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

• T = change in global mean temperature

• Q = radiative forcing (W m-2)

• = climate sensitivity (W m-2 K-1)

Q

T

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Sensitivity of Climate Models

• Sensitivity to doubled CO2 (~4 Wm-2)

Summer 2002

NC

AR

GF

DL

2xC

O2 S

ensi

tivity

(K

)

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Cloud Changes and Climate Sensitivity

=4.2 K Wm-2

=1.8 K Wm-2

% Change in low cloud amount for 2xCO2

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Change in Cloud Radiative Forcing

• Today’s Earth is cooler because of clouds (net -20 Wm-2 forcing (= 4*CO2 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 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– Location (solar intensity)

– Depth/thickness

– Coverage

– Drop/ice concentrations

Very similar SW forcingVery different LW forcing

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Cloud Radiative Forcing

0 50 100 150 -40

-20

0

20

40

liquid water path (g m-2)

Ts (

K)

Winter 5o N

low

med

high

cloud height

Equilibrium surface temperature due to presence of different clouds

0 50 100 150 -40

-20

0

20

40

liquid water path (g m-2)

Ts (

K)

Winter 65o N

low

med

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 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 Dynamics 1

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 Dynamics 1

Tropical Cirrus – A Proposed “Adaptive Infrared Iris Effect”

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 m 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

• 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 http://www.gsfc.nasa.gov/topstory/20020915iristheory.html

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 (see lecture 7)

• 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 ln

(opt

ical

dep

th)/

dT

Ocean low clouds

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