TOPIC III THE GREENHOUSE EFFECT

SOLAR IRRADIANCE SPECTRA 1  m = 1000 nm = m Note: 1 W = 1 J s -1

Solar radiation received outside atmosphere per unit area of sphere = (1370) x (  r e 2 )/(4  r e 2 ) = 342 W m -2 TOTAL SOLAR RADIATION RECEIVED BY EARTH Solar constant for earth: 1368 W m -2

EFFECTIVE TEMPERATURE OF EARTH Effective temperature of earth (T e ) Temperature detected from space Albedo of surface+atmosphere ~ % of incoming solar energy is reflected by clouds, ice, etc. Energy absorbed by surface+atmosphere = = % of 342 W m -2 = W m -2 Balanced by energy emitted by surface+atmosphere Stefan-Boltzman law: Energy emitted =  T e 4  = 5.67 x W m -2 K -4 Solve  T e 4 = T e = 255 K

GLOBAL TEMPERATURE Annual and global average temperature ~ 15 C, i.e. 288 K T e = 255 K --> not representative of surface temp. of earth T e is the effective temp. of the earth + atmosphere system that would be detected by an observer in space

ENERGY TRANSITIONS Gas molecules absorb radiation by increasing internal energy Internal energy  electronic, vibrational, & rotational states Energy requirements Electronic transitions  UV (< 0.4  m) Vibrational transitions  Near-IR (<  m) Rotational transitions  Far-IR (> 20  m) Little absorption in visible range (  m) Gap between electronic and vibrational transitions Greenhouse gases absorb in the range 5-50  m Vibrational and rotational transitions

GREENHOUSE GASES Vibrational transitions must change dipole moment of molecule Important greenhouse gases H 2 O, CO 2, CH 4, N 2 O, O 3, CFCs Non-greenhouse gases N 2, O 2, H 2, Noble gases

ATMOSPHERIC ABSORPTION OF RADIATION ~100% absorption of UV Electronic transitions of O 2 and O 3 Weak absorption of visible Gap in electronic and vibrational transition energies Efficient absorption of terrestrial radiation Greenhouse gas absorption Important role of H 2 O Atmospheric window between 8 and 13  m

A SIMPLE GREENHOUSE MODEL Incoming solar radiation = 70% of 342 W m -2 = W m -2 IR flux from surface =  T o 4 Assume atmospheric layer has an absorption efficiency = f Kirchhoff’s law: efficiency of abs. = efficiency of emission IR flux from atmospheric layer = f  T 1 4 (up and down) W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4

RADIATION BALANCE EQUATIONS W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Balance at top of atmosphere f  T (1-f)  T o 4 = Balance for atmospheric layer f  T f  T 1 4 = f  T o 4

THE GREENHOUSE EFFECT W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 T o = 288 K f = 0.77; T 1 = 241 K Greenhouse gases  gases that affect f As f increases, T o and T 1 increase

THE IPCC THIRD ASSESSMENT

CONCEPT OF RADIATIVE FORCING W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Consider increase in concentration of a greenhouse gases If nothing else changes  f increases  outgoing terrestrial radiation decreases Change in outgoing terrestrial radiation = radiative forcing

RADIATIVE FORCING AND TEMPERATURE CHANGE W m -2 absorbed = f  T o 4  T o 4 (1-f)  T o 4 f  T 1 4 Response to imbalance T o and T 1 increase  may cause other greenhouse gases to change  f  (positive feedback) or  (negative feedback)  T o and T 1 may  or    f   T  …  Rad. balance Radiative forcing is measure of initial change in outgoing flux

RADIATIVE FORCING Permits assessment of potential climate effects of different gases Radiative forcing of a gas depends not only on change in concentration, but also what wavelengths it absorbs Aerosols can exert a negative radiative effect (i.e. have a cooling effect) by reflecting radiation (direct effect) and by increasing reflectivity of clouds (indirect effect)

GLOBAL WARMING POTENTIAL Index used to quant. compare radiative forcings of various gases Takes into account lifetimes, saturation of absorption

FORCINGS AND SURFACE TEMPERATURE Climate sensitvity parameter ( ):  T o =  F Global climate models  = K m 2 W -1

THE TEMPERATURE RECORD

Trend differences due to differences in spatial av., diff. in sea-surface temps., and handling of urbanization Same basic trend over last 100 years Increase in T by C RECENT CHANGES IN SURFACE TEMPERATURE

POTENTIAL CAUSES OF TEMPERATURE CHANGES Variations in solar radiation at top of atmosphere Changes in albedo (e.g. due to changes in cloud cover) Changes in greenhouse gas forcing (i.e., change in f) W m -2 absorbed = f  T o 4

SOLAR VARIABILITY Changes in sunspots and surface conditions

CHANGES IN CLOUD COVER Incoming solar radiation = 0.7 x 342 W m -2 = W m -2 Consider albedo change of 2.5% Albedo = 0.3 x = Incoming solar radiation = x 342 W m -2 = W m -2 Radiative forcing = – = W m -2  Comparable but opposite to greenhouse gas forcing Clouds are also efficient absorbers of terrestrial radiation  Positive forcing Cloud effects are larege source of uncertainty in climate projections

MODEL SIMULATIONS OF RECENT PAST

CLIMATE PROJECTIONS

POTENTIAL IMPACTS

JULY HEAT INDEX FOR S.E. U.S.