Feedback Cycles in Global Warming

[excerpt from http://web.pdx.edu/~rueterj/courses/casestudies/globalwarming/]

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. 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.

Possible feedback cycles (loops) for global warming:

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 (reflectivity) 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.

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Both positive and negative feedback processes may also exist involving evaporation of water into the atmosphere:

• [positive feedback] water vapor (clouds) is a greenhouse gas so an increase concentration of water vapor increases temperatures.
• [negative feedback] water vapor may increase the number of clouds which could reflect more of the sunlight and reduce the temperature (cooling).

Below is an example of a flow diagram which represents a global warming negative feedback loop with regards to clouds.

TASK: Using a diagram, illustrate one of the global warming positive feedback cycles (loops) mentioned in the text above entitled “Feedback Cycles in Global Warming.” What main difference should you consider including in your feedback loop?

CHECK YOUR LEARNING:

What does a Steady-State System look like?

What does a Static-State look like?