Simple Sam learns about climate feedback cycles

Posted on August 20, 2010

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Today, we’re going to look at climate feedback cycles.  One of the most important things you have to know about climate and climate change, Sam, is that—what with a planet of oceans, continents, ice cover, changing seasons, not to mention a biosphere—we’re looking at a dynamic system here.  It’s complicated, but it’s a system.

“Is that a chart?” says Sam.

Yes, Sam, from a recent report by the Australian Academy of Science.

“Oh goody, loops and text boxes.”

All will be made clear, Sam.  First, notice that there are two diagrams here.  The top one refers to climate change during the ice ages.  The second one to human-induced climate change.

“Ah yes, the propaganda enters already.”

When scientists say it in peer-reviewed documents, it’s called science, Sam.  The report from which I got this diagram was peer-reviewed, and accepted as an accurate statement of the current science–or the 400 members of the Australian Academy would not have permitted it to be released.

“Uh huh,” says Sam.

Why the Academy members—and practically any other scientist anywhere—think this is good science will be dealt with as we come to it, Sam.  I can sense somehow that you are still sceptical.  But today is an overview.  Broad strokes.  We’ll get to the evidence when we get to the details.

“I’ll be looking out for the evidence.”

Oh, I hope so, Sam.  You deniers seem so evidence-challenged most times.

“Two diagrams,” says Sam stiffly, not catching my eye.  “Ice ages.  Human-induced.”

Right, Sam, and both diagrams are broadly the same.  Note, at the centre of each is a box that says “Change in temperature.”  Everything else in both diagrams ultimately flows towards that box.  These are things which cause a change in temperature.

“Shouldn’t that say “Rise in temperature”?” asks Sam.

No, Sam.  A lot of these things work both ways, as you will see.  Okay, start with the diagram on top.  It shows four things which influence a change in temperature.  Changes in greenhouse gases (written as GHGs in the diagram), changes in ice extent, changes in water vapour, and orbital fluctuations.

“What do you mean by orbital fluctuations?”

Well, this deserves a lesson all by itself, but briefly, in its dance about the sun, and because of its interaction with the gravity of the giant planets Jupiter and Saturn, sometimes the earth’s orbit wobbles.  This has an effect on the earth’s seasons.  Over time, again according to predictable physics, the wobble corrects itself, and the seasonal effects consequently reverse.  The wobble and the correcting of the wobble is, according to the prevailing scientific theory, what ultimately caused the ice ages, and also what caused them to end.  The top diagram sets out some of the mechanisms.

But first, note the diagram beneath, the one describing human-induced climate change.  How is it different?

““Orbital fluctuations” is gone.”

Right.

“Why?”

Because the climate change that is going on now has nothing to do with orbital fluctuations, Sam.  If we were staggering out of orbit, and veering in towards the sun to catch a few extra rays, I think a scientist here and there would have noticed, you think maybe?—Okay, what else is different about the bottom diagram?

“It has a little box that says “GHG emissions from human activities.”

Right, Sam.  Now the thing with these diagrams is that they’re describing interactions.  A change in temperature causes changes in water vapour, changes in ice extent, changes in greenhouse gas concentrations in the atmosphere, and all of these things loop back to cause more changes in temperature.

“That’s obvious.  But you’re going to have to prove all these things.”

Of course.  But one more thing, if this was a game and I was the top hat and you were the locomotive, and I was to start on one diagram and you were to start on another diagram, where would we put our pieces?

“What?”

Every cycle has a starting point, every action has a cause, Sam.  We’re talking about changes here.  So where do these changes start?

“The first one starts with orbital fluctuations.”

Right, which caused a change in temperature, which affected all the reinforcing feedbacks—ice extent, water vapour, greenhouse gases—which further changed the temperature, and so on.  And in the other diagram, the starting point is human greenhouse gas emissions.

“Aha!”

You keep on saying that, Sam, but the theory that greenhouse gases affect the mean temperature of the earth has been around since the nineteenth century.  An eminent Nobel Prize-winning Swedish physicist, the physicist who in fact developed the field of physical chemistry, Svante Arrhenius, published the idea in 1896 in a paper called, “On the influence of carbonic acid in the air upon the temperature of the ground.

“Carbonic acid?”

CO2, Sam.  The idea has been around for a long time, and it’s based on solid physics.  Arrhenius, remember, was not even a climatologist.  He was a physicist.  And while he overestimated the effect of CO2, and vastly underestimated the amount of greenhouses gases we were going to throw into the atmosphere, he correctly deduced greenhouse warming more than a century ago.

Yup, and since Arrhenius died in 1929, we’re pretty sure Al Gore didn’t enlist him in his global warming conspiracy, either.

“Are you usually this sarcastic towards your students?”

You’re special, Sam.  Okay, to return to the diagrams, the starting point, the initiation of each of these temperature change cycles is different, but once they get started, the machinery in operation is the same.

Add heat to the system by the changing the concentration of greenhouse gases in the atmosphere, as in our case; add heat to the system by correcting earth’s orbit, as in the case where the ice ages end—and certain reinforcing feedbacks come into operation.

Adding heat causes water to evaporate.  Water is a powerful greenhouse gas which pulls yet more heat into the system.

Adding heat causes ice to melt.  (This happens a lot slower than water evaporation, of course, and that’s why the loop on the diagram is bigger.)  Ice reflects light and thus energy better than the water and land which the ice covers.  Removing large amounts of ice changes the earth’s albedo, meaning that the earth absorbs a lot more energy than it otherwise would if the ice was still there reflecting it.  Another reinforcing system.

Finally, adding heat causes the release of greenhouse gases, from, for instance, the heating up of the ocean, from the melting of permafrost, etcetera, and these again reinforce warming.

All of these things happen regardless of what the initial forcing is.

“Forcing?”

A forcing in climatology is something that pushes along or initiates climate change.

“So what’s the point?”

The point, Sam, is that when we mix human-produced greenhouse gases into the earth’s climate system, it’s not just the effect of those gases themselves we have to worry about.  The reinforcing feedbacks amplify the greenhouse effect, and we have to worry whether we have set processes in motion that we won’t be able to stop.

Look at the diagram again, Sam.  There is only one of those boxes we have control over.  If we wait too long to act, those reinforcing feedbacks will take over the process, and then there won’t be a thing our civilization can do to prevent the consequences.

“What are those consequences, as if I didn’t know?”

Catastrophic climate change.

“Same old story,” says Sam.  “You’re going to have to prove it.”

Of course I will, Sam.

But meanwhile, if you want to read further about Svante Arrhenius, or read his original 1896 paper, here are some links, etc.

—–

About Arrhenius:

http://earthobservatory.nasa.gov/Features/Arrhenius/arrhenius.php

http://earthobservatory.nasa.gov/Features/Arrhenius/arrhenius_2.php

His paper:

Arrhenius, S. (1896) On the influence of carbonic acid in the air upon the temperature of the ground, Philosophical Magazine 41, 237–276.

You can find excerpts of the paper here:

http://web.lemoyne.edu/~giunta/arrhenius.html

And the full paper here:

http://www.rsc.org/images/Arrhenius1896_tcm18-173546.pdf

The feedback diagram is taken from:

“The Science of Climate Change: Questions and Answers”, Australian Academy of Science, Canberra,

Which can be found here:

http://www.science.org.au/reports/ClimateChange2010-highres.pdf

Posted in: Climate School