An interesting parallel exists between the growth and development of humans as individuals and humans as a species.  Like a human egg cell, the size of the world’s human population remained constant and small for a very long time.  Then, roughly 10,000 years ago, human beings discovered agriculture and the size of their species began to grow very rapidly (Southwick, 1996).  Population growth had slowed considerably as the 18th century approached, yet this all changed with the rise of the Industrial Revolution in the 1750’s.  Like a child entering adolescence, Homo sapiens began another period of exponential population growth (Dolan, 1971).  As shown in Figure 1, humans in the 21st century live at the awkward tail end of this developmental stage, not quite self-sufficient but by no means helpless.  
Concerns of a Future World
    Just like their prehistoric ancestors, modern humans face the constant challenge of extracting enough resources from the accessible environment to simultaneously support multiple generations of human life.  Until around 10,000 years ago, there were never more than a few million people living on Earth at once (Ponting, 1993).  For the next 9,700 years or so, the global human population grew to around 500 million.  Just as it began to stabilize at this new level, the Industrial Revolution allowed for another massive expansion of human population to its present day level nearing 7 billion (United Nations Department of Economic and Social Affairs/Population Division, 2009).  In order to do so, humans have devised numerous technologies that can access previously unused energy resources such that much larger crop yields can be attained from each square mile of fertile land (Gates, 2009) (see Figure 1).  
    The significant question, though, is for how long?  That is, have humans surpassed the carrying capacity of their planet in the long term?  Since most of the technological breakthroughs of the Industrial Revolution depend upon nonrenewable energy and these resources are running out, the short answer is yes (Adas, 1990).  On the other hand, Homo sapiens has adapted and survived in numerous harsh environments, and who is to say that it cannot adapt to the predicted changes in the global environment?  Since the number of humans living on Earth is still rising, the need for large-scale changes in energy use is of critical importance in maintaining a reasonable quality of life for people in the future.  By making educated guesses about what the world of the future will like look like, modern scientists can foresee potential threats and begin the long process of designing the technologies needed to sustain the future world.  
    Increased Urbanization.  Research suggests that, among other things, future humans will be even more urbanized than their 21st century counterparts.  In 1800, hardly 3 percent of the world’s population lived in cities (Grimond, 2007).  In 1900, that figure had risen to 14 percent (Whitehouse, 2005).  As shown in Figure 2, over 50 percent of contemporary humans live in urban environments, and research suggests that the vast majority of future world population growth will take root in urban areas, especially those in Asia and Africa (Grimond, 2007).  Nevertheless, many cities in these countries are already among the most densely populated regions in the world.  For instance, Mumbai, which already houses 18.2 million people, is expected to hold 26.4 million people in 2025 (Lewis, 2007). Similarly, Delhi is expected to grow from 15 million to 22.5 million.  In the same time, the population of Shanghai is projected to grow from 14.5 million to 19.4 million. Even more astounding are the forecasts for Dhaka, Bangladesh and Karachi, Pakistan, both of which are expected to nearly double in population from around 11 million to 20 million in 2025.  
Growth of Megacities.  While most of the aforementioned cities are  known as a result of their exceptional size, many other cities are anticipated to join the ranks of the “megacities” in the next few years.  The term megacity is typically used to describe cities with population exceeding 10 million (Population Reference Bureau, 2008).  According to UN statistics, there were only three such cities–New York, Mexico City, and Tokyo– in 1975 (United Nations Population Fund, 2007).  As shown in Figure 3, there were 16 in 2000, and it is projected that there will be around 27 by 2025.  Only 12 of these cities are in the Americas, Europe, and Africa, with all the rest found in Asia.  During this same period, estimates suggest that number of world cities to with at least 1 million residents will surpass 500.  
    Given the extremely high human population densities in large metropolises, the task of maintaining sanitary living conditions is extremely costly, but so too is failing to do so.  There is evidence that the number of new infectious diseases has been on the rise of recent, likely as a result of the still rising population density around the world.  Some researchers have described the megacities in the developing world as breeding grounds for new pathogens and mutant forms of old diseases (Jones et al., 2008; Tatem et al., 2008).  This, along with the projected shortages of potable water, could interact to allow for the simultaneous infection of millions of people in a small area (Brown et al., 1999).  
    The growth of urban areas in the developing world has been increasingly associated with the formation of expansive slums fraught by crime, drug addiction, alcoholism, poverty, and unemployment (Davis, 2004).  While many large cities in the developed world like New York and London are extremely prosperous, the emerging cities of tomorrow are among the most poverty-stricken regions in the world.  For example, over 90% of the rapidly growing urban populations of Ethiopia, Malawi and Uganda already live in slums, and their governments do not have adequate resources to provide them the basic resources to maintain health (United Nations Population Fund, 2007).  That being said, the need to feed the nearly 8 billion people expected to inhabit Earth in 2025 will be one of the most pressing concerns, especially as nations will be required to decrease usage of nonrenewable resources.  
    Energy and the Environment.  Humans living in 2010 make up around 6 percent of all humans that have ever lived on Earth, which is incredible considering that anatomically modern humans have been around for around 200,000 years (Haub, 2002).  This being so, neither worldwide population density nor per capita energy usage has ever been higher.  Most national governments acknowledge the need to reduce consumption of nonrenewable energy resources and develop more efficient methods for tapping sustainable energy sources like solar, wind, and hydroelectric power.  In addition to the environmental reasons for doing so, decreasing human dependence on nonrenewable resources is also of vast economic and social importance.  Most of the world’s supplies of coal and gasoline are expected to be depleted in the next few centuries.  Since these ‘cheap’ fuels were among the main factors allowing for Earth to support as many humans as it has since the Industrial Revolution (e.g., Adas, 1990), developments in renewable energy extraction will be of utmost importance to food production, water purification, and nearly every other future human need.  
    It is not just humans, however, who are vulnerable to elevating global population densities.  Researchers modeling the effects of human population growth on global biodiversity generally agree that the expected population growth in the coming years has the potential to drastically reduce biodiversity on Earth (Luck, 2007), perhaps even triggering a second mass extinction of carnivorous megafauna (Cardillo et al., 2004).  Between 2020 and 2050, McKee et al. (2003) predict that the number of endangered species will double.  Since habitat destruction and pollution are anticipated to be among the largest causes of this trend, researchers can model future population distributions so as to minimize the detrimental effects of humans on the environment.  
Urbanization and Conservation
    Although the growth of cities has long gone hand in hand with pollution, habitat destruction, and other perils of the modern world, recent projections made by UN researchers shed a new light on the potential role of cities in the future of humanity.  Given that we cannot act immediately halt population growth, research indicates that the further population of cities may be our saving grace in the years to come (Kariuki, 2007).  By packing more people onto each square mile of already urbanized land, we could contain the ecologically destructive effects of humans, and in doing so, prevent further destruction of non-urbanized land and the ecosystems it supports.  Humans are nowhere close to having all the technologies needed to sustain future megacities, but the world’s remaining biodiversity may hold the keys to a fully sustainable future.  If this is the case, then cities just may be the road to adulthood that Homo sapiens have no choice but to take.  
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Anthony Stigliani 2010