Case study : Global Warming

This case study on the potential damage from global warming consists of the following parts:

1. This page, which is a summary of the scenarios for the effects of global warming and a short discussion on how we should try to choose between the scenarios.

2. We will examine global warming and the alternate scenarios using two different views; the systems view and the games view. You have already used these two views previously. In the systems view, we will focus on comparing positive and negative feedback controls and in the games view we evaluate different scenarios in a game against nature.

3.A link to a website that describes the problem and costs

EPA Global Warming

4. A like to a website that describes abrupt climate change



Feedback Cycles in Global Warming

One of the causes of global warming, or more generally, global climate change is increased atmospheric CO2 that comes from anthropogenic sources. Human activity is increasing the release of CO2 into the atmosphere by burning fossil fuels, burning forests, deforestation and destruction of the soil, along with other activities. This pulse of CO2 into the atmosphere is a perturbation and the earth system will respond with some changes. Our focus is to attempt to identify important responses and determine whether these responses will counter the increase in CO2 or temperature, or whether the response will exacerbate the change.

In a systems view of this system, we are looking for feedback cycles that are either positive or negative (Figure 1). A negative feedback cycle will resist change with compensatory flows in other parts of the system. Conversely, a positive feedback will accelerate the rate of change.


Figure 1. Several possible feedback cycles for global warming. The
details are discussed in the text below.

  • There is a negative feedback cycle involving CO2, temperature and algae.
    • increased CO2 causes surface temperature to rise
    • which leads to increased algae growth rates in the ocean,
    • which depletes atmospheric CO2
    • thus countering the rise in atmospheric CO2.
  • There is a positive feedback cycle involving air temperature, CO2 and soil organisms.
    • increased CO2 causes surface temperature to rise
    • increased temperature causes soil organisms to respire faster
    • faster respiration converts more soil organics to CO2
    • thus accelerating the cycle of CO2 input.
  • There is another positive feedback involving surface albedo of glaciers and temperature.
    • increased temperature causes glaciers to melt
    • the loss of reflective surface of the glacier leads to more absorption of sunlight
    • more absorption leads to higher temperatures
    • thus accelerating the melting and temperature rise

It is crucial that we understand these cycles and the potential interaction between these cycles.The negative feedback cycles will lead to controlling or minimizing temperature gain, whereas positive feedback processes will contribute to acceleration of the problem. If we are very lucky, there may be very strong negative feedback controls that will buffer human impact. If we are less lucky, a slight anthropogenic change may trigger a set or processes that will cause a shift in the processes that control surface temperature. In terms of resilience; if the overall global system is very resilient, human perturbation may be quickly fixed, on the other hand, once we cross a threshold (exceed the resilience) there may be a dramatic and essentially irreversible shift in the fundamental processes of the system.


Systems View Simulations of the Possible Scenarios

One aspect of the systems view that is very useful is the construction of simulation models that will predict what will happen if the current processes continue at the same rate. A simple simulation is the projection of when oil or natural gas reserves will be depleted if they are consumed at the same rate as they are now compared to if there is the same rate of growth of the consumption. This is a very enlightening comparison that shows that a resource that might last 200 years at the current rate of consumption (barrels of oil per year for example) might only last 50 years if we project that the rate of growth of consumption continues at 5% per year.

We have seen other examples of this type of modeling when we studied population growth models. Pure exponential growth occurs if we assume that the growth rate remains unchanged, whereas the "logistic" equation is on example of the pattern of growth that factors in reduced growth rate as resources are depleted. Variations on the "logistic" model include boom and bust cycles or irruptive growth.

Similar, but more involved, simulations can be constructed for human population growth, energy resource depletion, pollution, and quality of life indicators. Donella Meadows and colleagues (Meadows et al. 1992) have created very large models that project future scenarios based on current consumption and growth rates and slight variations in those assumptions. They have used these to explore possible future scenarios and examine characteristics of systems that lead to global collapse compared to the characteristics of systems that lead to sustainable societies. Figure 2 presents a cartoon of one of their comparison.

Figure 2a. In this scenario the initial resources are lower which leads to a moderate rise in population. As resources are depleted, industrial output only creates a low level of pollution which is eventually reduced. The population goes through a minor correction as expected from the industrial transition.

Figure 2b. In this simulation, higher initial resources lead to more rapid population growth and the level of pollution reaches as level that is high enough to degrade natural resources, including food production. The drop in resources and pollution lead to a major correction, i.e. bust, in the population.

Both of these scenarios are equally probably but one is much more desirable. The simulation shows illustrates the importance of containing a potentially positive feedback between increased population leading to increased pollution which destroys food production capacity and leads to an overshoot in population and a crash. Although we may see a population crash as a "natural" correction in human population, the causes and circumstances (environmental degradation and starvation) would probably be considered very undesirable future for most people.


Choosing Between Scenarios

Each scenario represents a set of initial conditions and response parameters that are theoretically under human social control. The choice of strategy can be portrayed as a "game against nature", where each strategy that you choose has different outcomes depending on uncertain natural events. Figure 3 was presented earlier in the "games view" as a game against nature.

  No tornado Tornado comes right down your street
You - spend money to prepare for a bad tornado You "wasted" your money You suffered only minor damage and lived through the storm
You - spend the money on a new TV You didn't waste your money and you have a cool TV in front of your lounger Your house is wrecked and it isn't the same watching your TV from a folding chair

Figure 3. A simple game against nature.

In the present case study, you choices are more sophisticated strategies for managing natural resources and reducing pollution impact, and the natural uncertainty has to do with the global biogeochemical cycle response to resources, pollution and human population. For the purposes of this case study, the pollution is the general CO2 increase in the atmosphere caused by increased energy use and poor land use management. The choices might be represented by the game against nature shown in Figure 4.

  CO2 pollution is countered by healthy oceans CO2 pollution causes a downward spiral in natural critical natural resources
You - spend money to reduce CO2 output You "wasted" your money You suffered only minor damage and lived through the worst of global degradation
You - spend the money increasing industrial growth You didn't waste your money and you have a bigger economy with more TVs to sell. Your population crashes, your economy is wrecked, Hollywood is underwater and there's nothing good on TV.

Figure 4. The "game against nature" modified to show how dealing with CO2 pollution might be the best strategy. Many people would rationally choose the strategy that results in the least bad outcome, the "maximin" solution.




Meadows et al. 1992


John Rueter
July 22, 2003