ESS15 Lecture 14. End of the oceans, weather vs. climate, climate sensitivity & feedback. Please read Chapter 7 in Archer Textbook

Transcription

1 ESS15 Lecture 14 End of the oceans, weather vs. climate, climate sensitivity & feedback. Please read Chapter 7 in Archer Textbook

2 Midterm 1 grades will be released later today.

3 Midterm 1 grade distribution Number of students Grade (out of 27)

4 i-clicker question: The average grade was 17.5/27 which is 65% based on the UCI letter grade scale, this is a D so if you got a midterm score of ~ you should be worried about your final course grade. A: True B: False Grade (out of 27)

5 i-clicker question: The average grade was 17.5/27 which is 65% based on the UCI letter grade scale, this is a D so if you got a midterm score of ~ you should be worried about your final course grade. A: True B: False Grade (out of 27)

6 FALSE! The final grade will be curved so that the total average grade is a B. Please don t panic if you got a 16/27 or 19/27 on the midterm it means you are on track for a B* Number of students Grade (out of 27) *assuming you continue to perform similarly relative to the rest of the class on other exams, quizzes, etc

7 Review

8 Earth s Energy imbalances cause global patterns in the winds of the atmosphere. E.g. Midlatitude westerlies Subtropical trade winds.

9 Meanwhile, the winds push the surface of the ocean water, spinning up planetary patterns in ocean currents

10 Together, the winds and currents both help do sideways energy transport of moving energy from where it piles up (tropics) to the poles.

11 I-clicker review question: Why does energy constantly pile up near the equator, creating Earth s energy imbalances? A: Warmer temperatures lead to less cooling by glowing thermal radiation there. B: Sun rays strike the surface of the Earth at a more perpendicular angle there. C: There is more water vapor to amplify the greenhouse effect there. D: The distance to the sun is smaller there.

12 Thermohaline circulation (the last ocean current we need to discuss)

13 Thermohaline circulation, the great ocean conveyer belt.

14 i-clicker survey. The thermohaline circulation is what mixes the ocean vertically. How long do you think it takes for the ocean to mix? A: Less than 5 years B: Between 50 and 100 years C: Between 100 and 500 years D: Over 1000 years

15 How do we measure such a slow flow of water? How do we know it exists?

16 Research ships sample water at many depths along a regular grid pattern follow the isotopes!

17 The pattern of where the radioactive water has gone reveals the slow ocean circulations on our planet.

18 It may seem slow, but a lot of energy flows from the thermohaline circulation Upper limb inflow to North Atlantic ~ 10º C Lower limb outflow ~ 3ºC dq = c dt ~ 3 x 10 7 J of heat released by each m 3 of water during conversion from upper limb to lower limb water mass 20 Sv = 20 x 10 6 m 3 s -1 of water makes this transition, releasing 6 x J s -1 (= 0.6 Pw) of heat to the atmosphere This is 35% of solar heating of North Atlantic north of 40º N latitude!

19 Oceans - in a nutshell. Oceans have fast gyres confined to each basin. The atmospheric wind pattern of westerlies-polewardof-easterlies causes water to pile up in the center of ocean basins. Because the Ekman effect pushes water to the right of the wind (left in S.H.). Swirly currents result as water pushes out from the mound but feels a Coriolis deflection. Exceptions are on the equator and in the southern ocean (east-west flow instead of swirly flow).

20 Oceans - in a nutshell. El Nino - a coupled atmosphere/ocean cycle in the equatorial Pacific, every 3-7 years. When El Nino happens, a huge reservoir of very warm water spreads halfway across the Pacific. World wide weather ripples. e.g. changes jet stream direction, promotes Cali winter-storms. Recently happened! A pretty big one.

21 Oceans - in a nutshell. Oceans have a slow global circulation behind the gyres. Much more gradually, salty, cold water sinks in the N. Atlantic, flowing south across the equator to the Antarctic. Thermohaline Part of the great conveyer belt ocean current system. Takes 1000 s of years for water to complete the journey. Thus the adjustment of the ocean to climate change takes 1000 s of years to occur. Water in deepest oceans is inky black, 1000 s of years old, and doesn t know the industrial age is here yet. Despite being slow, transports a heck of a lot of energy.

22 Weather vs. climate What s the difference?

23 Climate vs. weather Climate is what you expect weather is what you get! Climate is an envelope of possibilities within which the weather bounces around Climate is determined by the properties of the Earth system itself (the boundary conditions), whereas weather depends very sensitively on the evolution of the system from one moment to the next

24 Predictability If they can t predict the weather, how can they possibly hope to predict the climate? Weather forecasts are only useful for a few days, maybe a week at best Forecasting is limited by modeling skill and inadequate observations, but even if these were perfect, the limit of predictability would be about 2 weeks This limit is a property of the atmosphere itself, not a failure of our science!

25 Limits to predictability. The butterfly effect Instability and scale interactions make longrange weather forecasting impossible (not just hard!) This is not true for climate!

26 Limits to predictability. The flow around an airplane wing is governed by the same strongly nonlinear Navier-Stokes equations that govern the atmosphere For the same reasons we will never forecast the weather a month in advance, we can never predict the details of the flow around the wing But given boundary values and parameters, we can predict with confidence the statistics of this flow, or flight would be impossible!

27 Long-term forecasting. Can t forecast the weather in Irvine on the day of the ESS15 final exam in March (Cloud? Sunshine? 60 F? 70 F?) Can forecast with complete confidence that 100 C < T max < +100 C, or even that March ocean will be colder than October Why? Boundary conditions! Solar constant, position of Earth in orbit Atmospheric composition Tilt of Earth s axis, Irvine latitude, Heat capacity, thermal inertia of ocean.

28 Weather Depends on time weather nearby (especially upwind!) weather yesterday which way the wind blows Changes a lot! from day to day from season to season from place to place on a given day Unpredictable more than a few days ahead

29 Climate Depends on where you live: Latitude! Altitude (mountains vs plains) What s upwind (ocean vs land) Location relative to global circulation features. Changes very slowly Very predictable We can predict that Miami is warmer than Minneapolis for precisely the same reasons that we can predict a warmer future!

30 Climate Sensitivitiy How Many Degrees of Warming per W/m2 of Heating?

31 Earth s climate as a black box F = S 0 (1 α) W m -2 Climate System 15 C T S 15 C Absorbed Sunshine In climate feedbacks Surface Temperature Out

32 Climate is full of feedbacks In science, positive feedback does not mean hey, good job! Feedback: A cycle where the output affects the input a forcing produces a response that can be either: Amplified (positive feedback loop), or Damped (negative feedback loop, stabilizing).

33 Feedback in the technical sense Example: Guitar feedback.

34 Another positive feedback loop

35 A negative feedback: Body temperature (stabilizing)

36 Feedback diagrams. Board interlude.

37 i-clicker: Shocks on a mountain bike can be thought of as what type of feedback? Forcing = bump on the trail, response =? A: Positive feedback B: Negative feedback C: Neither.

38 What types of feedback does the climate system have?

39 Liquid oceans throughout Earth s history imply stabilizing climate negative feedbacks must exist.

40 On the other hand, massive ice age swings imply positive feedbacks must exist in climate too.

41 Climate forcing, response, and sensitivity. Essential for thinking about climate change. Forcing (change in absorbed sunshine) Response: (Change in Surface Temperature)

42 Climate forcing, response, and sensitivity. Forcing (change in sunshine) Let s do the math Response: (Change in Surface Temperature) Solve for ΔF that produces a given ΔT

43 Baseline climate sensitivity Let s do the math (You do not need to be able to understand this math!) A 1 W m -2 change in absorbed sunshine produces about a 0.27 C change in Earth s temperature (if the Earth were a bare rock )

44 Climate feedback processes Positive Feedbacks (amplify changes) Water vapor Δ Lapse Δ high cloud Δ low cloud Ice-albedo High clouds ΔF ΔT S Δvapor Δ albedo Negative feedbacks (damp changes) Lapse rate Low clouds

45 Water vapor feedback. Radiative forcing warms surface Warmer surface evaporates more water Warmer air can hold more water Increased water vapor (GHG) absorbs more outgoing radiation, amplifying warming

46 Cloud feedbacks are special

47 If you go winter camping, you know high clouds are your friend because they help keep you warm at the surface.

48 But if you hang out at the beach, you know that low clouds are different

49 it gets cold when low clouds roll in on our coast. Unlike high clouds, low clouds cool the planet.

50 Cloud feedbacks - depend on the cloud type. High clouds act like greenhouse gases - trap longwave radiation, heat the surface. Low clouds act like ice sheets, reflect solar radiation, cool the surface. Additional water vapor makes more clouds Low clouds cool, but high clouds warm Cloud feedbacks can be both positive and negative.

51 Ice feedbacks.

52 i-clicker Warming leads to mel0ng ice, exposing darker surfaces that absorb more solar radia0on. This is: A: A posi@ve feedback process, amplifying warming. B: A nega@ve feedback process, resis@ng warming through added cooling. C: Could be either, but depends on where the ice is. D: Not a feedback process.

53 Ice - albedo feedback. Radiative forcing melts snow and ice Darker surface absorbs more radiation Amplifies warming or cooling

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55 Lapse rate feedback. You do not need to understand the inner workings of this process, but know that it is negative.. Greenhouse effect depends on emission to space from higher (colder) levels of the atmosphere If radiative forcing produces increased vertical mixing by convection, then more heat is mixed to higher levels Warm air aloft emits more radiation to space, compensating for original forcing

56 A lot of feedback processes - but what s the total climate sensitivity if you add them up?

57 Next time: past (natural) climate change. Thanks.