How Has the Human Population Grown Over Time?

7-12:4: Human Populations

[Keep the Dangerous Disease “My Planet Diary” feature on p. 98 (except, change the picture from a rat to a gerbil, which is now the leading culprit for the spread of the black plaque)]

How Has the Human Population Grown Over Time?

The oldest fossils of modern humans (homo sapiens) are about 195,000 years old. For most of human history, the human population stayed at fairly low levels. When the first cities were built, 10,000 years ago, there were about 5 million humans. Now there are over 7 billion humans, and the number is rapidly increasing, doubling every 40 to 50 years. The number of humans cannot increase forever. Humans already now use more resources than planet Earth can continue to provide. Scientists debate what is the maximum number of people Earth can hold.

The populations of most animals fluctuate over time,but stay at about the same level. When environmental conditions are good, the population may increase. When resources become scarce, or there is a spread of disease, the population may decrease. However, with modern science and technology, humans now control the conditions they live in. As a result, the human population has been increasing exponentially.Exponential growth occurs when a population grows at an ever-increasing rate. In exponential growth, the larger a population gets, the faster it grows.

The human population hasn’t always increased steadily. There have been times when the population has decreased. About 74,000 years ago, asupervolcano in Indonesia called Toba had a huge eruption. Some scientists think that this eruption might have darkened the skiesand killed most humans. Perhaps only 10,000s of humans survived. In the 13th century, Mongol warriors led by Genghis Khan waged wars across Asia and Europe that killed 10% of all humans. In the 14th century, plagues and famines caused the population of Western Europe to decrease by 70%. Throughout history, climate changes have triggered droughts or floods that have killed large numbers of people. However, the climates of the last 200 years have been fairly steady, and the human population has gone from 1 billion to 7 billion during this time. Scientists are trying to understand what the factors are that might cause the next decrease in human population.

Do The Math!

World Population Milestones

(1) Look at the pattern of the years between billion-year milestones. Do you think the number of years until there are 8 billion people will be more, less, or equal to 12 years?

(2) In what year do you think the world human population will reach 8 billion? 9 billion? 10 billion?

Assess Your Understanding

Now I know that the human population______

I need extra help with ______

What Factors Allow the Human Population to Increase?

Any population needs food and other resources to survive. Any population needs to stay healthy long enough to reproduce. Humans are no different. However, humans now significantly alter the world they live in, and this is allowing more people to live longer. Advances in medicine and technology have improved human health and increased resources, allowing for exponential population growth. Unlike other animals, however, humans can also plan for the future and make decisions on whether or not to have children. These decisions can be personal or part of cultural traditions. All of these are important factors that affect how the human population increases.

Birth and Death Rates A population’s birthrate is the number of babies born each year per 1,000 people. The death rate of a population is the number of people who die each year per 1,000 people. The global population grows because more people are born each year than die each year. Part of the reason for this is that, in most parts of the world, people now live longer than ever before. The human population is now increasing so fast that 7% of all humans who have ever lived are alive today. Even more amazing, because in the past most humans died as children, more than 20% of all human adults that have ever lived are alive today.

In the U.S., someone is born every 8 seconds and someone dies every 12 seconds. In addition, some people move into the U.S through immigration and some people move out of the U.S. through emigration. The difference between these is the net migration rate. For the U.S., the net migration rate is one personinto the country every 32 seconds. The result, combining these three rates, is that the U.S. population gains an additional person every 15 seconds. Globally, the world population increases by a rate of 2.5 people each second.

Apply It:

(1) Complete the table by calculating the Total Growth Rates in the right column.

(2) Circle the country or countries in the table with decreasing populations.

Medical Care For most of human history, the process of childbirth was very dangerous, both for the infant and mother. Advances in medical care now allow most children and mothers to survive childbirth, both significantly increasing birthrates and reducing death rates. The major part of this is sanitation – using soap to kill germs. Antibiotics, vaccines, medicines, and improved nutrition have also led to greatly reduced death rates and a resulting exponential rise in human population. Many more humans now live to be old enough to reproduce than ever before.

In previous centuries both the birthrates and death rates were very high. Traditions developed for mothers to have many children because most were going to die young. In some countries such as Mozambique [see the Apply It section], the birth rates are still very high because the traditions have not changed to keep pace with medical improvements. This is often the case in developing countries, where people do not have access to good education. A map of population growth rates shows that the countries with the highest rates are mostly in developing parts of Africa, South America, and Asia, where standards of living and access to education are very low.

Interactive Art: Examine this map of the yearly percentage increases in the populations of different countries. Circle the countries whose populations are decreasing. Draw an “X” through the countries whose growth rates are above 2.5% per year.

Natural Resources and Technology As the human population grows, so does the demand for natural resources. Humans use an enormous amount of “stuff” – over 80 billion tons of resources each year. This includes food to eat, trees for homes and paper, rock for roads and buildings, metal for cars and cities, and fuels like coal and oil for energy. This averages to more than ten tons of resources per person, per year. These resources need to be mined from the ground or grown and raised at the surface. These resources also need to processed, transported, and delivered to more then 7 billion people.

One of the most important resources for humans is food. Humans now use 40% of the land’s surface to raise or grow their food. This is currently close to the maximum limit, because much of rest of the land is either poor for growing food (deserts, glaciers, mountains, swamps) or places that live on. Because soils also lose their fertility over time, humans use huge amounts of fertilizers to continue to grow crops, year after year. The world used180 million tons of fertilizer in the year 2013, largely made with petroleum. The world also uses about 10% of its total energy consumption to make these fertilizers.

Advances in technology support our large and increasing population, powered by a huge consumption of energy. Most humans now live in cities, and a vast infrastructure of industry now exists to provide our resources. Powerful computers coordinate complex transportation systems and the distribution of electricity, natural gas to heat our homes, and water. Advances in technology are also required to process the garbage and waste left over from the consumption of resources. Humans generate about 4 million tons of solid waste every day, enough to fill a line of garbage trucks 5000 km long each day. Advances in technology are needed to process this waste in a variety of ways including recycling, reprocessing, burning it for energy, or burying it in landfills.

Identify the Main Idea: Between 1980 and 2010, global population increased by about 60%, going from 4.4 billion to 7.0 billion. Was the increase in resources consumed, from 1980 to 2010, more or less than 60%. Why do you think this might be?

Assess Your Understanding

Now I know that the human population has grown exponentially because of______

I need extra help with ______

What Factors Can Cause the Human Population to Decrease?

The human population cannot continue increasing at a rapid rate indefinitely. The Earth does not have enough resources. By some estimates, humans now use about a planet and a half’s worth of resources. How can this happen if there is only one Earth? This happens because we are borrowing against the future. We are using up resources in ways that are unsustainable. For example, by the year 2010 there were only half as many vertebrate animals (mammals, reptiles, amphibians, fish, birds) as there were in 1970. In just 40 years, we reduced the world’s population of vertebrate animals by 50%, by eating them, removing and polluting their habitats, and other human activities. Will they be all gone in another 40 years?

There are other factors besides shortages of resources that could be responsible for causing the human population to decrease. Some of these factors are natural, such as the sudden climate change from a large supervolcano eruption or from a large meteoroid impact. These catastrophic disasters happen rarely, but they do happen. The fossil record shows several times in Earth’s past when most living organisms died because of a natural catastrophe.

Some of the factors are human-caused. As shown before, countries with higher education rates tend to have lower population growth rates. Some developed countries in Europe with very high education rates even have shrinking populations. War between or within countries can also decrease the population, as with the two World Wars of the 20th century.

Often, factors that can cause the human population to decrease are a combination of both natural and human-caused factors. These include famines, the spread of plagues, the response to natural disasters, and the impacts of climate change.Human activities affect the impacts of these natural factors.

Figure: Increase in the human consumption of biological resources, shown in terms of the number of planet Earth’s resources we have consumed. When was the last year that humans consumed an amount of biological resources that planet Earth could sustainably provide?

Food and Water Shortages Some resources are non-renewable, such as coal and petroleum. There are limited amounts of them. Once we use them up, they will be gone. We will be able to use other energy sources such as solar and wind to make electricity and to heat our homes. However, petroleum is used to make fertilizers, and we could not feed 7 billion people without them. When we run out of petroleum, we will need to find another way to make fertilizers, or we will not be able to grow as much food as we do now.

Some resources, such as water, are renewable. However, the water isn’t always renewable at rates we would like them to be. We now use all of the water from some rivers for human needs. For example, for parts of the year the Colorado River is entirely dry at its mouth. There is no water left; it is all used for drinking water, agriculture, and other human uses.

The Colorado River delta is entirely dry for parts of the year, due to a combination of drought and overuse of water by humans.

We also use groundwater in many parts of the world to grow our food. For instance, much of the food grown in the Midwestern U.S., from Oklahoma to South Dakota, is grown with water from the underground Ogallala aquifer. However, groundwater takes thousands of years to accumulate, and in many places the groundwater is rapidly being used up in just decades.

The amount of rain that falls can also change over time as regional climates change. Some parts of the world, such as the western U.S., have been experiencing many decades of extended severe drought. Very little rain falls during a drought, making it hard to grow food. There have also been dry times in Earth’s climate history when global rainfall levels decreased significantly. The recent trend of globally warming temperatures has caused many parts of the world to experience increased droughts. Global populations will decrease if the amount of food that can be grown decreases.

Figure showing predictions for changes in the amount of precipitation.Can you find where you are on the map? In the future, will it be drier or wetter where you are? Which state of the United States is likely to become wetter than it is today?

Pollution and the Environment Human populations will decrease if resources become unusable because of pollution. Pollution is the release of contaminants into the land, water, or air that cause adverse change. A pollutant is any substance that causes pollution, often in the form of a chemical substance. For example, the garbage and waste that humans dump onto the land contain many different chemicals that can harm creatures living there or make water undrinkable. Some of the significant decreases in the numbers of the world’s animals are due to pollution of the animals’ habitats.

There are many forms of air pollution that contaminate water and food resources. Factories and coal-fueled electricity power plants release contaminants such as sulfur dioxide that react with gases in the atmosphere to form acid rain. The acid rain makes streams and lakes too acidic for many kinds of fish and water plants to live. The burning of gasoline by the more than 1 billion cars and trucks on the planet releases many harmful air pollutants, or emissions, into the air. Air pollution in cities has become a major source of human deaths from lung diseases.

Why do you think these people in Harbin, China, are wearing masks? Why do you think you can barely see the building on the other side of the street?

A significant pollutant from both cars and factories is carbon dioxide. Carbon dioxide in the atmosphere has increased by 50% in the past 200 years, which has caused an increase in the temperature of the atmosphere. This, in turn, has leads to increased droughts and forest fires around the world and rapid melting of the ice of glaciers. The melting glaciers are causing global sea levels to rise. Within a few centuries, many major coastal cities may need to be abandoned. Where will these people live?

Precious water resources can be polluted in many different ways. Animal wastes, fertilizers, and pesticides are sources of water pollution. Human wastes can contaminate water with bacteria that can spread diseases. Industry and mining wastes can contaminate rivers and lakes with harmful chemicals and metals. Oil and gasoline present a particularly harmful form of water pollution because they float on top of water. Oil spills in rivers or the ocean stay at the surface, coating and killing living organisms. Oil spills into groundwater float at the top of aquifers, coming into and contaminating water wells.

Dead fish on the waterways north of Point a la Hache Marina, Louisiana, following the Deep Water Horizon oil spill in the Gulf of Mexico

Water pollution on land often gets washed in the ocean where it can kill ocean life that humans rely on for food. Fertilizers washed down rivers sometimes cause blooms of ocean algae that create “dead zones,” killing off all other ocean life. Plastics dumped or washed into the ocean take a long time to break down, and are harming marine life. About one-fourth of the carbon dioxide released into the atmosphere bythe burning of fossil fuels has gone into the ocean, which has caused the ocean to become 30% more acidic. This acidity has begun to harm many different ocean life forms.

Spread of Diseases Some of the deadliest natural disasters have been the result of the spread of diseases. These diseases are usually caused by viruses or bacteria. Some disease outbreaks occur quickly and some spread slowly. In one year, 1918, global outbreaks of the Spanish Flu virus killed about 100 million people, which was about 3% of the world’s total population. The HIV virus, on the other hand, kills almost 2 million people each year, but this number has risen slowly over many decades, and has killed over 30 million people so far.