Greenhouse Effect Mechanism and Radiative Forcing

Transcription

1 Greenhouse Effect Mechanism and Radiative Forcing How does radiative energy balance help determine Earth s climate? How does the greenhouse effect work? What is radiative forcing? References AR4 Ch. 2 Hartmann,

2 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

3 Energy enters and leaves Earth system via electromagnetic radiation albedo = fraction of radiation reflected by object albedo = α =.3 for Earth mostly clouds, snow, ice, also aerosols, sand, pavement, etc. Addison-Wesley Longman

4 Radiation & Energy wavelength (μm)

5 Black Body Law wavelength (μm) based on Pierrehumbert, Principles of Planetary Climate, ch 3

6 Energy Balance Equilibrium temperature E out = E in (Equilibrium) E = total energy flux (W) Incoming E in = (1-α)SA c Outgoing E out = σt 4 A E S = solar radiation 1360 W/m 2 (S/4 = 340 W/m 2, (1-α)S/4 = 238 W/m 2 ) T = surface temperature, σ = Stefan-Boltzmann constant A c = area of Earth cross section = πr 2 A E = area of Earth = 4πr 2 r = Earth radius 6400 km 4σT 4 = (1-α)S T = [(1-α)S/4σ] 1/4 T = 255 K = -18 C Actual surface T = 288 K = 15 C Prediction is about 11% too cold. Why?

7 Much of the upward radiation ( IR or longwave ) is absorbed by the atmosphere Black Body Radiation downward solar upward longwave visible Absorption of Upward Radiation by Atmosphere Absorption has complicated dependence on wavelength. Often idealize gray atmosphere neglecting wavelength dependence. Hartmann (1994) from Goody & Yung (1989) Fig 1.1

8 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

9 The Greenhouse Effect Solar radiation passes through the atmosphere and warms the Earth s surface The Earth emits thermal radiation (also called infrared radiation) back to space, part of which is absorbed by the molecules of greenhouse gasses (water vapor, H 2 O; carbon dioxide, CO 2 ; some other micro gases) in the atmosphere and warms the atmosphere This warming effect of the greenhouse gases is called the Greenhouse Effect (Global Warming: The Complete Briefing by John Houghton)

10 The Greenhouse Effect 1827: Fourier pointed out the similarity between the blanketing effect of greenhouse gases in the atmosphere and what happens in a real greenhouse, hence the name: Greenhouse Effect (Not a good analogy) 1860: Tyndall measured the actual absorption of infrared radiation by CO 2 and H 2 O 1896: Arrhenius calculated the effects of increasing greenhouse gases in the atmosphere; estimated the magnitude of global warming due to doubling of CO 2 (Global Warming: The Complete Briefing by John Houghton)

11 The Greenhouse Effect Natural Greenhouse Effect: Due to greenhouse gases present for natural reasons; these gases (viz. CO 2 ) were in the atmosphere (except CFCs) long before human beings came on the scene. Enhanced Greenhouse Effect: The additional greenhouse effect caused by the additional greenhouse gases in the atmosphere due to human activities (fossil fuel burning; deforestation, etc) If there were no greenhouse gases (hence no greenhouse effect) the Earth s temperature would be -18 C (not +15 C as it is at present) Greenhouse effect is real; without it, the Earth would be uninhabitable. But how does it work?

12 Radiation in atmosphere can be: transmitted (continues unimpeded) absorbed (energy captured by matter) emitted (energy released by matter) scattered (changes direction) reflected (reverses direction) Radiation Transmission, Absorption, Emission Schematic shows typical behavior: short wave from sun mostly transmitted (scattering & reflection also important but not shown) longwave emitted as B.B. radiation by ground and different levels of atmosphere (colder levels of atmosphere emit longer wavelength, weaker flux) (radiation emitted equally in all directions, both upward & dnward) Longwave is also absorbed by atmosphere

13 Optical Depth Note: k depends on wavelength

14 Optically thick = large τ = large absorption Optically thin = small τ Earth atmosphere optically thick Longwave escaping into space mostly from high in the atmosphere Emission Level = altitude longwave escapes from actually a range of altitudes mostly upper troposphere Crude model of greenhouse effect: atmosphere transparent to solar completely absorbs longwave

15 TOA (Top Of Atmosphere) energy balance: (1-α)S/4 = σt A 4 surface energy balance: (1-α)S/4 + σt A4 = σt S 4 T A indep of atmospheric details also called Emission Temperature For Earth, T A = 255 K Surface temperature warmer, T S = 2 1/4 T A 1.19T A What s going on? Can describe as Downward radiation from atmosphere warms up surface relative to no-absorption case or Surface needs to heat up more to generate enough radiation to heat up atmosphere enough to emit at emission temperature Can generalize to n layers: T S = (n+1) 1/4 T A What does adding more layers mean? Each layer just thick enough to absorb most of radiation. More layers greater optical depth stronger greenhouse effect (Saturation argument isn t valid)

16 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

17 WAIT! Calculations were based purely on radiation. Actual vertical energy transport has another term: atmospheric motions. Convection warmer air goes up, colder air goes down water vapor in rising air condenses, rains & releases latent heat additional upward heat transport large scale weather tends to have similar effect lower θ Dry air will convect if potential temperature θ decreases with height. Moist air can convect even if θ increases with height if enough heat release from condensing water vapor Large scale motion has more complex conditions for overturning higher θ NOAA training material via Wikipedia Thunderstorms

18 Can produce model that calculates T(z) based on radiative balance (like layer model, but include details of absorption and emission & many layers Radiative equilibrium tends to create strong temperature gradients θ decreases with altitude Can represent atmospheric motion with adjustment : if dt/dz too big, automatically set it to given value. Lapse Rate = dt/dz. Dry adiabatic: uniform θ. 6.5C/km: observed dt/dz. Manabe & Strickler (1964, J. Atmos. Sci.)

19 If lapse rate is fixed in convective part of troposphere, and emission temperature is held fixed (because S and α fixed) increasing greenhouse gas (GHG) concentration increasing altitude of emission temperature & increasing surface T Calculations with fixed relative humidity go back to 1960s. These models made assumptions about humidity, lapse rate, clouds. Correctness of those assumptions still under discussion! pressure (mb) altitude (km) temperature (K) Manabe & Wetherald (1967, J. Atmos. Sci.)

20 Closer look at shortwave and longwave radiation at TOA and Surface Kiel and Trenberth (1997, Bull Amer Meteor Soc)

21 How well do we know S? In recent decades, solar irradiance measured directly by satellite. daily, 11-year, and longer timescale variability O(.1%) Instrument bias disagreement of a few W/m 2 (1.4 W/m 2 =.1%) Kopp and Lean (2011, Geophys Res Lett): about 1361 W/m 2 rather than old value of 1365 W/m 2

22 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

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24 Components of energy transfer estimated through observations & models Some things to think about: What determines all these numbers? How might they change? To what extent do they determine other climate features? surface temperature atmospheric temperature distribution hydrological cycle

25 Think of climate as a system. It is affected by inputs. It has components that affect each other Not always easy to separate these: One person s boundary condition is another person s thesis. Some factors may be obviously unaffected by system and can be external. Other factors may only be weakly or indirectly affected by system. Sometimes we want to specify internal factors as if they were external. the system inputs component 1 component 2 What are the elements of the climate system?

26 External inputs in red boxes

27 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

28 Radiative Forcing (RF) The change in net (down minus up) irradiance (solar plus longwave, W/m 2 ]) at tropopause (top of troposphere) after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values (Ramaswamy et al. 2001, IPCC Third Assessment Report) Some Alternative Definitions of Radiative Forcing Blue line: The unperturbed temperature profile Red line: the perturbed temperature profile

29 Why Radiative Forcing? ΔF = RF = net change in irradiance at tropopause, with stratospheric T in radiative equilibrium, and surface and troposphere state unchanged. RF is a way to measure direct effect of factors that change radiative equilibrium. Factors include: greenhouse gases aerosols (particles and drops floating in the atmosphere) solar irradiance surface albedo Strength of each factor measured differently RF is common denominator. Effects can be complicated: example: more greenhouse gas temperature change circulation change cloud change albedo change etc. RF measures direct effect on radiation, leaves out further resulting changes.

30 Radiative forcing may eventually create new equilibrium: no net change in downward irradiance (change in one component equal & opposite changes in other components) What is ΔT = change in surface temperature from old equilibrium to new equil.? ΔT = λδf λ = climate sensitivity parameter In general, depends on original state of system ΔF but we often assume λ constant (linear system). Estimating sensitivity λ is one of the big goals of climate change research.

31 Energy Balance of Earth Greenhouse Effect Vertical Radiative-Convective Balance Climate System and Energy Budget Radiative Forcing Stratospheric Temperature

32 How do greenhouse gases affect stratospheric temperature? stratosphere represented by 1 layer tropospheric emission coming up from below solar radiation coming down from above stratosphere emitting upwards & downwards IR absorption = IR emission (based on Hartmann, p 332)

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34 increasing GHG concentration cooling of stratosphere

35 Radiative-convective models show troposphere warming & stratosphere cooling Observations show stratosphere cooling but high-latitude ozone loss may be responsible for some cooling also. (Manabe & Wetherald, 1967, JAS)