Feedbacks

Spring 2012, Lecture 7

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Feedback Mechanism

• Any process that acts to oppose or amplify changes to a system that is in a steady state

• Feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input

• The response is often proportional o That is, as the system deviates more from the steady state

position, the faster the process works to counteract it.

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Homeostasis

• Property of an open system, especially living organisms, to regulate its internal environment so as to maintain a stable condition, by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms

• Term was coined in 1932 by Walter Cannon from the Greek roots homo- (same, like) and sta- (to stand or stay)

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Negative Feedback

• A type of feedback, during which a system responds so as to reverse the direction of change

• Since this process tends to keep things constant, it is stabilizing and attempts to maintain homeostasis

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Negative Feedback Example - A ball Rolling Inside a Curved Bowl

• Initially, the ball oscillated from side-to-side, rising far up the sides of the bowl

• With time, the amplitude of the motion decreases until the ball comes to rest at the bottom of the bowl

• Once stationary, the ball has reached a position of stability

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Positive Feedback – Microphone Screech

• Small sounds picked up by the microphone are amplified by the nearby speaker, where the microphone once again picks up the amplified sound and rebroadcasts it through the speakers

• This looping continues until the initially tiny sound is re-amplified repeatedly to a piercing squeal

Audio Feedback

• An example of microphone-speaker feedback7

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Nuclear Chain Reaction

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Cybernetic Systems

• From the Greek kubernetes, meaning a steersman

• Cybernetics means the branch of study concerned with self-regulating systems using communication and control in either mechanical devices or living biological organisms

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Cynernetics at Work -Walking

• Babies need to learn to control their balance

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Cybernetic System Components

• Sensor

• Amplifier

• Controller

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Instrument vs. Man

• Cybernetic control by electrical system

• Cybernetic control by human brain

Feedback Terminology

• State variable – the quantity that is being fed-back

• Feedback loop – the loop of causality

• Input perturbation – any disturbance to the system

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Climate Feedback Loops

• Ice Albedo

• Stefan-Boltzmann

• Water Vapor

• Hydrologic Cycle

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Ice Albedo Feedback

• Ice with snow cover has a high albedo of about 0.9, meaning that 90% of incident radiation is reflected into space – there is little heating of the ground

• Bare ice absorbs about 50% of the incident radiation

• Open ocean absorbs 94%

• As ice melts, more and more heat is retained, creating a positive feedback

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Ice Melting with Albedo Change

• Animation illustrates the effect of albedo change16

Result of Decreasing Albedo

• How climatologists have fun…..

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Extreme Albedo

• What would happen if the earth grew colder, and ice advanced from the poles toward the equator?o Increasing albedo would reflect more light into

spaceo Colder temperatures would reduce the amount of

water vapor in the atmosphere, reducing the Greenhouse effect

o Earth would go into an “Icehouse” state

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Icehouse

• Mikhail Budyko, Russian climatologist, produced the first energy-balance climate model in the 1960’s

• He showed if ice advanced within 30 degrees of the equator, Earth would become an icehouse

• The problem them becomes to how does earth emerge from this state?

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Ice Sheet Melting May Be Rapid

• As Ice sheets melt, they become darker, which speeds further melting

• Meltwater lakes on the surface have much lower albedo than ice

• As ice melts, it thins, and the remaining ice is at lower altitude, where it is warmer

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Arctic Sea Ice Changes

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• NASA’s Crysosphere Program Manager Tom Wagner explains the Importance of Arctic Sea Ice, and why NASA is studying it

Measuring Albedo on the Ice

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Another Albedo Related Effect

• A substance which affects the albedo feedback is Black Soot

• Soot is a product of incomplete combustion of carbon-containing fuels, usually petroleum-basedo Complete combustion would, in theory, produce

practically no soot or carbon monoxide, and is recognized by a blue flame.

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Black Soot

• Black soot particles, composed of black carbon and organic carbon, also decreases the albedo of ice

• The photo shows a soot particle under a microscope - Credit: D.M. Smith, University of Denver

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Sources of Soot

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• Soot may come from o Diesel engines on trucks, buses, heavy equipment, etc. o Airplanes and shipso Other Sources: Internal combustion engines, power plant

boilers, central steam heat boilers, waste incineration, local field burning, house or forest fires, fireplaces, and furnaces

Earth’s “Third Pole”• “Glaciers on the Tibetan Plateau, sometimes called

Earth's "third pole", hold the largest ice mass outside the polar regions.”

• “These glaciers act as a water storage tower for South and East Asia, releasing melt water in warm months to the Indus, Ganges, Brahmaputra and other river systems, providing fresh water to more than a billion people”

• “In the dry season glacial melt provides half or more of the water in many rivers”

• From James Hansen, Survival of Tibetan Glaciers

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Tibetan Glaciers

• “Tibetan glaciers have been melting at an accelerating rate over the past decade

• Glacier changes depend on local weather, especially snowfall, so glacier retreat or advance fluctuates with time and place

• Thus it is inevitable that some Tibetan glaciers advance over short periods, as has been reported

• But overall, Tibetan glaciers are retreating at an alarming rate”

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Chinese Glacier Study• The speed with which Tibetan glaciers are melting

suggests that, in addition to worldwide climate change, other factors may be at work

• A team of Chinese scientists measured black sooto During the 1950’s and 1960’s, they found BC values of 20-

50 ng/g for four stations that are downwind of European pollution sources

o BC and OC amounts decreased strongly in the early 1970s, probably because of clean air regulations in Europe

o Ice core data shows that, in the 2000’s, BC and OC began to increase again, and data suggest that increased black soot arises from Asian sources, especially the Indian subcontinent

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Effect of Snow Melting

• Measured concentrations of BC and OC refer to fresh snow

• As snow melts, black soot concentrations on the glacier surface increaseo Soot particles do not escape in the melt water as

efficiently as the water itself

• Soot noticeably darkens the glacier surface during the melt season, increases absorption of sunlight, and speeds glacier disintegration

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Other Effects of Soot

• Black Soot also has serious adverse health effects, and adversely affects agricultural output

• Soot contains Polycyclic Aromatic Hydrocarbons (PAH), compounds known to be mutagenic and carcinogenic

• December, 2008 audio presentation of KQED radio, San Francisco’s National Public Radio station (click speaker icon to play)

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Ice-Albedo Feedback Summary• The Ice-Albedo feedback can work in reverse

• If climate starts to cool, snow accumulates, turns to ice, and reflects more light back into space

• This further cools the surface and the air above it

• Either in forward or reverse, ice-albedo is a positive feedback, amplifying the input perturbation

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High-Latitude Climate Sensitivity• The high latitudes, particularly the Arctic, are predicted

by all the Global Circulation Models to be much more sensitive to climate change then the temperate and tropical regions

• This is currently observed

• It is largely a result of the ice-albedo feedback

• It is estimated that the Arctic will see 2-4 times the as much warming as the global average

• Sea ice changes have a very obvious effect on the large changes climate changes seen in the Greenland ice cores

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International Polar “Year” (IPY)

• During the period 2007-2009, a broad international effort was made to study the polar regions of the earth

• The predecessor for this study was the International Geophysical Year, in 1957-58

• Combining the efforts of many nations allows us to gain a much more complete view of the earth

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ARCTAS

• A major contribution to IPY by the United States was ARCTAS, Arctic Research of the Composition of the Troposphere from Aircraft and Satellites

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ARCTAS Goals

• How the climate in the Arctic is changing in response to human influence

• How pollution is transported to the Arctic

• How does pollution impact ecosystems and the regional climate

• The video on the next slide provides a summary of the 2008 ARCTAS mission – it is from the Earth Science Project Office (ESPO) of NASA

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NASA-ESPO ARCTAS Video

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Stefan-Boltzmann Feedback

• Recall the Stefan-Boltzmann equationo I = εσT4

• As the earth warms, T increases, and the intensity of the long-wave infrared radiation increases very rapidly

• This acts to cool the planet, opposing the change

• Stefan-Boltzmann is thus a negative feedback

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Water Vapor

• As climate warms, more water vapor is added to the atmosphere

• Recall that water vapor is responsible for most of the natural greenhouse effect

• This should further warm the earth, and thus be a positive feedback

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Hydrologic Cycle

• As more water vapor goes into the atmosphere, the relative humidity goes up, and saturation may be reached

• Water vapor then condenses, and precipitation often follows

• This reduces the water vapor in the atmosphere, and acts as a negative feedback, keeping the water vapor feedback in check

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Water Phase Diagram

• Superimposed on a phase diagram for water are “climate trajectories” created by introducing water to a planet, and letting it evaporate until equilibrium is reached

• As more water enters the atmosphere, temperature increases, because water is a GHG

• Therefore the trajectory curves to the right

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Water Trajectories

• When the trajectory reaches a phase boundary, equilibrium is achieved

• For Mars, the equilibrium is between ice and water vapor

• For Earth, the boundary is along the water-water vapor line, so Earth should have both

• Earth represents the “Goldilocks” situation, temperatures just right to have and retain liquid water

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Venus – Runaway Greenhouse Effect

• The curve for Venus never intersects the liquid line, so Venus got hotter and hotter

• Presumably, all the water on Venus entered the atmosphere

• UV light from the sun can split water molecules into hydrogen and oxygen

• Hydrogen is light and moved very fast, faster than the escape velocity, so it escapes into space and the water is gradually destroyed

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Why Has Earth Retained Its Water?

• On Earth, the tropopause acts to keep most water from getting to the stratosphere

• The temperature is about -70ºC, and the amount of water in the atmosphere at that temperature is vanishingly small

• Hence, earth has retained its water throughout its 4.5 billion years of existence

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Runaway Greenhouse Effect

• What would happen if a positive feedback were to continue unchecked until the temperature became very high?

• This happened on Venus

• The result is a planet whose surface is hot enough to melt lead

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A Runaway Greenhouse on Earth?

• Even if all the carbon in fossil fuels were converted to CO2, it would not be enough to push earth into the runaway greenhouse position

• The sun is warming with time, but it will be some billions of years before the solar output would push us into greenhouse conditions

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Hotter Earth

• Looking at the diagram, we can see an initial perturbation with no water vapor, perhaps from CO2, would push the line a bit to the right

• The water vapor greenhouse effect makes the difference larger on the liquid-vapor line, but no where near a runaway effect

• This also assumes that relative humidity is 100%

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Hadley Circulation

• Air at the equator is heated, and rises convectively

• As it approaches the tropopause, air cools and moisture is removed by precipitation

• Around 30º from the equator, the dry air mass sinks to the ground undergoing adiabatic compression, and flows back to the equator, picking up moisture as it moves

• This is the Hadley circulation

• It keeps the relative humidity below 100% most of the time – global average is about 80%

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Deserts

• The regions around 30º from the equator are the sites of many great deserts

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Sahara Mojave

Namib Sonoran

ArabianThar (NW India)Great Victoria (Australia)

ChihuahuanGreat Basin