Forests are one of the richest forms of expression of the earth’s three million years of evolution. They contain about 90 percent of all living animal and plant species on the planet and cover an area of 3.9 billion hectares, equal to 30 percent of the Earth's surface. Tropical and subtropical forests account for 56 percent of the world's forests, while temperate and boreal forests account for 44 percent. Forests are therefore essential for protecting the planet's biodiversity. Altogether, tropical, temperate and boreal forests offer a multitude of habitats for plants, animals and microorganisms, hosting the vast majority of the earth's species. They provide a wide range of goods and services, from wood products to non-wood products, and they also provide livelihoods and jobs for hundreds of millions of people around the world. The biological diversity of forests has an important economic, social and cultural role to play in the lives of many indigenous communities, and they also fill a fundamental role in global climate dynamics, playing a significant role in climate mitigation as carbon sinks. When forests are destroyed, especially as a result of tropical deforestation, they release large amounts of carbon, which reaches the atmosphere and contributes massively to the greenhouse effect.
As human society has evolved, the perception of the relationship between man and nature has changed enormously. In the Middle Ages, and even before, man was afraid of the forest. In the collective imagination, the forest represented the fear of the unconscious, of the unknown, and was represented in many paintings and stories as a place of mysterious presences (fauns, elves, witches and orcs) or dangerous animals, dreamlike and legendary creatures (dragons, griffins and centaurs). Many well-known fairy tales and legends still evoke that representation, witness the tale of Snow White. Over the last 50 years, however, men have altered ecosystems more quickly and more intensely than at any other time in human history, so much so that we can say we are no longer afraid of forests, indeed we have learned to destroy them even in the most remote corners of the planet. What artists and writers like Chretien de Troyes, Ariosto and Collodi saw, always paying great respect and attention to forests and nature, no longer exists today. The speed with which man has appropriated nature has led to a substantial and irreversible loss of many of his functions. More land has been converted to agriculture since the 1950s than in the eighteenth and nineteenth centuries, at the expense of the planet's natural capital, and we have gone from around 15 billion hectares of forests in the 1950s to four billion today. Population growth from 2.5 to 7.5 billion inhabitants in just 60 years and the consequent food requirement have resulted in our using 73 percent of dry land (with the exception of that covered by ice), putting a heavy burden on future generations, who will have only the remaining 27 percent of land available, an area insufficient to meet expected further population growth of around 2 billion by 2050 (IPCC-SRCCL, 2019).
Deforestation and CO2
The pace of tropical deforestation is currently running at around 13 million hectares a year, a huge amount if we consider that in Italy forests cover 10 million hectares of land. Tropical deforestation contributes 5.3 billion tons of CO2 emissions, an amount equivalent to 13 percent of total greenhouse gas emissions. This contribution increases greatly if we consider that deforestation is linked to the expansion of agriculture, which contributes 6.4 billion tons of CO2, equivalent to the global greenhouse gas balance with 11 percent of global emissions. Furthermore, if one considers the entire food chain (deforestation, agricultural production and food consumption), the contribution of the sector rises to 37 percent of global emissions (IPCC - SRCCL, 2019).
However, forests also balance the planet’s climate system. As shown in figure 2, while fossil fuel emissions amount to 34.4 billion tons (1Gt = 1 billion tons) of CO2 per year and the emissions from tropical deforestation amount to 5.3 Gt of CO2 per year, only 44 percent of these emissions remain in the atmosphere thanks to the role played by forests and oceans, which capture 11.6 Gt and 8.9 Gt of CO2 per year (29 and 22 percent of total emissions) respectively. If there were no forests and oceans, the quantity of atmospheric carbon dioxide would have almost doubled, resulting in very dramatic conditions for the global climate today.
The need to activate effective atmospheric carbon capture systems as quickly as possible is well illustrated by the emission scenarios contained in the fifth report and in the subsequent IPCC special report on global warming of 1.5 °C by the end of the century (IPCC SR1.5 , 2018, IPCC SRCCL 2019). In both cases, if we want to limit global warming by the end of the century to 2 °C or 1.5 °C, we must achieve zero emissions and then negative emissions by 2060 in the first case and by 2050 in the second. In any case, both scenarios envisage the achievement of negative emissions and the maintenance of atmospheric carbon sequestration far beyond the zero emission point until the end of the century. The term negative emissions is fairly unique from a scientific point of view but is used to indicate the absorption of atmospheric carbon, to make the complementarity of the two processes more understandable to policy makers. The achievement of negative emissions or carbon absorption can be implemented through atmospheric carbon storage technologies. The latter are receiving considerable attention and numerous studies and pilot projects that explore them are beginning to appear. For example, the pumping of emissions from large energy production plants and cement plants into geological cavities or the capture of CO2 from the air by chemical-physical processes are technologies currently being studied quite intensively, and pilot projects already exist. However, the scalability of these methods, costs and the permanence of stored-carbon are still critical elements that prevent their spreading more quickly.
Recourse to natural systems, a real option
For this reason, mitigation policies are increasingly looking at the possibility of increasing the capacity of natural systems to absorb excess atmospheric carbon due to human activities. Oceans, for example, have a fairly constant capacity in the medium term to capture carbon dioxide, but it is very difficult to increase their carbon absorption rate. Interesting suggestions about climate engineering (e.g., ocean fertilization) have been made, but, due to their size, volume and costs they are considered impractical. However, reducing tropical deforestation and increasing the forested area through reforestation is certainly more practicable for the sequestration of atmospheric carbon and is now a real mitigation option. Through the process of photosynthesis, net of the oxidative processes of decomposition of the organic substance, under normal conditions a forest can store 12 to 24 tons of CO2 per hectare per year. In general, tropical forests have a greater capacity to absorb atmospheric carbon, but decomposition processes and anthropic disturbances (deforestation) can reduce their contribution to atmospheric absorption to zero. Tropical forests can contribute about 3.7 Gt of CO2 per year in carbon sequestration, but unfortunately tropical deforestation, including regrowth after disturbance, produces emissions of around 5.3 Gt of CO2, negating the absorption role. For this reason, reducing deforestation could make a very significant contribution to the global carbon balance and does not require any particular investment. In boreal areas, the growth of forest biomass is limited by climatic conditions, although decomposition rates and the consequent release of carbon are slowed down. Their net balance is however positive in terms of carbon sequestration, particularly in view of their considerable size, contributing with about 1.8 Gt of CO2 seized per year. Temperate forests like Italy’s have good CO2 absorption capacity and contribute globally by absorbing about 2.8 Gt of CO2 per year. Overall the role of forests is significant in reducing the absorption of atmospheric carbon and protecting them is fundamental for the future of humanity.
In summary, what could be the most effective forestry-related measures to counter global warming? There are certainly several ways to increase the carbon absorption of forests. The most effective and inexpensive action, with significant environmental benefits, is to reduce tropical deforestation. The recent IPCC report on Land and Climate tells us that the mitigation potential of forest deforestation reduction is between 0.4 and 5.8 Gt of CO2 per year. By comparison, the energy sector is worth approximately 33 Gt of CO2 and that of coal alone contributes 10 GtCO2 per year. Secondly, much can still be done to improve the management of existing forests and protecting them from adversities and the risk of fires. This is the case in Italy, where, despite having a significant forested area (about 10 million hectares), our forests are abandoned and subject to various forms of degradation. Another very direct solution is to plant trees on degraded land that is not used for agriculture. A recent study (Bastin et al 2019) shows that it would be possible to globally reforest about 900 million hectares of degraded land and thus contribute to absorbing about 758 Gt of CO2 at maturity, or 25 percent of the current carbon content in the earth's atmosphere. Finally, significant contributions can be made by the use of forest plantations for bioenergy production, and therefore the replacement of fossil fuels, as well as the replacement of material produced with fossil energy, with renewable ones (for example the replacement of cement or steel in buildings with wood). In any case, attention must be paid to promoting large-scale reforestation work, given the potential conflict over land use for the necessary production of food. Food security is a topical issue due to global warming and the land available for agriculture today being very limited. Moreover, large-scale reforestation might affect the regional climate which, at high latitudes, might lead to localized heating of the climate due to changes in surface energy exchanges. In essence, the challenge is still that of making the energy transition to non-fossil sources, and it would be dangerous to consider forests as the only alternative to combating climate change. However, the mitigating role of forests is absolutely essential to limit global warming by the end of the century to 2 °C and even more if the target is 1.5 °C.
Riccardo Valentini is full professor at Tuscia University since 2000 and a scientist at the Euro-Mediterranean Center on Climate Change. Valentini's research activity is mainly concerned with ecology, forests and the problems connected with the implementation of international conventions for the protection of the global environment. Italian author of the IPCC report that won the Nobel Prize.