Planetary boundaries and the impact of human activities

Planetary boundaries refer to the concept involving Earth system processes that contain environmental limitations. Understanding the planet's ecological limits to sustain life and the impact of agricultural activities on ecosystems is essential to design and implement strategies to build resilient and sustainable farming systems, including homegardens.

2.1.1 The global state of the unprecedented environmental crisis

There is an urgent need for a paradigm that integrates the continued development of human societies and the maintenance of the Earth System (ES) in a resilient and accommodating state. Long-term global sustainability and human well-being depend primarily on the capacity of the ES to sustain ecological functions and provide ecosystem services. For instance, healthy ecosystems provide clean air, water, food, and other services or benefits to humankind, fundamental for its survival and well-being (Millennium Ecosystem Assessment [MEA], 2005).

11 But, since the new epoch of the Anthropocene, humans have become dominant drivers of planetary change (Crutzen, 2002 & 2006). Primarily, since the past three centuries, thanks to the advances made in science and technology, enormous economic progress has been achieved worldwide. However, much of humanity's growth has come at a considerable cost to the environment (FAO, 2017a). This is mainly due to the interactions between the coupled human and environmental systems: complex, non-linear, and reciprocal. The adverse impact among these systems influences each other, significantly contributing to the collapse of the ecosystems' vital ecological functions and poses a severe socio-economic challenge to humanity (Turner et al., 1990; Liu et al., 2007; Rockström et al., 2009a; Rockström, 2015; Steffen et al., 2015).

(Source: Steffen et al., 2015).

To be specific, human activities not only alter ecosystems (MEA, 2005) but cross certain biophysical thresholds of planetary boundaries. And scientists argue that humanity has already transgressed four (climate change, biosphere integrity, i.e., genetic and functional diversity, land-system change, nitrogen, and phosphorus

Figure 1.Planetary boundaries of the Earth's systems.

12 levels) out of nine planetary boundaries (Figure 1). Trespassing these boundaries could disrupt the known stability of the Holocene state of the ES, where living conditions to human development were favorable (Rockström, 2015; Steffen et al., 2015).

Due to the industrial revolution (that significantly contributed to the exponential growth of the human population), the added demand for natural resources to satisfy the needs of the growing population increased human activities that altered the natural environment where humanity thrives. Besides, the expected population growth of ten billion by 2050 will positively accelerate anthropogenic pressures and the competition for natural resources (Sachs, 2015).

Thus, in summary, the growing human impact on the environment is triggering abrupt and irreversible changes in the ES. These unacceptable changes have an immediate effect and could disrupt the social-ecological resilience of the ecosystems as well as the agroecosystems (human-modified ecosystems for agricultural purposes). Moreover, they pose a higher risk to humanity in the planet's transition from the Holocene to the Anthropocene (Rockström et al., 2009b; Steffen et al., 2015).

2.1.2 The impact of agriculture on ecosystems

The advent of agriculture, about 10,000 – 12,000 years ago (Trabanino, 2018), has not only facilitated human beings to produce their food and satisfy their basic needs but has given way to the rise of complex societies and human civilization.

However, since the industrial revolution, agriculture is one of the economic sectors with an equal or more significant adverse impact on natural resources than other industries. Therefore, advances in agriculture are inevitably associated with alterations of natural ecosystems (FAO, 2017a).

As the elements of agriculture and ecosystems are necessarily linked to each other by using the same natural resources (such as soil, water) based on the same biological processes (such as photosynthesis, biomass production), the

13 progressive control and intensification of natural resources used to increase food production have triggered negative feedback on the whole environment. For instance, Ramankutty et al. (2018) state that the current global environmental crisis such as deforestation, degradation of land, soil erosion, biodiversity loss, climate change, water scarcity, eutrophication, etcetera has resulted primarily from agricultural activities.

Ironically, recent technological progress aimed to increase global outputs of the food production systems and efforts to produce resource-intensive bioenergy (instead of using more sustainable energy sources) have led to further intensification of the competition for natural resources and the degradation of ecosystems (FAO, 2017a).

According to FAO (2020), about 37.6% (4,889 out of 13,003 million hectares) of the total land area is used for agriculture. In other words, agriculture dominates nearly half of the habitable land, i.e., around 51 out of 104 million km² (Ritchie, 2019). Remarkably, the intensification and expansion of agriculture in the last few decades, especially from the green revolution, triggering significant detrimental impacts on the Earth's physical systems. Specifically, the irrational land conversion for agriculture is one of the significant impacts on ecosystems that transform habitats and triggers immense biodiversity loss.

Nevertheless, by 2050, a growing population will need to produce twice as much food as 2000 but will have to use the same natural resources and other inputs.

Notably, an additional one billion tons of cereals will be needed annually by 2050.

Furthermore, the demand for grain to feed livestock and produce bioenergy also contributes to the above global scenario (FAO, 2019). Therefore, there is an urgent need to intensify food production levels in the near future.

Alexandratos and Bruinsma (2012) and FAO (2017a) conclude that the intensified competition for these resources for agriculture degrades the environment due to overexploitation and creates a negative feedback loop. This, in turn, increases further competition for the remaining available resources and triggers further

14 degradation of both resources and the environment. Thus, the persistence of current trends in the immediate future will lead to natural resource scarcity for agriculture in 2050 and undermine the overall sustainability of the environment.

Hence, the above-mentioned vicious cycle of the adverse impacts between the coupled human and environmental systems is inevitable unless strategies are employed or enforced to promote traditional agroecosystems and sustainable multifunctional agriculture. However, it is essential to remember that even though traditional practices could serve as one of the alternative strategies to face the current crisis, the satisfaction of human necessities also requires enhancing the agricultural frontier. Still, the crucial question is how to do it within the limits of the ES?