Lecture 14 Climate Sensitivity, thermal inertia

Climate Sensitivity The change in equilibrium temperature per unit of radiative forcing

Temperature Time Start in equilibrium Apply radiative forcing Temp. rises Change in equilibrium temp New Equilibrium Temp

Example Suppose Sensitivity = 2  C per unit of forcing (1 Wm -2 ) Radiative forcing = 3 Wm -2 Radiative forcing = 3 Wm -2 Then, eventual warming = 2 x 3 = 6  C

Differing Sensitivities Same radiative forcing applied at t= 0 System 2 is twice as sensitive 1  C 2  C

Comparing Models Double CO 2 content of model atmosphere Radiative forcing ~ 4 W/m 2 Radiative forcing ~ 4 W/m 2 IPCC has compared many climate models Results used to estimate actual climate sensitivity of Earth

Sensitivity Estimates Sensitivity Estimates Model sensitivities have a range of 2  C to 4.5  C for a doubling of CO 2 Model sensitivities have a range of 2  C to 4.5  C for a doubling of CO 2 (A technical point – don’t memorize.) (A technical point – don’t memorize.)

The Role of Feedbacks Model sensitivity is determined by the strength of the feedbacks in the model Positive feedbacks increase sensitivity Negative feedbacks decrease sensitivity

Differences in Model Sensitivity Main Cause of Variation: Cloud Feedbacks In most models, cloud feedback is positive However, magnitude varies a lot from one model to another However, magnitude varies a lot from one model to another

From IPCC Report Cloud Feedback in various models

Thermal Inertia Determines rate of temperature change

Rate of Warming Thermal inertia: resistance of system to temp. change Measured by heat capacity Measured by heat capacity Higher heat capacity  slower warming

System 1: 70% of warming has occurred at t = 1.2 Time Temperature Change (  C) System 2: 70% of warming has occurred at t = 2.4

Earth-Atmosphere System Most of the heat capacity is in oceans Presence of oceans slows down warming

Comparison Look at two systems with same radiative forcing and sensitivity, but different heat capacities

Compare Two Systems T = 20  C Low Heat Capacity High Heat Capacity T=20  C t = 0 Incoming radiation Outgoing radiation Net radiation

T = 22  C Low Heat Capacity High Heat Capacity T = 21  C t = 1 Systems have warmed  emission has increased  net radiation has decreased

T = 24  C Low Heat Capacity High Heat Capacity T = 22  C t = 2 Still warming

T = 26  C Low Heat Capacity High Heat Capacity T = 23  C t = 3 Back in equilibrium Still warming

T = 26  C Low Heat Capacity High Heat Capacity T = 24  C t = 4 Back in equilibrium Still warming

T = 26  C Low Heat Capacity High Heat Capacity T = 25  C t = 5 Back in equilibrium Still warming

T = 26  C Low Heat Capacity High Heat Capacity T = 26  C t = 6 Back in equilibrium Back in equilibrium, finally

Summary Positive (negative) radiative forcing causes warming (cooling) System warms (cools) until equilibrium is restored Amount of eventual warming (cooling) depends on radiative forcing and sensitivity Eventual warming (cooling) = sensitivity x rad. forcing Eventual warming (cooling) = sensitivity x rad. forcing Rate of warming is inversely proportional to heat capacity

More Realistic Situation Previous examples assumed radiative forcing applied instantaneously i.e., all g.h. gas and aerosols added instantaneously i.e., all g.h. gas and aerosols added instantaneously Real life: g.h. gas & aerosols added gradually More later