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The Bad Kind of Positive Feedback: Climate Change July 8, 2010

Posted by Jamie Friedland in Climate Change.
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This post is about feedback loops and the impact they have on our climate system.  This follow-up post explains the specifics of the positive feedback loops described below.

If you roll a small snowball down a snow-covered hill, it gets bigger and bigger as it rolls.  This kind of self-magnification is aptly called the “snowball effect.”  Ironically, the snowball effect encapsulates the urgency with which we must respond to global warming.

The technical description of the snowball effect is a positive feedback loop. Positive feedback loops are processes that make themselves stronger as they continue.  I will discuss these in much more detail below, but to understand positive feedback loops it is useful to understand their opposite: negative feedback loops.

Examples of feedback loops. One reinforces the initial action. The other counters it.

Negative Feedback Loops

Colloquially, “negative feedback” discourages one from repeating a certain action.  Negative feedback loops operate in the same fashion, reversing the initial action that triggered the feedback loop.

A negative feedback loop has an output that opposes the changes of the input.  It reverses and minimizes change.  A thermostat is a basic example:

A rising temperature (change in one direction) triggers the air conditioner to turn on, cooling the air (opposing that change).  Another negative feedback loop controls the heater.  If the temperature gets too cold, the heater will turn on and raise the temperature again.  Thus the air in a house stays within a comfortable, stable range; an equilibrium.

Negative feedback loops essentially fight change (…like Republicans).

Negative feedback loops are common in nature.  Even just within our bodies, negative feedback loops regulate blood pressure, body temperature, blood sugar, many hormone levels and explain the gag reflex.

Positive Feedback Loops

In contrast, positive feedback loops magnify change.  Let’s look at our snowball example: proper, construction-grade snow adheres to itself.  Each time the snowball rolls, more snow grabs on to the ball’s exposed surface area, increasing its size.  As the snowball grows larger, it gains more surface area and is thus able to grab even more snow and get even bigger and have even more surface area to grab even more snow…you get the idea.  The process is self-magnifying and continues until the bottom of the hill.

Now, just because such feedback loops are “positive” does not mean that they are good; the most famous positive feedback loop is a nuclear reaction.  That being said, there are plenty of beneficial positive feedback loops, including many in nature.

Using our bodies as an example again, positive feedback loops are responsible for contractions in childbirth, blood clotting, lactation, and sneezing just to name a few.

A flawed but entertaining positive feedback loop.

Negative Feedback Loops Normally Regulate Climate

Earth’s climate is influenced by a number of different variables, including changes in solar radiation, changes in Earth’s orbit and axial tilt, volcanic eruptions and asteroid impacts.  However, carbon dioxide plays a critical role in one of the primary negative feedback loops that normally regulates climate for our planet.

Carbon dioxide in the atmosphere traps radiated heat from the sun and warms our planet.  That’s the greenhouse effect.  More carbon dioxide in the atmosphere means more warming.  That is an indisputable scientific fact.  Period.

When temperatures warm, the air can hold more water vapor so there is more rain.  Rain and atmospheric carbon dioxide are the two things that plants need most (other than sunlight, so they grow more.  More plant growth removes more carbon dioxide from the atmosphere.  Additionally, more CO2 dissolves in rainwater and gets stored in the oceans (note: too much of this is a bad thing and causes ocean acidification).  Over time, this eventually reduces atmospheric CO2 levels and their corresponding greenhouse effect, cooling the climate.

The same thing works in reverse.  If the planet starts cooling (because of other factors), the carbon cycle tries to warm it back up.  Cooler temperatures mean less rainfall.  Less rainfall means less dissolved CO2 stored in the oceans.  It also means less plant growth and drier weather.  Drier weather can lead to forest fires, which return the carbon stored in plants back to the atmosphere.  More carbon in the atmosphere means more greenhouse effect and the planet warms back up.

Without our interference, the carbon cycle essentially acts as Earth’s thermostat.  However, the carbon cycle is only so strong, and like an air conditioner on a scorching day, it can be overwhelmed.

Positive Feedback Loops Can Overwhelm Earth’s Natural Thermostat

First of all, it is undisputable that burning fossil fuels and cutting down the forests adds CO2 to the atmosphere.  Obviously, this increases the amount of heat trapped by the greenhouse effect and warms the planet directly.  That is pretty simple.  However, there are also a number of indirect positive feedback loops on our planet that can be triggered by rising temperatures and threaten to make climate change irreversible.

Using our snowball analogy, if you start a tiny snowball rolling down on a huge hill, a person farther down the hill could theoretically stop it while it was the size of a baseball or a basketball or probably even the size of one of those big yoga balls.  But you wouldn’t be able to stop the snowball once it reached the size of a car or a house.  Once the snowball gets that big, it’s going to roll all the way to bottom of the hill no matter what you do.

Don't end up like Susie. Tell your senator to stop climate change today.

For climate change, the bottom of the hill is that endgame scenario you’ve probably heard about, 2-300 years from now when the planet is much warmer and the sea level is, according to the U.S. Geological Survey, ~260 feet higher than today.

To use a non-snowball analogy, think of our climate as a rowboat on a lake.  If you tip the boat a little bit to either side, its negative feedback loops will still steady it back in its equilibrium in the middle.  However, if you rock it hard enough in one direction, the negative feedback loops are overwhelmed and the boat flips to a new equilibrium – upside down.  With you, conveniently enough for climate change, underwater.

The moral of the story is: because of a number of positive feedback loops that will keep getting stronger, if we don’t stop climate change before a certain threshold of atmospheric CO2, we won’t be able to stop it later when its effects truly become devastating.

So we need to act.  Now.

I had intended to use this post simply to explain the different climate change positive feedbacks, but I decided to make this accessible to more people by starting from the very basics.  This post describes the actual climate positive feedback loops.

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