Energy Inputs and Outputs

Transcription

1 Energy Inputs and Outputs Sun Earth ultravilet visible infrared Bth Sun and Earth behave as blackbdies (absrb 100% incident radiatin; emit radiatin at all wavelengths in all directins) Earth receives energy frm sun in the frm f shrtwave radiatin with peak in the visible (λ = μm) Earth emits energy t space in the frm f lngwave radiatin in the infrared (λ = 5-20 μm) functin f Earth s temperature SOLAR RADIATION SPECTRUM: blackbdy at 5800 K Ttal Slar Radiatin Received By Earth Slar cnstant fr Earth: 1368 W m -2 (Nte: 1 W = 1 J s -1 ) Slar radiatin received tp f atmsphere unit area f sphere = (1368) x (π r e2 )/( π r e2 ) = 32 W m -2 1

2 A N-Atmsphere Earth Assume 30% f incming slar energy is reflected by surface (albed f surface = 0.3) Energy absrbed by surface = 70% f 32 W m -2 = 239. W m -2 Balanced by energy emitted by surface Stefan-Bltzmann law: Energy emitted = σ T ;(σ=5.67 x 10-8 W m -2 K - ) 239. W m -2 = σ T T = 255 K (-18 C) much less than average temperature f 288 K (15 C) What is missing? Absrptin f terrestrial radiatin by the atmsphere TERRESTRIAL RADIATION SPECTRUM FROM SPACE: cmpsite f blackbdy radiatin spectra fr different T Scene ver Niger valley, N Africa (nn) NORMAL VIBRATIONAL MODES OF CO 2 Greenhuse gases = gases with vib-rt absrptin features at 5-50 μm Majr greenhuse gases: H 2 O, CO 2, CH, O 3, N 2 O, CFCs, Nt greenhuse gases: N 2, O 2, Ar, Mlecules that can acquire a charge assymetry be stretching r flexing are greenhuse gases. 2

3 An Idealized Earth+Atmsphere W m W m W m -2 (1-f)σ T σ T absrbed = f σ T f=atmspheric absrptin efficiency Slar radiatin at surface = 70% f 32 W m -2 = 239. W m -2 Infrared flux frm surface = σ T Absrptin f infrared flux by atmsphere = f σ T Kirchhff s law: efficiency f absrptin = efficiency f emissin IR flux frm atmspheric layer = f σ T 1 (up and dwn) IR flux t space = f σ T 1 + (1-f)σ T Radiatin Balance Equatins W m W m -2 (1-f)σ T fσ T 1 absrbed = f σ T 239. W m -2 σ T Balance at tp f atmsphere f σ T 1 + (1-f) σ T = 239. Balance fr atmspheric layer f σ T 1 + f σ T 1 = f σ T Slve fr T and T 1 The Greenhuse Effect W m W m -2 (1-f)σ T fσ T 1 absrbed = f σ T 239. W m -2 σ T Fr f=0.77, T =288 K and T 1 =22 K (33 K warmer than the n-atmsphere case) As f increases, T and T 1 increase Greenhuse gases gases that affect f (absrptin efficiency f atms.) Earth has a natural greenhuse effect; human activities enhance effect 3

4 TERRESTRIAL RADIATION SPECTRUM FROM SPACE: cmpsite f blackbdy radiatin spectra fr different T Scene ver Niger valley, N Africa trpsphere surface tp f stratsphere Hw des the additin f a greenhuse gas warm the earth? 1. Initial state Example f a GG absrbing at 11 μm 2. Add t atmsphere a GG absrbing at 11 μm; emissin at 11 μm decreases (we dn t see the surface anymre at that λ, but the atmsphere) 3. At new steady state, ttal emissin integrated ver all λ s must be cnserved Emissin at ther λ s must increase The Earth must heat! 3. Cncept f Radiative Frcing Glbal mean radiative frcing f the climate system relative t 1750 AD Purpse: t quantify a radiative perturbatin assciated with an increase in a greenhuse gas IPCC, 2007

5 Aersl direct and indirect effects Figure 2.10 IPCC, 2007 Radiative Frcing and Temperature Change Increasing the abundance f a greenhuse gas by Δm crrespnds t an increase in Δf f the absrptin efficiency, and an initial decrease in IR radiatin emitted t space (by ΔF) Respnse f system t energy imbalance: T and T 1 increase may cause ther greenhuse gases t change T and T 1 may increase r decrease depending n internal climate feedbacks Δf ΔT etc Ultimately, the system gets back in balance Radiative frcing is nly a measure f initial change in utging terrestrial radiatin Climate mdels (GCMs) indicate ΔT surface = λ ΔF where λ (climate sensitivity parameter) ranges frm 0.3 t 1. K m 2 W -1 depending n the GCM Psitive Feedback Cause enhance Effect Effect enhances cause increase effect bm!!! Ex: Water vapr feedback Ex: warmer melting land ice lwer albed further warming 5

6 Negative Feedback Negative feedback Cause suppress Effect Effect suppresses cause decrease effect self-regulatin Example: warmer mre cluds higher albed cling effect (negative feedback) Feedbacks water vapr feedback: psitive. CO 2 Greenhuse effect Temperature Greenhuse effect Water vapr ice-albed feedback: psitive r negative?. clud feedbacks: psitive r negative? ptentially large (cluds can reflect slar radiatin r absrb IR radiatin depending n their height, thickness and micrphysical prperties) Largest uncertainty in current estimates f climate change. land surface feedback: psitive r negative? (surface albed affect, CO 2 fertilizatin, sil misture) GLOBAL WARMING POTENTIAL (GWP): fundatin fr climate plicy The GWP measures the integrated radiative frcing ver a time hrizn Δt frm the injectin f 1 kg f a species X at time t, relative t CO 2 : GWP = Gas t t t +Δt t +Δt ΔF ΔF 1 kg X 1 kg CO2 dt dt Lifetime (years) GWP fr time hrizn 20 years 100 years 500 years CO 2 ~ CH N 2 O CFC-12 (CF 2 Cl 2 ) HFC-13a (CH 2 FCF 3 ) SF