1. Clouds and Climate: Cloud Response to Climate Change SOEEI3410 Ken Carslaw Lecture 5 of a series of 5 on clouds and climate Properties and distribution of clouds Cloud microphysics and precipitation Clouds and radiation Clouds and climate: forced changes to clouds Clouds and climate: cloud response to climate change

2. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Content of this Lecture The importance of cloud feedbacks: Climate sensitivity Cloud radiative forcing Factors affecting clouds Cloud feedback in climate models

3. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Reading Section 7.2.2 Cloud Processes and Feedbacks of IPCC 2001 –http://www.grida.no/climate/ipcc_tar/wg1/271.htmhttp://www.grida.no/climate/ipcc_tar/wg1/271.htm

4. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Climate Sensitivity Climate sensitivity determines the global temperature when a radiative forcing is applied

5. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Climate Sensitivity  T = change in global mean temperature Q = radiative forcing (W m -2 ) = climate sensitivity (W m -2 K -1 )

6. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Sensitivity of Climate Models Sensitivity to forcing from doubled CO 2 (~4 Wm -2 ) Summer 2002 NCAR GFDL 2xCO 2 Sensitivity (K)

7. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Changes and Climate Sensitivity  =4.2 K Wm -2  =1.8 K Wm -2 % Change in low cloud amount per 1K temperature change

8. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Change in “Cloud Radiative Forcing” Clouds cause a net cooling effect on climate (net -20 Wm -2 forcing (equivalent to about 8*CO 2 ) –All models agree on sign (+/-) of CRF Cloud feedback is about how CRF changes as greenhouse gases increase –Models disagree greatly on this Some clouds warm, some cool.  T depends on which clouds change

9. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Humidity and Temperature Increased T Increased water vapour in atmosphere Increased cloudiness? NO Relative humidity is the relevant quantity Overall increase in atmospheric water vapour Overall increase in atmospheric water vapour and temperature 100% RH

10. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Radiative Forcing (CRF) Factors that determine CRF (or, what does a climate model need to get right?) –Cloud location (solar intensity) –Depth/thickness –Coverage –Drop/ice concentrations How would SW and LW impact on climate change for these two cloud field?

11. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 CRF, dependence on location, thickness and height 0 50 100 150 -40 -20 0 20 40 liquid water path (g m -2 )  T s (K) Winter 5 o N low med high cloud height Equilibrium surface temperature change due to presence of different clouds 0 50 100 150 -40 -20 0 20 40 liquid water path (g m -2 )  T s (K) Winter 65 o N low med high

12. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Reasons for Cloud Changes Large-scale dynamics/circulation –Global circulation changes in response to changes in ocean circulation, changes in ocean-atmosphere T contrast, etc Thermodynamic/cloud-scale changes Changes to: –vertical T profile, –atmospheric stability, –turbulence structure of boundary layer, –water substance transport –aerosol Very difficult to separate in observations

13. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Circulation/Dynamical Changes Tropical convection Tradewind cumulus Sub-tropical St/Sc Hadley/Walker circulation Equator30 o N Cloud fields are determined by large- scale circulation Non-local response El Nino

14. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Observed Clouds With Temperature Observations from the International Satellite Cloud Climatology Project Clouds become optically thinner (less reflective) at higher temperatures +ve or –ve feedback? -60 -40 -20 0 20 40 60 -0.15 -0.1 0.05 0 0.1 latitude d log(optical depth)/dT Ocean low clouds

15. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Observed cloud with temperature: Tropical Cirrus 25 26 27 28 29 30 0 0.05 0.1 0.15 0.2 sea surface temperature (K) Cloud Amount slope = 10-20% change per 1 K SST observations Japan’s Geostationary Meteorological Satellite 11 and 12  m wavelength radiometer 130 o E-170 o W, 30 o S-30 o N (Pacific) 260 K brightness temperature product is a measure of “high thin cloud” – cirrus Cirrus cover decreases with increasing SST Richard Lindzen, MIT

16. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1

17. 1 The Adaptive Infrared Iris as a Climate Change Regulator warm ocean cold ocean more IR to space less cirrus more rain less water transport less water vapour

18. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Problems With the Infrared Iris Idea A hotly debated climate feedback See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html http://www.gsfc.nasa.gov/topstory/20020915iristheory.html

19. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Net Cloud Feedbacks in GCMs -3 -2 0 1 2 3 Change in CRF (W m -2 ) Different models SW LW net COOLING WARMING Doubled CO 2 experiments

20. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Difficulties Different types of clouds have different effects and may change in different ways – many separate problems Some aspects of clouds (thickness, ice content) are difficult to observe Sub-grid scale problems Effects of temperature and circulation can be confused Changes observed on short time scales (e.g., El Niño) may not always be good indicators of climate change- induced changes

21. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Questions for this lecture On slide 6, what could explain the wide range of climate model sensitivities to doubling of CO 2 ? Based on slide 6, what would happen to our climate if the coverage of high thin cirrus clouds increased (a) at the equator, (b) 65 o N? What explains the difference? On slide 14, explain whether the data indicate a positive or negative climate feedback. For the first model shown on slide 19 explain what cloud changes could account for the changes in global mean SW and LW cloud radiative forcing.

22. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Competition Take a photograph of a cloudy scene. Send it to me with a detailed explanation of what the clouds are doing to climate. The winner will be decided based on beauty and complexity of the cloud scene and accuracy of the explanation Closing date: end of term Prize: A large tin of chocolates