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Climate Forcing and Physical Climate Responses
Theory of Climate Climate Change (continued)
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Content
• Concept of “forcing”• Climate sensitivity
– Stefan-Boltzmann response
• Feedbacks– Ice-albedo repsonse– Water vapour– Clouds
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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 longwave2Such as changes in CO2 concentration, land surface, cloud cover, solar
radiation, etc.
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Estimated Forcings since pre-industrial times (IPCC 2007)
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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 = T4
dE/dT = 4T3
E = 4T3T
E=1 Wm-2 implies T = 0.27 oC0.27 oC temperature increase required for Earth to emit
extra 1 Wm-2 to balance forcingIgnores feedbacks caused by T increase
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
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Climate Sensitivity
T= E
(lambda) = climate sensitivity (temperature change for a given applied forcing)
T = change in global mean temperatureE = global mean radiative forcing(With E in W m-2, will be in oC per Wm-2)
• Stefan-Boltzman sensitivity is = 0.27 oC per Wm-2
• This is the minimum temperature response expected because it ignores positive feedbacks in the climate system
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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.
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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
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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
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Ice-Albedo Feedback
response
response
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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
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Water Vapour Feedback
• Water vapour accounts for about 60% of atmospheric infrared absorption
• Carbon dioxide about 20%
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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
100% relative humidity
<100% relative humidity
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Atmospheric Water Vapour Abundance
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Water Vapour Feedback
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Clouds and Precipitation: A Limit to the Water Vapour Feedback
Water vapour
Rainfall
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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
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The Net Effect of Clouds on Earth’s Energy Balance
Basis Investigation LW warming
(W m-2)
SW cooling
(W m-2)
Net Effect
(W m-2)
SatelliteRamanathan et al. (1989)
31 -48 -17
SatelliteArdanuy et al. (1991)
24 -51 -27
ModelsCess and Potter (1987)
23 to 55 -45 to –75 -2 to -34
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Cloud Feedback
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Cloud Feedbacks: Which Direction?
• How might clouds change?– 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
Clouds formwhen water contentof the atmosphereis above this line
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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
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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• http://www.atmos.ucla.edu/csrl/global.html