Module Six (of Eight)

Global Warming, Understanding the Forecast

Chapter 5, What Holds the Atmosphere Up?Module SixHow the greenhouse effect works within the temperature structure of Earths atmosphereThe greenhouse effect is powered by the lapse rateAtmospheric scientists call the change in temperature of the air with altitude the lapse rateIt is about 6C colder per kilometer of altitudeThe lower part of the atmosphere is called the troposphereAtmosphereThe troposphere is the lower part of the atmosphereIt contains about 90% of the air It contains all of the weatherThe boundary of low temperature is about 17 km high on averageThe boundary where the air temperature reaches its coldest point is the tropopauseCommercial airplanes fly in the tropopause

Atmosphere with altitude

Atmospheric layersTroposphere about 10 km high, contains 90% of air and all of the weatherTropopause boundary where air is the coldest, commercial aircraft areaStratosphere air begins to warm up because of ozone content

Mesosphere not much effect on the weatherExosphere ditto

No temperature contrast, no greenhouse effectRemember the layer model with a skin temperatureThink of the skin altitude for the air column as some kind of average altitude from which the IR escapes to spaceThe idea of a skin layer in the atmosphere is fuzzier than using a glass pane in the layer model but it still a useful conceptLapse rate vs. strength of GH effectIf we increase the GHG concentration of the atmosphere, the IR radiation to space will originate from a higher altitude (skin altitude). The increase in skin altitude increases the ground temperature.If the temperature of the atmosphere was the same at all altitudes, then raising the skin temperature would have no impact on the ground temperature.

More CO2 higher skin altitude warmer groundPressure as a function of altitudeThe pressure in the atmosphere depends primarily on the weight of the air over your headThe weight of the overhead air at sea level is more than The weight of the overhead air at the top of a mountainThe pressure of the air is non-linear with altitude (unlike scuba diving, where the pressure is linear with depth)Pressure is a non-linear (exponential) function

What to rememberWhen a gas is depressurized (less pressure) the gas expands

When a gas expands, it cools

When you pressurize a gas it heats up

Expansion, Compression and HeatIf we had a gas inside a container with a piston, and pressurized the gas, it would heat up, even in an insulated container with no heat entering or leaving

A closed system with no heat coming in or out is called adiabatic

If gas is compressed adiabatically, it warms up. It takes work to compress a gas, the work energy is transferred to heat

When it expands, it cools, reversing the process and the gas cools downWater vapor and latent heatRemember chemistry and the phase change diagram, where energy is added, the substance stayed at the same temperature until it completely changed phase, solid to liquid, or liquid to gas. The energy that was added is called latent heatLatent heat of fusion between solid and liquidLatent heat of vaporization between liquid and gas

In one direction the heat is added, in the other direction the heat is released.Phase changesSolid + heat liquid (latent heat of fusion) meltingLiquid + heat gas (latent heat of vaporization) boiling

When the phase change goes in the other direction, the same amount of energy is released during condensation or freezing

Vapor liquid + heat released Liquid solid + heat released

Latent heatYou charge up an air parcel with latent heat when you evaporate water into it (vapor contains the latent heat-not sensible heat)You get the heat back when the water condenses and the latent heat is released

A thermometer does not measure latent heatA thermometer measures sensible heat (what you can sense)Equilibrium conditionsWhen water is in equilibrium between liquid and vapor, its called saturated, or 100% relative humidity, and the equilibrium vapor pressure of water will be high.

Undersaturated occurs when it is cold, the amount of water vapor is lower than the equilibrium value

Supersaturated occurs when vapor pressure is higher than equilibrium, and the vapor tends to condense into precipitationConvectionConvection occurs when you heat a fluid from below or cool it from above (either a liquid or a gas)

Fluid expands as temperature increases, density decreasesUnstable condition causes the fluid column to turn overWarm fluid rises to the topThe Atmosphere tends to mix when it convectsAir is compressibleThe air is not all the same temperaturePressure is higher at the bottom because o f the weight of the air columnCompressed air at the bottom heats upBecause the air is well mixed, the moving air will always find itself at the same temperature as the rest of the air in the columnThis is what static stability looks like in a column of compressible air the same temperature as the rest of the column

Convection in the atmosphereDriven by sunlight hitting the groundWarms the air at the bottom of the columnWarm air begins to rise, as it rises, it expands, and coolsWhile ascending, it remains lighter and warmer than the air around itIf it does not mix on the way up, the air can get all the way to the top of the columnIf it mixes on the way up, the whole column warms up uniformlyMoist ConvectionThe latent heat in water vapor drives most of the drama in our weatherMoist convectionAir at the surface of the Earth with a relative humidity of 100% rises due to convectionAs the temperature drops, the equilibrium amount of water vapor decreasesSupersaturation drives water to condense into droplets or iceThe story of cloud formation will continue in chapter 7Water vaporIt changes the temperature of the airIt systematically changes the lapse rateDry convection has a lapse rate of about 10C temperature change per km of altitudeAdd the latent heat in moist convection, the lapse rate decreases to about 6C per kmIt is possible that the lapse rate of the atmosphere could be different in a changing climateTake home points, Chapter 5Air in the upper troposphere is colder than air at the ground because of the process of moist convection. The process includes the following:

Convection is driven by sunlight heating the air near the groundThe air rises and cools because it expandsWater vapor condenses, releasing heat as the air risesContinuedThe moist convecting air gets colder with altitude, but not as much as if it were dry

If the air did not get colder with altitude at all, there would be no greenhouse effectRevisit the layers of the atmosphereTroposphereStratosphereMesosphereEntering outer space:IonosphereExosphere

Global Warming, Understanding the Forecast

Chapter 6, Weather and ClimateHow the Weather Affects the ClimateChaos10 days is the limit for predicting weather because weather is chaotic an extreme sensitivity to initial conditions, so that small differences between two states tend to amplify, and the states diverge from each otherThe butterfly effect, a puff of air from a butterflys wing eventually resulting in a giant storm somewhere that would not have happened if the butterfly had never existedButterfly effectFirst observed in a weather simulation modelThe model stopped runningEdward Lorenz restarted it by typing in the variables like temperature and wind speedHe had small, insignificant changes, such as rounding errorsThe model diverged completely from the results of the initial simulation

Edward Norton LorenzMathematicianEdward Norton Lorenz was an American mathematician and meteorologist, and a pioneer of chaos theory. He discovered the strange attractor notion and coined the term butterfly effect. WikipediaBorn: May 23, 1917, West Hartford, CTDied: April 16, 2008, Cambridge, MABooks: The essence of chaosEducation: Massachusetts Institute of Technology, Dartmouth College, Harvard University

WeatherForecasts rely on computer modelsSmall imperfections in the initial conditions and the model cause the model weather to diverge from the real weatherBy about 10 days the prediction is worthlessTo overcome the error, run the model may times with tiny variation in initial conditionsan ensemble of model runs

ClimateDefined as some time average of the weather

Climatological January (or any other month) would be the average of many Januaries

The weather is chaotic, but the climate generally is not

The weather would predict rain on a particular day, whereas the climatologist may predict a rainy seasonAveragingLayer ModelReal WorldWarm and coldSummers and wintersDay and nightCompletely balanced energy budgetAveraging is validSome places much hotterSome place much colderRadiative energy budget at some place could be wildly out of balanceWill averaging change the answer to something unreasonable?Averaging a non-linear system

Top panel averaging radiative energy flux (S-B equation) over a large temperature range introduces a large bias.

Bottom panel over the temperature range of normal Earth conditions, the blackbody radiation energy flux is closer to linear, so averaging over a small range would be less of a problemThe Fluctuating Heat BudgetNot stable like a model, but fluctuates widelySolar energy comes in only during the daySunshine varies seasonally and by locationInfrared is radiated day and nightThe energy budget is in balance over a 24 hour periodBut at any time in any spot on the planet, the energy is usually out of balance

Seasonal variationsThe seasons are caused by the tilt of the Earth relative to its orbit around the sun, the obliquity

Wintertime, days are shorter and the sun is lowerAdded over a day the winter hemisphere has less sunlight

Seasons are NOT caused by the Earths distance from the sunThe eccentricity cycle refers to the shape of the Earths orbit around the sunIt varies from elliptical, to circularCurrently we are in a near circular orbitThe Earth is actually closer to the sun in January than it is in JulySeasons are not caused by proximity to the sun

Earths seasons are caused by the tilt of the poles relative to the orbit, and not by its distance to the Sun

Incoming flux depends on latitude and day of the year

Northern hemisphere summer is in the middle of the plot, which shows flux as a function of latitude and time of the year.Interesting to note from the plotHighest daily fluxes are at the poles during the summerPoles get six months of sunlight Sun whirls around in a circle above the horizon (not overhead)Why isnt it a tropical garden in the summer?

Thermal InertiaDamps out the temperature swing between day and nightDamps out the temperature swing as the seasons changeEven damps out the temperature change of global warming

OceansHas a tremendous capacity to absorb and release heat from the atmosphereLand not so much diffusion through the soil is slow and only affects the first meter or twoCool water surface turns over and has convective mixing to about 100 metersMaritime areas have milder seasonsMiddle of large continents have more intense seasonal cycles Averaging a seasonal cycleOut of balance because of the heat distribution from the water and from the windThe outgoing heat in the tropics cant keep up with the incoming solar radiationThe heat is carried to cooler, higher latitudes by water and windsThe Earth can vent the excess heat to space from the higher latitudes

Heat carried to higher latitudes for venting to space

The Coriolis Accelerationhttp://www.youtube.com/watch?v=i2mec3vgeaI

http://www.youtube.com/watch?v=aeY9tY9vKgs

http://www.youtube.com/watch?v=iqpV1236_Q0

Two clips on the Coriolis Effect and one shows a Foucault Pendulum, demonstrating the rotation of the Earth.

Coriolis EffectThe water and the air feel the most effect at the poles (incredibly high tides in higher latitudes, nearly no tide difference at the equator)

At the equator there is no apparent rotation

The middle latitudes fall somewhere between these two extremes

Modeling the WeatherFluids are governed by Newtons Laws of Motion because fluid has mass and inertiaInertia is the sluggishness of matter to resist changes in motionTendency to keep moving if its moving, or remain stationary if it is already stationaryTo change speed or direction, motion requires a force such as gravity or a change in pressure (weather) Bathtub vs. EarthBathtub flows more quickly than the Earth rotates, so does not feel the Earths rotationFlows in the atmosphere and ocean persist long enough to feel the effect of a rotating EarthOcean flows can be driven by friction with the windCoriolis acceleration tries to deflect the flow to the right in the northern hemisphereAfter a few rotations, a steady state is reached where the fluid flows 90 degrees to the wind

The eventual steady state

Top the fluid initially flows in the direction of the wind. Middle after a while the Coriolis force swings the fluid to the right. Eventually, the fluid itself flows 90 degrees to the wind or pressure force, and the Coriolis force just balances the wind or the pressure force. Bottom the steady state where the flow stops changing and remains steady.Geostrophic FlowIn a rotating world the fluid will eventually end up flowing completely crossways to the direction that Its pushed. This condition is called geostrophic flow.

A geostrophic flow balances the forces on it against each other.Geostrophic cells on weather mapsCells of high pressure and low pressure with flow going around them

Low pressure, pressure force points inward, 90 to the right of that the winds flow counterclockwise in the N. hemisphere cyclonic direction of flow

High pressure, pressure force points outward, and the flow is clockwise around the high pressure anticyclonic direction

Surface wind field from a climate model (computer generated)

Parameterization, assumptions in modelsAssume that cloud formation is a function of humidity in the air, humidity is a parameter that would control cloudiness

Effects of turbulent mixingAir-sea processes such as heat transferBiology modeling

Take home points chapter 6The energy budget to space of a particular location on Earth is probably out of balance, fluctuating through the daily and seasonal cycles and with the weather, This is in contrast to the Layer Model.The annual average energy budget for some location on Earth may not balance either, because excess heat from the tropics is carried to high latitudes by winds and ocean currents.The global warming forecast requires simulating the effects of weather, which is a really difficult computational challenge.Global Warming, Understanding the Forecast

Chapter 7, Feedbacks

Complexity in the Earth system arises form the way pieces of it interact with each other

Positive and Negative FeedbacksA feedback is a loop of cause and effectAt the center of a feedback is a state variable (average temperature of the Earth)A positive feedback makes the temperature change larger than it would be without the feedbackA negative feedback counteracts some of the external forcing, and tends to stabilize the state variable

Feedbacks:A positive feedback is an amplifier

A negative feedback is a stabilizerStefan-Boltzmann FeedbackNegative feedback a stabilizer

The radiated infrared heat attempts to pull the temperature back down

Ice Albedo FeedbackPositive feedback an amplifier

Ice albedo feed works on the state variable of temperature. An input perturbation, such as a rise GHG, drives temperature up. Ice melts, reducing the albedo, and warming the ground up a bit. The direction of the input and the feedback loop agree with each other. It can also go in the other direction, perturbation cools things down and feedback agrees.

Water Vapor FeedbacksPositiveNegativeWater is involved in a positive feedback loop acting on global temperature Warming allows more water to evaporate before it rainsWater vapor is a GHGDoubles the climate impact of rising CO2 concentrationsWithout the water vapor feedback, climate would be less sensitive to CO2There is a negative feedback loop that controls the amount of water vapor in the atmosphere at any given temperature, having to do with rainfall and evaporation (the hydrological cycle)At the center of the feedback loop is a state variable

Runaway Greenhouse EffectIt is possible for the water-vapor feedback to feed into itselfMeans the end of a planets waterEarths climate uses the high latitudes as cooling fins to avoid the runaway greenhouse effectA runaway greenhouse effect stops if the vapor concentration in the air reaches saturation with liquid water or ice, so that any further evaporation would just lead to rainfall or snow

Phase diagram shows that Venus had a runaway GH effect, but not Earth and Mars

Triple point of water

Pressure: 0.006207 atm

Temperature:0.01C (273.16 K)Earth retained its waterEarth has its water because of the structure of the atmosphereThe tropopause acts as a cold trap, making sure that water vapor rains or snows out before getting too close to spaceThe oceans are protected by a thin layer of cold air for billions of years now The Hadley circulation controls the distribution of atmospheric water vapor warm air rises at the equator, it cools and water condensesCloudsCirrus high altitude thin and wispy, barely noticeable, and made of ice crystals

Cumulus clouds storm clouds are towers, the result of focused upward blasts of convection

Stratus clouds low clouds layered, formed by broad diffuse upward motion spread out over large geographical areasClouds:Interfere with both incoming visible light, and outgoing IR lightIn the IR, clouds act as blackbodies, warming the planetIncoming visible light is reflected back to space, cooling the planetThe overall impact of a cloud depends on which of these two effects is stronger, which in turn depends on what type of cloud it isEarths Energy BudgetThe difference between Earths energy budget between absorbed and scattered sunlight is that when light is scattered back to space, its energy is never converted to heat, so it never enters into the planets heat budget Clouds:Vary by meteorological conditions and human pollution

Cloud droplet size is an important factorThe smaller the drop, the better it scatters lightRain clouds look dark because they have large droplets, and are optically thick

Cloud droplets are affected by cloud condensation nuclei (seeds) that help droplets formSea salt, pollen, dust, smoke, and sulfur compounds from phytoplankton

Human footprintsSulfate aerosols from coal fired power plantsInternal combustion enginesForest fires, heating fires and cooking firesContrails (short for condensation trails) jet airplanes passing through clean air containing water vaporPersistent spreading contrails are thought to have a significant effect on global climateGeneralities - You cant see through low clouds meaning they are optically thickYou can see through high clouds, optically thinHigh clouds warm, low clouds coolClouds that form in dirty air tend to be better light scatterers with a higher albedo, cooling the planetClouds are the largest source of uncertainty in climate models

Ocean Currents, el Nio climate oscillationPeriodic flip flop between two states of the ocean called el Nio and la NiaOcean interaction with the atmosphere, corresponding atmospheric cycle called the Southern oscillationENSO el Nio Southern OscillationThe state of the ENSO affects climate patterns around the worldEl Nio climate oscillationLa NiaEl NioCool surface water

Productivity high

Fisheries good

Equatorial EW wind

Tilted thermocline

Wetter weatherWarm surface water

Less fertile

Fisheries collapse

Winds diminish

Thermocline collapses

Drier weather

Meridional overturning circulation in the North AtlanticGulf stream carries warm water from tropics to the North AtlanticWater cools and sinks, making more room for warm waterGreenland ice cores show instability in Meridional overturning synchronous with large temperature swings (~ 10C) within a few years8.2k event (8200 years ago) catastrophic freshwater release to the North AtlanticCirculation will slow down with melting iceTerrestrial Biosphere FeedbacksChanges in vegetation could alter the albedo of the land surface when ice meltsLand surface stores carbonTrees evaporate water through transpiration (a self-replicating cycle)Droughts, vegetation dies, soil dries, and the water shortage is a positive feedbackCarbon Cycle FeedbacksThe subject of the next three chapters (Module 7)Feedbacks in the Paleoclimate RecordModels tend to under-predict the extremes of climate variation in the real world climate

The future may surprise usTake home points chapter 7Positive feedbacks act as amplifiers of variability, whereas negative feedbacks act as stabilizers.

The water-vapor feedback doubles or triples the expected warming owing to rising CO2 concentrations.

The ice albedo feedback amplifies the warming in high latitudes by a factor of three or four.ContinuedClouds have a potentially huge impact on climate. Clouds are expected to exert an amplifying feedback to climate warming, although the strength of this feedback is uncertain. Clouds are the largest source of uncertainty in model estimates of the climate sensitivity.