Transcript Aerosols and Climate Change

The Role of Aerosols in Climate
Change
Eleanor J. Highwood
Department of Meteorology,
With thanks to all the IPCC scientists, Keith Shine (Reading)
and James Haywood (Met. Office)
Outline
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What are aerosols?
Importance in present day atmosphere
Estimates of past climate impact
Uncertainties
Estimates of future changes
What next?
What are aerosols?
• Small particles or droplets suspended in the
atmosphere
• Radius is 0.01 to 10 microns
• Many different types and sources
• Natural and man-made sources
• Important for both present day climate and
climate change
Sources - Natural
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sea salt
volcanic aerosols
mineral dust
Biomass burning
Sources - Man-made
• Fossil fuel burning
(produces several
different types)
• Biomass burning
• Mineral dust
Importance: Direct solar effect
• Aerosols scatter and absorb solar radiation
No aerosol
Scattering aerosol
Absorbing aerosol
Importance: Direct terrestrial
effect
• Large aerosols (e.g. dust or sulphuric acid in
the stratosphere) behave like greenhouse
gases.
No aerosol: ground emits to
space
Aerosol absorbs radiation from
ground and re-emits a smaller
amount up and down
Importance: Indirect effects
• Some aerosols can alter the properties of
clouds, changing their reflectivity or
lifetime
• Some aerosols can allow chemical reactions
between atmospheric constituents to take
place very rapidly
Measuring aerosol effects on
climate
• Measure effect on radiation at top of
atmosphere and surface.
• “Radiative effect” : effect of having aerosol
in the present day atmosphere
• “Radiative forcing”: effect of changes in
aerosol on radiation budget over a given
period of time
e.g. seasalt
GCM(no aerosols) - ERBE
GCM (aerosols) - ERBE
GCM (Aerosols + sea salt) - ERBE
e.g. radiative effect of Saharan
dust outbreaks
Figure courtesy of
SeaWiFs and OrbiImage
The solar radiative effect of Saharan
dust can be very large - measurements
from SHADE on 25th September 2000
between Sal and Dakar show:
3 times more solar radiation being
scattered back to space than in clear
sky (so a big reduction in the amount of
radiation that reaches the surface).
Figure courtesy of J.M. Haywood, Met. Office
AVHRR Ch4
AVHRR Ch5
Dust also affects our knowledge of other climate variables like
sea surface temperature because it absorbs outgoing terrestrial
radiation.
Figure courtesy of J.M Haywood, Met. Office
Change in SST (K) from AVHRR data when dust is present September 2000.
The SST anomaly over the Cape Verde Islands reaches -3.6K.
+2.4
+1.8
+1.2
+0.6
0
-0.6
-1.2
-1.8
-2.4
-3.0
-3.6
Figure courtesy of J.M. Haywood, Met. Office
Estimating climate change due to
changes in aerosols
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Emission sources and time history
Chemistry and transport model
Radiation code
Climate model
Radiative forcing
Global and annual mean radiative forcing can
be related to a global and annual mean
change in surface temperature using:
T = F
e.g. F over past 250 years
From IPCC TAR (2001)
Greenhouse gases
Dust
Sulphates
Indirect
From Shine and
Forster, 1999
Summary of issues
• Aerosols all much more uncertain than
greenhouse gases
• Can’t add up aerosols to cancel out
greenhouse gases
• Total aerosol forcing is unlikely to be a
linear combination of individual
contributions
• Indirect is holding us up.
What do we need to know about
aerosols?
5 key parameters to give us radiative forcing
– mass light scattering efficiency
– dependence of scattering on relative humidity
– Single scattering albedo (absorption vs
scattering)
– Asymmetry parameter
– change in mass burden over time
Distribution
Emissions
Processing
Chemistry
Transport
Background
Natural
aerosols
Optical properties
Uncertainties
Chemical
composition
Mixing
Size
Distribution
Uncertainty
in forcing
Other components
CLOUDS
Relative
humidity
Surface
albedo
Wavelengths
Transfer
scheme
Radiation code
Distribution: sulphates
• Formed from gases SO2 (from fossil fuel or
volcanoes) and DMS (from ocean algae)
Distribution: carbonaceous from
anthropogenic sources
• Fossil fuel burning
• Inventories have an uncertainty of a factor
of 2.
Distributions: Biomass burning
•Some biomass burning is natural.
•Episodic and regional in nature
Distribution: Mineral dust
50% of dust burden due to anthropogenic
sources due to land use change, overgrazing
etc.
Past Trends
From ice cores: very uncertain.
(From IPCC 2001)
1st indirect effect
Increase in aerosol

Increase in cloud
droplet number

Change in
reflectivity
(albedo)
From Brenguier et al (2000)
2nd indirect effect
• Aerosols affect precipitation efficiency and
therefore cloud lifetime.
• Also affect cloud reflectivity?
Semi-direct effect
Aerosol such as black carbon absorbs solar
radiation

Layer heats up

Cloud burns off or atmosphere is stabilised
and cloud prevented from forming.
Distribution
Emissions
Processing
Chemistry
Transport
Background
Natural
aerosols
Optical properties
Uncertainties
Uncertainty
in forcing
Other components
CLOUDS
Relative
humidity
Surface
albedo
Chemical
composition
Mixing
Size
Distribution
Climate
response?
Wavelengths
Transfer
scheme
Radiation code
Climate response 1
Climate sensitivity (Hansen et al 1997)
5
-10
=0
.8
5
w
T.
=1
.0
w
T.
w=
0.
85
S.
w=
1.
0
S.
O
3
So
So
-2
%
Sensitivity
-5
+2
%
2x
CO
2
0
-15
-20
-25
-30
Fixed cloud
All feedbacks
Adapted from Hansen (1997)
Is climate
response to
changes in
aerosol the
same as for
changes in CO2
or solar
constant?
Climate response 2
Reader and Boer
(1998): large scale
responses
surprisingly similar
Modelling climate change over
past 250 years
No aerosol
+ aerosol
Global Mean Temperature (Anomaly from 1961-1990)
Global Mean Temperature (Anomaly from 1961-1990)
1.50
1.20
1.00
1.00
0.80
1995
1990
1985
1980
1975
1970
1965
1960
1955
1950
1945
1940
1935
1930
1925
1920
1915
1910
1905
1900
1895
1890
1885
1880
1875
1870
1865
1860
1855
0.40
0.20
0.00
1995
1990
1985
1980
1975
1970
1965
1960
Pink - observations, blue - model
Year
1955
-0.80
Year
1950
-1.50
1945
-0.60
1940
-0.40
-1.00
1935
-0.20
1930
1925
1920
1915
1910
1905
1900
1895
1890
1885
1880
1875
1870
1865
1860
-0.50
1855
1850
Temp Anomaly (deg C)
0.00
1850
Temp Anomaly (deg C)
0.60
0.50
Future changes in aerosols
From IPCC (2001)
Future areas of research
• Mixing of aerosol types
• Remote sensing of aerosol properties and
amount using satellites, combination with
in-situ data
• Long term and consistent modelling of
aerosol profiles across globe
• Regional climate modelling
• Indirect effect and semi-direct effect
“Real knowledge is to know the extent of
one’s ignorance”
Confucius