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Chapter 1

The Greenhouse Effect

In this chapter the greenhouse effect will be discussed. First of all the energy balance of the earth will beoutlined followed by several models to describe the greenhouse effect.

1.1 The energy balance of the earth

So far the info on what drives our sun and how the energy of the sun reaches us. Now starting with theGreenhouse effect we can have a look at figure ??. When looking at the earths surface there is energy comingin and there is energy going out. There are two sources of energy of the surface: the sun and the heat comingfrom the earth core. However, the latter is with 0.06 W/ m2 negligible due to the insulating earth crust. Theleft hand side shows the incoming sunlight which equals 342 W/m2 on average on earth. 107W / m2 of that isreflected either by clouds/air/dust or by the earths surface. Because it is reflected it goes out with the samecolor of light as it went in. The middle and right hand side shows how the energy that is not reflected butabsorbed finds its way back to the universe. Most is radiated back by surface radiation. This is nothing elsethan black body radiation, but now the black body is the earth surface, with a temperature which is muchlower and therefore radiation with a much longer wavelength in the infrared. Now we saw above that infraredis heavily absorbed in the atmosphere. For that reason the atmosphere heats up and starts radiating in turn.Part of this radiation escapes to the universe, part comes back to earth.

Figure 1.1: The energy balance of the earth

1.1.1 Energy Absorption

This section explains how atmospheric molecules absorb infrared radiation. The atoms within the moleculescan vibrate, rotate and bend in various molecular structure dependent ways. Each special type of motion, ormode, has a special frequency. These frequencies are recognized in the transmission spectra: where there is lesstransmission there is a certain mode that can be excited by the infrared radiation.

There are also translations and rotations of the whole molecules possible. It happens to be the case that

vibrations and rotations obey quantum rules. This means that not all arbitrary frequencies can be excited, butonly frequencies obeying certain multiples of a quantum unit. The energy levels are quantized. Such quantizedenergy level spectrum can be observed for e.g. nitrogen. As strong lines in absorption or emission.

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1.2. TWO-LAYER MODEL OF GREENHOUSE WARMINGCHAPTER 1. THE GREENHOUSE EFFECT

Figure 1.2: The energy balance of the earth

For the Greenhouse effect it is essential to note that most incoming solar radiation actually passes theatmosphere as visible light because there are no strong absorbing molecules in the atmosphere. When theserays heat the surface this in turn radiates in infrared (IR). Then the atmosphere is not transparent for IR.

Next we will have a look at the absorption for different spectral bands (can also be seen in figure ??

H2O: strong band of absorption in the IR.

CO2: fewer strong bands.

O2 and O3: most important is the absorption in the ultra violet (short wavelengths)

CH4: this has only two distinct narrow absorption peaks. Total: the sum has strong absorption bands,where most appears to be due to H2O (except the UV part)

To sum up it can be said that emission of electromagnetic radiation both of a body like the earth or the sun orportions of the troposphere and stratosphere follows the Planck distribution (i.e. radiation is from a ‘thermalizedbody) The temperatures of sun and earth and composition of atmosphere are such that atmosphere is almosttransparent to solar radiation but not transparent to terrestrial (longwave length) radiation. The absorptionof radiation by the atmosphere happens at discrete wavelengths (frequencies) reflecting the quantized natureof molecular oscillations. Vibration-rotation transitions give rise to band structure of absorption regions. Andlastly large amount and a fine structure of absorption lines complicates climate simulations.

1.1.2 The Albedo coefficient

The albedo coefficient is the fraction of solar radiation (visual light) which is reflected. For different surfacesthe reflection varies. Obviously white surfaces reflect more light whereas black surfaces absorb it.

1.2 Two-layer model of Greenhouse warming

If we want to create a model of the greenhouse warming, we first need to set up some basic parameters.First of all a fraction α p is reflected back to space. Furthermore the atmosphere is transparent to the

remaining solar radiation. Next two layers are set up which are positioned at 0.5 km and 2 km height andacting as blackbodies. It is furthermore assumed that effects of dynamics (convection and large-scale circulationon temperature) is neglected. For the model which is first described in figure ?? we need to add some extraparameters:

L0 = total radiant energy emitted by the sun

Solar constant = Flux density at distance d from the sun (S d)

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1.3. C O2 CONCENTRATION IN HISTORY CHAPTER 1. T HE GREENHOUSE EFFECT

Figure 1.3: Diagram showing the shadow area of a spherical planet

The at mean distance of Earth to the Sun , S 0 is equal to 1367 W / m2 and the mean energy reveived byearth with radius r is 0.25 S 0

Hereafter we can go a step further and set up the two different layers above the surface which can be seenin figure ??. The layers are absorbing all IR going through it, but transmitting all solar light. When the

atmosphere layers become warm they radiate as black bodies with σ T 4

W/m2

. 0.25 S 0 equals 341 W/m2

,which is the value of around 12 sheets ago for the incoming solar radiation averaged over the earths surface.

Figure 1.4: Two layer model

For each layer the incoming radiation is in equilibrium with the outgoing. Only then it has a fixed temper-ature T. At the surface solar radiation comes in (α is reflected) and the IR from the first layer of atmosphere(proportional to its T 4). IR from the second layer does not reach the surface because layer 1 absorbs it. Outgoingfrom the surface is only black body radiation depending on the surface temperature T 4s .

1.3 CO2 Concentration in history

H 2O is the most important Greenhouse gas, and it is present in the atmosphere just because of the many oceansand land areas from which it evaporates. H 2O also spontaneously disappears due to precipitation (rain). CO2

is a Greenhouse gas which is much less abundant, but when present it hardly disappears from the atmosphere.It is changing its concentrations with time due to various influences, the burning of fossil fuels is one of them.

There are clear correlations (see figure ?? between temperature and CO2 concentrations. Note that suchcorrelations do not directly tell what is the consequence of what: higher temperatures are a consequence of higher CO2 concentrations or the other way around. These data go back to ¿400000 years.

How can one know temperatures and CO2 concentrations of that many years ago? It is based on measuringthe gas concentrations in air bubbles that are present in very old ice layers in old glaciers near the poles.

one needs the age of the bubbles: count ice layers/ look at markers of known age like volcano eruptiondust, radioactive dating methods.

one needs to know the temperature: this is done by looking at the isotopic composition of H/D in theice. The more D the warmer the climate was because D reaches the colder pole areas easier when the Tis higher

CO2 can just be measured

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1.4. GREENHOUSE GASES CHAPTER 1. THE GREENHOUSE EFFECT

Figure 1.5: Vostok Ice Core Data

Correlation between CO2 concentration and D concentration. Note that when looking at figure ?? theamount of D is negative because it is compared to warm areas where D concentrations are higher. Less negative

concentration difference with warm areas means less low Ts in Vostoks history.The most recent statement of the IPCC concerning the anthropogenic (human induced) Green House Gas

(GHC) emissions: it is likely that this has caused warming on a global scale. When looking at the earth itcan be seen that the predicted temperature changes (IPCC reports, also on BB) as a result of GHG emissions.Note that the largest T changes are present at the Northpole. There are two reasons for that: the north pole isonly a thin layer of ice on top of water (2-5 m). If this melts there is more water flow higher up north so moreheat coming in, AND ther is less sunlight reflection by ice. The Southpole does have land under it at elevatedaltitude, that will not have these warme water currents. The second reason is that most land area is above theequator, and land area heats up faster than water, so it becomes on average warmer during the day.

Some definitions needs to be set up first before starting on the actual reasons for the global warming.

Atmospheric Lifetime: clearly when a gas is present for a long time after release it has more influence onclimate.

Radiative forcing: at the tropopause ( around 11km high, see some sheets further) more heat is receivedwhen the GHG is released because the stratosphere (11-50 km high) with this GHG radiates down on itmore.

Global Warming Potential: GWP is the ratio of the time-integrated radiative forcing from the instanta-neous release of 1 kg of trace substance relative to 1kg of a reference gas (which is CO2). Values dependon radiative properties and lifetime of the substance, and on time horizon considered

Climate Sensitivity: Models suggest that the perturbation of the global average, equilibrium surfacetemperature, T S , is related to radiative forcing, RF, by: δT s = λRF where λ is the climate sensitivityparameter.

1.4 Greenhouse Gases

The sources of GHGs are mainly at surface. There are synthetic and non-synthetic GHGs. For syntheticGHGs sources may be accurately known from production data. All greenhouse gases except CO2 , H2O are

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1.5. OTHER FACTORS FOR THE CLIMATE CHANGE CHAPTER 1. THE GREENHOUSE EFFECT

removed from atmosphere primarily by chemical processes within atmosphere. CO2 is practically inert inatmosphere. Lifetime concept does not apply. H2O in atmosphere is mostly controlled by temperature throughthe Clausius-Clapeyron equation. The treenhouse gases are removed by reaction with hydroxyl radicals (OH) inthe troposphere: mainly those that contain H: CH4 , HFCs, HSFCs. The lifetimes of the order of years to several100 years. The greenhouse gases removed by photolysis in the stratosphere and mesosphere: N2O,PCFs,SF6,CFCs. The Lifetimes are in the order of several thousand up to 50000 - years.

After having given general specifications about GHG we will look at some of them in detail.

1.4.1 Methane

Let us start with looking at Methane. Methane is 21 times CO2. Other fluoro compounds are much worse(reason for ban on these compounds). Methane arises from natural resources (wetlands, termites, ocean) butalso from anthropogenic sources (Energy, Landfills and so on).

Methane has not such large IR absorption bands (see earlier sheets) but nevertheless it has more GHwarming potential than CO2. How can that be? There appears to be an indirect mechanism: CH4 can reachthe stratosphere, oxidize to water and CO2, and thus produce the strong greenhouse gas H2O at high altitude.In the process it consumes OH-.

Method to see if methane is released as a result of fossil fuel release or from vegetation. Fossil fuels havebeen underground for millions of years so there is no radioactive 14C present anymore. In recent vegetationthere is 14C because it is exposed to the atmosphere in which 14C is produced from nitrogen.

1.4.2 Water

Water vapor most important GHG as well as most important for anthropogenic caused perturbation of naturalgreenhouse effect. Water vapor feedback: increased temperature leads to higher water vapor pressure accordingto Clausius Clapeyron Equation (exponential increase) which increases Greenhouse warming . At high altitudenot much water is present: it freezes.

Water normally freezes and falls down before reaching the top of the troposphere. Methane can pass thatcold trap and produce H2O higher up.

1.5 Other factors for the climate change

Clearly there have to be other factors than human caused factors: there have been ice ages and hot periodsbefore.

Sun spots reflect a varying solar activity with a typical period of 11 year. This fluctualtion is of the orderof 1W/m2 . However, there also might be far longer timecales on which the sun fluctuates in activity.

Figure 1.6: Sun Periods

Fluctuation in cosmic particle intensity; this is mainly fast particles coming from the sun (protons, muons,neutrons, ) and therefore it also fluctuates mainly with solar activity. These fast particles can stimulate thecondensation of water in the atmosphere: cloud formation. This changes the albedo, which could have asignificant effect.

If you have fast charged particles from a radioactive source one can see these particles draw a track of smalldroplets in a saturated environment of a cloud chamber. This happens because the charged particles ionize the

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air in the chamber and ionized air happens to form droplets faster. So these cosmic particles really can causecloud formation.

Cause for ice ages, etc? These were caused without human intervention. The rotation of earth around thesun could have changed (due to the other planets). Also the earth rotation axis can change with time due toinfluence of other planets. Important to note is: whatever the cause for climate change the GH effect just workson top of that. More GH gasses means that climate changes are also multiplied. If there is only an increase inGH densities this will enhance the global warming because it changes the multiplification factor.

Feedback loops can increase the effect of global warming: higher temperatues lead to defrosting permafrostareas resulting in enhanced decomposition of vegetation remnants. This leads to more methane; a potent GHG.Another loop: warming up ocean waters releases CO2, leading to more warming, more CO2 release etc.

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