Clouds and Climate: Cloud Response to Climate Change

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. Content of this Lecture • The importance of cloud feedbacks: Climate sensitivity • Cloud radiative forcing • Factors affecting clouds • Cloud feedback in climate models

3. Reading • Section 7.2.2 Cloud Processes and Feedbacksof IPCC 2001 • http://www.grida.no/climate/ipcc_tar/wg1/271.htm

4. Climate Sensitivity • Climate sensitivity determines the global temperature when a radiative forcing is applied

5. Climate Sensitivity • DT = change in global mean temperature • Q = radiative forcing (W m-2) • l = climate sensitivity (W m-2 K-1)

6. Summer 2002 GFDL 2xCO2 Sensitivity (K) NCAR Sensitivity of Climate Models Sensitivity to forcing from doubled CO2 (~4 Wm-2)

7. Cloud Changes and Climate Sensitivity 1/l=4.2 K Wm-2 % Change in low cloud amount per 1K temperature change 1/l=1.8 K Wm-2

8. Change in “Cloud Radiative Forcing” • Clouds cause a net cooling effect on climate (net -20 Wm-2 forcing (equivalent to about 8*CO2) • 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. DT depends on which clouds change

9. Humidity and Temperature Overall increase in atmospheric water vapour and temperature • Increased T • Increased water vapour in atmosphere • Increased cloudiness? • NO • Relative humidity is the relevant quantity Overall increase in atmospheric water vapour 100% RH

10. 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. CRF, dependence on location, thickness and height 40 40 Winter 5o N Winter 65o N high 20 20 cloud height med low 0 0 DTs (K) DTs (K) high -20 -20 med low -40 -40 0 50 100 150 0 50 100 150 liquid water path (g m-2) liquid water path (g m-2) Equilibrium surface temperature change due to presence of different clouds

12. 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. Circulation/Dynamical Changes • Cloud fields are determined by large-scale circulation • Non-local response • El Nino Hadley/Walker circulation Tropical convection Tradewind cumulus Sub-tropical St/Sc Equator 30oN

14. 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? Ocean low clouds 0.1 0 d log(optical depth)/dT 0.05 -0.1 -0.15 -60 -40 -20 0 20 40 60 latitude

15. Observed cloud with temperature: Tropical Cirrus Richard Lindzen, MIT • Japan’s Geostationary Meteorological Satellite • 11 and 12 mm wavelength radiometer • 130oE-170oW, 30oS-30oN (Pacific) • 260 K brightness temperature product is a measure of “high thin cloud” – cirrus • Cirrus cover decreases with increasing SST 0.2 slope = 10-20% change per 1 K SST 0.15 observations 0.1 Cloud Amount 0.05 0 25 26 27 28 29 30 sea surface temperature (K)

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

18. Problems With the Infrared Iris Idea • A hotly debated climate feedback • See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html

19. Net Cloud Feedbacks in GCMs Doubled CO2 experiments 3 WARMING 2 SW 1 Change in CRF (W m-2) 0 LW -1 net -2 COOLING -3 Different models

20. 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. Questions for this lecture • On slide 6, what could explain the wide range of climate model sensitivities to doubling of CO2? • Based on slide 6, what would happen to our climate if the coverage of high thin cirrus clouds increased (a) at the equator, (b) 65oN? 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. 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