Download Chemical forcing of climate - Atmospheric Chemistry Modeling Group

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]
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:
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:
F   l d l
0
BLACKBODY RADIATION
•
•
Objects that absorb 100% of incoming radiation are called
blackbodies
For blackbodies, l is given by the Planck function:
Function of T
only! Often
denoted B(l,T)
F  sT 4
s  2p 5k 4/15c2h3 = 5.67x10-8 W m-2 K-4
is the Stefan-Boltzmann constant
lmax = hc/5kT
Wien’s law
lmax
KIRCHHOFF’S LAW:
Emissivity e(l,T) = Absorptivity
For any object:
Illustrative example:
Kirchhoff’s law allows
determination of the
emission spectrum of
any object solely from
knowledge of its
absorption spectrum
and temperature
…very useful!
SOLAR RADIATION SPECTRUM: blackbody at 5800 K
TERRESTRIAL RADIATION SPECTRUM FROM SPACE:
composite of blackbody radiation spectra for different T
Scene over
Niger 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+hn gO2+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
Δp  0
forbidden
Δp  0
allowed
Δp  0
IR spectrum
of CO2
asymmetric
stretch
bend
allowed
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
IR
VISIBLE
Incoming
solar
FS / 4
FS / 4
Reflected
solar
FS A / 4
Energy balance equations:
• Earth system
FS (1  A) / 4  (1  f )s To4 + f s T14
Transmitted
surface
• Atmospheric layer
(1  f )s To4
f s To4  2 f s T14
Solution: 
f s T14
Atmospheric
emission
f s T14
Atmospheric
emission
1
4
 To=288 K
 F (1  A)  e f=0.77
To   S

f
 4(1  )s  T1 = 241 K
2


Atmospheric layer (T1)
abs. eff. 0 for solar (VIS)
f for terr. (near-IR)
Surface emission
FS A / 4
s To4
Earth surface (To)
Absorption efficiency 1-A in VISIBLE
1 in IR
THE GRAY ATMOSPHERE MODEL
In a purely radiative equilibrium atmosphere T decreases exponentially with z,
resulting in unstable conditions in the lower atmosphere; convection then
redistributes heat vertically following the adiabatic lapse rate
Integrate over z
Absorption ~ ρ(z)dz
dz
σTo4
surface
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 presentday 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
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
sTcloud4 < sTo4
sTcloud4≈ sTo4
Tcloud≈ To
convection
sTo4
To
LOW CLOUD: COOLING
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?
Example of a GG absorbing at 11 mm
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.
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!
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
Fin
Incoming
solar
radiation
Fout
Reflected
solar radiation
(surface, air,
aerosols,
clouds)
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
Increasing greenhouse gases g DF > 0 positive forcing
• The radiative forcing changes the heat content H of the Earth system:
DT
dH
 DF  o
dt
l
eventually leading to steady state
DTo  lDF
where To is the surface temperature and l is a climate sensitivity parameter
• 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
IPCC [2007]
CLIMATE MODELS CAN EXPLAIN 20th CENTURY WARMING
AS DRIVEN BY ANTHROPOGENIC RADIATIVE FORCING
Colored and thin black lines: results from 13 different GCMs
Thick black lines: observations
Models including anthropogenic
forcing
Models not including anthropogenic
forcing
observed
models
Year
Year
IPCC [2007]
IPCC PROJECTED WARMING OVER 21st CENTURY
for different socioeconomic scenarios (A1, A2, B1, B2)
CO2 trend
Global temperature
Trend (GCM ensemble)
IPCC [2001]
Verification of past IPCC projections
actual
Surface temperatures
IPCC 1995
IPCC 1990
CO2 emissions
IPCC (2007)
EOCENE (55 to 36 million years ago): The last time in Earth
history when atmospheric CO2 was above 500 ppm.
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
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
RCP8.5
RCP6
RCP4.5
RCP3-PD
RCP6
RCP4.5
RCP3-PD
ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol: dispersed condensed matter suspended in a gas
Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)
Soil dust
Sea salt
Environmental importance: health (respiration), visibility, radiative balance,
cloud formation, heterogeneous reactions, delivery of nutrients…
SCATTERING OF
RADIATION
BY AEROSOLS:
“DIRECT EFFECT”
Scattering efficiency is
maximum when
particle diameter = l
eparticles in 0.1-1 mm
size range are efficient
scatterers of solar
radiation
By scattering
solar radiation,
aerosols
increase the
Earth’s albedo
2 (diffraction
limit)
AEROSOL OPTICAL DEPTH
MODIS satellite data
IPCC [2007]
EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:
+0.2
Observations
0
circulation model
-0.4
-0.2
NASA/GISS general
-0.6
Temperature Change (oC)
Temperature decrease following large volcanic eruptions
1991
1992
1993
Mt. Pinatubo eruption
1994
SCATTERING vs. ABSORBING AEROSOLS
Scattering sulfate and organic aerosol
over Massachusetts
Partly absorbing dust aerosol
downwind of Sahara
Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar
radiation
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
EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS
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
 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
SATELLITE IMAGES OF SHIP TRACKS
NASA, 2002
Atlantic, France, Spain
AVHRR, 27. Sept. 1987, 22:45 GMT
US-west coast
OTHER EVIDENCE OF CLOUD FORCING:
CONTRAILS AND “AIRCRAFT CIRRUS”
Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).
Radiative forcing by aerosols is very inhomogeneous
…in contrast to the long-lived greenhouse gases
Present-day annual direct radiative forcing from anthopogenic aerosols
Leibensperger
et al., 2012
global radiative
forcing from CO2
• 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
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
oC
Contiguous US
1930-1990 trend
• US has warmed faster
than 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?
GISTEMP [2010]
1950-2050 surface temperature trend in eastern US
Leibensperger et al. [2012]
1930-1990 trend
Observations (GISTEMP)
Model (standard)
Model without US anthropogenic aerosols
• 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