Global mean surface temperature trend [IPCC, 2014]

January 2014 temperature anomaly

NASA/GISS temperature analysis

Strongest warming in the Arctic [IPCC, 2014]

Trends of multiple indicators of climate change [IPCC, 2014]

Here DF is the radiation flux emitted in [ , + ]l l Dl

is the flux distribution function characteristic of the object

Total radiation flux emitted by object:

EMISSION OF RADIATION

• Radiation is energy transmitted by electromagnetic waves; all objects emit radiation

• One can measure the radiation flux spectrum emitted by a unit surface area of object:

0

d

BLACKBODY RADIATION

• Objects that absorb 100% of incoming radiation are called blackbodies

• For blackbodies, f l is given by the Planck function:

= F s T 4

s = 2 p 5k 4/15c2h3 = 5.67x10-8 W m-2 K-4

is the Stefan-Boltzmann constant

lmax = hc/5kT Wien’s law

Function of Tonly! Often denoted B( ,l T)

lmax

KIRCHHOFF’S LAW: Emissivity ( ,e l T) = Absorptivity

For any object: …very useful!

Illustrative example:

Kirchhoff’s law allowsdetermination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 K

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra for different T

Scene overNiger valley,N Africa

RADIATIVE EQUILIBRIUM FOR THE EARTH

Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2

Radiative balance c effective temperature of the Earth:

= 255 K

where A is the albedo (reflectivity) of the Earth

Questions

1. For an object of given volume, which shape emits the least radiation?

2. If the Earth were hollow, would it emit more or less radiation?

3. In our calculation of the effective temperature of the Earth we viewed the Earth as a blackbody.  However, we also accounted for the fact that the Earth absorbs only 72% of solar radiation (albedo = 0.28), so obviously the Earth is not a very good blackbody (which would absorb 100% of all incoming radiation).  Nevertheless, the assumption that the Earth emits as a blackbody is correct to within a few percent.  How can you reconcile these two results?

ABSORPTION OF RADIATION BY GAS MOLECULES

• …requires quantum transition in internal energy of molecule.

• THREE TYPES OF TRANSITION– Electronic transition: UV radiation (<0.4 mm)

• Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O3+h ngO2+O)

– Vibrational transition: near-IR (0.7-20 mm)• Increase in vibrational frequency of a given bond

requires change in dipole moment of molecule– Rotational transition: far-IR (20-100 mm)

• Increase in angular momentum around rotation axis

Gases that absorb radiation near the spectral maximum of terrestrial emission (10 mm) are called greenhouse gases; this requires vibrational or vibrational-rotational transitions

NORMAL VIBRATIONAL MODES OF CO2

forbidden

allowed

allowed

Δp 0

Δp 0

Δp 0

IR spectrumof CO2

bend

asymmetricstretch

GREENHOUSE EFFECT:absorption of terrestrial radiation by the atmosphere

• Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,…

• Not greenhouse gases: N2, O2, Ar, …

SIMPLE MODEL OF GREENHOUSE EFFECT

Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR

Atmospheric layer (T1)abs. eff. 0 for solar (VIS) f for terr. (near-IR)

/ 4SF

Incoming solar

/ 4SF

Reflectedsolar

/ 4SF A

/ 4SF A4oT

Surface emission

4(1 ) of T

Transmittedsurface

41f T41f T

Atmosphericemission

Atmosphericemission

Energy balance equations:• Earth system

4 41(1 ) / 4 (1 )S oF A f T f T

• Atmospheric layer4 4

12of T f T

Solution:14

(1 )

4(1 )2

So

F AT

f

To=288 K e f=0.77T1 = 241 K

VISIBLE IR

THE GRAY ATMOSPHERE MODEL

σTo4

surface

Integrate over z

dz

Absorption ~ ρ(z)dz

In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection thenredistributes heat vertically following the adiabatic lapse rate

The ultimate models for climate research

GENERAL CIRCULATION MODELS (GCMs)Standard research tools for studying the climate of the Earth

• Solve conservation equations for momentum, heat, and water on global 3-D atmospheric domain

• Horizontal resolution ~100 km

• Include coupling to ocean, land, biogeochemistry, atmospheric chemistry to various degrees

• Solution to equations of motion is chaotic, so that a GCM cannot simulate an observed meterorological year; it can only simulate climate statistics including interannual variability

• A GCM can be tested by its ability to simulate present-day climate statistics in a repeatable manner when run in radiative equilibrium (equilibrium climate simulation)

• A radiative imbalance (such as changing concentrations of greenhouse gases) will result in warming or cooling in the GCM

CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS

convection

To

Tcloud≈ To

Clouds reflect solar radiation (DA > 0) g cooling;…but also absorb IR radiation (Df > 0) g warming

Cloud feedbacks are the greatest source of uncertainty in climate models

sTo4

sTcloud4≈ sTo

4

LOW CLOUD: COOLING

sTcloud4 < sTo

4

sTo4

HIGH CLOUD: WARMING

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes

at different temperatures

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?

1.1. Initial state

2. 2. Add to atmosphere a GG absorbing at 11 mm; emission at 11 mm decreases (we don’t see the surface anymore at that , l but the atmosphere)

3. At new steady state, total emission integrated over all l’s must be conserved e Emission at other l’s must increase e The Earth must heat!

3.

Example of a GG absorbing at 11 mm

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING

The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 mm). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

RADIATIVE FORCING OF CLIMATE CHANGE

Incomingsolar

radiation

Reflected solar radiation (surface, air, aerosols, clouds)

Fout

Fin

IR terrestrial radiation ~ T4; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols

EARTH SURFACE

• Stable climate is defined by radiative equilibrium: Fin = Fout

• Instantaneous perturbation e Radiative forcing DF = Fin – Fout

• IPCC GCMs give l = 0.3-1.4 K m2 W-1, insensitive to nature of forcing; differences between models reflect different treatments of feedbacks

Increasing greenhouse gases g DF > 0 positive forcing

• The radiative forcing changes the heat content H of the Earth system:

oTdHF

dt

where To is the surface temperature and l is a climate sensitivity parameter

eventually leading to steady state oT F

IPCC [2007]

CLIMATE MODELS CAN EXPLAIN 20th CENTURY WARMING AS DRIVEN BY ANTHROPOGENIC RADIATIVE FORCING

Year Year

Colored and thin black lines: results from 13 different GCMsThick black lines: observations

Models including anthropogenicforcing

Models not including anthropogenic forcing

observed

models

IPCC [2007]

IPCC PROJECTED WARMING OVER 21st CENTURY

CO2 trend

Global temperatureTrend (GCM ensemble)

IPCC [2001]

for different socioeconomic scenarios (A1, A2, B1, B2)

Verification of past IPCC projections

IPCC 1995IPCC 1990

IPCC (2007)

CO2 emissions

Surface temperatures

actual

The Eocene climate was warm, even at high latitudes:-palm trees flourished in Wyoming-crocodiles lived in the Arctic-Antarctica was a pine forest-deep ocean temperature was 12°C (today it is ~2°C)-sea level was at least 100 meters higher than today

EOCENE (55 to 36 million years ago): The last time in Earth history when atmospheric CO2 was above 500 ppm.

Present models cannot reproduce this warm climate – missing processes?Positive feedbacks could cause abrupt climate change but this is not well understood

New IPCC AR5 Scenarios: Representative Concentration Pathways (RCPs)

• Defined by radiative forcing trajectories rather than socioeconomic storylines• Are representative of the Integrated Assessment Model (IAM) literature• Provide continuity with older IPCC scenarios: RP8.5 ≈ A2, RP6 ≈ A1B, RP4.5 ≈ B1• Introduce new “peak-and-decline” scenario – aggressive climate policy• RCP4.5 to be used for multi-decadal high-resolution simulations

RCP8.5

RCP6

RCP4.5

RCP3-PD

RCP8.5

RCP6

RCP4.5

RCP3-PD

ORIGIN OF THE ATMOSPHERIC AEROSOL

Soil dustSea salt

Aerosol: dispersed condensed matter suspended in a gasSize range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)

Environmental importance: health (respiration), visibility, radiative balance,cloud formation, heterogeneous reactions, delivery of nutrients…

SCATTERING OF RADIATION BY AEROSOLS:“DIRECT EFFECT”

By scattering solar radiation, aerosols increase the Earth’s albedo

Scattering efficiency is maximum when particle diameter = l particles in 0.1-1 mmsize range are efficient scatterers of solar radiation

2 (diffraction limit)

AEROSOL OPTICAL DEPTH

IPCC [2007]MODIS satellite data

Mt. Pinatubo eruption

1991 1992 1993 1994

-0.6

-

0.4

-0.2

0

+0.

2

Tem

pera

ture

Ch

ang

e (

oC

) Observations

NASA/GISS general

circulation model

Temperature decrease following large volcanic eruptions

EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:

SCATTERING vs. ABSORBING AEROSOLS

Scattering sulfate and organic aerosolover Massachusetts

Partly absorbing dust aerosoldownwind of Sahara

Absorbing aerosols (black carbon, dust) warm the climate by absorbing solarradiation

AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES

Clouds form by condensation on preexisting aerosol particles (“cloud condensation nuclei”)when RH>100%

clean cloud (few particles):large cloud droplets• low albedo• efficient precipitation

polluted cloud (many particles):small cloud droplets• high albedo• suppressed precipitation

Particles emitted by ships increase concentration of cloud condensation nuclei (CCN) Increased CCN increase concentration of cloud droplets and reduce their avg. size

Increased concentration and smaller particles reduce production of drizzle Liquid water content increases because loss of drizzle particles is suppressed

Clouds are optically thicker and brighter along ship track

N ~ 100 cm-3

W ~ 0.75 g m-3

re ~ 10.5 µm

N ~ 40 cm-3

W ~ 0.30 g m-3

re ~ 11.2 µm

from D. Rosenfeld

EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS

AVHRR, 27. Sept. 1987, 22:45 GMTUS-west coast

NASA, 2002Atlantic, France, Spain

SATELLITE IMAGES OF SHIP TRACKS

Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).

OTHER EVIDENCE OF CLOUD FORCING:CONTRAILS AND “AIRCRAFT CIRRUS”

Radiative forcing by aerosols is very inhomogeneous

…in contrast to the long-lived greenhouse gases

Present-day annual direct radiative forcing from anthopogenic aerosols

Leibenspergeret al., 2012

• Aerosol radiative forcing over polluted continents can more than offset forcing from greenhouse gases

• The extent to which this regional radiative forcing translates into regional climate response is not understood

global radiativeforcing from CO2

Radiative forcing from US anthropogenic aerosol

• Forcing peaked in 1970-1990

Cooling due to US anthropogenic aerosols

From difference of GCM simulations with vs. without US aerosol sources in 1970-1990, including aerosol direct and indirect radiative effects

• During the period of maximum aerosol pollution (1970-1990), the eastern US cooled by up to 1o C.

Leibensperger et al. [2012]

Observed US surface temperature trend

GISTEMP [2010]

• US has warmed fasterthan global mean, as expected in general for mid-latitudes land• But there has been no

warming between 1930 and 1980, followed by sharp warming after 1980

“Warming hole” observed in eastern US from 1930 to 1990; US aerosol signature?

1930-1990 trend

Contiguous USo C

1950-2050 surface temperature trend in eastern US

• US anthropogenic aerosol sources can explain the “warming hole”• Rapid warming has taken place since 1990s that we attribute to source reduction• Most of the warming from aerosol source reduction has already been realized

Leibensperger et al. [2012]

1930-1990 trend

Observations (GISTEMP)Model (standard)

Model without US anthropogenic aerosols