Theory of Climate Climate Change (continued)

Climate Forcing and Physical Climate Responses

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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 radiation 1 balance at the top of the atmosphere that results from a change in the climate system 2 , 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: 1 Radiation includes shortwave and longwave 2 Such as changes in CO 2 radiation, etc.

concentration, land surface, cloud cover, solar

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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 = s T 4 dE/dT = 4 D E = 4 s T 3 s D T T 3 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 D E=1 Wm -2 implies D back into radiative energy balance T = 0.27 o C 0.27 o C temperature increase required for Earth to emit extra 1 Wm -2 to balance forcing Ignores feedbacks caused by T increase

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Climate Sensitivity

D T= l D E l (lambda) = climate sensitivity (temperature change for a given applied forcing) D T = change in global mean temperature D E = global mean radiative forcing (With D E in W m -2 , l will be in o C per Wm -2 ) • Stefan-Boltzman sensitivity is l = 0.27 o C 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 o C/7.1 W m -2 = 0.7 o C 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 o C.

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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

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

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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 o C 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

Clouds and Precipitation: A Limit to the Water Vapour Feedback Water vapour

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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

Satellite

Satellite Models Investigation LW warming (W m -2 )

Ramanathan et al. (1989) Ardanuy et al. (1991) Cess and Potter (1987) 31 24 23 to 55

SW cooling (W m -2 )

-48 -51 -45 to –75

Net Effect (W m -2 ) -17 -27 -2 to -34

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Cloud Feedback

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Cloud Feedbacks: Which Direction?

Clouds form when water content of the atmosphere is above this line

• 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

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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 CO 2 : 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