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“Positive” isn’t always good: Cilmate Change Feedback

March 25th, 2010 · No Comments

This guest post comes from DCoronata covering a quite important set of issues on feedback systems, starting off with some basic education and ending with basic (and critical) questions. 

 

There are so many tangible and complicated parameters to our planet’s climate that after centuries of examination and decades of high-level accurate predictions about what the future may bring.  This is a topic so vast, so magnificent in its scope and so vital to the future of all ecological niches on Earth that I can’t possibly do it justice, and can only examine the most important potential details.  

Climate is the interplay of dozens of inputs and outputs, of energy radiated and transmitted through several different strata.  But what makes it so damned hard to figure out, is when a change in one value leads to unexpected changes in another.  This isn’t comprehensive, this isn’t even close to being a full analysis and I won’t pretend it is.  But it is a start into the wonderful world of feedback and control systems.

Providing a Glossary of Key Terms

 

Every so often I do one of these, and it often necessitates a glossary.  So here goes…

  • Feedback is a system of events where the value of one output causes the value of an input to change.
  • Input(s) are various parameters which effect the value of the output.  In this case we have solar radiation (insolation), heat reflected by clouds and the atmosphere back to space, heat radiated by the earth back into space, and the various heat stored by the earth (or atmosphere) and then later radiated back into space.  Each of these parameters are directly affected by AGW in one way or another, either minimally or significantly.
  • Output in this case, is average global temperature. I will only examine global averages, as the differences in AGW’s effect on day and nightime temps, seasonal changes, and differences based on latitude and altitude would take a dozen diaries.
  • Amplification is the amount that feedback signal is multiplied by the system and reapplied back to the input.
  • Positive feedback amplifies part of an outgoing signal, and injects it back to the input in a way which increases the outgoing signal.  In this case, positive feedback increases average global temperatures.
  • Negative feedback amplifies an outgoing signal and applies it to the input in a way which reduces the output.  In our case, negative feedback reduces the amount of possible temperature change.
  • Closed-loop.  A system where the amount of energy in is always the same.  In actuality, it really doesn’t exist, or if it does it doesn’t exist for long.  Pipes leak, walls corrode, freon comes shooting out, nature abhors permanence.
  • Open-loop.  More like the real-world; where outside forces can influence or change the state of the system.

Having fun yet?  

Okay, now let me get some myths out of the way first.  

Feedback is an analog system.  Unless you are talking at a quantum scale (and then there is that whole problem with probabilities being analog) a signal of X leads to a fractional signal Y.  In a fixed system, the multiplier is the same, but there are systems where the multiplier gets smaller the smaller the input is.  This is how automatic gain control used to work in your old radios.  The louder the output, the more negative feedback was applied to reduce the volume.  It made the “quiet periods louder” and the louder parts less loud.

Examples of Feedback

The easiest way to demonstrate positive feedback, is with rock music!  Put the guitar near enough to the amplifier, and the vibrations of the amp cause the strings to vibrate, which then makes the amp louder, and the strings vibrate some more.  Put a mike too close to the speakers, and you get a permanent feedback loop the instant any sound is generated in the room.

The first use of feedback in control systems was nothing more glamorous than the earliest precursor to the water-closet (or toilet) float valve.  Water reaches a certain level, shuts off the valve, no more water coming in.  Water goes below that level, valve opens, water goes back up.

My favorite early feedback mechanism was used by Watt to regulate the speed of his steam engine.  The first use of steam engines were to pump water from mines. Watt had a problem with the engine going too fast and destroying itself, so he improvised a valve which would go faster and faster along with the engine, shutting off power if it went too fast.  Two balls were mounted on a scissor-like assembly, and as they moved faster and faster the centrifugal force would pull themselves up and out, and as they rose they lifted the valve.  The two balls would move closer and closer to the wall of the cylinder.  I know that everybody says “balls to the walls” is an aviation term, but it isn’t, its a steam term used well before the pilots did. 

So how does feedback occur in nature?  Well in a myriad of ways, but the easiest way to demonstrate it is with the population of hunters and prey.  When you’ve got a lot of rabbits, the hawks have a banquet.  More hawks means less rabbits, which eventually means less food for the hawks to eat.  Their populations rise and fall with similar periods, with the rabbit population always at its highest first.

Climate Feedback

The primary source of all heat is the sun.  There is a small amount of residual heat of formation of the earth, and we still have a molten core which slowly leaks heat out towards the surface, and we still have some radioactive decay which heats up rocks as the particles spread outwards.  For all practical purposes, they amount to a specific amount which scientists call “diddly”.  The sun is not a constant, its output does vary with time and there is a significant component of this variation embedded in climate, but the sun’s output is not influenced by the earth (except for perhaps a tidal component which might account for a few parts per trillion, and not worth mentioning.)  Our main “input” is solar insolation.

Assume that the sun’s power doesn’t change very much (and it appears that our best data shows no more than a part in a thousand change).  If the sun were the only source of energy then our planet would be roughly the same exact temperature as the moon.  But it isn’t.  The moon averages about -23°C, or about -9°F (in Kelvins, 250).  The earth should also average the same -23°C, but it doesn’t.  Not even in this, the coldest winter on record!

That was a joke, you see…

The earth averages about 14°C, give or take a half degree.  Alright, give a half degree, it ain’t been below 14 for ages.  Now the earth isn’t a balanced perfect sphere, and there is a lot more land mass on the northern hemisphere than the southern. And there still is that heat of formation slowly percolating upwards, So it would be a bit warmer than the moon, or about -19°C.

That means our temperature is roughly 33 and a third degrees higher than it should be, just due to atmospheric and oceanic effects.

To keep it simple, there are two things that cause this temperature difference; things that reflect and things that absorb.  Anything that reflects more light from the surface of the earth or the atmosphere back out into space, the colder we will be.  The more energy absorbed by other elements of our system, the warmer we will be.  

  • Increased Absorption => warmer
  • Increased Reflection => cooler

And there is one more critical note- when we talk about “temperature” and climate change, we talk about surface temperature which is really air temperature.  Even if we walk on the ground, our bodies experience air temperatures primarily.  Air temperatures determine if it rains or snows, ground temps determine if it sticks.  The ground changes temperature very slowly, and water changes somewhere in between.  Dig deeply enough into the ground, and you start to get seasonal variations which relate to the near past.  It takes time for the thermal wave to push downwards.  Measure the tiny temperature differences for each of these regions and you can actually tell if the last winter was milder than the one before that, or the one before that!  Water temps vary greatly with season, and has a tempering effect on the land nearby.  That’s generally why interior regions get much hotter in the summer, colder in the winter than areas near the coast.

So now we actually get into the climate feedback portion of the diary.  There are several different mechanisms which will change their effectiveness as the earth warms.  In no particular order they are reflectivity (albedo), cloud cover, sea level increases, water vapor content, effectivity of CO2 absorption by the biosphere, effects of CO2 on certain ecological niches, and last but not least CH4 release mechanisms.  There are other natural factors in climate such as seismology and tectonics, but unless we learn how to force volcanoes into erupting there is no way a warming climate can induce them.

Reflectivity:  As the planet warms, ice cover reduces.  Ice is a brilliant reflector, and most of the sunlight striking ice is returned back into the atmosphere or into space.  But as the ice melts it exposes (most often) dark black soil.  This is stuff that might not have seen the light of day for thousands of years, potentially hundreds of thousands.  As that dirt and rock get exposed, the light striking them will no longer be reflected back, it will get absorbed.  In the Arctic summer, you might see as much as 900 or so watts per square meter striking the ground.  Instead of most of that being reflected, most of that will now be absorbed.  As you get nearer the pole, there is less ground cover but water reflects light much worse than ice does, about as well as soil.

To put some numbers on it, ice reflects about 35% back up.  In the Arctic there is actually little ice, most of it is covered by snow which (if clean and fresh) is very reflective, often as much as 80% or more!  Water reflects back very little, less than 10% of direct light.  (Because of refraction and wavelength effects, some colors are more reflected than others.  And at certain angles the water reflects much more, absorbing less heat.)  This could add a few hundred watts per meter, for a very significant portion of the globe.

Cloud cover:  This is where it gets tricky; some clouds are great reflectors, some clouds are fantastic at blocking IR from escaping the earth and going back into space.  So clouds and water vapor have a dualistic effect, sometimes heating, sometimes cooling.  But one thing is for certain, clouds can not reflect the sun at night.  So increased cloud cover at night will cause nighttime temperatures to go up, while in the daytime they will cause temperatures to go down.

Sea Level: As water very gradually inches up the coastlines, it has significant effects on the local environment.  There is no consensus yet about the effect of billions of tons of cold water pushing into the northern Atlantic and Hudson Bay.  There are some who suggest that the change in salinity will redistribute heat away from the coasts of the eastern north Atlantic, causing colder climates in Europe and perhaps the Maritimes.  In recent years about the only region of North America which hasn’t had increased average annual temperatures are the areas directly east of the Great Lakes.  Warmer weather west of the lakes has increased the amount of humidity transferred to winds blowing across them.  This has caused more “lake-effect” snow.  When the lakes are iced over, less moisture is available and the winds are drier.  

Water Vapor.  Now this is where it gets very interesting!  Stratospheric water vapor levels are down about 10% since 2000, which has very slightly reduced the level of warming.  Water vapor and methane go hand in hand, rising levels of CH4 is always followed by rising levels of H2O.  But is there a natural feedback mechanism in place tempering climate change, or is this based on our attempts at mitigation?  We’re certainly trying harder to prevent methane emissions in landfills.  Caping or burning CH4 at oil wells prevents their release into the atmosphere.  And there has been an effort to reduce (kid you not) bovine excretions into the atmosphere.  But is this a long-term abatement, or just a blip on the charts?  Water vapor is not the most potent greenhouse gas, but it is the most plentiful.  And as a kicker, since H2O absorbs different wavelengths than CO2, their mutual abundance synergistically act to increase global temperatures.

CO2 Absorption and effects on plant life.  It don’t take a rocket magician to know that plants breath in CO2, and spit out O2.  Along the way, they also create sugars through the process of photosynthesis.  That’s actually the way most scientists now define plants- their ability to convert sunlight into food.  We’ve been told that as CO2 concentrations increase, plant growth rates increase.

This is true, provided that the temperature remains the same.  But some studies have shown that there are other requirements that if not met, act to reduce plant growth rates.  This is a simple manufacturing/engineering problem- if you have ten different parts in an assembly, with ten different amounts in your inventory, you can only build as many completed items as the smallest inventory item on your list.  In other words, “the chain is only as strong as its weakest link” etc…  So even if higher CO2 rates will lead to more plant growth and more CO2 absorption (a negative feedback for GHGs) this might only be true as long as other essential nutrients or growing conditions are met.  So some plants may indeed thrive, some may be harmed.

CO2 Absorption in water.  For all practical purposes, the oceans have been absorbing so much CO2 that the concentration of H ions has gone up 30 percent.  We’ve been astonishingly lucky that about half of all the excess CO2 that has been emitted by man has been absorbed by our oceans.  But the oceans haven’t been that lucky.  The growth rate of calcium carbonate, the living material of most sea-life skeletons is severely reduced by oceanic acidity.  The greatest source of O2 on earth is oceanic algae, which are under threat if our oceans become more acidic.  More CO2, less potential O2, which means plants will do poorer (as will we) and this might act as a weak but dangerous form of positive feedback.

And now I’ve saved the worst for last- CH4 or methane.  Methane is a very powerful greenhouse gas, a potent displacer of oxygen at sea level (it pushes oxygen away, which isn’t too good if you need to breath it) and a synergistic companion to the another potent ghg, water vapor.  Methane also eventually breaks down into CO2, still one more ghg.

Methane is a natural by-product of all plant decomposition.  It is the simplest and lightest carbon molecule on the planet, each of its four available bonds are tied to a single hydrogen molecule.  It is also relatively stable, and has a longer life-expectancy than most energetic molecules.  It also burns like the dickens!  Compress it and put it in a gas tank, and you could power a car with relatively little modification to rubber seals and compression chamber components.  And uncounted tons of it exist on this planet, trapped in ice or deep under seas in pockets of not-quite solid methane.

There have been times in recent history when a deposit “burps” and sends potentially billions of cubic feet of nearly pure methane into the atmosphere.

It is lethal.

In 1986, a methane burp killed about 1800 in The Cameroon.

The greatest real concern (at least in the short run) is the trapped organic material in the Siberian taiga, and North American tundra.  As these are the very parts of the globe where we are currently seeing the greatest amount of global warming, the potential for a positive feedback effect could be quite strong.  Warmer northern no-longer “permafrost” leads to massive releases of methane and CO2, and a few other byproducts of decomposition.  Mixed with water, and energized by the long summer days of the Arctic, there will be other ghg generated which will create conditions for significantly greater warming.

There is still the unknown threats of underwater methane.  As the ice packs separate and violently collide, the physical expression of so much energy could lead to more turbulence at the great depths and pressures, and normally quiet stillness of hydrate lakes.

There are still a few more feedback mechanisms which might play very great roles.  The single greatest is human activity.  We have the potential to dramatically reduce our use of greenhouse-gas generating fossil fuels.  And we can reduce our consumption of meat, which is another topic entirely and one which generates more greenhouse-gases than personal transportation!  (You drive that Prius, but you ate two or three hamburgers every week- a vegan Ford Explorer driver might contribute less to greenhouse gases, and use less fossil fuels than you!)  We might be able to at the very least put a small brake on the process.

Part of what makes the science of climate so difficult, is trying to piece together how all of these various effects combine to either enhance or retard climate change.  There are so many equations in the mix, so many non-linear combinations that might perform differently when we actually have a much larger laboratory to work on.  Test plots or small-sample environments can’t give us enough data to draw absolute conclusions, which is why the error bars on any prediction are so great.  And the possibility for dramatic human behavioral change is just as great as that of the behavior of the climate itself.  Will differing rainfall patters force us to change what crops we grow, which fields we plant?  Will economic conditions favoring certain crops change the way we exploit our farmland?  Will we discover that certain crops which we’ve used in great abundance aren’t favorable to health (corn, hint, hint…) and maybe we should go back to more traditional foodstuffs?  Stupid little things like taxes on drinks, or ethanol subsidies might change which crops we grow, and what we burn.  And lastly will human endeavor and intelligence match our almost insatiable capacity for growth?

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Tags: climate change · Global Warming