Systemic 1: Understand current ecological issues and planetary limits

The object of study in ecology is the ecosystem. This term designates a whole formed by a community of living beings (biocenosis) within their environment (biotope). The richness of an ecosystem will therefore depend on the quality of the environment and the populations there. When an animal or plant population (denoted N) develops within an environment with favorable conditions (physicochemical properties favorable to its development, no predation ...) this population will follow an exponential growth until reaching a certain threshold called the carrying capacity of the environment (denoted by K). This carrying capacity K is the maximum size of the population of an organism that a given environment can support (in view of available resources, space, etc.).

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When we exceed this carrying capacity, if the population grows too quickly for example, we observe an overshooting phenomenon leading to the degradation of the environment and therefore the decline of the population.

In the case of the human species, the environment considered is planet Earth. Our species is seeing its population evolve exponentially with a population explosion in the 1950s, going beyond the 5 billion people mark and nearly 8 soldiers today.

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This population increase is accompanied by an increase in a large number of parameters such as the average income per capita or the consumption of energy and resources. This phenomenon is thus called the great acceleration.

Steffen et al. (2015) The trajectory of the anthropocene: the great acceleration

This exponential curve of human activities translated by the explosion of our economic activities and human demography exerts a massive effect on the Earth system, thus modifying the geological era of the planet. We thus switch from the Holocene, a geological period lasting 11,700 years, to the Anthropocene, a geological era which would have started the last 50 years. The Holocene separates us from the end of the previous Ice Age is characterized by a relative stability of conditions on Earth and creates conditions conducive to the flourishing of agriculture and great civilizations. The change of geological era therefore irreversibly leads to a modification of the terrestrial state that is most favorable to our activities and to the survival of our species.

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Since infinite growth in an environment with limited resources is not possible, it is therefore necessary to ask what is the real capacity that the Earth can support and what are the planetary limits not to be exceeded. These limits have been defined thanks to the work of Johan Rockström et al., 2009 and represent the thresholds that humanity must not exceed in order not to compromise the favorable conditions in which it has been able to develop and to be able to continue to live. sustainably. To date, of the 9 limits that have been defined, 4 of them have already been crossed.

The first two limits of climate change and the erosion of biodiversity are enough on their own to change the geological era.

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The sun is the greatest source of energy on Earth. Part of solar radiation is absorbed by the earth's surface (50%). The other part of the radiation is absorbed by the atmosphere (20%) or reflected in space (30%) thanks to the clouds and the clear surfaces of the Earth (glaciers, snow, deserts ...). The absorbed rays heat the earth's surface.

The more the surface heats up, the more it emits significant infrared thermal radiation. This radiation carries heat, part of which is returned out of the atmosphere.

The other part of the radiation is trapped by clouds and certain gases and gets stuck on the surface, this is called the greenhouse effect.

The temperature of the Earth adjusts to find a balance between the energy of the sun absorbed and that re-emitted in the form of infrared radiation, thus allowing an average temperature of 15 ° C. Without greenhouse gases, the global average temperature would be -18 ° C.

The greenhouse effect allowed the appearance of life on Earth. However, the increase in greenhouse gases as a result of human activities has greatly increased the proportion of infra red radiation retained, resulting in an increase in the global temperature of more than one degree.

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Over the past 2 billion years, Earth's climate has alternated between “hot” periods and ice ages during which ice could cover the entire planet.

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The concentration of CO2 in the atmosphere is a good indicator of climate change. It is evaluated in ppm CO2 (parts per million) which is the fraction equal to 10–6, ie one millionth. 1ppm of CO2 is equivalent to saying that there is 1 molecule of CO2 for 1 million molecules of other gases (= 1mg of CO2 / 1 kg of gas).

On the first graph, we see the evolution of the CO2 concentration over the last 800,000 years. From the beginning of human civilization (-3500) until 200 years, our atmosphere oscillated between 180 and 280ppm of CO2. This level allowed the maintenance of living conditions conducive to the development of our species. In order to contain the rise in temperatures to less than + 2 ° C, it is estimated that the safety level would be 350ppm of CO2 in the atmosphere.

The second graph shows the evolution of the CO2 concentration between 1960 and 2020. We then notice a very clear increase in the concentration in recent years with a rate of 320 ppm in 1960 which exceeds the threshold of 405 ppm in 2017.

Global CO2 emissions per year now exceed 40 GtCO2 per year (41.5 GtCO2 in 2018).

The IPCC also assesses how climate change will translate in the medium and long term. He plans :

Aggravated climatic phenomena: changes in the climate are modifying the frequency, intensity, geographical distribution and duration of extreme weather events (storms, floods, droughts).

A disruption of many ecosystems: with the extinction of 20 to 30% of animal and plant species and significant consequences for human settlements.

Crises linked to food resources: in many parts of the world (Asia, Africa, tropical and subtropical zones), agricultural production could fall, causing serious food crises, sources of conflict and migration.

Health dangers: climate change will likely have direct impacts on the functioning of ecosystems and on the transmission of animal diseases, which may present pathogenic elements that are potentially dangerous for humans.

Population displacements: the increase in sea level (26 to 98 cm by 2100, depending on the scenarios) should cause the flooding of certain coastal areas (in particular the deltas in Africa and Asia), or even disappearance entire island countries (Maldives, Tuvalu), causing significant migrations.

Source: https://www.ecologie.gouv.fr/comprendre-giec

Since the 19th century, humans have dramatically increased the amount of greenhouse gases in the atmosphere. As a result, the natural climate balance is altered and the climate is readjusted by a warming of the earth's surface. We can already see the effects of climate change. This is why we must mobilize and act. Everyone is concerned: elected officials, economic players, citizens, to reduce our greenhouse gas emissions, but also to adapt to the changes already underway.

Biodiversity plays a fundamental role in regulating the climate. Indeed, forests, wetlands and oceans have the ability to store atmospheric carbon, and thus help to mitigate global warming. On the other hand, it also helps to mitigate the effects of global changes, for example by protecting the coastline from erosion or by reducing the intensity of floods and floods.

Conversely, current climate change is modifying the interactions between species and their living environments in ecosystems. For a global warming of 2 to 3 ° C, experts predict an increase in the risk of disappearance for 20 to 30% of animal and plant species.

• Protection of species thanks to terrestrial and marine protected areas (represent 13% of emerged lands). However, the creation of these spaces sometimes generates conflicts with the development of certain human activities.

• Regulation of the exploitation of certain species thanks to international conventions such as: The Washington CITES convention which regulates the trade of a large number of animal and plant species. The International Whaling Commission. The Bern Convention which defines the list of protected species. These laws also clash with cultural, social and economic interests (poaching, illegal trade, etc.).

• “Ex situ” conservation of the most endangered species by placing them outside their natural habitat in zoological parks or botanical gardens. However, this approach includes several constraints such as the loss of genetic diversity linked to the small numbers or the habituation of species to captivity posing a problem for the long-term maintenance of these populations.

• Restoration of the population thanks to the recesses of existing populations or the reintroduction of extinct species into an environment. The main constraint of these actions is the length of time required for them to be carried out.

Nitrogen is an essential nutrient for plant growth. Reactive nitrogen, emitted in abundance in the environment, can however constitute a surplus compared to the needs of the plants. It then contributes to the pollution of water by nitrates. Associated with other nutrients such as phosphate, and depending on particular physicochemical conditions, it is responsible for the phenomenon of eutrophication. The main sources of nitrogen emissions into the environment are nitrogen fertilizers and the combustion of fossil resources and industrial processes. Nitrogen from polluting emissions of nitrogen oxides (NOx) to the atmosphere from transport and industry is not included in the global limit.

The challenge is to prevent an excessive release of reactive nitrogen into water and natural aquatic environments in order to prevent their eutrophication. The threshold not to be exceeded has been set between 62 and 82 million tonnes (Mt) per year, i.e. 41 to 55 kg of excess nitrogen (surplus) per hectare per year (kg / ha / year) on average at the global scale. In 2015, nitrogen losses to the environment are estimated at 150 Mt.

Like nitrogen, phosphorus is also an essential nutrient for plant growth. The modification of its biogeochemical cycle, caused by agriculture (fertilizers, livestock manure) and urban wastewater (excrement and detergents), affects the ability of the biosphere to sequester it and leads to eutrophication of fresh water.

The challenge initially envisaged was to prevent a major ocean anoxic event from occurring (an episode of strong reduction of oxygen in the oceans) with impacts on marine ecosystems. During the revision of the conceptual model in 2015, a two-level geographic approach is proposed.

Globally (asphyxiation of the oceans), the threshold is estimated at 11 Mt per year of phosphorus released into water (agricultural surplus and insufficiently purified wastewater). In 2015, it was exceeded with 22 Mt of phosphorus actually released into the water.

At a more localized level, an additional threshold is defined to address eutrophication of inland freshwater. This threshold relates to the phosphorus surplus resulting from excessive inputs during the fertilization of agricultural soils. These phosphorus surpluses must not exceed, each year, a range of between 6.2 and 11.2 Mt (i.e. 4.1 to 7.5 kg / ha / year) to avoid Eutrophication of water systems. pure water. In 2015, the limit was crossed with around 14 Mt.

In France, the excessive supply of nitrogen and phosphorus, mainly from agricultural activity for the first, and from urban wastewater for the second, corresponds to the surplus in the environment, to discharges at sea and in fresh water, which can lead locally to eutrophication phenomena and the proliferation of green algae. Surpluses are calculated on a regional scale without taking into account treatments and exports to other regions, if any, particularly in the breeding regions. The nitrogen and phosphorus surplus tends to decrease in metropolitan France between 2000 and 2015

A quarter of CO2 is absorbed by the ocean through dissolution or photosynthesis. The dissolved CO2 is transformed into carbonic acid and releases H + ions responsible for the acidification of the oceans. The drop in pH affects the ability of phytoplankton to grow and renew itself. Phytoplankton is the primary producer of oxygen on Earth. It is at the base of the food chain for all living things in the oceans. Its disappearance therefore leads to a great loss of marine biodiversity.

Like plankton, coral has difficulty developing in an acidic environment. Their degradation also leads to the decline of a very rich biodiversity.

The reefs also guarantee coastal human communities many services (protection of the coast against storms, underwater tourism and recreation, food supply, water purification, etc.) and contribute to local development. Conversely, other environmental impacts resulting from human activities on the coast (overfishing, land pollution, urban development along the coasts, etc.) combine with the acidification of the oceans and the rise in sea level. sea ​​to weaken coral reefs.

The disappearance of coral reefs in favor of more banal ecosystems dominated by algae is a marker of the vitality of the oceans and their ability to continue to function as carbon sinks.

https://ree.developpement-durable.gouv.fr/

The "land use change" limit is understood in terms of the percentage of the total land area converted to agricultural land. The threshold not to be exceeded is set at 15% of agricultural land. In 2009, around 12% of the world's land surface was cultivated.

Two indicators are defined within the framework of this planetary limit:

The first concerns, at the global level, the area forested in relation to the area covered by forest before human intervention, ensuring that at least 75% of the land once forest remains forested. In 2015, only 62% of previously forested land was forested, so the limit was exceeded. This reduces the Earth's ability to serve as a carbon sink.

The second concerns the area of ​​the three main forest biomes (tropical, temperate and boreal forests) with regard to the potential forest cover. Among forest biomes, tropical forests converted to non-forest systems, have significant effects on climate (evapotranspiration), while boreal forests affect soil albedo (reflective power of a surface) and therefore the exchange of regional energy. The limit, at the biome level for these two forest types, was set at 85% of the potential forest cover. It was set at 50% for temperate forests, as changes would have a lower impact.

Contrary to the global situation, agricultural land is declining there, in particular under the effect of the artificialization of soils. However, given its high level of consumption, associated with population growth, and the insufficiency of raw materials on the national territory, France imports significant quantities of agricultural and forestry raw materials from the deforestation of tropical forests. It thus indirectly uses land located in other regions of the world, and contributes to exerting strong pressure on foreign land resources: consumption of resources, disappearance of natural habitats.

According to WWF, France imports and consumes large quantities of agricultural and forestry raw materials from tropical forest deforestation: soybeans (4.8 million tonnes), palm oil (970 kilotons), cocoa (460 kilotons) . France's ecological footprint linked to these imports represents 14.8 million hectares, or more than a quarter of the metropolitan area and half of the French agricultural area. About 5.1 Mha are in countries with a high risk of deforestation (Argentina, Brazil, China, Ivory Coast, Indonesia, etc.).

Forests are home to a great diversity of species and help mitigate climate change by storing large amounts of carbon. Their role is central in adapting to climate change, protecting soil and providing fresh water. Thus, deforestation has a strong influence on the environment: disappearance of natural habitats, loss of biodiversity and ecosystem services, increase in the intensity and frequency of weather extremes amplifying climate disasters, decrease in water resources, increase in greenhouse gas emissions responsible for global warming.

Human activities disrupt the hydrological cycle and alter the available freshwater resource. For example, withdrawing more water than its natural renewal allows risks drying up or salinizing groundwater tables (Dalin et al., 2017), rivers or the disappearance of lakes and wetlands. Human activities are the source of polluting emissions, which are even more concentrated during the event of water resources reduction. Finally, climate change should lead to a decrease in the volumes of fresh water renewed annually in certain regions of the world, in particular the Mediterranean rim, southern Africa, part of North America and Central America (Milly, 2005 ).

The challenge is therefore to have enough good quality water for all uses, leaving some for the proper functioning of natural ecosystems, on which human activities also depend. Limit values ​​for the use of fresh water not to be exceeded have been defined on a planetary scale and on the scale of the watersheds. They represent the part of the renewable water resource that human activities can use without compromising ecosystems in the long term. Globally, this share is estimated at 4000 km3 of fresh water consumed per year (Rockström et al., 2009).

The water resource exploitation index (WEI +) is defined by the AEE as the share of water consumed in relation to the renewable water resource, over a period and a given geographical area (ETC / ICM, 2016). It varies according to the year, the season and the level of water demand linked to human activities. In watersheds modified by humans, the renewable water resource is renaturalized by adding the volume of water consumed to the volume flowing into the rivers. The flows considered in this document take into account the inflow of water reservoirs into the rivers. The volumes transferred artificially from another sub-basin are not identified.

The water footprint is a measure of humanity’s appropriation of fresh water in volumes of water consumed and/or polluted. There are 3 different kind of water footprint:

• Green water footprint is water from precipitation that is stored in the root zone of the soil and evaporated, transpired or incorporated by plants. It is particularly relevant for agricultural, horticultural and forestry products.

• Blue water footprint is water that has been sourced from surface or groundwater resources and is either evaporated, incorporated into a product or taken from one body of water and returned to another, or returned at a different time. Irrigated agriculture, industry and domestic water use can each have a blue water footprint.

• Grey water footprint is the amount of fresh water required to assimilate pollutants to meet specific water quality standards. The grey water footprint considers point-source pollution discharged to a freshwater resource directly through a pipe or indirectly through runoff or leaching from the soil, impervious surfaces, or other diffuse sources.

Since the 1980s, observations have shown significant seasonal decreases in the stratospheric ozone layer over the Arctic and the Antarctic continent. These declines can reach 50% in late winter and early spring, and are also reflected, to a lesser extent, in mid-latitudes. The thinning of the ozone layer affects the polar regions during the spring.

The depletion of the ozone layer, one of the nine planetary limits (Rockström et al., 2009), is understood by measuring the concentration of stratospheric ozone evaluated in Dobson units (DU). Knowing that the average value of the ozone column is 300 DU, the limit is set at 275 DU, i.e. 95% of its pre-industrial level (290 DU). While this limit was broken in the 1980s, the trends have since reversed, and the average thickness of the ozone layer is gradually increasing. In 2009, the concentration amounted to DU 283. In the 1980s, during the end of the southern winter (September-October), when the sun appeared, the thickness of the ozone layer could approach 100 DU.

Research on the subject has shown that the depletion of the ozone layer follows complex chemical reactions occurring in the stratosphere and mobilizing brominated or chlorinated compounds requiring very low temperatures, reached during polar winters, then significant solar radiation, from the following spring. These compounds are emitted by human activities. The most famous are :

• chlorofluorocarbons (CFCs) used in refrigeration systems, air conditioning, aerosol cans, solvents, etc. ;

• hydrochlorofluorocarbons (HCFCs) developed to replace CFCs due to their shorter lifetime in the atmosphere;

• halons used for extinguishers and fire protection systems;

• carbon tetrachloride used in particular as an industrial cleaning solvent;

• methyl bromide used for the treatment of plants, premises and agricultural soils by fumigation.

To preserve the ozone layer and in particular to help restore it, the international community has been committed since 1987 to the Montreal Protocol on Substances that Deplete the Ozone Layer. This international agreement aims to gradually stop the production and consumption of the substances in question. 197 countries have signed this protocol. Since its entry into force, global emissions of these products have fallen by more than 80% and almost all chemicals controlled by the Protocol have been phased out.

Since the pre-industrial era, human activities have doubled the global atmospheric concentration of most aerosols (Rockström et al., 2009). Moreover, as the IPCC shows in its successive reports, aerosols have a strong influence on the climate system by disturbing the Earth's radiation balance. Indeed, the increased concentration of aerosols in the atmosphere leads to an increase in the level of opacity of the atmosphere and can cause a decrease of 10% to 15% of solar radiation on the surface of the Earth. The impact of aerosols on cloud formation and life is one example of this.

Because of their potentially harmful effects on climate and health, the increase in aerosols in the atmosphere is one of the nine global critical processes. The planetary limit is understood in terms of the overall concentration of particles in the atmosphere, on a regional basis. However, the complexity of aerosols and the spatio-temporal variability of particles, sources and impacts, did not make it possible to define an overall threshold.

In France, four main sectors of activity share PM10 particle emissions (fine particles with a diameter of less than 10 µm): the residential and tertiary sector (mainly due to the combustion of wood), industry, agricultural activities (spreading, storage of effluents, resuspension during plowing, burning) and transport. Their emissions have decreased, in total, by 41% over the period 2000-2017 following the progress made in all sectors of activity (improvement of dust removal techniques in industry, improvement of the performance of wood heating installations...)

In Europe, outdoor air pollution due to fine particles is the cause of more than 400,000 premature deaths each year, including nearly 40,000 in France (European Environment Agency, 2018). Since October 2013, ambient air particles have been classified as carcinogenic to humans by the International Agency for Research on Cancer on the basis of a sufficient level of evidence of an association between exposure and increased risk of lung cancer.

France is thus regularly confronted with episodes of national pollution. Over the 2013-2016 period, these episodes are mainly due to particles with a diameter of less than 10 µm (PM10). At the start of winter, episodes are marked by a large amount of organic matter linked to combustion phenomena such as wood heating or the burning of green waste. In spring, the episodes observed are distinguished by the influence of emissions linked to agricultural activities (fertilizer spreading) which are added to and interact with the pollutants emitted by industry and transport.

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Two main factors have led to consider chemical pollution as a planetary limit: on the one hand, because of its harmful effects on human physiological development and on the functioning of ecosystems; on the other hand, because it acts as a slow variable that affects other planetary boundaries. Indeed, chemical pollution can have an impact on the limit "erosion of biodiversity" by reducing the abundance of species and potentially increasing the vulnerability of organisms to other threats (climate change). It also interacts with the "climate change" limit by the releases of mercury into the environment (via the combustion of coal) and by the emissions of CO2 due to industrial chemicals.

Three types of pollutants are treated here: plastic waste at sea, nuclear waste, varieties tolerant to herbicides.