The T&P system can be used as a tool for concrete learning because of the metaphorical mapping between the model and the real-world system. Participants can manipulate the T&P system (the conceptual source domain with the water, tubs and pumps), and then make valid inferences in, and construct an understanding of, the target domain (the carbon cycle). The explicit mapping of the concepts in the conceptual source domain to the conceptual target domain is shown in Table 4.3.
Table 4.3: Conceptual mapping of the T&P metaphor. The components in the conceptual source domain are mapped to the conceptual target domain, allowing participants to construct an understanding of one system (the carbon cycle) in terms of another (tubs and pumps). Note: * the arrows “ ” represent the expressions “corresponds to” or “maps to”.
Conceptual source domain: tubs & pumps
Conceptual target domain: the carbon cycle
The water in each tub * The accumulation of carbon at time t
The amount of water in the atmospheric tub
The amount of carbon accumulated at time t in the atmosphere; the atmospheric carbon concentration The amount of water in the
terrestrial tub
The amount of carbon accumulated at time t in terrestrial stocks, including oceanic and geological stocks.
Water entering the tub through the ‘natural additions’ pump
The natural processes that increase the amount of carbon in the atmosphere, such as decomposition of organic matter
Water entering the tub through the ‘anthropogenic additions’ pump
The processes that increase the amount of carbon in the atmosphere due to human activity, such as the burning of fossil fuels
Water leaving the tub through the
‘natural removals’ pump
The natural processes that decrease the amount of carbon
in the atmosphere, such as afforestation
As described in §3.2.3, the mapping between the conceptual source and target domains is partial. For participants, this partial mapping may lead to inferences that become areas of confusion. Potential areas of confusion are described in Table 4.4. I do not necessarily believe that the issues listed in Table 4.4 are problems to be fixed in future versions of the activity, but rather are listed for the purpose of further describing areas of potential confusion.
The simplification of the carbon cycle in this representation does, however, make it easier to see the importance of the balance between the key flows of the carbon cycle, which is an important goal of the activity. The model as presented here is not designed to be a simplified representation of the complex carbon system, but rather to demonstrate the effect of unbalanced carbon flows.
Table 4.4: Areas of potential confusion in the conceptual mapping due to both physical limitations and conceptual mapping. Efforts to reduce the effect of these limitations have been made to reduce the opportunity for confusion.
Concept Description
Physical limitations of the T&P system
Size of stocks The terrestrial carbon stock is many orders of magnitude larger than the
atmospheric stock. In the T&P system, it is only marginally larger
Feedback Feedback effects and mechanisms from the conceptual source domain are not
present, such as the effect of temperature on carbon storage in the ocean.
There is, however, feedback in the system, such as the change of participant behaviour due to the level of the water or the sound of the pumps as they are
manipulated at different speeds
Changing rates The natural carbon cycle fluctuates due to a number of subprocesses and
feedback mechanisms; the rate of anthropogenic additions continues to increase. In the T&P system, these changes are not controlled in a regulated way
Homogeneity The terrestrial carbon stock is not a homogenous entity as it is represented in
the T&P system. For example, in the real world, the oceanic stock has a
different relationship with the atmospheric stock than with the terrestrial
stock. Conceptual mapping limitations
Colour of the water The water is intentionally coloured green to aid visualisation. Some participants remarked that the green was chosen to represent greenhouse gases
Water leaking out of the system
On occasion, water leaves the system through leaks in the connections.
Upper limit of the atmospheric tub
The ‘overflow’ level of the atmospheric tub. When exceeded, water leaves the system.
Feedback in the system
The natural carbon cycle is influenced by the natural capacity for carbon flux; for example, the more trees available to remove carbon from the atmosphere will also result in higher levels of carbon additions in the future as the organic matter decomposes. There are no such built-in feedback mechanisms in the model.
Reversibility of pumps
Pumps in the T&P system can go in both directions without regard of the feedback processes. To mitigate this possibility, inflow pipes have been placed at the tops of stocks, and outflow pipes have been placed at the bottom of stocks to reduce the likelihood of this behaviour
The physical limitations of the equipment used means that it is not practical for the model to be scaled accurately. For example, a terrestrial stock that is 50 times larger in volume than the atmospheric stock, or an anthropogenic pump that is 30 times smaller than the natural pumps, is impractical (see Figure 2.1 for relative sizes). This is not a significant limitation, given that the main aim of the T&P activity is to help students understand the impact of unbalanced flows. Removing these elements and considerations from the simple model also means that less prerequisite knowledge about the relationships and behaviour of the system is required to understand the activity, and in turn the model.