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Monitoring Climate Change from Space
Richard Allan
Department of Meteorology, University of Reading
Why Monitor Earth’s Climate from Space?
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Global
Spectrum
Current
Detection
Understanding
Prediction
The problem...
IPCC: www.ipcc.ch/ipccreports/ar4-wg1.htm
Earth’s Radiation balance in space
4πr2
S
πr2
Thermal/Infra-red or
Outgoing Longwave
Radiation (OLR)=σTe4
Absorbed Solar or Shortwave
Radiation (S/4)(1-α)
• There is a balance between the absorbed sunlight
and the thermal/longwave cooling of the planet:
(S/4)(1-α) ≈ σTe4
• How does it balance? Why is the Earth’s average
temperature about 15oC? e.g. Lacis et al. (2010) Science
Earth’s global annual average energy balance
Solar
240 Wm-2
Thermal
240 Wm-2
Efficiency
~61.5%
390 Wm-2
Surface Temperature = +15oC
Radiating Efficiency, or the inverse of the Greenhouse Effect, is strongly
determined by water vapour absorption across the electromagnetic spectrum
Now double CO2 or reduce suns output:
a “radiative forcing”
Solar
240 Wm-2
Thermal: less cooling to space
236 Wm-2
Efficiency
~60.5%
390 Wm-2
Surface Temperature = +15oC
Radiative cooling to space through longwave emission drops by
about 4 Wm-2 resulting in a radiative imbalance
The climate system responds by warming
Solar
>
Thermal
240 Wm-2
236 Wm-2
Efficiency
~60.5%
Heating
390 Wm-2
Surface Temperature = +15oC
The climate system responds by warming
Solar
240 Wm-2
=
Thermal
240 Wm-2
Efficiency
~60.5%
397 Wm-2
Surface Temperature = +16oC
The 2xCO2 increased temperature by about 1oC in this simple
example. So what’s to worry about?
But it’s not that simple…
IPCC (2007)
Climate forcing and feedback : a natural experiment
29/3/06
11.05am
29/3/06
12.26pm
• Clouds affect radiation fluxes
• Radiation fluxes affect clouds
Feedback loops or “vicious circles” amplify or
diminish initial heating or cooling tendencies
e.g. Ice “albedo” Feedback
 CO2
Melting ice
and snow
Temperature
Reduced reflection
of suns rays
Additional
surface heating
One of the strongest
positive amplifying
feedbacks involves
gaseous water vapour
 CO2
 Water
vapour
Temperature
 Net
Heating
 Greenhouse
effect
Cloud Feedback: a
complex problem
• Clouds cool the present climate
• Will clouds amplify or reduce
future warming?
Monitoring Climate From Space
file:///C:/Documents%20and%20Settings/Richard%20Allan/My%20Documents/CONTED/ANIMATIONS/200603_60min_DUST.mov
Satellite measurements
(1970, 1997) confirm
the effect of increasing
greenhouse gases
Stronger greenhouse effect
IRIS/IMG spectra: Harries et al. 2001, Nature
CH4
CO2
O3
1/wavelength
Monitoring Natural forcings: The Sun
ACRIM/VIRGO
IPCC WG1 2.7.1 (p.188-193)
Implied changes in global temperature
0.2
0.1
0.0
Lean (2000)
Y.Wang (2005)
See also: http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant
Monitoring Sea level
IPCC 2007 Fig. 5.13 (p. 410)
Recontructed (proxy)
Coastal tide gauges
Satellite
altimetry
Current rises in global sea level
Is sea level rising faster than projections made by
numerical climate simulations?
Research by Rahmstorf et al. (2007) Science, 4 May
Monitoring
sea surface
temperature
Monitoring Land Ice
From Space
Above: results from
Gravity Recovery And
Climate Experiment
(GRACE) mission
Right: NASA's
ICE-Sat satellite
- Ice, Cloud and
land Elevation
Satellite
Arctic sea ice:
recovery from 2007
minimum but robust
downward trends in
extent since 1979
measured by SSM/I
satellite instruments
NSIDC : http://nsidc.org/news
Remote sensing clouds and aerosol
from space: Cloudsat and CALIPSO
Cloudsat radar
CALIPSO lidar
Target classification
• Radar: ~D6,
detects large
particles (e.g. ice)
• Lidar: ~D2, more
sensitive to thin
cirrus, low-level
liquid clouds and
aerosol pollutants
but signal is
attenuated
Insects
Aerosol
Rain
Supercooled liquid cloud
Warm liquid cloud
Ice and supercooled liquid
Ice
Clear
No ice/rain but possibly liquid
Ground
Work by Dr. Julien Delanoë and Prof. Robin Hogan, University of Reading
How will the water cycle change?
Work by Dr. Ed Hawkins and Prof. Rowan Sutton, University of Reading
Trenberth et al., work published in the Bulletin of the American Meteorological
Society (2009) and Intergovernmental Panel on Climate Change (2007)
Physical basis: water vapour
• Physics: Clausius-Clapeyron
• Low-level water vapour concentrations increase with
atmospheric warming at about 7%/K
– Wentz and Shabel (2000) Nature; Raval and Ramanathan (1989) Nature
Extreme Precipitation
• Large-scale rainfall events fuelled by moisture convergence
– e.g. Trenberth et al. (2003) BAMS
 Intensification of rainfall (~7%/K?)
Global Precipitation is
constrained by energy balance
Allen and Ingram (2002) Nature
Precipitation 
Changing character of rainfall events
Temperature 
See discussion in Trenberth et al. (2003) Bulletin of the American Meteorological Society
Climate model projections (see IPCC 2007)
Precipitation Intensity
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Increased Precipitation
More Intense Rainfall
More droughts
Wet regions get wetter,
dry regions get drier?
• Regional projections??
Dry Days
Precipitation Change (%)
Precip.
(%)
Using microwave measurements from
satellite to monitor the water cycle
Allan and Soden (2008) Science
Precipitation change (%)
The rich get richer…
Wet
Dry
Observations
Models
Allan et al. (2010) Environmental Research Letters
Conclusions
• Earth’s radiative energy balance drives climate change
• It also provides a rich spectrum of information
 Monitoring and detecting climate change
 Understanding physical processes
 Enabling and evaluating prediction
• Challenges...
 Clouds & Aerosol
 Precipitation
 Regional impacts
Earth’s global average energy balance:
no atmosphere
Solar
240 Wm-2
Thermal
240 Wm-2
Efficiency
= 100%
240 Wm-2
Surface Temperature = -18oC
Earth’s global average energy balance:
add atmosphere
Solar
>
Thermal
240 Wm-2
Heating
240 Wm-2
Temperatures rise
Earth’s global average energy balance:
present day
Solar
240 Wm-2
Thermal
240 Wm-2
Efficiency
~60%
390 Wm-2
Surface Temperature = +15oC
The greenhouse effect helps to explain why our planet isn’t frozen.
How does it work?