Vietnam part 6 (optional)

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Ngày đăng: 01/12/2016, 18:46

1 From the best estimates of temperature over the last millennium, the mean global surface temperatures observed in the last decade are thought to be warmest, although, given the uncertainty in estimates of mean global surface temperature, warmer decades could well have occurred (the grey areas give an indication as to the uncertainty) More reliable estimates of mean global temperatures are thought to occur after about 1600 Certainly since 1900 a sharp increase in mean global surface temperature can be seen The extent of anthropogenic influence on mean global temperature is still uncertain, as there also appears to be large natural temperature variations That over the last 100 years, the increase in mean global temperature corresponds to increases in, or introduction of, certain trace gases that can strongly increase absorption of infrared radiation is not in debate However, increase in absorption of infrared radiation is only the first step in a more complex interaction that apparently occurs in the Earth’s system The influence of many of these interactions is still uncertain and there are likely other important factors still to be uncovered Here we concentrate on what is known about the direct effect of trace gases on the radiative balance of the atmosphere Most of the large studies undertaken to construct the global temperature record over the last 100 years are in relatively good agreement The main consensus is that, since around the year 1910 there has occurred a rather abrupt increase in the Earth’s lower atmosphere temperature and that this increase was interrupted between 1940 and 1980, but since then has continued until today Abstracts from this publication: ABSTRACT (Xoplaki et al 2005): We evaluate variability, trends, uncertainties, and change of extremes of reconstructed and observed European spring and autumn temperature back to 1500 Spring and autumn temperature experienced systematic century-scale cooling compared to present conditions The coldest springs appeared during the Maunder Minimum (DT = -1 K wrt 1901–2000) The amplitude of spring temperature variations at decadal and multidecadal scales doubles that of autumn and is most expressed in northeastern Europe The decade 1995–2004 was very likely the warmest of the last half millennium Anomalously warm springs and autumns have generally become more extreme in recent decades However, the recent changes are statistically not significant with respect to the pre-industrial period ABSTRACT (Luterbacher et al 2004): Multiproxy reconstructions of monthly and seasonal surface temperature fields for Europe back to 1500 show that the late 20th- and early 21st-century European climate is very likely (>95% confidence level) warmer than that of any time during the past 500 years This agrees with findings for the entire Northern Hemisphere European winter average temperatures during the period 1500 to 1900 were reduced by ~0.5°C (0.25°C for annual mean temperatures) compared to the 20th century Summer temperatures did not experience systematic century-scale cooling relative to present conditions The coldest European winter was 1708/1709; 2003 was by far the hottest summer This slide is a reminder of an earlier slide presented in the course It shows that the average power input (think of this as Joules per second) to the Earth’s Atmosphere from the Sun is 1366 Wm-2, where the unit area is the cross-sectional area of the Earth This is a yearly average value that changes very little from decade to decade, implying that any small variations here cannot account for the observed change in Earth’s average surface temperature over the last 100 years This slide shows some types of reflective surfaces on the Earth that contribute to the overall average surface albedo of 0.31 Noteworthy are the effects of snow and cloud How would the surface albedo be effected by a temperature change of the Earth’s surface? Some of the electromagnetic radiation of the Sun that is intercepted by the Earth is reflected back into space On average the fraction reflected is found to be 0.31 (31%) The fraction is called the albedo,  Thus, taking this into account, we know the rate of energy absorption by the Earth (which includes absorption by its atmosphere) One must also note that other sources of energy are negligible (such as heating from the Earth’s interior and combustion of fossil fuels) If we assume that a steady-state is reached for which the rate of absorption of electromagnetic energy equals the rate of emitted electromagnetic energy, we can use the Stefan-Boltzmann relation to calculate the temperature of the Earth’s surface When this is done, the predicted average temperature of the Earth’s surface is 253 K, which is about 35 K less than the experimentally-determined average value of about 288 K The explanation for this discrepancy is quite simple and not at all surprising: the Earth’s atmosphere is insulating One is not referring here to conductive insulation (as one cannot conduct heat into space), one refers to radiative insulation Though it is not surprising that the outer part of a body (the atmosphere) provides some form of heat insulation, what is slightly unusual is that the part of the body that is mainly heated by the Sun lies within the insulating sheet It is the Earth’s surface and not the outer atmosphere that receives most of the Sun’s energy The Sun’s output energy maximum happens to correspond to a spectral region (visible) in which atmospheric molecules (especially O2, H2O, and N2) not absorb significantly Much of the radiated photons – at longer wavelengths - from the Earth’s surface however are absorbed by the atmosphere If one looks at the emission spectrum of the Earth from space one finds that it is equivalent to a black body at about 253 K It is just that it is slightly warmer under the ‘blanket’ The more absorbing the blanket, the warmer will the Earth’s surface become The Sun has a surface temperature of about 5800 K but just below the surface it is warmer than this Likewise the surface of the Earth/atmosphere system is 253 K, but it is difficult to define exactly at which height this surface is situated – as it varies with time and position 71 For calculations of the radiative forcing of due to ozone, one has to consider stratospheric ozone and troposphere ozone separately Stratospheric ozone is more complicated to treat since it has two roles Firstly a reduction of stratospheric ozone will result in less solar energy being deposited in the stratosphere Secondly, a reduction will decrease the amount of outgoing IR radiation from the from the troposphere (but only in those none saturated bands) 72 73 There are three characteristics that distinguish compounds containing C-F bonds from the other greenhouse gases so far discussed The first of these is the very large absorption crosssections for these molecules owing to the highly polar C-F bond The second is that the most important of these compounds absorb in the atmospheric windows and are thus not spectroscopically influenced by the other greenhouse gases Thirdly they are produced almost entirely by human activity All these amount to a very large relative contribution of these gases even though their concentrations are relatively low 74 75 Although there are quite a number of potential greenhouse gases, the most important of these are H2O, CO2, CH4, O3, N2O, CFC's and HCFC's This is due to a complex combination of factors that includes long atmospheric lifetime, large absorption cross sections, spectral position of absorptions, and abundance 76 A closer look at the emission spectrum of the Earth reveals the prominent absorption lines of H2O, CO2, O3, and CH4 A fit to the bottom of these absorption bands reveals the temperature of the absorbing molecules For H2O and CO2 this is about 215 K corresponding to the top of the troposphere The only relatively free spectral region, where emission comes directly from the Earth's surface lies between 800 cm-1 and 1000 cm-1 and 1070 cm-1 and 2100 cm-1 Any strong absorbers in this region will contribute to global warming 77 As can be seen from this table, the global warming potentials for CFCs and their substituent's are huge compared to the other greenhouse gases Naturally then, those molecules that were introduced as substitutes for CFCs to relieve pressure on stratospheric ozone need also to be phased out and replaced with compounds with lower GWPs 78 Overall then, halocarbons make a relatively large contribution to radiative forcing, but it is envisaged that this will become less as these substances are now being phased out 79 80 The development of substances suitable especially for refrigeration has gone in several directions, one of these is the production of Hydro-fluoro-olefins (HFOs) These molecules still poses polar C-F bonds, but they are endowed also with one or more C=C double bonds, which are readily attacked by OH radicals This severely limits their atmospheric lifetime preventing their long-term build-up in the atmosphere Their tropospheric degradation pathways are not fully researched 81 A discussion of the various other contribution to radiative forcing was presented in the lessen You are encouraged to look these up for yourselves You won’t be required in the examination to know the details of those not already discussed in the text, but you should be aware of why some are negative and why some are positive Notice that the total radiative forcing is calculated (modeled) to be around 1.6 Wm-2 To put this value in perspective consider how much atmosphere this 1.5 Watts has to heat If the atmosphere were compressed so that overall it was atmospheric pressure it would extend to 7.4 km (Remember the expression for the change of pressure with height P = P0exp(-H/7400), where H is in meters) Say that the troposhere would extend to about km as a first guess if it were all atmospheric pressure, then above every square meter of surface would be a column of air km in height A typical room in a house probably has a width of 3m and a height of 3m (let us say 10m2 width x height) Thus a room of this dimension would have to be 500 m long to contain the same amount of air A good radiator in a room is probably 1000 W But even using such a radiator, it would take many hours to increase the temperature of the room by K For the radiative forcing of the Earth we only have a heater equivalent to 1.6 W (note that an average light bulb gives out 50 W of power) 1.5 W is probably equivalent to a power input from a small bicycle lamp How long to you think you would have to wait to heat up a room 500 m long by K using the energy from a bicycle lamp? This can be easily calculated if you know the specific heat capacity of air It would probably take about a year, but owing to the very small heating rate much of the heat will be absorbed by the walls of the room (this does not happen if the heating rate is relatively rapid) Equivalently, in order to convert radiative forcing to heating rate in the troposphere one has to take into account then the warming of the oceans and, to a lesser extent, the land This makes the calculations more complex, but suffice 82 it to say that the response of the troposphere to radiative forcing is slow and a new equilibrium is expected to be reached over one or two decades only By contrast, the stratosphere adjusts to radiative forcing changes in a matter of about a month only 82 This part of the course has been devoted to radiative forcing, R, mainly by atmospheric gases In order to convert this to temperature change, one needs to model the various climate feedback mechanisms This is difficult and models can never account for every mechanism, even if they are known But an important conclusion nevertheless remains There exists a definite mechanism involving atmospheric gases, who’s concentration has increased due to human activity, that provides a positive forcing on the atmosphere These forcing may be tempered by feedback mechanisms but this is not at all sure So to end with an analogy, we know that adding coal to the fire of our steam engine locomotive will cause it to accelerate, what we don’t know yet for sure is whether or not the brakes will work 83 You may have noticed that a prominent dip in the Earth’s average temperature was determined during 1950 to 1980 This is now thought to be due to sufate aerosols released into the atmosphere by industry which served to reflect solar radiation back to space Since sulfur emissions have now been drastically reduced it seems that the Earth is back on its warming direction 84 [...]... lost from the troposphere, but expended eventually as heat (to the troposphere) So, in fact, only 66 Wm-2 from the sun goes into heating the Earth’s surface, but (66 + 78 + 24 =138) Wm-2 is deposited in the troposphere by the sun - all of this causes heating Additionally, the troposphere will receive (67 /2 = 33.5) Wm-2 of ‘band’ IR radiation from the stratosphere That is, half of the deposited energy... reflected to space; the greater part being reflected from the atmosphere (clouds and aerosols mainly) Though a significant fraction of the remaining light (67 Wm-2) is absorbed in the atmosphere – mainly by O3 and O2 below 350 mn – the greatest flux is absorbed by the Earth’s surface ( 168 Wm-2), which comprises direct radiation and scattered radiation from clouds and particles as well as Rayleigh scattering... m2 The length of the tube is not specified If you look down the tube using one particular wavelength you would in principle (but not in practice) observe the individual molecules moving around and having an area that is not their physical (collisional) area, but an area corresponding to the absorption cross-section (at that particular wavelength) of the molecule Is this picture correct ? Let us see Suppose... is entirely equivalent to treat it as a spectral separation of energy where some wavelengths can penetrate unabsorbed through higher surfaces These wavelengths then have a partial black-body spectrum of the warmer surface beneath 16 Previously, the radiative transfer was discussed in terms of several blocks contained in a perfectly insulating box with only the bottom block having an internal source... (neglecting those places effected by direct CO2 emissions and the temperature change with altitude) The main CO2 absorption band that blocks IR emission from the Earth's surface occurs at around 15 m (66 0 cm-1) This band corresponds to the ro-vibrational transitions associated with the CO2 bending mode The other main CO2 band lies outside the emission spectrum of the Earth and also outside the main... direct sunlight 26 The height of the emission from gasses in the atmosphere is not actually very well defined, since at each wavelength there is always a contribution to the observed intensity in space from various layers 27 Calculating the height at which a molecule essentially emits light to space is critical in calculating the radiative balance of the Earth This information can be partly gained from... initial assume that the change of one molecule being in front of another molecule is negligible In other word, we can see all of the molecules if we look down the box How many are there? There are 1 x 10 16 cm-3 x 100 cm (length) x 1002 cm2 (area) = 1 x 1020 molecules Each of these has an area, according to the photon, of 5 x 10-20 cm2 So the total area covered by the molecules is 5 x 10-20 cm2 x 1 x 1020... The total area of the box is 1002 cm2 = 1 x 104 cm2 So the fraction of light that can pass through is (1500/1 x 104) = 0.95 exactly Know let us use Beer-Lambert Law : Itr = Io x exp (-5 x 10-20 x 1 x 10 16 x 100) = 1 x exp(-0.05) = 0.951 So this picture is quite accurate (For large cross-sections or large concentrations the assumption that molecules do not 'hide' behind each other is not correct) An important... separation of energy, but it is entirely equivalent to treat it as a spectral separation of energy where some wavelengths can penetrate unabsorbed through higher surfaces These wavelengths then have a partial black-body spectrum of the warmer surface beneath 15 This slide and the previous one illustrate that when viewing a body via its radiative emission one must take account that perhaps some of the... energy range A gas though has a relatively few energy levels per unit energy range and it is therefore cannot produce a continuous black-body emission spectrum But how much power (J s-1) is emitted by a particular transition? According to what has been mentioned before about gasses in the atmosphere we would expect the power to be the same as that corresponding to a black body radiator, but this time
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