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