Climate Forcing and Physical Climate Responses

1. Theory of Climate Climate Change (continued) Climate Forcing and Physical Climate Responses

2. Content • Concept of “forcing” • Climate sensitivity • Stefan-Boltzmann response • Feedbacks • Ice-albedo repsonse • Water vapour • Clouds

3. Radiative Forcing • Radiative forcingis 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 CO2 concentration, land surface, cloud cover, solar radiation, etc.

4. Estimated Forcings since pre-industrial times (IPCC 2007)

5. 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 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 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

6. Climate Sensitivity DT=lDE • (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

7. 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.

8. 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

9. 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

10. “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

11. Ice-Albedo Feedback response response

12. 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

13. Water Vapour Feedback • Water vapour accounts for about 60% of atmospheric infrared absorption • Carbon dioxide about 20%

14. 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

15. Atmospheric Water Vapour Abundance

16. Water Vapour Feedback

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

18. 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

19. The Net Effect of Clouds on Earth’s Energy Balance

20. Cloud Feedback

21. Range of atmospheric humidities Overall increase in atmospheric water vapour Overall increase in atmospheric water vapour and temperature Cloud Feedbacks: Which Direction? • How might clouds change? • Increase in water vapour content of the air and increase in temperature (=> RH constant?) Clouds form when water content of the atmosphere is above this line

22. 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

23. 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

24. 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