Climate SensitivityClimate Sensitivity
The change in equilibrium temperature The change in equilibrium temperature per unit of radiative forcingper unit of radiative forcing
Tem
per
atu
re
TimeStart in equilibrium
Apply radiative forcing
Temp. risesChange in equilibrium temp
New Equilibrium Temp
ExampleExample
Suppose Sensitivity = 2Suppose Sensitivity = 2C per unit of C per unit of forcing (1 Wmforcing (1 Wm-2-2))
Radiative forcing = 3 WmRadiative forcing = 3 Wm-2-2
Then, Then, eventualeventual warming = 2 x 3 = 6 warming = 2 x 3 = 6C C
Differing SensitivitiesDiffering Sensitivities
Same radiative forcing applied at t= 0
System 2 is twice as sensitive
1 C
2 C
Comparing ModelsComparing Models
Double CODouble CO22 content of model atmosphere content of model atmosphere Radiative forcing ~ 4 W/mRadiative forcing ~ 4 W/m22
IPCC has compared many climate modelsIPCC has compared many climate models
Results used to estimate actual climate Results used to estimate actual climate sensitivity of Earthsensitivity of Earth
Sensitivity EstimatesSensitivity Estimates
Model sensitivities have a range of 2Model sensitivities have a range of 2C to C to 4.54.5C for a doubling of COC for a doubling of CO22
(A technical point – don’t memorize.)(A technical point – don’t memorize.)
The Role of FeedbacksThe Role of Feedbacks
Model sensitivity is determined by the Model sensitivity is determined by the strength of the strength of the feedbacksfeedbacks in the model in the model
PositivePositive feedbacks feedbacks increase increase sensitivitysensitivity
NegativeNegative feedbacks feedbacks decreasedecrease sensitivitysensitivity
Differences in Model SensitivityDifferences in Model Sensitivity
Main Cause of Variation: Cloud Main Cause of Variation: Cloud FeedbacksFeedbacks
In most models, cloud feedback is In most models, cloud feedback is positivepositive However, magnitude varies a lot from one However, magnitude varies a lot from one
model to anothermodel to another
Cloud
Albedo
Water Vapor
0.4
0.3
0.2
0.1
0.0
Fig. 2. Boxplot of feedback strengths in 12 climate models.
Feedback Strength
Thermal InertiaThermal Inertia
Determines Determines raterate of temperature of temperature changechange
RateRate of Warming of Warming
Thermal inertiaThermal inertia: resistance of system to : resistance of system to temp. changetemp. change Measured by Measured by heat capacityheat capacity
Higher heat capacity Higher heat capacity slower warming slower warming
System 1: 70% of warming has occurred at t = 1.2
Time
Tem
per
atu
re C
han
ge
(C)
System 2: 70% of warming has occurred at t = 2.4
Earth-Atmosphere SystemEarth-Atmosphere System
Most of the heat capacity is in oceansMost of the heat capacity is in oceans
Presence of oceans slows down warmingPresence of oceans slows down warming
ComparisonComparison
Look at two systems with same radiative Look at two systems with same radiative forcing and sensitivity, but different heat forcing and sensitivity, but different heat capacitiescapacities
Compare Two SystemsCompare Two Systems
T = 20C
Low Heat Capacity
High Heat Capacity
T=20C
t = 0
Incoming radiation Outgoing
radiation
Net radiation
T = 22C
Low Heat Capacity
High Heat Capacity
T = 21C
t = 1
Systems have warmed Systems have warmed emission has increased emission has increased
net radiation has decreased net radiation has decreased
T = 26C
Low Heat Capacity
High Heat Capacity
T = 26C
t = 6
Back in equilibrium
Back in equilibrium, finally
SummarySummary
Positive (negative) radiative forcing causes Positive (negative) radiative forcing causes warming (cooling)warming (cooling)
System warms (cools) until equilibrium is System warms (cools) until equilibrium is restoredrestored
Amount of eventual warming (cooling) Amount of eventual warming (cooling) depends on radiative forcing and sensitivitydepends on radiative forcing and sensitivity Eventual warming (cooling) = sensitivity x rad. Eventual warming (cooling) = sensitivity x rad.
forcingforcing
RateRate of warming is inversely proportional to of warming is inversely proportional to heat capacityheat capacity
More Realistic SituationMore Realistic Situation
Previous examples assumed radiative Previous examples assumed radiative forcing applied instantaneouslyforcing applied instantaneously i.e., all g.h. gas and aerosols added i.e., all g.h. gas and aerosols added
instantaneouslyinstantaneously
Real life: g.h. gas & aerosols added Real life: g.h. gas & aerosols added graduallygradually
More laterMore later