitigate the impact of global warming in our cities. And that will soon be reality. However, a doubt begins to emerge when these actions are announced:

The cause of global warming is in cities

More than half of the world’s population (about 55%) lives in cities, which, even if they cover only 3% of the global surface in size, are the centers responsible for 70% of global greenhouse gas emissions. The process of urbanization of humanity is unstoppable and continuous: it is estimated that in 2050, seven out of ten people will live in cities. As is now also evident in Italy, heat waves in the Mediterranean are increasingly frequent, especially in urban areas, where we speak of “heat islands”: a phenomenon, as he explains Greenpeaceaccording to which cities as a result of land consumption present temperatures air both day and night even 12°C higher than surrounding natural areas. This is why we start from interventions in urban agglomerations to deal with the effects of the climate crisis.

Trees are used to cool cities

According to the data reported by Lina Fusaroresearcher at the Bioeconomy Institute of the National Research Council (Cnr-Ibe), in a deepening dedicated to the theme by Greenpeace, planting trees in the city helps. First of all because the treetops guarantee thermal comfort that derives both from the direct shading of the artificial surfaces, both by the process of perspiration.

Through the evaporation of water from the foliage, the air temperature is lowered, thus partially replacing the use of air conditioners. Here because in the last 15 years the availability of urban green spaces in major cities around the world has increased by over 4% as well as accessibility to urban green spaces (+7% in the same period of time). Yet we are still far from minimum standard of presence of greenery in cities: according to the World Health Organization it should be 9 square meters of green spaces per inhabitant, with the ambition in the best of all possible worlds to be able to reach around 50 m² per capita. From this point of view, Rome boasts a ratio of 39m² per capita, Madrid stands at 21 m². But the big cities of the world have relationships that are far from even the slightestsuch as Mexico City (6 m² per capita) or Bogota (5 m² per capita). For this, the 50 m² WHO ideals are truly a mirage, or rather a distant prospect. This is why there is still a need to populate our cities with trees (and above all with greenery).

Planting trees is not enough

A tree is not a solution, but a living being. Planting trees matters little compared to taking care of their development and taking care of their location. Planting hundreds of thousands of trees could prove not only an ineffective but even harmful choice if this does not happen within a strategic planning of urban regeneration. This is because given the speed with which environmental changes occur especially within cities, the difficulty of species to adapt to the rapidly changing environment could decrease their mitigating functionality while requiring important maintenance. As Fusaro summarizes:

Several recent reports, also at the Italian level, have underlined the importance of integrating information on the state of conservation of the “Natural Capital” into the decision-making processes of urban development and of the country in general. With this expression we refer to an overview that frames trees within an overall strategy to make cities greener through different actionsas explained by a Wired Federica Ferrario, agriculture campaign manager and special projects of Greenpeace Italy: “On the one hand it is important to plant and increase the green surface in the city: we know very well what the difference is between a paved road and one shaded by trees. Having said that, what is generally missing is adequate upstream planning of green spaces, and not placing them after where there is leftover space”. The urban strategy also includes the space protection available, which must no longer be subtracted for purely speculative reasons. “The other issue to keep an eye on is overbuilding: before identifying spaces for planting new trees, let’s stop the consumption of soil, which is a significant problem”. And finally Ferrario puts the emphasis on added value of a forest compared to planting many trees: “These are two different worlds. Recreating forests has an added biodiversity value, for example, compared to planting a large number of trees” which are not a synergistic living system. And even to recreate a forest, a precise and generally shared strategy is required with respect to the needs of the territory.

It is more important to create ecological services

Be careful though: trees are not the only green solution that a city must find. In fact, urban forestation ensures various benefits, in addition to the removal of CO2, when it is planned in a synergistic way with the area in which it is intervened. But as Fusaro explains again, the numerous ecological services provided by the vegetation depend on the full functionality of the species that are planted. A tree in itself is not functional for the mitigation of the urban climate: this is instead the effect of urban planning where to create, or regenerate, an ecosystem of which the tree is only one of the protagonists. Furthermore, according to the researcher, it must be chosen the right tree in the right place:

Trees against global warming? An unsustainable solution

In the public sphere, announcing the planting of thousands or millions of trees follows the slippery slope of the private compensation projects. The most polluting companies in the world continue to plant trees. In 2020 Chevron said it planted 30,000 trees in a brownfield site in British Columbia (Canada) and Gazprom more than 60,000 trees in Russia.

In 2021 Total in collaboration with Forêt Resources Management announced its intention to plant acacias in a 40,000-hectare forest on the Bateké plateaus in Congo. And the world’s largest issuer of all oil companies, Saudi Aramco, has filed 5.3 million mangroves along the Persian Gulf coast as reforestation. These are often choices dictated by brand reputation reasons or even by a greenwashing intention that have little to do with climate mitigation intervention. Among others, Oxfam explains it in a report: to offset your own emissions the four sisters of oil and gas (BP, Eni, Shell and Total Energies) alone should consume an area twice the size of the UK to plant trees and achieve zero emissions by 2050. And if the whole energy sector – whose emissions continue to grow – were to set targets close to zero, a region the size of the entire Amazon forest would be needed, the equivalent of a third of all arable land on the planet. At Cop26, one of the most significant agreements was the halt to deforestation by 2030. For this reason, before replanting lost trees, we need to curb the causes for which they were destroyed.

Reforest? Let’s stop deforestation first

If planting is considered as a suitable measure to offset one’s emissions, this effort bears fruit poor results. Given the amount of CO₂ emitted by major polluters and given the fact that reforestation involves an average of one thousand plants per hectare, it will take decades to offset just a fraction of the global emissions emitted. Also according to the Oxfam study it would be necessary reforest 1.6 billion hectares, equivalent to 5 times the size of India to absorb all the carbon that the big polluters continue to emit. And in any case, the planting in place so far does not in the least compensate for what has been lost.

Forests on the earth’s surface exceed 4 billion hectares: in the last twenty years they have been 386 million hectares of forest lost in the world, while in the same period they were recovered through reforestation and spontaneous regeneration only 59 million. And what breaks down cannot be easily replaced because quantity is not quality: just think that in the last 250 years the surface of woods and forests has increased by 10% in Europe, yet absorbs 3 billion tons of CO₂ in less. Because the coniferous forests that have replaced the deciduous are better for economic purposes, but absorb less carbon dioxide.

A national plan, not individual local initiatives

When it comes to climatethere are no borders: local actions may be ineffective, expensive or even counterproductive if the context is not considered. The same goes for urban tree planting. It is necessary instead “redesign urban and peri-urban areas in order to achieve widespread naturalness in which biodiversity at all scales (local and landscape) is encouraged and protected – remember Fusaro – A reforestation plan should start from these networks and strengthen them by investing in planning that involves the whole national territory, acting more incisively in critical areas such as metropolitan cities, but aiming to restore widespread environmental quality, the only one that can guarantee long-term resilience. This condition must permeate the whole city fabric from the center to the suburbs“.