Day15_iceageFeedbacks_post.pdf

Day 15: Ice age cycle feedbacks Class learning goals: CG1. Derive the climate sensitivity associated with orbital forcing and ice age cycles CG2. Describe how the ice-albedo feedback helps explain the mismatch between the amplitude of insolation forcing and the climate response. CG3. Describe the two main pathways by which CO 2 gets from the atmosphere to the deep ocean (solubility pump and biological pump) CG4. Explain how feedbacks between CO 2 and temperature can amplify glacial- interglacial climate cycles

Upcoming deadlines Prairie-Learn Quiz 3: Nov 1st at 11.59pm Assignment 4 (paleoclimates and isotopes): Nov 6 th at 11.59pm (Extra credit) Mid-term survey: Let me know what helps you learn in this course, what could be better – I’ll try to implement some of your suggestions and feedback into the rest of the course. Anonymized survey on canvas, due Nov 1 st at 11.59pm

Isotopes and temperature: summary In Ice Cores: - Heavier d 18 O (less negative/more positive) = warmer temperatures over the ice sheet WHY? If the air over the ice sheet is warmer, there is a smaller temperature difference between evaporation in the tropics and precipitation over the ice sheet, so less condensation, so less fractionation (and fractionation makes the remaining water vapour, and thus the precipitation from that water vapour, lighter) In Sediment Cores: 1. Heavier d 18 O (less negative/more positive) relative to the ocean d 18 O = colder ocean temperatures that the foram grew in WHY? Fractionation during shell formation (colder temperatures, less energy available, heavier isotope stronger preference for most condensed format, in this case solid in shells)* AND/OR… 2. Heavier d 18 O (less negative/more positive) = heavier d 18 O of sea water = lower sea levels (and thus we infer a colder climate generally as we’re forming large ice sheets) WHY? Snow in high latitudes is lighter than ocean water due to fractionation. If we have large ice sheets, we have a lot of light isotopes locked up in the ice and so a higher ratio of heavy isotopes in the sea. More ice on land = lower sea levels. * You don’t need to know the text in purple, but the reasoning might help you remember the direction warmer heavier colder heavier (difference) d 18 O ice = -30 ‰ More ice Heavier ocean water

CLICKER: What orbital conditions would MOST FAVOR growth of an ice sheet over Canada? A. Low tilt angle, June 21 st at perihelion B. Low tilt angle, June 21 st at aphelion C. High tilt angle, June 21 st at perihelion D. High tilt angle, June 21 st at aphelion Let “June 21” = summer solstice for the northern hemisphere

CLICKER: What orbital conditions would MOST FAVOR growth of an ice sheet over Canada? A. Low tilt angle, June 21 st at perihelion B. Low tilt angle, June 21 st at aphelion C. High tilt angle, June 21 st at perihelion D. High tilt angle, June 21 st at aphelion LOW seasonal contrast: cool summers (and warmer winters) Greater tilt = greater seasonal contrast, so want low tilt Cooler NH summers when NH summer is when the Earth is furthest away from the sun, i.e. aphelion: June 21st at apheliion

Recap on climate sensitivity: How much is the planet warming for a given change in radiative forcing? The climate sensitivity ( ! ) is defined as the equilibrium temperature change per unit forcing (either 1W/m 2 or a doubling of CO2 = 3.8W/m 2 ) Current best estimates are that ! = 3°C (2.5 to 5 °C) for a doubling of CO 2 Converting units: " = 3.0/3.8 = 0.8K/(Wm -2 ) " = ∆$ ∆% Recall that the climate sensitivity is related to the feedback parameter, f : " = − ∆& ∆' = −1/! CG1

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