The natural environment, commonly referred to simply as the environment, is a term that comprises all living and non-living things that occur naturally on Earth or some part of it (e.g. the natural environment in a country). This term includes a few key components:
The natural environment is contrasted with the built environment, which comprises the areas and components that are heavily influenced by man. A geographical area is regarded as a natural environment (with an indefinite article), if the human impact on it is kept under a certain limited level (similar to section 1 above). This level depends on the specific context, and changes in different areas and contexts. The term wilderness, on the other hand, refers to areas without any human intervention whatsoever (or almost so).

Complete landscape units that function as natural systems without massive human intervention, including all plants, animals, rocks, etc. and natural phenomena that occur within their boundaries.
Universal natural resources and physical phenomena that lack clear-cut boundaries, such as air, water, and climate, as well as energy, radiation, electric charge, and magnetism, not originating from human activity.
Natural features which occur within areas heavily influenced by man (such as wild birds in urban gardens). Challenges
It is the common understanding of natural environment that underlies environmentalism—a broad political, social, and philosophical movement that advocates various actions and policies in the interest of protecting what nature remains in the natural environment, or restoring or expanding the role of nature in this environment. While true wilderness is increasingly rare, wild nature (e.g., unmanaged forests, uncultivated grasslands, wildlife, wildflowers) can be found in many locations previously inhabited by humans.
Goals commonly expressed by environmentalists include reduction and clean up of man-made pollution, with future goals of zero pollution; reducing societal consumption of non-renewable fuels; development of alternative, green, low-carbon or renewable energy sources; conservation and sustainable use of scarce resources such as water, land, and air; protection of representative or unique or pristine ecosystems; preservation and expansion of threatened or endangered species or ecosystems from extinction; the establishment of nature and biosphere reserves under various types of protection; and, most generally, the protection of biodiversity and ecosystems upon which all human and other life on earth depends.
More recently, there has been a strong concern about climate change such as global warming caused by anthropogenic releases of greenhouse gases, most notably carbon dioxide, and their interactions with humans and the natural environment. Efforts here have focused on the mitigation of greenhouse gases that are causing climatic changes (e.g. through the Climate Change Convention and the Kyoto Protocol), and on developing adaptative strategies to assist species, ecosystems, humans, regions and nations in adjusting to the Effects of global warming.
A more profound challenge, however, is to identify the natural environmental dynamics in contrast to environmental changes not within natural variances. A common solution is to adapt a static view neglecting natural variances to exist. Methodologically this view could be defended when looking at processes which change slowly and short time series, while the problem arrives when fast processes turns essential in the object of the study.