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Lecture 14

Lecture 14. Climate Sensitivity, thermal inertia. Climate Sensitivity. The change in equilibrium temperature per unit of radiative forcing. New Equilibrium Temp. Temperature. Change in equilibrium temp. Temp. rises. Start in equilibrium. Time. Apply radiative forcing. Example.

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Lecture 14

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  1. Lecture 14 Climate Sensitivity, thermal inertia

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

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

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

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

  6. Comparing Models • Double CO2 content of model atmosphere • Radiative forcing ~ 4 W/m2 • IPCC has compared many climate models • Results used to estimate actual climate sensitivity of Earth

  7. Sensitivity Estimates Model sensitivities have a range of 2C to 4.5C for a doubling of CO2 (A technical point – don’t memorize.)

  8. 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

  9. 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

  10. From IPCC Report Cloud Feedback in various models

  11. Thermal Inertia Determines rate of temperature change

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

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

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

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

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

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

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

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

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

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

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

  23. 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 • Rate of warming is inversely proportional to heat capacity

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

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