Global Warming &Climate Sensitivity:

Climate Feedbacks in the TropicsProfessor Dennis L. Hartmann

Department of Atmospheric Sciences

University of Washington

Seattle, Washington

Berkeley Atmospheric Sciences SymposiumNovember 8, 2002

U.C. Berkeley

Two approaches to understanding climate change.

• Top Down Approach - Take observed climate record and attempt to extrapolate intelligently into the future.

• Bottom Up Approach - Attempt to understand and model the critical climate processes, then use the resulting detailed model to predict how future climates might respond to specified forcing like CO2 increase.

Greenhouse gas trendsare large and can be associated directly withhuman actions.

Carbon dioxide trendsCan be uniquely associatedwith fossil fuel burningthrough isotopes of carbonlike 14C and 13C.

The Instrumental Record of Global Temperature Anomalies.

IPCC - 2001

Model of Global Temperature Anomalies through time.

Q CTt

1

T

Energy Equation:

Climate = Heat + HeatForcing Storage Loss

In Equilibrium, temperature is constant with time and so,

is a measure of climate sensitivity; ˚K per Wm-2 of climate forcing

T Q

To Project future climates byusing the observed record of climate over the past century, we need to know three thingsto interpret the temperature time series:

Q CTt

1

T

Climate Forcing = Q (Wm-2)

Heat capacity = C (J oK-1 m -2)

Climate sensitivity = (oK per Wm-2)

Heat Storage: Mostly the Oceans

1955-1996; Levitus et al. 2001: Science

World Ocean = 18.2 x1022 JoulesAtmosphere = 0.7 x1022 JoulesLand Ice = 0.8 x1022 Joules

observed

Modeled

Model includes forcing from Greenhouse Gases, Sulfate AerosolsSolar irradiance changes, and volcanic aerosols.

Model minus solar irradiance changesand volcanic aerosols.

Model -

Top-Down Approach:

Determine sensitivity of climatefrom observed record over past130 years. Use simple modelto extrapolate into future.

Problems: Need to know:

No two of these are known with enough precision to usefully constrain uncertainty in the third, with the data available, although it is possible to fit the observations with fair precision using even a simple model.

• Climate forcing - uncertain, especially solar and aerosol forcing.

• Heat storage - somewhat uncertain.

• Climate sensitivity - also uncertain.

IPCC 2001

1850-2000 ~0.6oC Warming; 0.4oC per century

*mostly warming from CO2 already in atmosphere

2000-2030 ~0.6oC Warming; 2.0oC per century*

IPCC - 2001 Predictions for the year 2100

1.4oC < T < 5.8oC

Between 1990 and 2100 global mean surface temperature will increase by

This large range of uncertainty arises in equal measure from two principle sources:

• Uncertainty about how much climate forcing humans will do, principally through fossil fuel consumption. (Depends on political decisions, economic events, technical innovation and diffusion.)

• Uncertainty about how the climate system will respond to climate forcing by humans - Climate Sensitivity. (Depends on natural processes.)

Bottom-up approach

Understand and model keyphysical processes that affectclimate sensitivity.i.e. Feedback Processes

• Water vapor feedback• Cloud feedback

• Ice-albedo feedback

• Many more

Water Vapor Feedback:

• Water vapor is the most important greenhouse gas controlling the relationship between surface temperature and infrared energy emitted from Earth.

• Saturation vapor pressure increases about 20% for each 1% change in temperature (3 oC).

• Therefore, assuming that the relative humidity remains about constant, the strength of the greenhouse effect will increase with surface temperature.

0

10

20

30

40

50

60

70

80

-30 -20 -10 0 10 20 30 40

Saturation Vapor Pressure (hPa)

Sa

tura

tion

Va

po

r P

ress

ure

(h

Pa

)

Temperature (ÞC)

Infrared GreenhouseEffect:The amount by which the atmospheric reduces the longwave emission from Earth.

Greenhouse effect = Surface infrared emission - Earth infrared emission

= 390 Wm-2 235 Wm-2-155 Wm-2

Greenhouse effect = Surface longwave emission - Earth emission

To a first approximation,the clear-sky greenhouseeffect is proportional to the surface temperature.

Greenhouse Effect Ts4 Earth Emission

Sea Surface Temperature

Sea Surface Temperature

Upper Troposphere Water Vapor

And the Greenhouse Effectis related to the amount of water vapor.

Mount Pinatubo EruptionAs a test of Water Vapor FeedbackSoden, et al., Science, 26 April 2002

PhilippinesJune 1991

Year

Wat

er V

apor

Observed and Simulated Water Vapor

Observed and Simulated Temperature

Soden, et al., Science, 2002

TestingWaterVaporFeedback

Why is fixed relative humidity a good approximation during climate change?

• The relative humidity RH is required to be between 0 and 1.

• The mass of air moving upward (RH ~1), must be equal to the mass of air moving downward (RH~0+)

• The RH in the free atmosphere should be about 0.5

• The inadequacies in this theory for relative humidity do not change that rapidly with climate, compared to the saturation vapor pressure dependence on temperature.

Water Vapor Feedback

is a measure of climate sensitivity; oK per Wm-2 of climate forcing

T Q

o = for fixed absolute humidity = 0.25 oK/(Wm-2)

Effect on long-term response to doubled CO2

RH = for fixed relative humidity = 0.50 oK/(Wm-2)

(NRC, 1979, still good?)

Q2CO24Wm 2 gives

1.6C T 2.7C

RH 1 2.0 0.5Wm 2 K 1

Ice-Albedo Feedback

• As the Earth warms, ice melts in high latitudes and altitudes

• This lowers the albedo of Earth and leads to further warming.

• Ice reflects more solar radiation than other surfaces

Add Ice-Albedo Feedback to Water Vapor Feedback

(NRC, 1979 still good)

Add these changes to the basic relative humidity feedback and get

Q2CO24Wm 2 now gives

as the uncertainty range for the long-term response to CO2 doubling.

IPCC - 2001gives

NRC - 1979 gave

ice 1 0.1 to 0.9 Wm 2 K 1

RH ice 1 0.6 to 2.4 Wm 2 K 1

1.7C T 6.7C

2.1C TRH ice 3.6C

1.5C T 4.5C

T 3.0 1.5 C

Conclusions:

• Uncertainties in projections of global warming are closely related to uncertainties in climate sensitivity to external forcing.

• Official scientific estimates of climate sensitivity have remained constant for 20 years, but so have the uncertainties in sensitivity, which are large.

• Increased efforts to understand the underlying physical processes behind the key climate feedback processes are needed, and many are underway.

• For the time being, however, policymaking on climate will need to be conducted in the presence of large uncertainty about the exact consequences of greenhouse gas emissions.

1.5C T2CO2 4.5C

Part II:

The Tropics and Climate Sensitivity

1. The net radiative effect of tropical convective clouds.

2. The Fixed Anvil Temperature (FAT) Hypothesis.

GMS-5IR image

Cloud FeedbackCloud Feedback

IR E

mis

sion

Tem

pera

ture

(˚K

)

The Greenhouse Effect of Clouds:

Clouds absorb all IR radiation and emit like black bodies at their temperature. Usually they are cold and emit less energy than clear skies would in their absence.

z

Temperature

Cirrus

Convective Anvil

CAT

4

cirrusT

4

cirrusT

4

Clear Sky

T(z)

TClear Sky TConvective Anvil TTropopauseCirrus

Cloud Radiative Effect

Amount by which cloudsaffect the energy balanceat the top-of-atmosphere

High Cloud (p<440mb)in the tropicsis most common over warmest SST,or over land.

22%

9%

10%

> 1

Tropical clouds in the convectiveregions can have strongly positive or negative effects on the top-of-atmosphere energy balance, depending on their top altitudeand albedo (optical depth).

But, their populations tend toarrange themselves so that their net effect on the radiation balanceis small

Ri = Ricloudy - Riclear

i•Ri

M

M

R R

T T21

1 2

Convective Clear

Conceptual Model:A convective box with upper-level clouds, and a clear box.

Connect the two with a large-scalemass flux, M.

Assume that the albedo of theconvective cloud is proportional tothe strength of the mass flux, and keep the emission temperature ofthe convective clouds fixed.

Make the mass flux proportional tothe difference in SST between the two boxes T = T1 - T2.

M

M

R R

T T21

1 2

Convective Clear

Solve for the equilibrium using columnenergy balance.

cdT1

dtR1

MA1

c pe Exp1

cdT2

dtR2

MA2

c pe Exp2

300

302

304

306

308

310

60

70

80

90

100

110

120

130

0 2000 4000 6000 8000

Tem

pera

ture

(K

)

Net

Rad

iati

on (

Wm

-2)

Lambda

b) OLR =150

T1

T2

R2

R1

Hartmann, Moy and Fu, 2001, J. Climate

Ocean

AtmosphereCloud

Net Radiation in convective and nonconvective regions of the tropics

must be the same if:•Albedo of convective cloud responds sensitively to circulation or SST gradients.

•Column energy export is uniform in domain of interest.•Equilibrium conditions hold.

•Circulation responds sensitively to SST gradients.

•Then if cloud radiative forcing in non-convective regions is small, it must be small for convective clouds also.

Within the warm SSTregion, the correlationbetween high cloudamount and net radiationis small.

Finally, the FAT HypothesisFAT Hypothesis, Fixed Anvil Temperatures for All Climates.

This was assumed in previous ‘zero net radiative effect of convective cloud theory’.

Now, I want to argue that tropical anvil clouds appear at a fixed temperature given by fundamental considerationsof:

• Clausius-Clapeyron definition of saturation vapor pressure dependence on temperature.

• Dependence of emissivity of rotational lines of water vapor on vapor pressure.

Clear-sky Radiative Cooling and Relaxation:

In the tropical atmosphere, and the in the global atmosphere,radiative cooling approximately balances heating by latent heat release in convection.

The global mean precipitation rate is about 1 meter per year,

which equals an energy input of about 80 Watts/sq. meter,

Requiring a compensating atmospheric radiative cooling of about 0.7 ˚K/day, averaged over atmosphere.

for tropical climatological conditions

Rotational LinesRotational Linesof Water Vaporof Water Vaporand Upper-and Upper-Tropospheric Tropospheric CoolingCooling

Total Beyond 18.5m -->

Hypothesis: 200 hPaConvective outflow andassociated large-scale divergence near 200 hPaare both associated byradiatively-driven divergencein clear skies.

Fact: The radiatively-driven divergence in the clear regions is related to the decrease of water vapor with temperature following the Clausius-Clapeyron relation and the consequent low emissivity of water vaporat those low temperatures.

Further Conjecture: The temperature at which the radiatively-driven divergenceoccurs will always remain the same,and so will the temperature of the cloud anvil tops.

Larson and Hartmann (2002a,b) Model Study:

MM5 in doubly periodic domain

a) 16x16 box with uniform SST (297, 299, 301, 303K)

b) 16x160 box with sinusoidal SST

Clouds and circulation are predicted

Cloud interact with radiation

Basically, a radiative-convective model in which the large-scale circulation is allowed to play a role by dividingthe domain into cloudy (rising) and clear (sinking) regions.

Testing the FAT Hypothesis in a model.

Radiative Coolingin non-convectiveregion for SST’sranging from297K to 303K.From Larson &Hartmann (2002a).

Cooling profilemoves up inupper tropospherewith increasing SST.

But what happensto the temperatureat the top of the cooling profile?

The temperature at whichthe radiative cooling reaches-0.5 K/day remains constant at about 212K.

The temperature at which the visible optical depth of upper cloud reaches 0.1 remains constant at about 200K.

The temperature of the200 hPa surface increasesabout 13K, while thesurface temperature rises6K.

Conclusions:Net radiation in convective regions should be the same as that in adjacent non-convective regions of the tropics. Hartmann, Moy and Fu, J. Climate, 2001. This would mean that the net radiative effect of tropical convective clouds must remain small.The favored temperature for tropical anvil cloud tops should remain approximately constant during climate changes of reasonable magnitude. FAT Hypothesis. Hartmann and Larson, GRL, 2002.

The emission temperature of the rotational lines of water vapor should also remain approximately constant during climate change. Hartmann and Larson, GRL, 2002.

These assertions imply relatively strong positive water vapor and IR cloud feedback, but small net convective cloud radiative feedback.

Remaining Questions:

Will the area occupied by tropical convection change with climate? If so, how? IRIS?

Will the area, or optical properties of boundary layer clouds change with climate? Feedback looks negative.

To what extent is what I just said correct?

What will happen at the tropical tropopause? Will it get warmer or colder and what will this mean for climate?

Fin