Clouds and Climate: Cloud Response to Climate Change ENVI3410 : Lecture 11 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Content of Lecture 11 The importance of cloud feedbacks: Climate sensitivity Cloud radiative forcing Factors affecting clouds Cloud feedback in climate models

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Reading Section Cloud Processes and Feedbacks of IPCC 2001 –

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

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 )

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

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 for 2xCO 2

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Change in Cloud Radiative Forcing Today’s Earth is cooler because of clouds (net -20 Wm -2 forcing (= 4*CO 2 doubling effect) –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

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Radiative Forcing (CRF) Factors that determine CRF –Location (solar intensity) –Depth/thickness –Coverage –Drop/ice concentrations Very similar SW forcing Very different LW forcing

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Radiative Forcing liquid water path (g m -2 )  T s (K) Winter 5 o N low med high cloud height Equilibrium surface temperature due to presence of different clouds liquid water path (g m -2 )  T s (K) Winter 65 o N low med high

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 Very difficult to separate in observations

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Thermodynamic Changes Influence on water vapour feedback –water vapour is much more effective GHG in the upper troposphere than near the surface –Deep Cb clouds transport water vertically high feedback low feedback

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Tropical Cirrus – A Proposed “Adaptive Infrared Iris Effect” 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Problems With the Infrared Iris Idea Observations of cloud IR radiance are not directly related to cirrus coverage Other observations from TRMM (Tropical Rainfall Measuring Mission) show that warm clouds rain more, but they also transport more water vertically See

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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Observed Clouds With Temperature Observations from the International Satellite Cloud Climatology Project (see lecture 7) Clouds become optically thinner (less reflective) at higher temperatures +ve or –ve feedback? latitude d ln(optical depth)/dT Ocean low clouds

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

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