The biosphere consists of the living components (e.g., plants, animals, microbes, etc.) of Earth. In other words, biosphere is the part of Earth that supports life and is at the intersection of the abiotic components. The abiotic components of our Earth consist of the lithosphere, hydrosphere and atmosphere. The interaction of the biotic and abiotic components takes place in the form of matter and energy. An example is the carbon dioxide concentration in the atmosphere (abiotic component), which affects the growth rate of plants (biotic component), who in turn consume a certain amount of CO2. Similarly, climate and soil conditions may also affect living components, who in turn affect these components. The Gaia hypothesis is generally used to explain the interactions between the biosphere and abiotic components. It assumes the entire Earth to be a living organism, with the abiotic components serving as cooperative systems.
Possibilism is the geographical theory that even though the environment may set certain conditions, the culture is determined by social conditions. This theory is in direct contrast to determinism (which provides more significance to environmental conditions). For example, environmental conditions such as extreme weather may limit our ability to work in certain places. However, using our intelligence and skills, we can make machines and tools to help us overcome this limitation.
Sustainability in simplest terms means living within our resources. The environment or abiotic components have certain limits within which we can survive on this planet. If the biotic components exceed that capacity, there may be irreversible damage to the environment. For example, given the quantity of non-renewable resources, we can only generate so much energy before exploring renewable options.
An unsustainable ecosystem is the one in which biotic components exceed the carrying capacity of the abiotic components. An example is the amount of carbon dioxide we are generating today. It is more than what plants and trees can absorb and hence this extra amount is leading to climate change.