Please note that the problem has increased since a couple of my sources were written. For instance, my last source was written when the population was at 5.5 billion. It is now stated at, I think, 7 billion.
Sustainability is best defined as: "meeting the needs of the present without compromising the ability of future generations to meet their own needs."
The Water Supply
http://www.earthportal.org/news/?p=959
That is short enough to just put a link.
Climate Change
http://www.nytimes.com/2011/06/05/science/earth/05harvest.html?pagewanted=all
Basically, due to climatic change many scientists believe that we are soon going to have to genetically modify crops to survive.
Carbon Footprint
A few links supporting the following:
http://planetgreen.discovery.com/work-connect/demystifying-sustainability.html
http://co2now.org/
The magic number for this planet's carbon footprint, which is the biggest threat to our planet's survivability, is figured at 350 ppm, according to most scientists. This is the point where the oceans, trees, and other life forms (plant, mostly) can absorb the CO2 fast enough to keep the life forms balanced. Currently it sits at about 390.02 ppm. So this, the biggest factor in our planet's sustainability is past it's survivability point.
You mention living in places like the areas of Russia that are unpopulated or underpopulated. You are not taking into account this carbon footprint. The more land populated by humanity, the less plant life there is (proven throughout history, look at the rain forests and how they are being destroyed for farms, providing much less plant life). Mankind is no longer able to live in an untamed wilderness. We are no longer hunter/gatherers. We have to beat this planet into submission to survive. In doing so, we are using the natural resources at an alarming rate (non-sustainable), increasing our carbon footprint by putting more CO2 into the air (breathing and other emissions by animal life) while destroying the factories that balance that (plant life).
As I am stating current worldwide data, I am not saying any type of lifestyle, American or otherwise. I am saying the lifestyle we, as a planet, currently use. Factor in how many are starving, you can see how we have exceeded our sustainability.
Several points, not including the above
I am copying and pasting directly from the website itself:
http://www.dieoff.org/page112.htm
**"One measure of the impact of the global population is the fraction of the terrestrial net primary productivity (the basic energy supply of all terrestrial animals ) directly consumed, co-opted, or eliminated by human activity. This figure has reached approximately 40% (Vitousek et al. 1986). Projected increases in population alone could double this level of exploitation, causing the demise of many ecosystems on whose services human beings depend."
**"Ecologists define carrying capacity as the maximal population size of a given species that an area can support without reducing its ability to support the same species in the future. Specifically, it is "a measure of the amount of renewable resources in the environment in units of the number of organisms these resources can support" (Roughgarden 1979, p. 305)
For human beings, the matter is complicated by two factors: substantial individual differences in types and quantities of resources consumed and rapid cultural (including technological) evolution of the types and quantities of resources supplying each unit of consumption. Thus, carrying capacity varies markedly with culture and level of economic development.
Given current technologies, levels of consumption, and socioeconomic organization, has ingenuity made today's population sustainable? The answer to this question is clearly no, by a simple standard. The current population of 5.5 billion is being maintained only through the exhaustion and dispersion of a one-time inheritance of natural capital (Ehrlich and Ehrlich 1990), including topsoil, groundwater, and biodiversity. The rapid depletion of these essential resources, coupled with a worldwide degradation of land (Jacobs 1991, Myers 1984, Postel 1989) and atmospheric quality (Jones and Wigley 1989, Schneider 1990), indicate that the human enterprise has not only exceeded its current social carrying capacity, but it is actually reducing future potential biophysical carrying capacities by depleting essential nautral capital stocks. [1]
Where resources in high demand and in short supply are overharvested, a positive feedback cycle is established, thereby sequentially depleting the stocks and lowering the MSUs. For example, overharvesting of fuelwood, the primary source of energy for more than half of the world's population, has created severe local and regional shortages. To supply domestic energy, these shortages are countered by overharvesting increasingly distant supplies and by burning animal dung and crop residues, important inputs to the maintenance of soil productivity (WRI 1992b).
