Transcript Global Warming and Climate Sensitivity

Global Warming
and
Climate Sensitivity
Professor Dennis L. Hartmann
Department of Atmospheric Sciences
University of Washington
Seattle, Washington
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.
The Instrumental Record of Global
Temperature Anomalies.
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.
IPCC - 2001
Model of Global Temperature Anomalies through time.
Energy Equation:
T
1
Q  C
 T
t

Climate =
Forcing
Heat +
Storage
Heat
Loss
In Equilibrium, temperature is constant with time and so,
T   Q
 is a measure of climate sensitivity;
K per Wm-2 of climate forcing
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:
T
1
Q  C
 T
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 Joules
Atmosphere = 0.7 x1022 Joules
Land Ice
= 0.8 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.
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.
IPCC -2001
IPCC 2001
1850-2000 ~0.6oC Warming; 0.4oC per century
2000-2030 ~0.6oC Warming; 2.0oC per century*
*mostly warming from CO2 already in atmosphere
IPCC - 2001
Predictions for the year 2100
Between 1990 and 2100 global mean surface temperature
will increase by
1.4oC < T < 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.)
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
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). 80
Saturation Vapor Pressure (hPa)
• Therefore, assuming that the
relative humidity remains
about constant, the strength
of the greenhouse effect will
increase with surface temperature.
70
Saturation Vapor Pressure (hPa)
60
50
40
30
20
10
0
-30
-20
-10
0
10
20
Temperature (ÞC)
30
40
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
Greenhouse effect = Surface longwave emission - Earth emission
GreenhouseEffect  Ts4  Earth Emission
To a first approximation,
the clear-sky greenhouse
effect is proportional to
the surface temperature.
Sea Surface Temperature
Sea Surface Temperature
And the Greenhouse Effect
is related to the amount of
water vapor.
Upper Troposphere Water Vapor
Mount Pinatubo Eruption
As a test of Water Vapor Feedback
Soden, et al., Science, 26 April 2002
Philippines
June 1991
Observed and Simulated Water Vapor
Testing
Water
Vapor
Feedback
Year
Observed and Simulated Temperature
Soden, et al., Science, 2002
Water Vapor Feedback
Effect on long-term response to doubled CO2
T   Q
 is a measure of climate sensitivity;
oK per Wm-2 of climate forcing
o = for fixed absolute humidity = 0.25 oK/(Wm-2)
RH = for fixed relative humidity = 0.50 oK/(Wm-2)
2 1
1
2.0

0.5Wm
K
RH
Q2CO2  4Wm 2
gives
1.6C  T  2.7C
(NRC, 1979, still good?)
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.
Add Ice-Albedo Feedback to Water Vapor Feedback
1
ice  0.1 to  0.9 Wm
2
K
1
(NRC, 1979 still good)
Add these changes to the basic relative humidity feedback and get
1
RHice 0.6 to 2.4 Wm
Q2CO2  4Wm 2
2
K
1
nowgives
1.7C  T  6.7C
as the uncertainty range for the long-term response to CO2 doubling.
2.1C  TRHice  3.6C
IPCC - 2001gives
1.5C  T  4.5C
NRC - 1979 gave
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.
1.5C  T2CO2  4.5C
• 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.
Estimated Strength of Water Vapor Feedback
Earliest studies suggest that if the absolute humidity increases
in proportion to the saturation vapor pressure (constant relative
humidity), this will give rise to a water vapor feedback that will
double the sensitivity of climate compared to an assumption of
80
fixed absolute humidity.
Most observational and
modeling studies have
supported this conclusion.
Saturation Vapor Pressure (hPa)
70
Saturation Vapor Pressure (hPa)
60
50
40
30
20
10
0
-30
-20
-10
0
10
20
Temperature (ÞC)
30
40