ESS15 Lecture 13Weather vs. climate, climate sensitivity & feedback.

Please read Chapter 7 in Archer Textbook

What’s the difference?

Weather vs. climate

“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

Climate vs. weather

“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

Predictability

This limit is a property of the atmosphere itself, not a failure of our science!

• Instability and scale interactions make long-range weather forecasting impossible (not just hard!)

• This is not true for climate!

Limits to predictability.

“The butterfly effect”

• 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!

Limits to predictability.

• 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 < Tmax < +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.

Long-term forecasting.

• 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

Weather

• 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

Climate

We can predict that Miami is warmer than Minneapolis for precisely the same reasons that we can predict a warmer future!

Climate Sensitivitiy

How Many Degrees of Warming per W/m2 of Heating?

Absorbed Sunshine In

Surface Temperature Out

F = S0 (1−α)4

Earth’s climate as a “black box”

15 C

Climate System 15 °C

TS 240

W m-2

climate 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).

Climate is full of “feedbacks”

“Feedback” in the technical sense Example: Guitar feedback.

https://www.youtube.com/watch?v=fJgneekeb00

Another “positive feedback” loop

A negative feedback: Body temperature (stabilizing)

Board interlude.

Feedback diagrams.

• Forcing = bump on the trail, response = ?…

• A: Positive feedback

• B: Negative feedback

• C: Neither.

i-clicker: Shocks on a mountain bike can be thought of as what type of feedback?

What types of feedback does the climate system have?

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

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

Forcing(change in absorbed sunshine)

Response:(Change in

Surface Temperature)

Climate forcing, response, and sensitivity.Essential for thinking about climate change.

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“Let’s do the math …”

Solve for ΔF that produces a given ΔT

Climate forcing, response, and sensitivity.

“Let’s do the math …”

A 1 W m-2 change in absorbed sunshine produces about a 0.27 °C change in Earth’s temperature

Baseline climate sensitivity (You do not need to be able to understand this math!)

• Positive Feedbacks (amplify changes)

• Water vapor

• Ice-albedo

• High clouds

ΔF ΔTS Δvapor

Δ albedo

Δ LapseΔhighcloud

Δlowcloud

• Negative feedbacks (damp changes)

– Lapse rate

– Low clouds

Climate feedback processes

• 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

Water vapor feedback.

Cloud feedbacks are special…

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

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

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

• Additional water vapor makes more clouds• Low clouds cool, but high clouds warm• Cloud feedbacks can be both positive and negative.

Cloud feedbacks - depend on the cloud type.

Low clouds act like ice sheets,reflect solar radiation, cool the surface.

High clouds act like greenhouse gases -trap longwave radiation, heat the surface.

Ice feedbacks.

• Warmingleadstomel0ngice,exposingdarkersurfacesthatabsorbmoresolarradia0on.Thisis:

• A:Aposi)vefeedbackprocess,amplifyingwarming.

• B:Anega)vefeedbackprocess,resis)ngwarmingthroughaddedcooling.

• C:Couldbeeither,butdependsonwheretheiceis.

• D:Notafeedbackprocess.

i-clicker

• Radiative forcing melts snow and ice

• Darker surface absorbs more radiation

• Amplifies warming or cooling

Ice - albedo feedback.

https://www.youtube.com/watch?v=fJ999LIWvJk

• 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

Lapse rate feedback.You do not need to understand the inner workings of this process, but know that it is negative..

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

Thanks.

Next time: past (natural) climate change.