AGU Pressconference, Dec 15, 2015
Global Dimming and Brightening
Decadal changes in sunlight at the Earth's surface
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phenomenon and potential implications
Martin Wild
ETH Zürich
martin.wild@env.ethz.ch
Energy Balance of the Global Climate System
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Greenhouse gases
Units Wm-2
Wild et al. 2013 / IPCC AR5
Energy Balance of the Global Climate System
Sun as ultimative energy source
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Greenhouse gases
Units Wm-2
Wild et al. 2013 / IPCC AR5
Energy Balance of the Global Climate System
Anthropogenic
Perturbations
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Greenhouse gases
Units Wm-2
Wild et al. 2013 / IPCC AR5
Energy Balance of the Global Climate System
Anthropogenic
Perturbations
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Greenhouse gases
Units Wm-2
Wild et al. 2013 / IPCC AR5
Energy Balance of the Global Climate System
Anthropogenic
Perturbations
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Aerosols
Air pollution
Units Wm-2
Greenhouse gases
Wild et al. 2013 / IPCC AR5
Decadal changes in surface solar radiation
Insolation anomalies (Wm-2)
56 sites in Europe, 1939 – 2012
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Data from Global Energy Balance Archive (GEBA)
Sanchez-Lorenzo et al. (2015) JGR
Wild (2012) BAMS
Wild et al. (2005) Science
Decadal changes in surface solar radiation
Insolation anomalies (Wm-2)
56 sites in Europe, 1939 – 2012
“dimming”
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Data from Global Energy Balance Archive (GEBA)
Sanchez-Lorenzo et al. (2015) JGR
Wild (2012) BAMS
Wild et al. (2005) Science
Decadal changes in surface solar radiation
Insolation anomalies (Wm-2)
56 sites in Europe, 1939 – 2012
“dimming”
“brightening”
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Data from Global Energy Balance Archive (GEBA)
Sanchez-Lorenzo et al. (2015) JGR
Wild (2012) BAMS
Wild et al. (2005) Science
What causes dimming and brightening?
Variations in solar output
measured from satellites
Sunspot numbers
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Sun cannot explain
dimming / brightening
Variations in solar output more than an
order of magnitude smaller than
dimming/brightening and uncorrelated
What causes dimming and brightening?
Global Anthropogenic
Sulfur Emissions 1950-2000
“dimming”
“brightening”
Data source: Stern,
2005
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increase/decrease of air pollution in line with
dimming and brightening
Wild 2012, BAMS
What causes dimming and brightening?
Global Anthropogenic
Sulfur Emissions 1950-2000
“dimming”
“brightening”
surface solar radiation
Data source: Stern,
2005
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increase/decrease of air pollution in line with
dimming and brightening
Wild 2012, BAMS
What causes dimming and brightening?
Direct and indirect aerosol effects
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Aerosol from air pollution reduce sunlight at surface:
- directly through scattering and absorption of sunlight
- indirectly though the modification of cloud properties
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Relevance for global warming
Solar dimming versus greenhouse warming
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°C
Data:
NASA-GISS
Wild et al. 2007 GRL 34
Solar dimming versus greenhouse warming
1958-1985: 0.004°C/year
°C
Data:
NASA-GISS
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Dimming supresses
global warming
Wild et al. 2007 GRL 34
Solar dimming versus greenhouse warming
1958-1985:0.006°C/year
0.004°C/year
1958-1985:
1985-2002: 0.038°C/year
°C
Data:
NASA-GISS
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Dimming supresses Brightening enhances
global warming
global warming
Wild et al. 2007 GRL 34
Asymmetric hemispheric pollution
Anthropogenic sulfur emission 1950-2000
Source: IPCC AR4,
based on Stern (2005)
Globe
Northern Hemisphere
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Southern Hemisphere
•
•
Air pollution shows trend reversal in NH, but not in SH
Negligible air pollution in SH comparted to NH
Wild, BAMS 2012 / Wild 2015 WIRES Clim. Change: early online release
Asymmetric hemispheric warming
Northern Hemisphere warming
“dimming”
Southern Hemisphere warming
“NO dimming”
“brightening”
“NO brightening”
Obsdata:
CRU
Hemispheric warming rates
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Units °C/decade
1958-1985
“dimming”
1985-2014
“brightening”
Northern Hemisp.
-0.02
+0.24
Southern Hemisp.
0.07
+0.09
Strong changes in NH warming rates with strong pollution changes
< > steady warming rates on SH with little pollution
Wild 2015 WIRES Clim. Change, early online release
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Relevance for other environmental issues
Consequences of dimming and brightening
• Impact on the intensity of the water cycle
• Dimming > less energy to drive the water cycle
=> attenuation of water cycle
• Brightening > more energy to drive the water cycle
=> intensification of water cycle
• Impact on glacier melt
• Switzerland: no glacier retreat during dimming,
only started with brightening
• Impact on biosphere growth
• Plant growth more effective with more diffuse light
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• Impact on solar power generation
• Long term changes can affect productivity of
renewables
Wild 2012, BAMS
Wild 2015, WIRES Climate Change
Observed rainfall
northern hemisphere land
Additional Reading
1.) Popular overview paper:
2.) Paper in press:
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“The latest on dimming and brightening”
PUBLISHED ONLINE Dec 2015, open access
Summary
• Solar radiation (sunlight) at Earth’s surface is not stable
over the years, but undergoes substantial changes on
decadal timescales
• Caused by changes in the transparency of the atmosphere,
not by the Sun itself
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• This may affect various aspects of environmental change
(e.g., global warming, intensity of the global water cycle,
melting of ice, photosynthesis and plant growth) as well as
solar power production and agriculture
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Backup slides
Impact on carbon uptake and plant growth
Dimming / brightening affects quantity & quality of radiation
Total radiation= Direct + Diffuse
Diffuse fraction= Diffuse / Total
Total
Direct
Diffuse
Increased diffuse light during dimming
due to scattering from aerosol and clouds
Dimming
total radiation
Odessa, Abakumova et. al. 1996
=> less photosyntheis
diffuse fraction => more photosynthesis
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Brightening
total radiation => more photosyntheis
diffuse fraction => less photosyntheis
Mercado et al. 2009 Nature
Impact on carbon uptake and plant growth
Why is plant productivity enhanced with more diffuse?
Direct radiation: Can only be
used by uppermost leaves for
photosynthesis
Diffuse radiation: penetrates
deeper into canopy - can be more
effectively used for photosynthesis
Global-Dimming period
total radiation ---
reduces plant photosynthesis
diffuse fraction ---
enhances plant photosynthesis
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Modeling studies: Effect of diffuse fraction increase dominates
 Increased carbon uptake and plant growth during dimming
Mercado et al. 2009 Nature
Impact on northern hemispheric rainfall
Observed rainfall northern hemisphere land
Data source:
GHCN
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Decreasing solar
energy for evaporation
Increasing solar
energy for evaporation
variations in rainfall in line with
solar dimming and brightening
Wild 2012, BAMS
Impact on mountain glaciers
Swiss Glaciers
area reduction:
“Dimming phase“
1973 - 1985: -1 %
“Brightening phase“
1985 - 2000s:-18 %
Paul et al.
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Retreat of Swiss glaciers only evident
after solar dimming stopped
Impact on mountain glaciers
94-year time series of
annual glacier melt at
four high elevation sites
in the European Alps
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Claridenfirn,
Aletschgletscher
Silvrettagletscher
Maximum melt in the late 40s, due to maximum
insolation, despite lower temperatures
Huss and Funk GRL 2009
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Impact on solar energy production
Source: Berner Fachhochschule Burgdorf
Impact on evaporation
Evaporation from evaporation pans
decreased from 1950s to 1980s,
consistent with surface solar dimming
85 sites in China 1950 - 2000
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Close connection
between insolation
and evaporation
changes
Roderick, Farquhar 2002 Science
Ohmura and Wild 2002 Science
Wild et al. 2004 Geophys.Res.Let.
Qian et al. 2006 Geophys.Res.Let.
Dimming/Brightening and 20th century warming
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Global land surface temperatures, 20th century
Modulations of global warming in line with dimming/brightening
Wild JGR 2009
Impact on diurnal temperature range
Correlation of surface insolation and DTR in China
DTR:
Diurnal
Temperature
Range
1955-2000
From Liu et al. 2004,
J. Climate
= T max – T min
Correlation of surface insolation and DTR in Europe
R=0.87
1970-2005
Daily maximum temperature
dominated by surface solar
radiation
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Daily minimum temperature
dominated by thermal radiation
From Makowski et al. 2009, JGR
Impact on diurnal temperature range
Observation data:
CRU dataset
Wild et al. 2007, GRL
Linear regression slopes land mean DTR
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Evidence for large
scale change in
surface radiative
forcings
Wild et al. 2007,
Geophys. Res. Lett.
Units
1958-1985 1985-2002
°C/decad “dimming” “brightening”
e
T max
-0.04
+0.37
T min
0.11
+0.40
Observed DTR 20th century over NH land
DTR as proxy for surface solar radiation
Data source:
CRU dataset
Wild 2009, JGR
Brightening
Dimming
Brightening
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Indication for surface solar dimming mainly in
second part of 20th century (1950s-80s), and for two
brightening periods in the 1980s/90s and 1930s/40s
Wild 2009, JGR
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Recent developments since 2000
Wild 2012
BAMS
Global dimming/brightening beyond 2000
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Numbers: literature values for changes in Wm-2/decade
Wild, BAMS (2012)
Global dimming/brightening beyond 2000
Global SO2 emissions since 1970
?
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China
SO2 emissions increase again since 2000
Future of global dimming/brightening
Coal consumption in China 1980-2007
China coal consumption
3,500
3,000
Million Tons
2,500
2,000
1,500
1,000
2007
2002
500
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06
20
04
20
02
20
00
20
98
19
96
19
94
19
92
19
90
19
88
19
86
19
84
19
82
19
19
80
0
Coal consumption in China has doubled
from 2002 to 2007!
Source: US Energy Information Administration
Global dimming/brightening beyond 2000
Satellite-derived surface solar radiation
GEWEX SRB
vers. 2.8
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Global monthly mean anomalies 1983 -2005
Hinkelman et al. 2009 JGR
Future of dimming/brightening in ECHAM5-HAM
Surface clear sky insolation in NH midlatitudes
1950 - 2020
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From model simulations with ECHAM5 HAM
=> Interesting to see coming years in surface observations
Wild 2009a JGR
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RCP emissions
Sulfur emissions
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Z Klimont et al. ERL 2013