Martin Wild

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Global dimming and brightening variations in
solar radiation reaching the Earths surface
Earthshine Workshop Lund, Sweden, January 14

• Martin Wild
• Swiss Federal Institute of Technology ETH
• Zurich, Switzerland

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The radiation balance of the Earth
Units Wm-2
3
The radiation balance of the Earth
TOA radiation balance determines energy content
of climate system
Units Wm-2
4
The radiation balance of the Earth
TOA radiation balance determines energy content
of climate system
Units Wm-2
Surface radiation balance driver of surface
climate and intensity of hydrological cycle
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The radiation balance of the Earth
Units Wm-2
Anthropogenic Perturbations
Air pollution
GHGs
Land use change
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What do we know on changes in solar fluxes on
decadal timescales?

• Top of atmosphere
• Satellite observations
• global coverage, no continuous observations
  (ERBE 1985 - 1989, ScaRaB 1994, CERES since 2000,
  GERB since 2002), calibration problems
• Earthshine observations
• Surface
• Station observations
• no global coverage, data widespread available
  since the 1960s, Some sites further back in time
  (mainly in Europe)

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Surface radiation networks at ETH

• Worldwide measurements of energy fluxes at the
  surface (2500 sites)
• Solar radiation data for the second half of the
  20th century
• Monthly mean values

Ohmura, Gilgen, Wild 1989

• Highest measurement quality at selected sites
  worldwide (currently 38 anchor sites)
• Starting in 1992
• Minute Values
• Ancillary data for radiation interpretation

Ohmura et al. 1998
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Surface solar radiation change 1960 - 1990

• Decrease in surface solar radiation 1960 - 1990
• Global dimming
• Gilgen, Wild, Ohmura (1998) -9 Wm-2
• Stanhill, Cohen (2001) -10 Wm-2
• Liepert (2002) -7 Wm-2
• 2 decrease per decade,
• 6 over period 1960-1990

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Support from pan evaporation measurements
Evaporation from evaporation pans decreased
between 1960 and 1990, consistent with global
dimming
? Independent evidence for reduced surface solar
radiation
Data from 936 evapo- ration pans
Peterson et al. 1995 Nature Roderick, Farquhar
2002 Science Ohmura and Wild 2002 Science Wild
et al. 2004 Geophys.Res.Let.
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Global dimming potential causes
Stanhill and Cohen (2001)
Increase in fossil fuel emissions between 1960 -
1990, In line with decrease in solar radiation at
the surface
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Extending the records beyond 1990

• All studies on global dimming used data only
  prior to 1990
• gt Extend observational records from 1990 to
  present

?
Surface solar radiation (MJm-2)
Surface Solar Radiation in Estonia
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Extending the records beyond 1990

• All studies on global dimming used data only
  prior to 1990
• gt Extend observational records from 1990 to
  present

Surface solar radiation (MJm-2)
Surface Solar Radiation in Estonia
Wild et al. 2005 From dimming to brightening
Decadal changes in solar radiation at the Earths
surface. Science 308
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Changes in surface solar radiation after 1990
Observed changes at BSRN and GEBA sites since 1990
Yellow Increase Brown Decrease
Wild et al., 2005, Science 308
No evidence for global dimming after 1990 -
rather brightening
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Surface solar brightening and Earth reflectance
Earth Shortwave reflectance
Palle et al. 2005, GRL
Increase in surface insolation during 1990s
consistent with decrease in Earth reflectance
from Earthshine and other estimates
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Cloud amount from ISCCP 1983 - 2002
From NASA/ Bill Rossow
International Satellite Cloud Climatology Project

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Changes in atmospheric transmission 1960-2000
Wild et al., 2005, Science 308
Recent recovery in atmospheric transmittance
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Changes in surface solar radiation at BSRN sites
Wild et al., 2005, Science 308
Clear sky 1992- 2002 Clear sky
detection Algorithm Long Ackermann (2002)
Increase of clear sky surface solar fluxes at all
sites
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Trends in anthropogenic SO2 and BC emissions
1980 - 2000
From dimming to brightening Causes
Sulfur
Black Carbon
From Streets et al. GRL, 2006
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From dimming to brightening Causes
Aerosol optical depth 1981 - 2005
Mishchenko et al. Science 2007
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From dimming to brightening Causes
Reduction of Arctic haze pollution over 1990s
Decrease in BC 1989-2002 60
Measured at Alert, Canadian Arctic
Decrease in Sulphate 1989-2002 29
Sharma et al. (2004), JGR
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Simulation of observed trends

• Cooperation with Max Planck Institute for
  Meteorology,
• Hamburg, Group of J. Feichter
• Global Climate Model ECHAM5-HAM with
  sophisticated aerosol scheme (Stier, P. et al.
  2005, ACPD) installed at ETH Zurich
• Transient Simulation 1950-2000 with ECHAM5-HAM
  and realistic anthropogenic emission histories

BC Emissions Eastern Europe
Reduced emissions due to economic breakdown of
former communist countries included
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Simulation of observed trends
Changes in Clear Sky Insolation (linear trends)
dimming period
brightening period
Period 1950 -1990
Period 1990 - 2002
Transient Simulation with ECHAM5 HAM
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Simulation of observed trends
ECHAM5 HAM 1990-2002
Observations 1990-2002
Wild et al. 2005 Science 308
Model changes in qualitative agreement with
observations in most areas
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Simulated TOA SW clear sky changes
Changes in Clear Sky absorption TOA
Period 1950 -1990
Period 1990 - 2002
Transient Simulation with ECHAM5 HAM
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What are the consequences of the radiative
changes for climate change?
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Surface solar versus greenhouse forcings
-

• 1960s - 1980s
• Surface solar dimming counter-balances increasing
  thermal downward radiation
• Surface radiative heating is not increasing
• Wild et al. (2004) GRL 32

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Impact on global warming
C
Wild et al. 2007 GRL 34
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Impact on global warming
1958-1985 0.0036C/year
C
Solar dimming
Wild et al. 2007 GRL 34
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Impact on global warming
1958-1985 0.0036C/year
1958-1985 0.006C/year
1985-2002 0.038C/year
C
Solar dimming
No solar dimming
Wild et al. 2007 GRL 34
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Impact on daily temperature range
Observation data CRU dataset
Wild et al. 2007, Geophys. Res. Lett.
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Impact on mountain glaciers
Swiss Glaciers area reduction Dimming phase
1973 - 1985 -1 Brightening phase 1985 -
2000 -18 Paul et al. 2004, GRL
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Impact on pan evaporation
85 sites in China
Qian et al. 2006, GRL
Pan evaporation recovers during 1990s after
decades of decline
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Impact on soil moisture
45 years of summer soil moisture observations in
Ukraine
averaged over 22-40E, 46-52N
Soil moisture 1960-2000
Li, Robock, Wild (2007), JGR
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Summary

• Knowledge on the radiation balance both at the
  surface and top of atmosphere required to
  understand climate change
• Surface radiation observations allow to trace
  back surface insolation at widespread locations
  to the 1960s and further back at selected sites
• Evidence for significant decadal variations in
  surface solar radiation (dimming/ brightening),
  suggest also decadal variations in planetary
  albedo
• Surface solar brightening during 1990s in line
  with Earthshine reconstructions of a decreasing
  planetary albedo
• Evidence for aerosol variations as major
  contributor to variations in surface solar
  radiation
• Variations in solar radiation substantially
  influence climate change (greenhouse warming,
  diurnal temperature range, glacier retreat,
  evaporation, soil moisture, intensity of the
  hydrological cycle)

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Surface solar radiation Stockholm 1923 - 2005
Data from GEBA (Ohmura et al. 1989)