Models to Scenarios



From Climate Models to Climate Change Scenarios
What is a Climate Model?

Climate models are simplified mathematical representations of the Earth’s climate system. Complex equations are solved using a 3-D grid over the globe. Climate models try to simulate or imitate the components and processes at work in an actual climate system. Teams of meteorologists, mathematicians and computer specialists have been developing climate models for forty years. Models are designed to respond to changes internal changes in the climate system as well as external changes that are based on assumptions made by the scientists using the models.

The basic model is known as the General Circulation Model or GCM. Factors from atmosphere (A), oceans and sea-ice linkages (O) are linked or coupled to the GCM. These models are then called AOGCMs. The coupled models provide a more comprehensive simulation of a climate system. The latest Canadian Global Coupled Model (CGCM2) has improved ocean-mixing limits and has added sea-ice dynamics. The CGCM2 was used to run the 2001 IPCC report as well as the ongoing Arctic Climate Impact Assessment. The more factors that are linked or coupled for consideration by the model, the closer the projections are to the natural conditions and events that affect climate.

Scientists first include the physical properties of Earth especially its oceans, ice and atmosphere with their processes, reactions, interactions and feedback loops into the mathematical model. Climate models will become even more complex to include chemical and biological processes. (See chart historical development of climate models.

Based on actual collected climate data, climate model predictions try to produce the best estimate of a future climate. Climate models are tested by using data from past climate records to predict known present observations. E.g. the model uses 1900-1920 temperatures to make projections of 1970-1990 temperatures. Such testing or validation increases the reliability of predictions made by the model.

The accuracy of the predicted results reflects the reliability the model will have. Climate models allow scientists to move from known to unknown conditions using a validated model built on solid data and tested performance.

Earth Systems are the result of 5 major components and their interactions. See Section 3.

Looking at the diagram of climate systems below, the 5 major components of earth’s systems are:

    1. atmosphere
    2. ocean
    3. land surface
    4. cryosphere (ice, snow and permafrost)
    5. biosphere (all living organisms)As of 2004, the biosphere is not really dealt with in the mathematical representations of climate systems such as the General Circulation Models (GCMs). The GCMs simulate the Earth’s climate system by tracking radiated energy, energy transfers and transformations, especially energy that drives the water cycle. The AOGMs, which couple or link data from the atmosphere, ocean and sea-ice, provide a more complete representation of the climate system.

    Climate modeling includes 3 major climatic processes:

    1. Radiative the transfer of energy radiated through the climate system
    2. (e.g. absorption and reflection etc)

    3. Dynamic the horizontal and vertical transfer of energy
    4. (eg. advection, convection, diffusion etc.)

    5. Surface Processes surfaces involving land/ocean/ice and

    surface-atmosphere energy exchange (e.g. effects of albedo, emissivity)

ACTIVITY 1 Questions

Can you find examples of the 3 climatic processes in the above Climate System diagram?
Can you find one reference to the biosphere included here?

Climate models vary in focus and degree of complexity spatially (space) and temporally (time).

Each component of the climate is first looked at separately, and then combined or coupled to increase the complexity, and thus better represent the actual climate change. As computers have become more powerful, so too has the ability to compute equations of greater complexity while trying to simulate the complexity of natural processes. See chart above for the evolution of models over the last 25 years.

There are two broad methods of model construction – equilibrium or transient models. Both include both the ocean’s liquid and the gases and vapours of the atmosphere to provide better predictions. Both models can be global or the regional (nested) models.

Around the World

The chart below is a sample of some 30 available models. Why are there so many models? Although model designs are increasing in efficiency and accuracy they cannot mimic the incredible complexity of the global climate. Thus the greater the number of perspectives or model agreement, the greater is the scientific validity and understanding. The IPCC has 6 international teams working on model systems.

Country of Origin Institute Model Name Website
Canada Canadian  Centre for  Climate  Modeling CGCM1,  CGCM2*
Australia Commonwealth  Scientific and  Industrial  Research  Organization CSIRO, Climate  Change  Research, Program  (CCRP)
United Kingdom Hadley Centre HadCM2  AOGCM,  HadCM3  AOGCM  change
United States Geophysical  Fluid Dynamics  Laboratory GFDL

*The new model shows an improved response to increased greenhouse gas projections. The CGM2 was used in 2001 by IPCC for the Arctic Climate Impact Assessment. Simulations of the climate 18, 000 years ago were used to show the last glacial maximum (LGM).

Why are climate models useful? Because they are used for Predictions, Projections and Scenarios!!


Climate models are used made by meteorologists to predict or forecast monthly, seasonal and annual climate as part of their regular operations. Predictions assume no new variables in the climate system.


Check out the weather reports or weather channels for predictions.How many days do the average predictions cover?Which day of a 5-day weather forecast is probably the most accurate forecast?


Projections are the responses of a climate system model to assumed future changes or scenarios in emissions, concentration or radiation. Climate models are useful in research because their simulations make it possible to study how the Earth’s climate system works and to make projections about the future. Assumptions regarding human impacts on sociological, economic and technological development can be made when running the model. It is understood that these assumptions may never be realized or come true. They are, however, based on evidence seen in present trends. Projections are often the raw material for climate scenarios.

CLIMATE CHANGE RESEARCH is based on understanding the present climate system in the context of understanding the earth’s energy budget and climate systems response to changes. Climate facts and their analysis for patterns and relationships allow for the development of models then scenarios.

Assumptions, also based on research, must be made in order to move from the known data to the unknown future. The steps below outline the logical development of the body of climate change research and the implications of the findings for our world.

9 step sequence from research/ assumptions to projections and understandings to taking action!

Socio-Economic Scenarios

– Emission Scenarios ->
– Concentration Scenarios->
– Radiative Forcing->
– Temperature Change->
– Sea-level Rise->
– Impacts/Reasons for Concern->
– Vulnerability/Adaptation->

SCENARIOS are reasonable descriptions of how the future may develop. Scenarios are based on assumptions about key conditions or driving forces such as prices, populations or the rate of change of technology. Scenarios are neither predictions nor forecasts- they may be even based on a storyline. Scenarios may be based on projections but they usually need to include more information about other factors.

The scenario assumptions for future human activities result in projections for different levels of emissions of greenhouse gases and aerosols. These emissions projections are used to change their projected atmospheric concentrations. Concentration projections are then used for radiative forcing which is the basis for earth’s temperature change projections. Projected changes in temperature then make it possible to project sea-level rise and to consider impacts and reasons for concern such as changes in land ecosystems and ocean’s circulation patterns.

SOCIO-ECONOMIC SCENARIOS are different social and economic assumptions made in order to make projections using the models of emissions of greenhouse gases and aerosols. The IPCC chose 4 storylines and their scenarios for our future world; all are dependent on how humankind responds to climate change. Because there are 3 versions of A1 references are also made to 6 scenarios.

This a diagram of 4 basic scenarios and 3 versions of A1:







– very rapid economic growth and introduction of new technologies, global population peaking 2050.
Subset ( A1Fl= fossil fuel intensive, A1T= non-fossil energy A1B =balanced different energy sources)
A2 – world emphasizing self-reliance and preservation of local identities and economic growth, with global population slowly increasing continuously, and slower, more fragmented technological change.
B1 – rapid change to service and information economy, cleaner, more efficient technologies, global population peaking 2050, emphasizing global solutions to sustainability, improved equity.
B2 – local solutions to economic, social, environmental sustainability, intermediate levels of economic development, less rapid technological change, global populations continuously increasing.

There is however a full set of 35 SRES scenarios used for scientific research for IPCC 2001 report.

EMISSIONS SCENARIOS describe the possible effects of the future release or emissions of substances, which could affect the energy balance of the climate system. Emissions of the major sources of disturbance to the naturally balanced atmosphere were found to be CO2, CH4, and N2O. Scenarios for the emissions of these molecules are based on assumptions made about driving forces of change such as changes in population, socio-economic development or technology.

CONCENTRATION SCENARIOS are derived from emission scenarios. Concentration scenarios are used in climate models to compute radioactive forcing which is the basis of climate change projections. The IPCC used a set of 1992 emission scenarios as a basis for their 1996 climate projections.

THE EARTH’S ENERGY BUDGET is the total energy received from the sun’s radiation. Since the only source of energy in the earth’s climate system is from the sun, there must be a global radiation balance between the sun’s incoming radiation and the earth’s outgoing radiation. The earth’s outgoing radiation includes the energy reflected from the earth and the infrared radiation from the earth’s climate

“FORCING” is the term used for simulations assuming specific disturbances or driving forces of change. E.g. Climate Models programmed for Natural Forcing to simulate changes in temperature caused by natural driving forces of change used only data from solar variations and volcanic activity. Changes driven by humans such as greenhouse gas and sulfate aerosols were the only disturbances assumed in the temperature change simulation of Anthropogenic Forcing.

RADIATIVE FORCING is a disturbance of the global radiation balance. Radiative forcing can be caused by a natural change such as ash from a volcano. Radiative forcing can be also be caused by human activities such as emissions of substances, e.g. Greenhouse gases or aerosols that affect the radiation balance. Radiative forcing results are the basis of projections of earth’s surface temperature change and hence for projections for rise in sea level.

CLIMATE SCENARIOS describe how the future may develop if the present climate system continues to function while it responds to possible human impacts. To “run a climate scenario” is to project the future effect of the assumption on a climate model. E.g. the response of a climate model to the effect of an assumption of doubled concentration of atmospheric C02. Other information needed to run this scenario could include current fossil fuel consumption per person per year or population growth for example.

CLIMATE CHANGE SCENARIO is the difference between a climate scenario and the data for current climate. Conditions or variables, such as CO2 levels, can be changed or “forced ” so the model can run a scenario for projections based on these new conditions. We can then predict how the climate may be affected in the future. Therefore, we might see how our current actions might affect our future!












Source:  Modified by Urquizo from IPCC (1990 and 1995) reports ww.utoronto/imap

Note: BAU =Business As Usual

The same graph appears below in different presentation for analysis and ready for publication.

Source:  Modified by Urquizo from IPCC (1990 and 1995) reports ww.utoronto/imap

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