Theory of Climate Climate Change (continued)
Climate Forcing and Physical Climate
Responses
Content
• Concept of “forcing”
• Climate sensitivity
– Stefan-Boltzmann response
• Feedbacks
– Ice-albedo repsonse
– Water vapour
– Clouds
2
Radiative Forcing
• Radiative forcing is the change in the radiation1
balance at the top of the atmosphere that results from
a change in the climate system2, assuming that all
other components of the system are unaffected
• It is defined in such a way that positive forcing
corresponds to heating (more incoming than
outgoing radiation)
Footnotes:
1Radiation includes shortwave and longwave
2Such as changes in CO concentration, land surface, cloud cover, solar
2
radiation, etc.
3
Estimated Forcings since pre-industrial times (IPCC 2007)
4
Stefan-Boltzmann Response to Radiative Forcing
How does the atmospheric temperature respond to
increased trapping of outgoing longwave radiation?
Outgoing energy (W m-2) is E = sT4
dE/dT = 4sT3
DE = 4sT3DT
Increased trapping of 1 Wm-2 outgoing LW
radiation leads to an increase in Earth’s
temperature, which leads to more LW
radiation being emitted, bringing the Earth
back into radiative energy balance
DE=1 Wm-2 implies DT = 0.27 oC
0.27 oC temperature increase required for Earth to emit
extra 1 Wm-2 to balance forcing
Ignores feedbacks caused by T increase
5
Climate Sensitivity
DT=l DE
l (lambda) = climate sensitivity (temperature change
for a given applied forcing)
DT = change in global mean temperature
DE = global mean radiative forcing
(With DE in W m-2, l will be in oC per Wm-2)
• Stefan-Boltzman sensitivity is l = 0.27 oC per Wm-2
• This is the minimum temperature response expected because it
ignores positive feedbacks in the climate system
6
Climate Sensitivity from the Historical Record
• Examination of the historical temperature record between
glacials and interglacials together with a knowledge of the
change in radiative forcing of the climate enables the climate
sensitivity to be computed.
• For example, from the last glacial to interglacial transition the
climate sensitivity is approximately 5 oC/7.1 W m-2 = 0.7 oC per
Wm-2. This is somewhat higher than that estimated taking into
account the Stefan-Boltzmann response and the water vapour
feedback and implies that there are further feedbacks of
importance.
• Based on this sensitivity, a 4 W m-2 radiative forcing from a
doubling of carbon dioxide would produce a surface temperature
change of 3 oC.
7
Concept of Feedback
• A response of the system that either amplifies
or damps the effect
• Positive feedback: increases the magnitude
of the response (e.g., temperature)
• Negative feedback: decreases the magnitude
of the response
process
process
feedback
8
Climate Feedback Factor
• The climate feedback factor is the ratio of
temperature change including feedbacks to
the temperature change with no feedbacks
• Approx 1.2 to 3.75 for Earth based on climate
models and observations
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“Response” and “Feedback”
• Response is a change in the climate system
due to an imposed forcing
• Feedback is a response that amplifies or
damps the effect of the original forcing
10
Ice-Albedo Feedback
response
response
11
Ice-Albedo Feedback
• Feedback definitely
positive
• Exact magnitude not
precisely known in climate
models:
–
–
–
–
melt-ponds
snow cover
open water in leads
ice thickness (affects albedo
for depth < 2m)
– ice age
12
Water Vapour Feedback
• Water vapour accounts for about 60% of atmospheric
infrared absorption
• Carbon dioxide about 20%
13
Water Vapour Feedback
• Temperature of
ocean surface
determines water
content of the
atmosphere
• 1 oC increase in
water T causes
7% increase in
atmospheric
water vapour
14
100% relative
humidity
<100% relative
humidity
Atmospheric Water Vapour Abundance
15
Water Vapour Feedback
16
Clouds and Precipitation: A Limit to the Water Vapour
Feedback
Water vapour
Rainfall
17
The Effect of Clouds on Earth’s Energy Balance
• Clouds reflect incoming solar radiation
(cooling effect)
• They absorb outgoing longwave radiation
(warming effect)
clouds absorb IR in the
window region
18
The Net Effect of Clouds on Earth’s Energy Balance
19
Basis
Investigation
LW warming
(W m-2)
SW cooling
(W m-2)
Net Effect
(W m-2)
Satellite
Ramanathan et
al. (1989)
31
-48
-17
Satellite
Ardanuy et al.
(1991)
24
-51
-27
Models
Cess and Potter
(1987)
23 to 55
-45 to –75
-2 to -34
Cloud Feedback
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Cloud Feedbacks: Which Direction?
• How might clouds
change?
Clouds form
when water content
of the atmosphere
is above this line
– Increase in water
vapour content of the
air and increase in
temperature (=> RH
constant?)
Range of atmospheric humidities
Overall increase in atmospheric
water vapour
Overall increase in atmospheric
water vapour and temperature
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Cloud Feedbacks: Complications
• Increased surface
heating leads to
more vigorous
convection, greater
water vapour
transport, changes
in cloud particles,
precipitation, etc.
• Some upper level
clouds (cirrus) can
heat the
atmosphere
22
Climate Model Simulations of Cloud Changes
• Very uncertain model prediction – large
spread between models
• Double CO2: roughly 50-50% spread between
models of positive and negative feedback
• Large uncertainties regarding boundary layer
and deep convective clouds
• Remain largest source of uncertainty in
feedback calculations
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Further Reading
• Climate sensitivity
• http://en.wikipedia.org/wiki/Climate_sensitivity
• Some advanced further reading. A review of
current state of knowledge
•
http://www.atmos.ucla.edu/csrl/publications/Hall/Bony_et_al_2006.pdf
• Discussion of snow-albedo feedback
•
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http://www.atmos.ucla.edu/csrl/global.html