Clouds and Climate: Cloud Response to Climate Change
SOEEI3410Ken 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 Dynamics 1
Content of this Lecture
• The importance of cloud feedbacks: Climate sensitivity
• Cloud radiative forcing• Factors affecting clouds• Cloud feedback in climate models
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Reading
• Section 7.2.2 Cloud Processes and Feedbacks of IPCC 2001– http://www.grida.no/climate/ipcc_tar/wg1/271.htm
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Climate Sensitivity
• Climate sensitivity determines the global temperature when a radiative forcing is applied
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Climate Sensitivity
• DT = change in global mean temperature• Q = radiative forcing (W m-2)• l = climate sensitivity (W m-2 K-1)
lQT D
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Sensitivity of Climate Models
Sensitivity to forcing from doubled CO2 (~4 Wm-2)
Summer 2002
NC
AR
GFD
L
2xC
O2 S
ensi
tivity
(K)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Cloud Changes and Climate Sensitivity
1/l=4.2 K Wm-2
1/l=1.8 K Wm-2
% Change in low cloud amount per 1K temperature change
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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 Dynamics 1
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?
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
CRF, dependence on location, thickness and height
0 50 100 150 -40
-20
0
20
40
liquid water path (g m-2)
DTs (
K)
Winter 5o N
lowmed
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)
DTs (
K)
Winter 65o N
lowmed
high
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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 changesChanges to: – vertical T profile, – atmospheric stability, – turbulence structure of boundary layer,– water substance transport– aerosol
• Very difficult to separate in observations
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Circulation/Dynamical Changes
Tropicalconvection
Tradewindcumulus
Sub-tropical St/Sc
Hadley/Walkercirculation
Equator 30oN
• Cloud fields are determined by large-scale circulation
• Non-local response
• El Nino
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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 lo
g(op
tical
dep
th)/d
T
Ocean low clouds
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Observed cloud with temperature: Tropical Cirrus
25 26 27 28 29 300
0.05
0.1
0.15
0.2
sea surface temperature (K)
Clo
ud A
mou
nt
slope = 10-20% change per 1 K SST
observations
• 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
Richard Lindzen, MIT
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
The Adaptive Infrared Iris as a Climate Change Regulator
warm ocean cold ocean
more IR to space
less cirrus
more rain
less watertransport
less water vapour
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Problems With the Infrared Iris Idea
• A hotly debated climate feedback• See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
Net Cloud Feedbacks in GCMs
-3
-2
-1
0
1
2
3
Cha
nge
in C
RF
(W m
-2)
Different models
SW
LW
netCOOLING
WARMING
Doubled CO2 experiments
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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.
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1
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