The Effect of Population
on Global Climate Change

Tonalee Carlson Key


Table of Contents

The Greenhouse Effect and Global Warming
Why Do We Suspect That Human Activities Are Affecting Global Change?
Greenhouse Gases and Their Sources

Commercial Energy Production/Usage

Industrial Usage
Residential Usage
Commercial Usage
World Energy Usage


The Effect of Population on CO2 Emissions
The Controversy
The Predicted Consequences of Global Climate Change
Actions That Can Be Taken to Mitigate Global Warming

List of Tables

Table 1 Greenhouse Gas Sources
Table 2
Atmospheric Concentrations of Greenhouse Gases
Table 3
Estimated 1985 Global Greenhouse Gas Emissions From Human Activities
Table 4
U.S. Energy Supply Mix, 1996
Table 5
Carbon Coefficients At Full Combustion, 1996
Table 6
Car Ownership, 1991
Table 7 Estimate of Carbon Emissions From Fossil Fuel Burning During 1994
Table 8
Estimates for 1985 & Assumed Trends Through 2100 in Population Size, CO2 Emissions Per Capita and Total CO2 Emissions for the Developing & Developed Worlds
Table 9
Estimated Contribution of Population Growth to the Increase in CO2 Emission Between 1985 and 2100 Under the Assumptions in Table 8

List of Figures

Figure 1 Monthly Atmospheric CO2 Concentrations - Mauna Loa Observatory, Hawaii, 1958-1995
Figure 2
CO2 Emissions Generated by End Use Energy Sector, 1990
Figure 3
Determinants of Growth of CO2 Emission from Fossil Fuels

There are many facets to the subject of global climate change. While this presentation includes basic information about the subject, the primary focus is on human activities and how these activities affect global climate change.


In the fall of 1995, scientists with the Intergovernmental Panel on Climate Change, the international body charged with studying global climate change, reached a conclusion in their Second Assessment Report, which summarizes the current state of scientific knowledge on climate change. For the first time ever, the Panel concluded that the observed increase in global average temperature over the last century "is unlikely to be entirely natural in origin" and that "the balance of evidence suggests that there is a discernible human influence on global climate."1

As with many discoveries, scientists did not set out to determine if man was destroying the viability of the Earth. By the mid 1950s scientists were suspicious about rising carbon dioxide levels, however, the presence of a greenhouse effect, first discussed by Swedish scientist Arrhenius at the turn of the century, had not gained much attention. It was not until the development of supercomputers that investigation of global climatic change came of age. John von Neumann, a mathematics professor at Princeton University, was interested in applications for computers, such as weather forecasting and persuaded the United States (U.S.) Weather Bureau to fund such research. The task of developing a numerical model to study climate dynamics fell to Syukuro Manabe, a recent Ph.D. graduate from Japan.

Early efforts concentrated on one dimensional atmospheric models and were conducted in conjunction with geochemists and focused on trying to understand the last ice age. Over time computer simulations were created to study long-term climate change. By adjusting the model variables, such as the sun’s energy and greenhouse gas concentrations the group began to develop a picture of the earth’s climate and the effects that various parameters have on long-term climate. 2, 3

The Greenhouse Effect and Global Warming

"Greenhouse Gases" let sunlight through to the earth’s surface then impede the escape of energy (heat) into space. These gases act in much the same way as glass panels in a greenhouse which allow sunlight through and trap heat inside, thus the term greenhouse effect. Without naturally occurring greenhouse gases it is estimated that the earth’s average temperature would be nearly 33 oC colder.4 This would result in a planet much less suitable for human life.

Global warming is the term used to refer to the possible increase in global temperature due to increased atmospheric concentrations of greenhouse gases. Over the last 100 years, average global temperature has increased between 0.3 o and 0.6 o C while atmospheric greenhouse gas concentrations have increased significantly due to human activities. At this time, scientists believe that the increase is unlikely to be entirely natural in origin and that scientific evidence suggests there is a discernible human influence on global climate.

Why do we suspect that human activities are affecting global change?

Although Arrhenius first calculated the effect of an increasing concentration of greenhouse gases almost a century ago, it was not until the advent of computers that scientists were really able to start describing the effect quantitatively and discovering the factors which affect it. Scientists use numerical models to quantify the greenhouse effect and to predict future global climate changes. Early climate models were very simple mathematical descriptions which contained few parameters, as opposed to some of the models being run in the 1990s. Runs of these early models primarily looked at what happened when various parameters were changed or as parameters were added or deleted, called "diagnostic modeling", as opposed to being able to predict, called "predictive modeling".. This very simplified approach was due to several factors: first, numerical modeling of systems was very new - how could anyone have known how complex and interwoven systems affecting global climate would be, second, computers lacked the computing power to run "elaborate" models, and third, there was insufficient data to test the models against existing global conditions. In the early 1960s and 1970s, independent of a unified global climate research effort, researchers in meteorology, oceanography and atmospheric chemistry began to collect volumes of data on parameters such as global temperature, cloud density, solar radiation, atmospheric and oceanic carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations, oceanic circulation patterns, carbon budgets, just to name a few. This data has allowed scientists to develop more sophisticated models that are more closely explaining existing global climate conditions and will therefore be more likely to predict future global climate changes. The success of recent models can certainly be seen in the improved accuracy of daily weather forecast, tornado and hurricane predictions and El Nino weather effects.

Although scientists have learned an incredible amount about the Earth’s global climate system and the factors that affect it from their modeling efforts, they have a long way to go. Today’s climate models are better able to compensate for missing or insufficient data. However additional information is still needed for many factors, including: changes in greenhouse gases and aerosols, sources and sinks of CO2, feedbacks associated with clouds, water vapor , oceans, sea ice and vegetation, and source budgets for key gases including CH4, carbon monoxide (CO), N2O, and other nitrogen/oxygen compounds (NOx) and the relationship between energy use activities and emissions of these gases.

Another piece of the climate change puzzle deals with factors which appear to have a cooling effect on surface climate. For example, aerosols, like the small particles which produce smog, tend to produce a net climate cooling effect. These aerosols and solid particles, which are the result of fossil fuel burning among other sources, primarily have a local or continental to hemispherical effect on climate. And, unlike greenhouse gases which are long-lived in the atmosphere, aerosols are short-lived in the atmosphere. Therefore, they may have an ability to mitigate warming changes rapidly with increasing or decreasing emissions.5

Recent studies in which the modeled climate is compared with observed patterns of atmospheric temperature change show a correlation between temperature increases over time and an increasing human signal. However, our ability to quantify the human influence on global climate is currently limited because the expected signal is still emerging from the noise of natural variability and because there are uncertainties in key factors.5 Recent advancement in computer model simulations, called coupled atmosphere-ocean climate models, have increased our confidence in using numerical models to project future climate change. Although uncertainties remain, they have been taken into account in the model runs. Currently, using best available data, these models project an average increase in global mean surface air temperature, relative to 1990, of about 2 oC by 2100.,a 5 Confidence of the model projections is higher in the hemispheric-to-continental scale than for regional scales.

Greenhouse gases and their sources

The major greenhouse gases include carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and nitrous oxide (N2O).b Most greenhouse gases have two sources: natural processes and human activities. The major greenhouse gas sources are outlined in Table 1:

Table 1 Greenhouse Gas Sources

Greenhouse Gas

Natural Sources

Human Activity Sources

Carbon Dioxide



Rotting wood6

Decomposition of organic material in soil6

Burning fossil fuels for transportation & commercial energy5

Tropical deforestation4

Cement manufacture7

Other land use changes7





Grazing animals5

Losses to the atmosphere of natural gas during oil and gas production, transportation and use5

Coal mining7

Cattle raising5

Rice cultivation5

Tropical deforestation4


Animal waste6

Domestic sewage treatment6

(CFC-11 & CFC-12)

None Chemical products and processes, including refrigeration, industrial solvents and blown foam insulation7

Nitrous oxide

Biological processes in soil5 Coal combustion5

Industrial production of nylon5

Fertilizer use4

Tropical deforestation4

Burning wood and industrial biomass4

Agricultural wastes4

Cultivated land4

Nitric acid production7

Automobiles with 3-way catalysts7


The atmospheric concentration of these gases have increased in recent history. We know this because scientists have been able to determine the concentration of these gases in ancient atmospheres. For some gases, scientists are able to analyze the tiny gas bubbles found in ice cores from areas like Antarctica and Greenland. In more recent times, scientists have measured atmospheric CO2 concentrations from on top of Mauna Loa in Hawaii. As of 1992, that data shows a 12% increase in CO2 concentration for the past 33 years or approximately .5% a year (Figure 1). A comparison of greenhouse gas concentrations for the preindustrial and today’s atmospheres is provided in Table 2.


Figure 1 Monthly Atmospheric CO2 Concentrations -
Mauna Loa Observatory, Hawaii, 1958 - 1995


Table 2 Atmospheric Concentrations of Greenhouse Gases







Preindustrial atmospheric concentration

280 ppmv

0.8 ppmv



288 ppbv

Current atmospheric concentration (1998)9

363 ppmv

1.75 ppmv

261 pptv

534 pptv

312 ppbv

Percentage increase in concentration






ppmv: parts per million volume ppbv: parts per billion volume
pptv: parts per trillion volume NA: not applicable


Scientists consider CO2 the most important greenhouse gas being produced by human activities. However, the other major greenhouse gases, especially CH4 and the CFCs, play significant roles in greenhouse gas affects. Various human activities and the greenhouse gas emissions resulting from them are highlighted in Table 3.


Table 3 Estimated 1985 Global Greenhouse Gas Emissions from Human Activities 4


Greenhouse Gas Emissions (Mt/yr)

CO2-Equivalent Emissions (Mt/yr)

Percentage of Total Emissions (of CO2 Equivalent Emissions)

CO2 Emissions      
Commercial Energy




Tropical Deforestation












CH4 Emissions      
Fuel Production




Enteric Fermentation




Rice Cultivation








Tropical Deforestation












CFC-11 & CFC-12 Emissions      




N2O Emissions      
Coal Combustion




Fertilizer Use




Gain of Cultivated Land




Tropical Deforestation




Fuel Wood & Industrial Biomass




Agricultural Waste











Mt/yr: million metric tons per year


Since each greenhouse gas has different radiative properties and atmospheric chemistry, each has a different efficiency of trapping heat. In the second column of Table 3, CO2 Equivalent Emissions, each greenhouse gas has been converted to its CO2 equivalent in order to provide a direct comparison of "warming potential". For example, each ton of CFC-12 is approximately 5,750 times more efficient at trapping heat than each ton of CO2.4 The information in this table shows that primary human activities which add greenhouse gases to the atmosphere are: commercial energy production/usage (57%), CFC usage (10%) and tropical deforestation (10%).


Commercial Energy Production/Usage

In the U.S. and elsewhere in the industrialized world, energy use contributes more to global warming than any other human activity. This is because most of our energy comes from carbon-based fossil fuels like coal, oil, and natural gas (Table 4).


Table 4 U.S. Energy Supply Mix, 19967

Energy Source






Natural gas









Fossil fuels provide energy for a variety of purposes, including transporting goods and people, manufacturing products, heating and cooling buildings, lighting spaces, and cooking food. When we burn these fuels, the carbon they contain combines with oxygen from the air to form CO2. Coal produces the most carbon per unit of energy (British thermal units (Btu)), 26.00, and natural gas the least, 14.47 (Table 5).


Table 5 Carbon Coefficients at Full Combustion, 1996 7

Energy Source

Metric Tons of Carbon per Billion BTU


Natural Gas


.0326 pounds of Carbon per cubic foot of natural gas

Jet Fuel


Motor Gasoline




Crude Oil


244 pounds of Carbon per barrel of petroleum

Coal - Used in Electrical Generation


1474 pounds of Carbon per ton of coal

Coal- Used by Residential & Commercial Groups




What do these carbon coefficients mean in real life terms? Take for example a 2,500 square foot home in the northeast which uses natural gas for cooking, heating and making hot water. For an average winter month the family in this home uses 12,300 cubic feet of natural gas. Their natural gas usage in this month contributed approximately 400 pounds of carbon or 1467 pounds of CO2 to the atmosphere. Over a year the contribution is approximately 2,800 pounds of carbon or 10, 267 pounds of CO2 to the atmosphere. Therefore, over the course of a year, the contribution of greenhouse gases from this single household would be approximately 5 tons of CO2 or 1.3 tons of carbon.

U.S. CO2 emissions from fossil fuels are divided almost equally between transportation, industry, and homes/businesses (Figure 2). Of these emissions, more than a third are the result of generating electricity using fossil fuels. Each year U.S. energy use pours more than five and a half billion tons of CO2 into the atmosphere. This results in a per capita CO2 production of more than 20 tons a year (or 5 tons of carbon).11

Figure 2 CO2 Emissions Generated by End Use Energy Sector, 1990 12



The movement of goods and people in the U.S. accounts for one-third of the nation's CO2 emissions. Transportation's carbon emissions result from a combination of three factors: number of miles traveled, amount of fuel used, and amount of CO2 released when a particular fuel is consumed.13 On average, U.S. cars emit 20 pounds of CO2 for every gallon of gasoline they burn.1 Currently, the U.S. leads the world in per capita car ownership (Table 6) thus making the U.S. transportation end user energy sector one of the largest global transportation contributors to CO2 emissions.

Table 6 Car Ownership, 1991 6

Countries Persons per Car
U.S. 1.7
Italy 2.0
United Kingdom 2.4
Japan 3.3
Czechoslovakia 4.8
Malaysia 8.4
Republic of Korea 15.5
Former USSR 17.0
India 121.4
China 680.0


Industrial Usage

In the U.S., industries consumed 39% of the nation's end-use energy in 1990, including 35% of electricity generated. Two-thirds of the sector's electricity use is for motors. Industrial motors use over 20% of all U.S. electricity generation. Including the emissions from electricity generation, the industrial sector accounted for 33% of U.S. CO2 emissions. A small number of major manufacturing groups -- primary metals, petroleum refining, chemicals, pulp and paper -- account for about 70% of industrial energy use.12

Residential Usage

In 1990, 15% of all U.S. end-use energy, accounting for 34% of U.S. electricity demand, was consumed in homes. Including the fossil fuels used to generate electricity for homes, the residential sector contributed 19% of U.S. CO2 emissions. This energy was expended primarily on heating and cooling, home appliances and lighting.12

Commercial Usage

U.S. commercial buildings accounted for nearly 11% of total end-use energy consumption in 1990 and consumed over 30% of all electricity, primarily for lighting, heating, cooling, and air handling. Including the fossil fuel used to generate the electricity, the commercial sector accounts for over 15% of U.S. CO2 emissions.12

World Energy Usage

The current worldwide energy usage breakdown is approximately 20% of primary energy for transportation, 40% by industry and 40% by commercial businesses and in homes. Road transport accounts for approximately 80% of transportation in industrialized countries with air transport second at 13%. 6

About one third of worldwide primary energy goes to make electricity at an average efficiency of conversion of about one third. Efficiency of coal fired power stations have improved from ~32% 20 years ago to about 36% today. About half of the world’s population relies wholly on traditional fuels and does not currently have access to commercial energy in any form. In these areas much cooking is still carried out in open fires where only ~5% of the heat reaches the inside of the cooking pot. 6

The world consumption of energy was about 8,730 tons of oil equivalent (toe) in 1990. In physical energy units this equals 12 Terawatts or 12x1012 watts or, on average, 1.65 toe or 2.2 kilowatts(kW) for each person in the world.6 Of course this usage varied greatly from country to country. In North America it was 8 toe or 11kW and in India it was .4 toe or .5kW (mainly in the form of traditional fuels).6 Interestingly, the amount spent per year by the average person for the 1.65 toe of energy used, is about 5% of annual income. Despite the very large disparity in world incomes, the proportion spent on primary energy is much the same in developed and developing countries.6


CFCs are non-toxic, non flammable and chemically unreactive. This was one of the reasons they were considered ideal candidates as coolants in appliances like refrigerators and as product propellants in spray cans. But these properties also mean that they will remain in the atmosphere for an extended period of time, estimated to be 60-130 years, before being destroyed. Each CFC molecule has a greenhouse effect 5,000-10,000 times greater than that of a molecule of CO2.6 This means that although the total atmospheric CFC is small relative to CO2, it accounts for approximately 10% of CO2 equivalent emissions.

It should be noted that before CFCs were associated with global warming, they were associated with stratospheric ozone depletion. In the upper atmosphere molecules of ozone are destroyed in a natural process when they absorb solar ultraviolet radiation. This radiation would otherwise be harmful to life on earth. The addition of CFCs to the atmosphere due to human activity affected the natural formation/destruction process of ozone.6


Forests are a very important part of the ecosystem and carbon cycle. They are home to a vast amount of the plant and animal life and their loss has far reaching effects on biodiversity. As part of the carbon cycle, forests are huge carbon reservoirs. Trees and plants require CO2 to sustain themselves much the same way that humans require oxygen. The carbon from the CO2 becomes "fixed" or a part of the tree or plant and is thus removed from the carbon cycle for the life of the tree, i.e. less carbon in the cycle, less CO2 in the atmosphere. Loss of forests affects the amount of carbon in the cycle in two ways: first, in that they are no longer available to remove carbon from the cycle and second, in that the burning or decay of the trees adds carbon back into the cycle.

Worldwide forest coverage is approximately 80% of what it was 3,000 years ago when agriculture began to expand.14 Activities resulting in deforestation include logging, farming, ranching, use as fuel wood and lumber, mining, building of hydroelectric dams and urban expansion. In temperate zones, logging roughly balances growth of new trees. Abandonment of farmlands due to economic factors have increased in recent decades resulting in new woodlands. Tropical forest clearing for fuel wood, farming and ranching was estimated at ~1% per year for the past decade.14 In many cases, development of tropical forests is seen as the only possibility that some populations have for survival. However, often the soils and other conditions on the cleared land do not allow sustainable production, they are abandoned and serious land and soil degradation begins.6 In addition, since the 1970s the increased demand for tropical hardwoods has seen wood production in Asia and South America rise dramatically which suggests further deforestation.14 It is estimated that every square kilometer of tropical forest contains from 20,000-50,000 tons of total living material which contains from 10,000-25,000 tons of carbon. This means that since the 1970s approximately 1.3 gigatonsd of carbon, in the form of CO2, have entered the atmosphere due to deforestation.6


1 Intergovernmental Panel on Climate Change, 1995, Climate Change 1995: The IPCC Second Assessment Report, on the web at

2 Regan, T.L., 1997, "Understanding Climatic Change", The Trenton Times, page 1.

3 Caffrey, M., 1997, "Greenhouse effect "godfather" retires", Princeton Weekly Bulletin, 87(6).

4 National Academy of Sciences, National Academy of Engineering, Institute of Medicine, 1991, Policy Implications of Greenhouse Warming, 127p.

5 Quayle, R. and T. Karl, 1996, "The state of the climate - 1996", Earth System Monitor, March.

6 Houghton, John, 1994, Global Warming, The Complete Briefing, Lion Publishing, 192p.

7 Houghton, J.T., Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment.

8 Keeling, C.D., 1997, Atmospheric CO2 Concentrations - Mauna Loa Observatory, Hawaii, 1958 - 1995, Scripps Institution of Oceanography, on the web at; references listed as:

Keeling, C.D., J.F.S. Chine, and T.P. Whorf (1996), Increased activity of northern vegetation inferred from atmospheric CO2 measurements, Nature, Vol. 382, p. 146-149.

Keeling, C.D., et. al., Aspects of Climate Variability in the Pacific and the Western Americas (ed. Peterson, D. H.) 165-236 (Geophys. Monogr. 55, Am. Geophys. Union, Washington, D.C., 1989).

Keeling, C.D., T.P. Whorf, M. Wahlen, and J. van der Plicht (1995), Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980, Nature, Vol. 375, p. 666-670.

9 Weiss, R., 1998, personal communication.

10 U.S. Department of Energy, 1997, Emissions of Greenhouse Gases in the United States 1996, DOE/EIA-0573(96);

11 Union of Concerned Scientist, 1997, "Energy and Global Warming", website article,

12 Global Change Research Information Office, 1997, "Energy Demands Actions", on the web at

13 Union of Concerned Scientist, 1997, "Cars & Trucks", website article,

14 Ausubel, J.H., D.G. Victor and I.K. Wernik, 1995, "The Environment Since 1970", Consequences, 1(3).

15 The Worldwatch Institute uses data from British Petroleum and the Oak Ridge National Laboratory to estimate carbon emissions from fossil fuel burning during 1994. Table found on the Union of Concerned Scientists website,

16 U.S. Bureau of the Census, International Data Base, on the web at

17 Marquette, C. and R. Bilsborrow, 1997, "lation and Environment Relationships in Developing Countries: A Select Review of Approaches and Methods", The Population, Environment, Security, Equation, editors: B. Baudot and W. Moomaw, New York: Macmillian.

18 Bongaarts, J., 1992, "Population Growth and Global Warming", Population and Development Review, 18(2), 299-319.

19 George C. Marshall Institute, 1996, "Are Human Activities Causing Global Warming?", website article,

20 Rosenwieg, C. and D. Hillel, 1995, "Potential Imapcts of Cliamte Chnage on Agriculture and Food Supply", Consequesneces, 1(2), 22-32.

21 Fletcher, S.R., 1996, "International Environment: Current Major Global Treaties", Congressional Research Service, 96-884 ENR.

a This projection is based on the mid-range Intergovernmental Panel on Climate Change emission scenario, asuming "best estimate" values of the variable.

b It should also be noted that the concentration of water vapor, considered to be the most important greenhouse gas in the atmosphere, has increased probably indirectly due to increased greenhouse gases.

c The reservoirs in the Earth (i.e., soils), oceans, atmosphere and the biomass (i.e. humans) where carbon is stored and the transfer of carbon between these reservoirs is known as the carbon cycle.

d giga= 109

e The author notes that, "The specific contribution of population growth is calculated here as the proportional reduction in the average annual CO2 emission growth rate that would occur if population size is kept constant after 1985 and if the projected future trend in the per capita emission rate remains unaffected. It should be emphasized that the resulting estimates of the contribution of population are rather crude and should be used with caution because the assumption of independence between population growth and per capita CO2 production is at best a rough approximation of reality."

f This projection is based on the range of intergovernmental Panel on Climate Change emission scenarios.

g Biomass covers domestic, industrial and agricultural dry waste material, wet waste material and crops, all of which can be used as fuel to power electricity generators and some of which are appropriate to use in the manufacture of liquid or gaseous fuels. 5

h Net benefit is a cost less than or equal to zero; low cost is $1 to $9 per ton of CO2 equivalent; low to moderate cost is $10 to $99 per ton of CO2 equivalent. 3

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