8.4
LE 1 Human Population and Resource Consumption
(new) What Factors May Limit Future Human Population Growth? (4 pages if needed)
Earth cannot sustain the rapid increase in the human population. We are now adding a billion people to the planet every 12 years. Earth does not have enough resources to sustain this many people indefinitely. There are many factors, however, that could limit future human population growth, both natural and human-caused. These factors often work together as part of a system of factors with many feedbacks. For example, excessive rains could cause death by flooding. The flooding could also damage crops, causing starvation and the spread of diseases. Understanding the factors that limit human population growth requires understanding both the individual factors and the feedbacks among them.
Human factors can limit population growth through both reducing the birth rate and by increasing the death rate. Human birth rates can be reducedthroughsocial practices and traditions, levels of education, economic factors, and government regulations. For example, China has limited birth rates through laws that regulate the number of children that parents can have. Human death rates can be increased bylowering the standard of living, reducing the availability of health care, orincreasing the extent of warfare.
Natural factors limit population growth through the occurrence of natural disasters, such as sudden climate changes, and the availability of natural resources. If there are global or regional decreases in resource availability, death rates can increase. Primary among these are resources for energy, minerals, land, food, and water. (For more on energy and mineral resources, see the Topic on Resources). It is unclear what Earth’s largest sustainable human population is because it remains unclear what the long-term availability of these resources will be. The field of sustainability involves the study of the factors that would allow the long-term survival of the human population.
We can learn a lot about future sustainability by looking at the past. Human populations don’t always increase; sometimes they decrease. Some factors operate suddenly, such as disease epidemics. In 1918, about 100 million people (~3% of the world’s population) died from the Spanish Flu, making it the most fatal disaster in history. The second most fatal disaster was the killing during World War II, which resulted in over 66 million deaths. By percentage, however, the worst disaster in history was the conquests of the Mongols in Asia and Europe, led by Ghengis Khan. The Mongols killed about 40 million people, which at that time was about 10% of the world’s population. Sometimes the populationsof certain regions are affectedmore than others. In Western Europe, the population dropped by more than half during the century from 1300 to 1400 due to a combination of disease and famine.
[Graphic: photo of dead people from the Spanish Flu; or Mongols]
As with the past, a major key to future population levels will be the ability of humans to feed themselves. To do this, we will need land, agriculture, and water.
Land AvailabilityHumans now control about half of Earth’s land surface. More importantly, we now use 34% of the land to supply our food: 11% for growing crops and 23% for raising livestock. Humans live on an additional 6% of the land. There is not a lot of extra land to expand into, however, because most of the remaining land is not suitable for crops or livestock. Glacier ice covers 13% of the land, and inhospitable regions such as mountains, deserts, and tundras cover another 16%. Currently, forests cover 24% of the land, but this number is rapidly shrinking to make room for agriculture. Forests contain most of the world’s biodiversity and create most of the world’s oxygen, so cutting them down is not a good idea. The conclusion from looking at the current pattern of global land use is that there is not a lot of remaining land with which to grow or raise additional food. [Note: these numbers will be in a pie chart, so maybe they don’t all need to be called out in the text]
[Graphic: a pie chart with these numbers. They are taken from but I redid the percentages to include lands covered with ice, which brings the total land area from 130.1 million km2 to 148.94 million km2.]
There are factors that can change the ability to use the land for different purposes, positive and negative. Governments can provide needed regulations and incentives to encourage good land use, but wars between countries can damage land and make it unusable. There has been a steady shift of the world’s population away from rural areas into more concentrated urban areas. Just recently, for the first time, more people on Earth lived in cities than in rural areas. In the U.S., about 80% of people live in urban areas, leaving more land available for agriculture. Even so, in the U.S., roads and parking lots now cover more land than the size of the state of Georgia,and the land used for towns and cities is now larger than the size of the state of California. These areas are rapidly increasing, leaving even less room for agriculture.
The total area of land doesn’t always stay constant. The sea level is currently rising, due to increasing global temperatures. In places where the coastland is fairly flat, such as Florida and Louisiana, small rises in sea level can cause the shoreline to move inland large distances. Currently, 40% of the world’s population lives within 100 km of the ocean. This will significantly affect future land use. Some ocean island nations expect to be entirely flooded by the end of the century.
[Graphic: show a photo of a major metropolitan area at sea level, like a view of Manhattan from the south]
Food Availability Providing food for humans not only requires land but also requires fertilizer, sunlight, and a stable climate to raise the crops. In general, soil quality decreases over time, so over time you can’t grow as much food on it. However, the use of fertilizers can increase soil fertility. Currently, most of our fertilizers are chemically made from petroleum. However, petroleum resources are being used up, so this will not be able to provide future fertilizers. However, the efficient treatment of animal and human wastes can produce rich organic fertilizers that can restore nutrients back into the ground.
We think of sunlight as being constant, year to year. But it can change over time. In the 1600s (a period known as the “Little Ice Age”), the sun gave off slightly less sunlight. This contributed to colder climates and shorter growing seasons for much of the world, leading to famines and starvation. There can also be geologic events that can affect sunlight. In 1815, Tambora Volcano in Indonesia had an enormous eruption. About 100 cubic kilometers of ash and dust wereejected upward into the atmosphere. Even more significant were the large erupted amounts of tiny sulfur dioxide liquid droplets called aerosols. The aerosols and dust blocked out some of the sunlight that would normally have reached Earth. The following year was very cold and was called the “year without a summer” – it even snowed in places in New England in the summer. Large famines occurred around the world, including in the U.S., causing starvation that reduced the global population.Crops are also susceptible to attacks from insects, fungi, and bacteria. For example, a change in Ireland’s climate toward higher levels of humidity during the mid-1800’s led to the spread of a parasitic microbe called the potato blight which destroyed much of Ireland’s potato crop. This led to the death of millions of Irish, and the immigration of millions more to the U.S.
Food availability also depends upon human ability and interestin transporting food to areas that need it. Climates vary regionally as well as globally. One part of the world might be suffering from severe drought and food shortages, while another area might have an abundance of food. Satellites in space now can continuously monitor crop conditions around the world, and food can be shipped or flown into areas of severe famine.
[Graphic: relief food being airlifted in to a region of starvation]
Water Availability Producing food also requires water: a lot of water. To grow a pound of rice can require 250 - 600 gallons of water. A pound of hamburger require 12,000 gallons—for both the cow and the food the cow eats. The world has a lot of water, but most of it is undrinkable: it is either ocean salt water or it is frozen in glaciers. Growing food and raising livestock requires either sufficient rain or reliable sources of surface or groundwater. In cases where the rain falls for only part of the year, reservoirs can be used to store water for the dry times. In cases where there are extended droughts, however, existing water supplies can be insufficient to support the population. For example, in the 1930s a severe drought in the southcentral U.S. caused a total failure of crops in places like Oklahoma, and people had to abandon their homes and flee westward.
You can also have too much of a good thing. Excessive rainfall can lead to flooding that can destroy crops and reduce population. For example, excessive flooding in China in 1931 caused up to 4 million deaths, from both flooding and the typhoid and cholera epidemics that followed. Future food supply was affected because there were few people left alive to continue agriculture. (More about water later in this Topic).
The conditions required to grow food on Earth have been very favorable for the past 200 years. There haven’t been any major volcanic eruptions or changes in sunlight. Climates have been relatively stable. It took 200,000 years for modern humans to reach a population of 1 billion (which occurred in the year 1804), but only 200 years to add another 6 billion. If history is any indication, however, geologic factors over the next 200 years may provide significant challenges to human sustainability.
[Graphic: photo of the Oklahoma dustbowl; photo of flooding in China (an assessment question should address the fact that both of these are happening at roughly the same time period, but in different parts of the world, and what this says about regional climate variations)(BTW, this photo of the China flood was taken by Charles Lindbergh)]
[Note: I am assuming that the next B-head will include a discussion of limiting population growth voluntarily, by social norms, economic factors, health care, or government regulations. If not, then we will need to talk about it here in this B-head.]