We wish to Learn
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
Table 2 provides information on the two principal concerns we always have when discussing a trace gas, namely:
1.what is its the "Greenhouse
Table 2. Ozone Depletion Potential and Greenhouse Potential for various gases
* ozone depletion
potential (CFC-11 = 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.
We wish to learn:
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:
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.
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:
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
We wish to learn:
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:
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.
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:
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
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.
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: