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U4735x Environmental Science for Decision Makers

Lecture 2: Human population history & early river-dependent civilizations; global distribution of precipitation and discharge.

Jim Simpson & Martin Stute

OUTLINE OF DISCUSSION:

  1. History of world population over thousands of years.
  2. Population growth rates since 1000 AD.
  3. Early human civilizations on major river systems.
  4. Global distribution of precipitation and discharge

List of Projections:

1 - World pop (Cohen, 1995): 1-2000 AD, linear pop vs linear time.
2 - Exponential growth: 1000-1650; 1950 -1995; 1000 ¯1995.
3 - World population estimates (Cohen, 1995): 1000-1650; 1950-1995; 1000 -1995; log(base 10) population vs linear time, 3 separate exponential growth periods assumed.
4 - Locations of early human civilizations (Huang Ho, Indus, Tigris/Euphrates, Nile) were critically dependent upon rivers.
5 - Mean annual precipitation by region: <0.25 to >2.0 meters/yr.
6 - Arid areas of the world: < 200 mm precipitation per year.
7 - Schematic general circulation of the atmosphere: Hadley cell convergence of surface winds near the equator leads to high mean annual precipitation.
8 - Water vapor pressure as a function of temperature.
9 - Average annual precipitation in the United States, 1961 - 1990.
10 - Mean annual precipitation vs river discharge by continent.
11 - Locations of large river basins: World Resources Institute.

 

OUTLINE OF THE HISTORY OF HUMAN POPULATION GROWTH.

Although it will never be possible to reconstruct accurate records of the human population over many thousands of years, enough is known to describe first order trends. From integration of a variety of written historical information and other data, global population has been estimated to have risen from about 4 million in 10,000 BC to ~6 billion in 1999, an increase of three orders of magnitude over 12,000 years, which roughly corresponds to the time interval since melting of the huge continental glaciers that marked the end of the last glacial maximum. The rates of mean annual population growth have been estimated to be less than 0.2 percent per year up to the middle of the 18th century, but since have rapidly increased to mean annual growth rates of 1.3 to 1.9 percent for the past five decades. The change in rates of annual growth by about a factor of 10 in the second half of the 20th century, compared to the maximum before the mid 18th century has resulted in an explosion in global human population that is unprecedented in the history of our species.

By the middle of the 18th century, global population was about 0.8 billion as the industrial revolution began to gain momentum. Shortly after the end of World War II, the population was about 2.5 billion (1950). By 1999, total population had reached 6 billion. Thus global population numbers increased by more since 1950 than the entire history of the species prior to 1950! The rise in population from 1 AD to 1750, was only about 0.5 billion, an increase in numbers accomplished at the end of this century in only about 6 to 7 years. From 10,000 BC, just prior to the beginning of organized agriculture, through 1 AD, the total increase in population was less than 0.2 billion. Thus today our population numbers rise at an annual rate that is more than 4000 times faster than that typical of the 10,000 years of human history after the end of the last glacial period. The annual increase today is about 80 million per year (about 1.3%/year), while from 10,000 BC to 1 AD, it was less than 20 thousand per year.

A recent book by Joel Cohen (How Many People can the Earth Support?) summarized the history of human population growth in considerable detail. Using a combination of estimates by a number of authors, we can construct a population vs year plot for the period since 1 AD. The increase in world population between 1950 and 1995 was about 3.2 billion, compared to a total population of about 2.5 billion in 1950, more than doubling over this period of 45 years (Projection #1). On this linear plot of population, the period of greatest mass deaths from infectious diseases (especially bubonic plague) during the 13th and 14th centuries is evident in an extended period (several centuries) of low positive and/or negative growth. The trend of population as a function of time has the general appearance from this linear plot of what is often referred to as exponential growth for which the increase in population is proportional to the population already present. However, if the logarithm of population is plotted against a linear increase of time (semi-log plot) for the period since 1000 AD, the trend is not a single straight line as a function of time (Projection #2), as it would be if a single exponential growth function could be fit to the data. Instead the semi-log plot is concave upward, indicative of what is sometimes referred to as "super-exponential" growth, indicating that the annual rate of growth also increased through most of this period of about 1000 years, with the exception of the middle-age period of black death in the 13th and 14th centuries. Note how difficult it is to detect the cumulative effects of deaths from combat and disease during World Wars I & II on the rate of growth of global population, despite the loss of tens of millions of lives in these catastrophic periods. Clearly even widespread wars in the 20th century did not arrest the rapid growth of global population since 1900.

One simple approach to describing population trends between two points in time is to assume a single exponential growth rate over that period. Taking the difference between the natural log of the population at the end and beginning of this period, and dividing by the length of time involved yields the rate of exponential growth (r) over this period. This parameter is mathematically equivalent to the rate of interest paid on a bank account, and refers to the fraction of growth which occurs during a fixed period of time. Three examples of time intervals over which a single exponential function is assumed to apply are: 1000 - 1650 AD, 1950 - 1995 AD, and 1000 - 1995 AD. The percentage growth rates for these three periods are 0.11%, 1.84% and 0.31%, respectively. If we approximate each of these periods with a single straight line on a semi-log plot of global population vs year since 1000 AD, the period since 1950 can be seen as having a percentage rate of growth of the order of 15 - 20 times that for the first 7 centuries after 1000 AD (Projection #3). The mean percentage rate of annual population growth since 1950 (1.8%), multiplied by a world population of 6 billion would yield an annual increase of more than 100 million, compared to only 6 million if the rate were only 0.1% per year.

Current annual growth of global population on the order of 80 million is equivalent to adding another New York City each month. Thus over the duration of a one semester class, the equivalent of almost four New York Cities will be added to the world population.

As human populations rise, a number of essential resources become stressed. One of the most important of these is available fresh water, a resource whose distribution will be examined here primarily through discussion of a few examples of river basins that illustrate some major environmental issues. Global patterns of mean temperature and precipitation have had great influence on the history of human civilization. Nearly all early urban population centers in Asia and North Africa, that rapidly evolved as skills in agriculture accumulated, were associated with important rivers in today's China, India, Pakistan, Iraq and Egypt (Projection #4). All of these river systems (Yellow, Indus, Tigris, Euphrates and Nile Rivers) have major current water resource management issues. In the headwaters of the Tigris and Euphrates Rivers in southern Turkey, major new investments in dams and irrigation diversions will substantially impact the downstream countries of Syria and Iraq. The international community generally does not have very effective processes for influencing these types of serious water resource conflicts.

Global distribution of precipitation and discharge

In our modern world, some of the most urgent human food supply crises are associated with areas of limited rainfall and river discharge. A map of mean annual precipitation amount (P), expressed in cm per year (Projection #5), shows a number of distinct geographical patterns. Most of the large human populations live in areas with relatively plentiful rainfall ( P > 50 cm/yr). Conversely, very few major population centers are found in arid regions (P < 50 cm/yr). In the western hemisphere and the longitudes including Europe and Africa, highest rainfall rates are at low latitudes (tropics), while further from the equator, there tend to be large arid areas, especially in North Africa. This distinct pattern in both the northern & southern hemispheres of dependence of rainfall on latitude does not extend, in a simply way, to South Asia due to unique geography of that region. If we consider the regions with the most limited amounts of rainfall, some indication is provided of those geographical areas where one might expect water supply difficulties if appreciable human populations are present (Projection #6). Here, areas classified as arid (P < 200 mm/year) and hyper arid (P < 50 mm/year ) are shaded. The largest areas of land with these low mean annual rainfall rates are those within 10 degrees latitude of the Tropics of Cancer and Capricorn, located at 23° north and south of the equator. North Africa, the Arabian Peninsula, south central Asia and Australia have the largest continuous expanses of very dry climate of any land areas outside of high polar latitudes. The remainder of the smaller arid regions are also primarily found at similar latitudes, often strongly influenced by the presence of large mountain ranges which impede the transport of moisture inland from the adjacent ocean. This geographical distribution of arid lands is a direct result of the global pattern of atmospheric circulation that is dominated by flow of air towards the equator from both hemispheres (Projection #7) . Rising air at the thermal equator, sometimes referred to as the Inter Tropical Convergence Zone (ITCZ), results in very high mean precipitation rates in a narrow width of equatorial latitudes, and a band of clouds that are often continuous around much of the globe. Air rises in areas near the equator in part because heating by the sun is more intense there relative to high latitudes. The latitudes centered about 30° away from the ITCZ are zones where mean air motion from the upper troposphere is downward (opposite to those of equatorial areas), resulting in very low precipitation rates. These patterns of precipitation are controlled primarily by the fact that rising air cools, can then hold less water at lower temperature, so more precipitation occurs. Large amounts of heat are released as rain forms, causing air to continue rising (positive feedback). Conversely, as air sinks it warms, can hold more water at higher temperature, and less rain falls, on the average.

Consider the following thought experiment. A closed bottle is halfway filled with water. Water will evaporate until the air in the bottle is completely saturated with water vapor. The amount of water vapor in saturated air, or the water vapor pressure, is a strong function of temperature [Projection #8].  Consider a parcel of atmospheric air with a temperature of 20oC (right x on the projection). The air is undersaturated in terms of water vapor. When the air rises, it expands and cools, which means the air hits the saturation curve at some point. Further cooling will result in condensation of moisture (clouds, fog), because  the air cannot hold any more water vapor. If enough moisture condenses and forms little cloud droplets, drops of water will combine to form larger drops, and eventually rain will fall. If this air parcel sinks, the reverse process takes place. Air warms up, droplets will evaporate, and no precipitation can form, because now the air can hold a lot more moisture.

This phenomenon explains also why on a mountain range precipitation is significantly higher on the side facing the winds than on the other side. Air masses have to climb and cool, moisture condenses, and precipitation forms. On the other side, air sinks, warms and any existing clouds evaporate. A typical example for this so-called orographic effect can be seen in the Pacific Northwest of the United States [Projection #9].

Average annual precipitation on the continents is primarily a function of:

The world average of annual precipitation amount over continental areas is about 800 mm. Thus the land areas defined here as arid have less than 25% of the mean. Average river runoff over a year for all land areas is about 300 mm, or almost 40% of precipitation amount delivered. The remainder of the water that falls as rain and snow returns to the atmosphere by evaporation (E) and transpiration (T) through plants, the sum of which (E + T) is often called evapotranspiration (ET). We will use the symbols P and Q to represent annual precipitation and annual river runoff, respectively, in subsequent discussion. Assuming groundwater recharge & outflow in a drainage basin are approximately in balance, then P = E + T + Q. River discharge per unit area of land is not the same for all continents, primarily because of their locations with respect to the global atmospheric circulation and precipitation patterns (Projection #10). Thus South America, with a large fraction of its land mass beneath the Inter Tropical Convergence Zone (ITCZ) has the highest global mean P (1600 mm/yr) and Q (700 mm/yr). At the other extreme, Australia has the lowest global mean P (400 mm/yr) and Q (<40 mm/yr). Note that lower P by a factor of about 4 translates into lower Q by a factor of more than 15. Clearly mean annual river discharge is not linearly proportional to mean annual precipitation rate on the scale of continents. All of the other continents are clustered much closer to the average of P and Q for all land area. Africa falls fairly close to the global continental mean Q per unit area because it contains large areas of both very wet equatorial river basins (Zaire) and very dry subtropical lands with no rivers at all (Sahara Desert).

Much of modern human use of fresh water involves diversions or control of large rivers to provide irrigation water for agriculture, hydroelectric dams, cooling of fossil fuel electric generating stations, and domestic water supplies for cities. Some major watersheds of the world which will be discussed in subsequent classes are shown schematically in a World Resources Institute map (Projection #11). A watershed or catchment is an area of land, bounded by a divide, in which water flowing across the surface will drain into a stream or river and flow out of the area through a specified point on that stream or river.

Approximate ages of a few important events in the history of the Earth and life on Earth (including humans) are listed in table form (scientific notation: A x 10b) to illustrate the central importance of the time-scale of natural processes in their possible influence on human activities (Projection 12). The entries are listed in order of increasing time scales from about one week (H2O vapor recycling time) to the age of the Earth (4.6 billion years). Scientific notation (powers of ten) of years was used to represent these time scales.

Updated September 10, 2003
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