The Chemistry of Climate Change. Reading: Chapter 8 Environmental Chemistry, G. W. vanloon. S. J. Duffy

Transcription

1 The Chemistry of Climate Change Reading: Chapter 8 Environmental Chemistry, G. W. vanloon. S. J. Duffy

2 The Science of Global Climate There's a lot of differing data, but as far as I can gather, over the last hundred years the temperature on this planet has gone up 1.8 degrees. Am I the only one who finds that amazingly stable? I could go back to my hotel room tonight and futz with the thermostat for three to four hours. I could not detect that difference. -Dennis Miller

3 Solar Radiation top of atmosphere q: zenith angle h q L Earth

4 Solar Radiation cosq h L L h cosq hsecq m length of path of direct solar radiation through th e atmosphere length of vertical path throu gh the atmosphere m L h m: air mass

5 Solar Radiation I I 0 tm e t: attenuation coefficient t t sg t ag t sp t ap Light scattering by gases Light absorption by particles Light absorption by gases Light scattering by particles

6 10 1 Solar Radiation Attenuation Coefficient Aerosol Extinction Rayleigh Scattering 10-2 Ozone Absorption Wavelength (nm)

7 Solar Radiation F sun 2 earth 2 earth 2 ( r ) (1389Wm )( r ) W Global energy use by population ~ 1x10 13 W 1 part in % for U.S. Urban Areas

8 Solar Flux solar flux r 2

9 100 Units Incident Albedo Clouds Aerosols 6 ( ) Albedo

10 max 5 2 / ) / exp( 1 2 ) ( K Wm c h k T emitted energy total F T kt hc hc J Solar Radiation peak wavelength (m) Wein s Law Planck s Relation

11 Temperature at the Surface of Three planets: D=T actual -T calculated Calculated T/K Actual T/K D Earth Mars Venus

12 Greenhouse Effect 300 nm

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14 Greenhouse Gases Species Absorption Region Estimated Global Warming H 2 O 2.5, 3.5, 5-7 mm ~110 Wm -2 CO mm ~50 Wm -2 CH 4 3-4, mm ~1.7 Wm -2 O mm ~1.3 Wm -2

15 Global Warming Potential The GWP of a greenhouse gas is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas (IPCC 2001). Direct radiative effects occur when the gas itself is a greenhouse gas. Indirect radiative forcing occurs when chemical transformations involving the original gas produces a gas or gases that are greenhouse gases, or when a gas influences other radiatively important processes such as the atmospheric lifetimes of other gases. The reference gas used is CO 2, and therefore GWP weighted emissions are measured in teragrams of CO 2 equivalents (Tg CO 2 Eq.) All gases in this executive summary are presented in units of Tg CO 2 Eq. The relationship between gigagrams (Gg) of a gas and Tg CO 2 Eq. can be expressed as follows: Tg CO2 Eq (Gg of Gas) (GWP) (Tg/1,000 Gg)

16 Global Warming Potential GWP 0 H 0 H a ( t) C a i C ( t) C i C ( t) dt ( t) dt Due to species i Due to CO 2 a ( t) : i Radiative forcing (net change in the flow of radiative energy per unit mass) C i ( t) : Concentration of Greenhouse gas

17 Relative Contributions to Global Warming Residence Radiative Global Warming time/y Forcing Potential CO CH N 2 O CFC CFC HCFC

18 GAS GWP Carbon dioxide (CO 2 ) 1 Methane (CH 4 ) 21 Nitrous oxide (N 2 O) 310 HFC-23 11,700 HFC-32 2,800 HFC-125 1,300 HFC-134a 3,800 HFC-143a 140 HFC-152a 2,900 HFC-227ea 2,900 HFC-236fa 6,300 HFC-4310mee 1,300 CF 4 6,500 C 2 F 6 9,200 SF 6 23,900 Global Warming Potentials

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21 Increase in CO 2 Atmospheric CO 2 levels have increased 26% since the industrial revolution Total Dry Mass of Atmosphere: 5.13x10 18 kg Average molar mass of the atmosphere: 29 g/mol or 1.8x10 20 moles Mass of Carbon from fossil fuel combustion per year: 6x10 12 kg or 5x10 14 moles increase of 5x10 14 /1.8x10 20 =2.8 ppmv

22 Effect of Greenhouse Gases An excess of 1% of absorbed over emitted flux for one year would be equivalent to a heating of about 7 C of the entire atmosphere.

23 For the first time in my life, I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light - our atmosphere. Obviously, this was not the ocean of air I had been told it was so many times in my life. I was terrified by its fragile appearance. Ulf Merbold, German Astronaught

24 Evolution of Trace Gas Mixing Ratios Preindustrial CO 2 (ppmv) CH 4 (ppbv) N 2 O(ppbv) CFC-11(pptv) CFC-12(pptv) CFC-113(pptv) CFC-114(pptv) CFC-115(pptv) CCl 4 (pptv) CH 3 Cl(pptv)

25 Combustion Coal C + O 2 CO 2 DH= kj Natural Gas CH 4 + 2O 2 CO H 2 O DH= kj Heavy Oil C 20 H ½O 2 20 CO H 2 O DH= kj

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30 Climate Climate is characterized by the statistical properties of the weather over a period of time, including averages of local variables such as temperature, winds, humidity, pressure, and precipitation.

31 Feedback Feedback is the influence of system responses on the operation of greenhouse gas forcing either tending to reinforce the main process (positive feedback) or resisting the main process (negative feedback). Water vapor feedback. Due to greenhouse warming, more water evaporates and more water vapor is present in the atmosphere. But water is a greenhouse gas, so one result of greenhouse gas emissions is to increase the concentration of another greenhouse gas, a positive feedback.

32 Feedback Snow/ice albedo feedback. A warmer climate results in less snow and ice cover and therefore the albedo is reduced, a positive feedback. However, because atmospheric circulation and polar atmospheric stability may also be affected, the situation is difficult to analyze.

33 Feedback Cloud feedback. Clouds cause warming by closing the window to the escape of earthlight, a positive feedback; but they also increase the albedo by efficiently reflecting solar radiation, a negative feedback. The net result averaged over the earth s surface is calculated to be a cooling, a negative feedback. However, if radiative forcing increases substantially, the analysis is more complicated. Although warming increases humidity, higher temperatures may reduce cloud cover. Similarly, warming may increase the water content of clouds, increasing the efficiency of solar reflection, but also increasing the efficiency of infrared absorption.

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37 Virtually certain facts not dependent on the accuracy of climate models: 1. Greenhouse gases are increasing as a result of human activity. 2. The increasing concentration of greenhouse gases produces an increase in radiative forcing. 3. Anthropogenic greenhouse gases remain in the atmosphere from decades to centuries. 4. Over the past century, global surface temperatures have increased by 0.5 C (±0.2 C). 5. Increases in CO 2 have produced about 1 C stratospheric cooling. The cooling is caused by increased infrared-emitting CO 2 in the stratosphere, where the emissions are not intercepted by an absorbing atmosphere. 6. Natural climate variability makes it difficult to detect long term trends. 7. Key uncertainties about the effects of water in the system, especially clouds, are likely to remain a modeling problem for a decade or more.

38 Very probable projections. These projections are likely to be true (~90% probability) within stated ranges: 1. The increase in global average surface temperature over the past century is consistent with model projections. No viable alternative explanations for the warming have been identified. 2. If atmospheric CO 2 is doubled compared with pre-industrial levels (which is likely to occur in the next century), equilibrium global warming is 1.5 C to 4.5 C. 3. Because the effect of CO 2 saturates, the forcing does not increase in proportion to CO 2 concentration. 4. By 2100 AD, sea level will rise by 50 cm (±20 cm). The pace is slow because the main mechanism, simple warming of the water, is slow. 5. Global mean precipitation will increase by 2% (±0.5%) for each 1 C of warming, due to increased evaporation and enhancement of the hydrologic cycle. 6. There will be substantial changes at high latitudes in the Northern Hemisphere. Temperature increases will be far above the global average, sea ice will be reduced, and precipitation increased.

39 Probable projections. These projections have a greater than even chance of being correct: 1. In northern mid-latitudes, soil moisture will be reduced in summer. 2. Around Antarctica the oceans are resistant to warming and little change is expected for a century or more. 3. Formation of deep ocean currents at high latitudes of the North Atlantic will be attenuated. Thus the Gulf Stream which carries warm water into the North Atlantic and the associated deep ocean return current may be threatened by increased fresh water in the Arctic, which would reduce salinity and thus the descent of water which initiates the return current. 4. Once formed, tropical storms will become more intense on average. 5. The range of natural temperature fluctuations will remain about the same.

40 Unsubstantiated projections. These projections are plausible, but are unsupported at present by climate science: 1. The number of tropical storms, hurricanes, and typhoons will increase. 2. The winds in mid-latitude cyclones (low pressure areas) will become more intense. There is no evidence for either projection.

41 Websites of Interest