System Dynamics and Climate Change

Posted on October 17, 2013

6


Father Theo goes to climate school 4.

Continued from part 1, part 2 and part 3.

Today we talk about stocks, flows and feedbacks.  These are terms used in systems dynamics, which in its details can be complicated stuff but like many complicated things is built around a number of quite common-sense ideas.  In an earlier post, I used water flowing in and out of a bathtub to help explain climate change.  We can apply systems dynamics to that example to start with.

Flow

Water flows in from the tap and out through the drain.  This represents the idea of flow.  Stuff coming in, stuff going out.

This idea can be directly applied to the carbon cycle.  Human burning of fossil fuels instigates an additional flow of carbon into the atmosphere, like turning up the tap.  This is an example of a change in inflow.  Plants pluck some of this carbon out of the atmosphere; some of it dissolves in the oceans, ans so on.  These are examples of outflow.

Energy Flow

There are a number of factors affecting the inflow and outflow of energy in Earth’s climate system.

We can also apply this idea to energy entering and escaping Earth’s climate system, the basic issue underlying climate change.  Energy pours in from the Sun, as usual.  Inflow doesn’t change.  But a build up of greenhouse gases impairs the escape of infrared energy into space, plugging the drain up, so to speak.  Outflow does change.

Stock

In our bathtub, depending on flow, there will be an increase or decline in the amount of water contained in it over time.  The quantity at a particular time is known as the stock.  If the amount of water pouring in at a given time exceeds the amount draining out, then the stock rises; if, contrarywise, the amount pouring in is less than that draining out, the stock goes down.  The history of inflow and outflow determines the stock at a given moment.

Stock can refer to the quantity of greenhouse gases in the atmosphere or to the amount of energy circulating in Earth’s climate system at a point in time.  In reference to our climate system, when the stock of energy rises, that’s called global warming.

Feedbacks

Affecting stock and flow in a complex system are feedbacks.  There are feedbacks which amplify change and feedbacks which work to stabilize it.  Such feedbacks can operate simultaneously.

Stabilizing feedbacks.

Even our simple bathtub model yields an example of a feedback.  If we increase the amount flowing in from the tap slightly, the stock of water in the tub rises, which in turn increases the pressure on the drain.  The increased pressure speeds the rate that water passes out the drain.  At a certain point, when the accumulating weight of the water is sufficient, the pressure on the drain, if there are no other complications, should cause the outflow to match the new inflow. At that point the water level will stabilize.

A feedback that tends to counteract a change in flow is known as a stabilizing feedback.  As in our example of the bathtub, a stabilizing feedback is not necessarily strong enough by itself to erase the effect of a change in flow.  In our bathtub example it only determined a new level of stock.

A similar stabilizing feedback operates in Earth’s climate system with the stock of energy.  With greenhouse gases increasingly trapping energy before it escapes into space, the stock of energy in the climate system grows.  But the more energy in stock, the more energy there is to escape Earth’s system into space, and thus the faster it escapes.  Outflow thus increases also, as with pressure on a drain.  When the amount of energy coming in matches the amount of energy going out, the system stabilizes at a new higher level.  But that new higher level represents energy now permanently trapped in the system, and the Earth is now permanently warmer.

Welcome to climate change.

Amplifying feedbacks

The bathtub example is too simple to provide easy examples of amplifying feedbacks.  Amplifying feedbacks are ones which push the system in the same direction as whatever is destabilizing the system.

An avalanche provides an example of amplifying feedback.  A small perturbation high on the mountain results in a large perturbation at the foot of the mountain.  Whoops!  If yours is the footstep which caused that perturbation which launched the avalanche, it doesn’t do much good to decide afterwards not to put your foot there again.

There’s the rub.

There are also amplifying feedbacks, you see, in the climate system, which is why I brought it up the subject of footprint remorse.  Warmer air means melting sea ice and melting glaciers.  Sea ice and glaciers are splendid at reflecting sunlight back into space.  Their melting means that the underlying dark areas are absorbing energy which once had been warded off, turning on an extra tap of energy which pours in causing still more warming and still more melting of ice.

Humanity names the tune.  Nature sings along.

There are also amplifying feedbacks which affect the carbon cycle.  Warmer temperatures caused by an increase of carbon in the atmosphere result in more wildfires which place still more carbon in the atmosphere.  Warmer temperatures instigate the melting of the Arctic permafrost, releasing large amounts of methane into the atmosphere—a hugely potent greenhouse gas—which further fuels increases in temperature.

And so on.

In the case of Earth’s climate system, runaway feedbacks can trigger runaway climate change, until we reach a point where human effects on the system stop being the driving force, a point where the changes are happening of their own momentum.  At that point, we could stop deforestation and fossil fuel use entirely and the system would continue changing.  From the human point of view, and the point of view of most species on the planet, the climate would be broken.

Once amplifying feedbacks become dominant, if we allow it to get that far, stopping it by changing our ways would be like blowing out the match that set our house on fire.

That’s why we don’t want it to get that far.  But we’re almost there already.

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My usual acknowledgements to Coursera and the University of British Columbia and the course Climate Literacy: Navigating Climate Change Conversations, taught by Sarah Burch and Sara Harris.  Any errors are my own.  Image from the University of Washington.

Posted in: Climate School