Climate Change

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We wish to Learn

  • What is the global climate system?
  • What is the evidence of modern climate change?
  • What is the role of greenhouse gases?
  • What is the political response to recent evidence?

The Climate System

Compendium of Trace Gases

In the past twenty years, it has become increasingly evident that certain trace gases play a major role in determining the climate system - far in excess of what might be thought based on their small numbers. Carbon Dioxide is perhaps the principal culprit for potential global warming, but it is by no means the only one. 

The above shows the relative contribution to tropospheric warming due to the greenhouse effect of various gases. This plot, taken from model calculations, contains two surprises. Firstly, the Chloro-fluorocarbons (CFC's) taken as a whole (there are several members of this family of gases) represent the second most important gas for global warming - even though their concentrations are measured in the parts per trillion, as opposed to parts per billion for carbon dioxide and methane. The CFC's are entirely of anthropogenic origin. 

Secondly, we see that both ozone and nitrous oxide (N2O or "laughing gas") are significant greenhouse gases. In fact, most gases that are made up of three or more atoms are effective greenhouse gases. This is because they have the ability to absorb and emit infra-red radiation  via processes of rotational and vibrational excitation (think, for example, of the three atoms making up CO2 as being connected by springs - infra red light is emitted and absorbed in association with the jiggling and spinning of the springed molecule). 

For a full study of the issues relating to Global Change, therefore, we need to account quantitatively for the sources and sinks of all these greenhouse gases, incorporating a discussion of the extent to which their presence in the atmosphere can be attributed to human activities and a projection of their future abundances. 

Tables 1 and 2 provide more detailed summaries of some of the attributes of important trace gases that are found in the Earth's atmosphere. 

Table 1 lists the major anthropogenic sources for each trace gas, as well as the mean residence time and the projected change in abundance with time. The last column of Table 1 provides an estimate for the projected concentration of the gas in the year 2030 in parts per billion (ppb), based on a conservative assumption for future global industrial development. 

Table 1. Compendium of Trace Gases in the Atmosphere



Total Emmision
/yr(M of Tons)




YEAR 2030 (PPB)


Fossil-Fuel Combustion,
Biomass Burning



?, N. Hem.
40-80, S. Hem.
(Clean Atmospheres)

100-200, N. Hem.
40-80, S. Hem.
(Clean Atmospheres)

Probably increasing


Fossil-Fuel Combustion, Deforestation


100 Years





Rice Fields, Cattle, Landfills,
-Fuel Production


10 Years





Fossil-Fuel Combustion, 
Biomass Burning



.001 to ?
(Clean to Industrial)

(Clean to Industrial)

(Clean to Industrial)


Fertilizers, Deforestation, 
Biomass Burning


170 Years





Fossil-Fuel Combustion, Ore Smelting


Days to Weeks

.03 to ?
(Clean to Industrial)

(Clean to Industrial)

(Clean to Industrial)


Aerosol Sprays, Refrigerants,


60-100 Years


About 3
(Chlorine atoms)

(Chlorine atoms)

Table 2 provides information on the two principal concerns we always have when discussing a trace gas, namely: 

    1.what is its the "Greenhouse Potential" (GP)? 
    2.what is its the Ozone Depletion Potential (ODP)? 

Table 2. Ozone Depletion Potential and Greenhouse Potential for various gases

Trace Gas


Primary Source

Average Life in Atmosphere (Years)





Refrigerant/AC, Plastic Foams, Aerosols






Refrigerant/AC, Plastic Foams, Sterilants










Halon 1211


Fire Extinguishers




Halon 1301


Fire Extinguishers




Carbon Tetrachloride


Industrial Processes




Methyl Chloroform


Industrial and Natural Processes




Nitrous Oxide


Fossil Fuels






Biogenic Activity, Fossil Fuels




Carbon Dioxide


Fossil Fuels




Carbon Monoxide


Motor Vehicles




* ozone depletion potential (CFC-11 = 1.0)
** greenhouse potential (CFC-12 = 1.0)

For convenience, both GP and ODP are measured on a per molecule basis, using as reference the potentials of specific CFC molecules. Thus, for example, we see from Table 2 that a molecule of methane has only 0.001 times the effectiveness of a molecule of CFC-12 for greenhouse warming. Similarly, we see that Carbon Dioxide is not a particularly effective greenhouse gas on a per molecule basis (GP = 0.00005), but since it is much more abundant than the others, it still comes out on top (see Figure 1).

In addition to these gases, clouds and aerosols also play important roles in radiative forcing.  A discussion of the physical processes involved in the adsorption and/or scattering of radiation by aerosols and clouds can be found in the relevant GC1 lecture.

Additional information about the Earth’s natural greenhouse and the role played by anthropogenic emissions can be found at the relevant GC1 web page.


  • Climate Change 2001: The Scientific Basis; IPCC 2001
  • Climate Change 2001: Synthesis Report; IPCC 2001
  • NRC, Improving the Effectiveness of U.S. Climate Modeling, 2001
    J. T. Houghton et al., eds. Climate Change 1995: The Science of Climate Change, published for the IPCC, in collaboration with WMO and UNEP

Modeling the Climate System

We wish to learn:

  • How are climate models built and applied?

  • What are the types and sources of uncertainty?

  • How credible are the projections?

  • What are the driving forces of greenhouse gas emissions?

  • What are scenarios? Why use them?

Climate models

Major components of the climate system must be represented in sub-models (atmosphere, ocean, land surface, cryosphere and biosphere), along with the processes that go on within and between them.  General circulation models (GCMs), such as Atmosphere GCMs and Ocean GCMs, include equations that describe the large-scale evolution of momentum, heat and moisture. An important consideration for these models is their resolution, which represents their accuracy.  An atmosphere GCM has a resolution of approximately 250 km in the horizontal direction, and an ocean GCM of about 125 - 250 km in the horizontal and about 200 to 400 m in the vertical. 

Many physical processes (e.g., related to clouds or ocean convection) take place on much smaller spatial scales than the model grid and therefore cannot be modeled and resolved explicitly. Their average effects are approximately included in a simple way by taking advantage of physically based relationships with the larger-scale variables. This technique is known as parameterization.

Quantitative projections of future climate change require models that simulate all the important processes governing the future evolution of the climate atmosphere, land, ocean and sea ice developed separately and then gradually integrated considerable computing power to run comprehensive "AOGCMs." Simpler models (e.g., coarser resolution and simplified dynamics and physical processes) are widely used to explore different scenarios of emissions of greenhouse gases to assess the effects of assumptions or approximations in model parameters

Together, simple, intermediate, and comprehensive models form a “hierarchy of climate models”, all of which are necessary to explore choices made in parameterizations and assess the robustness of climate changes. 

Source: Climate Change 2001: The Scientific Basis; IPCC 2001

One common test of model accuracy is their ability to predict past temperatures.  The following graphs show comparisons of actual temperature data and GCM model predictions of what those temperatures should have been.  One main criticism of this approach is that the models themselves were built using past data, and relationships between the variables under consideration could change in the future.


 There are different types of possible uncertainty in climate models. First, there is uncertainty in the quantities used as inputs.  The different values are obtained from experiments conducted by different experts, who may disagree about the results.  It is difficult to resolve these issues because conflicting data can come from equally well-designed experiments.  There is also uncertainty about model structure, or how the data inputs are combined together to form a complete picture.  The following bullets summarize primary sources of uncertainty:

  • Data uncertainties arise from the quality or appropriateness of the data used as inputs to models.

  • Modeling uncertainties arise from an incomplete understanding of the modeled phenomena, or from approximations that are used in formal representation of the processes.

  • Completeness uncertainties refer to all omissions due to lack of knowledge. They are, in principle, non-quantifiable and irreducible.

Credibility of Projections

Assessment of the credibility of GCM projections of climate change indicates that there are a number of processes and feedbacks requiring sustained research.  These include cloud-radiation-water vapor interactions, ocean circulation and overturning, aerosol forcing, decadal to centennial variability, land-surface processes, short-term variability affecting the frequency and intensity of extreme and high impact events (e.g., monsoons, hurricanes, storm systems), interactions between chemistry and climate change and improved representations of atmospheric chemical interactions within climate models.  The image below is a diagram of our current level of understanding of these interactions, which is given to illustrate the complexity of interactions which must be taken into consideration.

Source: Global Environmental Change:Research Pathways for the Next Decade; NRC 1999

Greenhouse Gas (GHG) Emissions

The primary driving forces of greenhouse gas emissions are demographic change, social and economic development, and the rate and direction of technological change.

Emissions Scenarios

Scenarios of emissions are neither predictions nor forecasts; they are alternative images of how the future might unfold.  These are used as tools with which to analyze how driving forces may influence future emission outcomes and to assess the associated uncertainties.

Scenarios are given descriptive names and have different part to them.  The storyline is a coherent narrative which describes a particular demographic, social, economic, technological, environmental, and policy future.  All interpretations and quantifications of one storyline together are called a scenario family.  Each scenario family includes a storyline and a number of alternative interpretations and quantifications of each storyline to explore variations of global and regional developments and their implications for greenhouse gas and sulfur emissions.  Storylines were formulated in a process that identified driving forces, key uncertainties, possible scenario families, and their logic.

Source: IPCC Special Report on Emissions Scenarios

Scenarios also have uncertainties associated with them. The sources of these uncertainties include the choice of storylines and the authors' interpretation of those storylines.  Another important source of uncertainty is the translation of the understanding of linkages between driving forces into quantitative inputs for scenario analysis.  Furthermore, there are differences in methodology, sources of data, and inherent uncertainties which result from assumptions made to simplify models.  Now let us look at the actual scenarios in detail:

  • A1 Storyline and Scenario Family 

These assume a world with very rapid economic growth, low population growth, and rapid introduction of new and more efficient technologies.  The major underlying themes are convergence among regions, capacity building, increased cultural and social interactions, and substantial reduction in regional differences in per capita income.

Alternative directions of technological change are represented as A1FI (high coal, oil and gas), A1B (balanced, or even distribution among options) and A1T (predominantly non-fossil fuel).

  • A2 Storyline and Scenario Family

A highly heterogeneous world with: high population growth (due to slow convergence of fertility patterns across regions), regionally-oriented economic development, per capita economic growth, and more fragmented/slower technological change. The major underlying themes are self reliance preservation of local identities.

  • B1 Storyline and Scenario Family

A convergent world with: rapid changes in economic structures toward a service and information economy, low population growth (same as A1), reductions in material intensity, and introduction of clean and resource-efficient technologies. Major Underlying Themes are the global solutions to economic, social, and environmental sustainability (without additional climate initiatives), as well as improved equity.

  • B2 Storyline and Scenario Family

A world with: intermediate levels of economic development, moderate population growth, reductions in material intensity, and less rapid and more diverse technological change. Major Underlying Themes are local solutions to economic, social, and environmental sustainability (without additional climate initiatives), social equity (with local/regional focus), and environmental protection (with local/regional focus).

The following chart is a summary of the qualitative directions of these scenarios for different indicators:

Source: Climate Change 2001 Mitigation Technical Summary

Now compare the chart above to the next set of graphs, which contain predictions for the global climate based on the above assumptions:

Anthropogenic emissions of Greenhouse Gases Under Different Scenarios


Source: Working Group I Summary for Policy Makers; IPCC 2001



The Potential 
Impacts of Climate Change 
on the United States

We wish to learn:

  • In what ways is the United States vulnerable to climate variability and change?
  • What steps can be taken to contribute to sustainable solutions to the greenhouse problem?

The Global Change Research Act of 1990 [Public Law 101-606] highlighted early scientific findings that human activities were starting to change the global climate.  It found that:

“(1) Industrial, agricultural, and other human activities, coupled with an expanding world population, are contributing to processes of global change that may significantly alter the Earth habitat within a few generations; 

(2) Such human-induced changes, in conjunction with natural fluctuations, may lead to significant global warming and thus alter world climate patterns and increase global sea levels. Over the next century, these consequences could adversely affect world agricultural and marine production, coastal habitability, biological diversity, human health, and global economic and social well-being.” 

Congress established the US Global Change Research Program (USGCRP) to address these issues, and mandated that the USGCRP: 

“ shall prepare and submit to the President and the Congress an assessment which 
integrates, evaluates, and interprets the findings of the Program and discusses the scientific uncertainties associated with such findings; 

analyzes the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity; and analyzes current trends in global change, both human-induced and natural, and projects major trends for the subsequent 25 to100 years.

A National Assessment Synthesis Team (NAST), comprised of government, academic, industry, and non-government organization experts, was formed, and NAST, in collaboration with the Federal agencies that make up the USGCRP, completed an assessment of the potential consequences of climate variability and change on the United States.  The results of the assessment were provided in the form of two reports, a Foundation Report, and an Assessment Overview.  Cambridge University Press published both reports in 2000.

The focus of NAST, in this first assessment for the United States, was on geographic regions (Northeast, Southeast, Midwest, Great Plains, West, Pacific Northwest, Alaska, and the Islands of the Caribbean and the Pacific) and on sectors (water, agriculture, forests, coastal areas and marine resources, and human health).  The assessment team also attempted to identify potential adaptation measures, although there were insufficient resources to complete an evaluation of their costs, practicality or effectiveness.  The NAST used state of the science climate models to generate a variety of climate change scenarios (plausible alternative futures) and hydrologic, ecological and socioeconomic system models to assess responses to the different scenarios.  It is common to vary such parameters as population and economic growth, and technological development.  They also used historical climate records to assess regional and sector sensitivity to climate variability and extremes and to learn about how adaptation occurred in the past.  They also completed a number of studies to determine the extent to which the climate would have to change in order for major regional and sector impacts to occur (e.g., the extent to which temperature would have to increase to cause a negative effect on soybeans in the South). 


The NAST used two state-of-the-science climate models from the Hadley Centre in the United Kingdom and the Canadian Centre for Climate Modeling and Analysis and data from historical observations to generate a variety of climate scenarios.  Both assume mid-range emissions and no major changes in policies to limit the emissions of greenhouse gases.  They also assume that, by the year 2100: world population will increase to about 11 billion people; the global economy will continue to grow at about the current average rate, which translates to an increase of more than a factor of 10; increased use of fossil fuels will result in an atmospheric CO2 level of just over 700 ppmv and increased SO2 emissions; and total energy produced each year from non-fossil sources will increase more than 10 times, to provide > 40% of the world’s energy needs.  The NAST emissions projections fall in the middle of other Intergovernmental Panel for Climate Change (IPCC) emissions scenarios. 

Sector Overview

The NAST decided to focus on five issues of national importance:  agriculture, water, human health, coastal areas and marine resources, and forests.  The key issues identified for the agriculture sector are crop yield changes and associated economic consequences, changing water demands for irrigation, surface water quality, increasing pesticide use, and climate variability. The key issues identified for the water sector are competition for water supplies, surface water quantity and quality, groundwater quantity and quality, floods, droughts, and extreme precipitation events, and ecosystem vulnerabilities. The key issues identified for the health sector are temperature-related illnesses and deaths, health effects related to extreme weather events, air pollution-related health effects, water- and food-borne diseases, and insect-, tick-, and rodent-borne diseases. The key issues identified for the coastal areas and marine resources sector are shoreline erosion and human communities, threats to estuarine health, coastal wetland survival, coral reef die-offs, and stresses on marine fisheries. The key issues identified for the forest sector are effects on forest productivity, natural disturbances such as fire and drought, biodiversity changes, and socioeconomic impacts.

The assessment concludes that:

Agriculture:  “Overall productivity of American agriculture will likely remain high, and is projected to increase throughout the 21st century, with northern regions faring better than southern ones.  Though agriculture is highly dependent on climate, it is also highly adaptive.  Weather extremes, pests, and weeds will likely present challenges in a changing climate.  Falling commodity prices and competitive pressures are likely to stress farmers and rural communities.”

Water:  “Rising temperatures and greater precipitation are likely to lead to more evaporation and greater swings between wet and dry conditions.  Changes in the amount and timing of rain, snow, runoff, and soil moisture are very likely.  Water management, including pricing and allocation, will very likely be important in determining many impacts.”

Human Health:  “Heat-related illnesses and deaths, air pollution, injuries and deaths from extreme weather events, and diseases carried by water, food, insects, ticks, and rodents have all been raised as concerns for the US in a warmer world.  Modern public health efforts will be important in identifying and adapting to these potential impacts.”

Coastal Areas and Marine Resources:  “Coastal wetlands and shorelines are vulnerable to sea-level rise and storm surges, especially when climate impacts are combined with the growing stresses of increasing human population and development.  It is likely that coastal communities will be increasingly affected by extreme events.  The negative impacts on natural ecosystems are very likely to increase.”

Forests:  “Rising CO2 concentrations and modest warming are likely to increase forest productivity in many regions.  With larger increases in temperature, increased drought is likely to reduce forest productivity in some regions, notably in the Southeast and Northwest.  Climate change is likely to cause shifts in species ranges as well as large changes in disturbances such a fire and pests”


1. Increased warming
Assuming continued growth in world greenhouse gas emissions, the primary climate models used in this Assessment project that temperatures in the US will rise 5-9ºF (3-5ºC) on average in the next 100 years..  A wider range of outcomes is possible.

2. Differing regional impacts
Climate change will vary widely across the US.   Temperature increases will vary somewhat from one region to the next.  Heavy and extreme precipitation events are likely to become more frequent, yet some regions will get drier. The potential impacts of climate change will also vary widely across the nation.

3. Vulnerable ecosystems
Many ecosystems are highly vulnerable to the projected rate and magnitude of climate change. A few, such as alpine meadows in the Rocky Mountains and some barrier islands, are likely to disappear entirely in some areas. Others, such as forests of the Southeast, are likely to experience major species shifts or break up into a mosaic of grasslands, woodlands, and forests. The goods and services lost through the disappearance or fragmentation of certain ecosystems are likely to be costly or impossible to replace.

4. Widespread water concerns
Water is an issue in every region, but the nature of the vulnerabilities varies. Drought is an important concern in every region. Floods and water quality are concerns in many regions. Snowpack changes are especially important in the West, Pacific Northwest, and Alaska.

5. Secure food supply
At the national level, the agriculture sector is likely to be able to adapt to climate change. Overall, US crop productivity is very likely to increase over the next few decades, but the gains will not be uniform across the nation. Falling prices and competitive pressures are very likely to stress some farmers, while benefiting consumers.

6. Near-term increase in forest growth
Forest productivity is likely to increase over the next several decades in some areas as trees respond to higher carbon dioxide levels. Over the longer term, changes in larger-scale processes such as fire, insects, droughts, and disease will possibly decrease forest productivity. In addition, climate change is likely to cause long-term shifts in forest species, such as sugar maples moving north out of the US.

7. Increased damage in coastal and permafrost areas
Climate change and the resulting rise in sea level are likely to exacerbate threats to buildings, roads, powerlines, and other infrastructure in climatically sensitive places. For example, infrastructure damage is related to permafrost melting in Alaska, and to sea-level rise and storm surge in low-lying coastal areas.

8. Adaptation determines health outcomes
A range of negative health impacts is possible from climate change, but adaptation is likely to help protect much of the US population. Maintaining our nation's public health and community infrastructure, from water treatment systems to emergency shelters, will be important for minimizing the impacts of water-borne diseases, heat stress, air  pollution, extreme weather events, and diseases transmitted by insects, ticks, and rodents.

9. Other stresses magnified by climate change
Climate change will very likely magnify the cumulative impacts of other stresses, such as air and water pollution and habitat destruction due to human development patterns. For some systems, such as coral reefs, the combined effects of climate change and other stresses are very likely to exceed a critical threshold, bringing large, possibly irreversible impacts.

10. Uncertainties remain and surprises are expected
Significant uncertainties remain in the science underlying regional climate changes and their impacts. Further research would improve understanding and our ability to project societal and ecosystem impacts, and provide the public with additional useful information about options for adaptation. However, it is likely that some aspects and impacts of climate change will be totally unanticipated as complex systems respond to ongoing climate change in unforeseeable ways.

Source:  Climate Change Impacts on the United States, NAST Overview, 2000.

Several issues that are shared by a number of geographic regions were identified.  Concerns regarding water are widespread and provide an excellent example of the need for and importance of the adoption of adaptive strategies in the area of water resources.

The following table addresses concerns regarding flooding, drought, loss of snowpack, groundwater quantity and quality, freshwater resources and water quality.

Water Issues in the United States






Lake, river, and reservoir levels 























Great Plains

































Source:  Climate Change Impacts on the United States:  The Potential Consequences of Climate Variability and Change, National Assessment Overview, Cambridge University Press, 2000.

As well, a many of the US’s ecosystems face potentially disruptive climate changes.  Concerns regarding forests, grasslands, semi-arid and arid regions, tundra, freshwater ecosystems, and coastal and marine ecosystems are shared by a number of regions as shown in the following table.

Ecosystem Type











Changes in tree species composition and alteration of animal habitat 










Displacement of forests by open woodlands and grasslands under a warmer climate in which soils are drier










Displacement of grasslands by open woodlands and forests under a wetter climate










Increase in success of non-native invasive plant species









Semi-arid and Arid

Increase in woody species and loss of desert species under wetter climate 










Loss of alpine meadows as their species are displaced by lower-elevation species










Loss of northern tundra as trees migrate poleward










Changes in plant community composition and alteration of animal habitat










Loss of prairie potholes with more frequent drought conditions












Habitat changes in rivers and lakes as amount and timing of runoff changes and water temperatures rise









Coastal & Marine

Loss of coastal wetlands as sea level rises and coastal development prevents landward migration










Loss of barrier islands as sea-level rise prevents landward migration










Changes in quantity and quality of freshwater delivery to estuaries and bays alter plant and animal habitat 










Loss of coral reefs as water temperature increases










Changes in ice location and duration alter marine mammal habitat









Source:  Climate Change Impacts on the United States:  The Potential Consequences of Climate Variability and Change, National Assessment Overview, Cambridge University Press, 2000.

Although an evaluation of the practicality or feasibility of adaptation strategies was not the focus of this assessment, the NAST did provide the following recommendations: 

  1. Develop a more integrated approach to examining impacts and vulnerabilities to multiple stresses; 

  2. Develop new ways to assess the significance of global change to people; 

  3. Improve projections of how ecosystems will respond; 

  4. Enchance knowledge of how societal and economic systems will respond to a changing climate and environment;

  5. Refine our ability to project how climate will change; 

  6. Extend capabilities for providing climate information.  They also defined a number of areas that could provide needed information in the near term, as listed in the box below.

Areas with High Potential for Providing Needed Information in the Near-Term

  • Expand the national capability to develop integrated, regional approaches of assessing the impacts of multiple stresses, perhaps beginning with several case studies.

  • Develop capability to perform large-scale (over an acre) whole-ecosystem experiments that vary both CO2 and climate.

  • Incorporate representations of actual land cover and land use into models of ecosystem responses.

  • Identify potential adaptation options and develop information about their costs, efficacy, side effects, practicality, and implementation.

  • Develop better ways to assign values to possible future changes in resources and ecosystems, especially for large changes and for processes and service that do not produce marketable goods.

  • Improve climate projections by providing dedicated computer capability for conducting ensemble climate simulations for multiple emission scenarios.

  • Focus additional attention on research and analysis of the potential for future changes in severe weather, extreme events, and seasonal to interannual variability.

  • Improve long-term data sets of the regional patterns and timing of past changes in climate across the US, and make these data-sets more accessable.

  • Develop a set of baseline indicators and measures of environmental conditions that can be used to track the effects of changes in climate.

  • Develop additional methods for representing, analyzing, and reporting scientific uncertainties related to global change.

Source:  Climate Change Impacts on the United States, Assessment Overview, 2000.

Climate Change Self Test

Climate Change Self Test 2

Climate Change Self Test 3

Climate Change Self Test 4

Suggested Readings:

  • Climate Change Impacts on the United States:  the Potential Consequences of Climate Variability and Change, Overview, 2000.

  • Preparing for a Changing Climate: the Potential Consequences of Climate Variability and Change, Great Lakes Overview, 2000.

All materials © by the University of Michigan unless noted otherwise.