1. Global Warming & Climate Sensitivity: Climate Feedbacks in the Tropics
Professor Dennis L. Hartmann
Department of Atmospheric Sciences
University of Washington
Seattle, Washington
Berkeley Atmospheric Sciences Symposium
November 8, 2002
U.C. Berkeley

2. 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.

3. Greenhouse gas trends
are large and can be
associated directly with
human actions.
Carbon dioxide trends
Can be uniquely associated
with fossil fuel burning
through isotopes of carbon
like 14C and 13C.

4. The Instrumental Record of Global Temperature Anomalies.

5. IPCC

6. Model of Global Temperature Anomalies through time.
Energy Equation:
Climate = Heat Heat
Forcing Storage Loss
In Equilibrium, temperature is constant with time and so,
l is a measure of climate sensitivity;
˚K per Wm-2 of climate forcing

7. To Project future climates by
using the observed record of
climate over the past century,
we need to know three things
to interpret the temperature
time series:
Climate Forcing = DQ (Wm-2)
Heat capacity = C (J oK-1 m -2)
Climate sensitivity = l (oK per Wm-2)

9. Heat Storage: Mostly the Oceans
; Levitus et al. 2001: Science
World Ocean = 18.2 x1022 Joules
Atmosphere = x1022 Joules
Land Ice = x1022 Joules
Model -
observed
Modeled
Model includes forcing from Greenhouse Gases, Sulfate Aerosols
Solar irradiance changes, and volcanic aerosols.
Model minus solar irradiance changes
and volcanic aerosols.

10. Top-Down Approach: Determine sensitivity of climate
from observed record over past
130 years. Use simple model
to extrapolate into future.
Problems: Need to know:
• Climate forcing - uncertain, especially solar and aerosol forcing.
• Heat storage - somewhat uncertain.
• Climate sensitivity - also uncertain.
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.

11. 1850-2000 ~0.6oC Warming; 0.4oC per century
IPCC 2001
~0.6oC Warming; 0.4oC per century
~0.6oC Warming; 2.0oC per century*
*mostly warming from CO2 already in atmosphere

12. Predictions for the year 2100
IPCC
Predictions for the year 2100
Between 1990 and 2100 global mean surface temperature
will increase by
1.4oC < DT < 5.8oC
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.)

13. Bottom-up approach Understand and model key
physical processes that affect
climate sensitivity.
i.e. Feedback Processes
• Water vapor feedback
• Cloud feedback
• Ice-albedo feedback
• Many more

14. 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.

16. - Infrared Greenhouse Effect: The amount by which
the atmospheric reduces
the longwave emission
from Earth.
Greenhouse effect = Surface infrared emission - Earth infrared emission -
155 Wm-2 =
390 Wm-2
235 Wm-2

17. Greenhouse effect = Surface longwave emission - Earth emission

18. To a first approximation, the clear-sky greenhouse
effect is proportional to
the surface temperature.
Sea Surface Temperature

19. And the Greenhouse Effect is related to the amount of water vapor.
Sea Surface Temperature
And the Greenhouse Effect
is related to the amount of
water vapor.
Upper Troposphere Water Vapor

20. Mount Pinatubo Eruption As a test of Water Vapor Feedback
Soden, et al., Science, 26 April 2002
Philippines
June 1991

21. Testing Water Vapor Feedback Observed and Simulated Water Vapor
Year
Observed and Simulated Temperature
Soden, et al., Science, 2002

22. 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.

23. Effect on long-term response to doubled CO2
Water Vapor Feedback
Effect on long-term response to doubled CO2
l is a measure of climate sensitivity;
oK per Wm-2 of climate forcing
lo = for fixed absolute humidity = oK/(Wm-2)
lRH = for fixed relative humidity = oK/(Wm-2)
(NRC, 1979, still good?)

24. Ice-Albedo Feedback
• Ice reflects more solar radiation than other surfaces
• As the Earth warms, ice melts in high latitudes and altitudes
• This lowers the albedo of Earth and leads to further warming.

25. Add Ice-Albedo Feedback to Water Vapor Feedback
(NRC, 1979 still good)
Add these changes to the basic relative humidity feedback and get
as the uncertainty range for the long-term response to CO2 doubling.
IPCC gives
NRC gave

26. 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.

27. Part II: The Tropics and Climate Sensitivity
The net radiative effect of tropical convective clouds.
The Fixed Anvil Temperature (FAT) Hypothesis.

28. GMS-5
IR image
Cloud Feedback
IR Emission Temperature (˚K)

29. 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.

31. Cloud Radiative Effect
Amount by which clouds
affect the energy balance
at the top-of-atmosphere

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

35. t > 1 9%
22%
10%

36. DRi = Ricloudy - Riclear
Tropical clouds in the convective
regions can have strongly positive
or negative effects on the top-of-
atmosphere energy balance,
depending on their top altitude
and albedo (optical depth).
Ai •DRi
But, their populations tend to
arrange themselves so that their
net effect on the radiation balance
is small

37. Conceptual Model:
A convective box with upper-
level clouds, and a clear box.
Connect the two with a large-scale
mass flux, M.
Assume that the albedo of the
convective cloud is proportional to
the strength of the mass flux, and
keep the emission temperature of
the convective clouds fixed.
Make the mass flux proportional to
the difference in SST between the
two boxes DT = T1 - T2.

38. Solve for the equilibrium using column energy balance.
Cloud
Atmosphere
Ocean
Hartmann, Moy and Fu, 2001, J. Climate

39. 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.
Circulation responds sensitively to SST gradients.
Column energy export is uniform in domain of interest.
•Equilibrium conditions hold.
•Then if cloud radiative forcing in non-convective regions is small, it must be small for convective clouds also.

40. Within the warm SST
region, the correlation
between high cloud
amount and net radiation
is small.

41. Finally, the FAT 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 considerations
of:
• Clausius-Clapeyron definition of saturation vapor pressure dependence on temperature.
• Dependence of emissivity of rotational lines of water vapor on vapor pressure.

42. Clear-sky Radiative Cooling and Relaxation:
for tropical climatological conditions
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.

43. Rotational Lines
of Water Vapor
and Upper-
Tropospheric
Cooling
Total Beyond 18.5mm -->

45. 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 vapor
at those low temperatures.
Hypothesis: 200 hPa
Convective outflow and
associated large-scale
divergence near 200 hPa
are both associated by
radiatively-driven divergence
in clear skies.
Further Conjecture:
The temperature at which the
radiatively-driven divergence
occurs will always remain the same,
and so will the temperature of
the cloud anvil tops.

46. Testing the FAT Hypothesis in a model.
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 dividing
the domain into cloudy (rising) and clear (sinking) regions.

47. Radiative Cooling in non-convective region for SST’s ranging from
297K to 303K.
From Larson &
Hartmann (2002a).
Cooling profile
moves up in
upper troposphere
with increasing SST.
But what happens
to the temperature
at the top of the
cooling profile?

48. The temperature of the
200 hPa surface increases
about 13K, while the
surface temperature rises
6K.
The temperature at which
the 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.

49. 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, 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.

50. Remaining Questions:
To what extent is what I just said correct?
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.
What will happen at the tropical tropopause? Will it get warmer or colder and what will this mean for climate?
Fin