ESS15 Lecture 16Past climates, Part 1

• Post solutions to practice exam questions & i-clicker questions.

• Slow down on especially important conceptual diagrams.

• Do more math problems during lecture.

• Continue rapping*

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Review.

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

Forcing(change in absorbed sunshine)

Response:(Change in

Surface Temperature)

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

• The “climate sensitivity” is a number that quantifies how many degrees of warming/cooling occur per W/m2 addition/removal of energy to the climate system.

• Last time we calculated that if the Earth were a bare rock its climate sensitivity would be:

ΔT / ΔF ~ 0.27 K/W/m2

Climate sensitivity - an important definition.

Of course the Earth is not a bare rock.

So the actual climate sensitivity is not 0.27 K/W/m2.

It is determined by multiple competing feedbacks.

• 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.

• Warmingleadstomel0ngice,exposingdarkersurfacesthatabsorbmoresolarradia0on.Thisis:

• A:Aposi)vefeedbackprocess,amplifyingwarming.

• B:Anega)vefeedbackprocess,resis)ngwarmingthroughaddedcooling.

• C:Couldbeeither,butdependsonwheretheiceis.

• D:Notafeedbackprocess.

i-clicker - ice albedo feedback

• Warmingleadstomel0ngice,exposingdarkersurfacesthatabsorbmoresolarradia0on.Thisis:

• A:Aposi)vefeedbackprocess,amplifyingwarming.

• B:Anega)vefeedbackprocess,resis)ngwarmingthroughaddedcooling.

• C:Couldbeeither,butdependsonwheretheiceis.

• D:Notafeedbackprocess.

i-clicker - ice albedo feedback

• Radiative forcing melts snow and ice

• Darker surface absorbs more radiation

• Amplifies warming or cooling

Ice - albedo feedback.

• 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.

In the last decade climate scientists have discovered that high cloud feedbacks are very likely positive.

But the sign of the low cloud feedback is uncertain.

(A major frontier of modern climate science)

"Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds”

— IPCC AR5, Ch. 7

In other words, we happen to live next to one of the most climatically mysterious cloud systems on the planet.

http://icons.wunderground.com/data/wximagenew/j/jrkerns/3.jpg

Think about that next time you witness our coastal clouds….

http://icons.wunderground.com/data/wximagenew/n/NickGeorge/4-800.jpg

The “marine layer” clouds we see at the coast are just the edge of a massive brilliant white sheet of low clouds extending over 100,000 square kilometers!

San Diego, CA

View out the airplane window on your way to Hawaii.

The Californian stratocumulus system is one of several massive low cloud decks in other regions of the Earth.

R. Wood, Monthly Weather Rev., 2012

low cloud cover

Summary - Low clouds and climate change

• If Earth’s low cloud decks shrink it will amplify global warming in a positive feedback, like shrinking ice sheets.

• But if low cloud decks grow in size with climate change this could partially offset global warming, helping stabilize the planet!

• Climate science cannot yet predict what they will actually do - an active research frontier.

• Some geo-engineering enthusiasts want to try to artificially brigthen low clouds to offset global warming.

The trillion dollar question…

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

1. Paleoclimate analogs: how much has climate changed in the past when forcing of known strength was applied?

• Advantage: all feedbacks included • Disadvantage: hard to know exactly how much

forcing & global temperature response

2. Calculation from physical principles including feedback processes (complex global climate models)

• Advantage: Physical insight • Disadvantage: “All models are wrong …”

Estimating total climate sensitivity

1. Geologic past (100’s of millions of years)

2. Deglaciation analog (18,000 years ago to preindustrial time)

3. Last Millennium analog (Medieval Warm Period to Little Ice Age)

4. Modern Climate Record (20th Century changes)

Learning from the past

The further back we go, the less data we have to work with. Using modern data, we have only brief transients to study.

Climate changes in the recent past provides a way to estimate how sensitive climate is.

1. High albedo2. Low CO2

Ice age world Two main causes of fewer Watts:

Vostok (400k yr) Ice Core data (Petit et al, 1999)

iceiceice

ice

CO2

• Over the past 420,000 years atmospheric CO2 has varied between 180 and 280 ppm, beating in time with the last four glacial cycles

CO2 and the ice ages

• Recall climate sensitivity is the change in temperature per W / m2 change in forcing

• So, if we can estimate how much colder it was 20,000 years ago….

• …And if we can estimate how many less Watts the Earth system was forced by compared to today….

• Then we can estimate a climate sensitivity from observations.

Paleoclimate observations of the last glacialcan tell us something about climate sensitivity.

Source: Hansen and Sato (2011)

Reconstructed forcing of the last ice age.

This is three times higher than the bare rock model predictsimplies climate sensitivity of 0.75 K/ W / m2.

• Stonger positive feedbacks than negative feedbacks.

• Stronger negative feedbacks than positive feedbacks.

• Equally strong positive and negative feedbacks.

i-clicker: The fact that the climate sensitivity of Earth is 3x stronger than the bare rock model predicted means our climate has:

• Stonger positive feedbacks than negative feedbacks.

• Stronger negative feedbacks than positive feedbacks.

• Equally strong positive and negative feedbacks.

i-clicker: The fact that the climate sensitivity of Earth is 3x stronger than the bare rock model predicted means our climate has:

But 20,000 years was a long time ago….

What can we learn from the more recent past?

What processes“force” changes in climate by changing the Watts absorbed by the system?

Changing sun

Changing GHGs

Our variable star.

A: < 0.25 W/m2 B: 0.25-0.5 W/m2 C: 1-3 W/m2 D: 4-8 W/m2 E: > 8 W/m2

i-clicker: How big a change in incoming energy (Watts per m2), happens from the crest to the trough of the 11-year solar cycle?

Our variable star.

A: < 0.25 W/m2 B: 0.25-0.5 W/m2 C: 1-3 W/m2 D: 4-8 W/m2 E: > 8 W/m2

i-clicker: How big a change in incoming energy (Watts per m2), happens from the crest to the trough of the 11-year solar cycle?

Climate forcing

= ΔS(1-α)/4 ~ 2 W m-2 x 0.7 / 4 = 0.35 W m-2

How big a change in absorbed Watts, peak vs. valley of 11-yr solar cycle?

What processes“force” changes in climate by changing the Watts absorbed by the system?

Changing sun

Volcanic aerosols

Changing GHGs

BOOM!• Volcanoes release

huge amounts of SO2 gas and heat

• SO2 oxidizes to SO4 particles (“aerosol”) and penetrates to stratosphere

• SO4 aerosol scatters solar radiation back to space

Mt. Pinatubo, 1991

Volcanic aerosol amount over the past 50 years.

After major eruptions, the world cools for a few years.

What processes“force” changes in climate by changing the Watts absorbed by the system?

Changing sun

Volcanic aerosols

Pollution aerosols

Changing GHGs

People make aerosolparticles too… fromair pollution.

• Ship tracks off west coast

• Cloud droplets condense on tiny particles

• Makes more/smaller cloud drops

• Clouds are brighter (higher albedo )

Human aerosol pollution can change cloud albedos in certain conditions.

Which of these has been increasing Watts most significantly over the past 1000 years?

Changing sun

Changing volcanic aerosols

Changing pollution aerosols

Changing GHGs

(A)

(B)(C)

(D)

i-clicker

Which of these has been increasing Watts most significantly over the past 1000 years?

Changing sun

Changing volcanic aerosols

Changing pollution aerosols

Changing GHGs

(A)

(B)(C)

(D)

i-clicker

1.0 W m-2 Tambora “year w/out a summer”

18

16

19

91

Pinatubo

0.5

Reconstructed climate forcing since 1000 AD

i-clicker: Over the past 500 years, the solar Watts have been:A: generally decreasing B: generally increasingC: neither decreasing nor increasing D: Insufficient data to say.

Which has been adding more Watts recently … sun or people?

Sun brightened by ~ 1 W/m2 gradually over past 500 years……GHGs have already added even more than this (1.5 W/m2), more rapidly.

During next 100 years people could add a heck of a lot more Watts!

sun

people

Global climate during the past 2000 years

0.8

K

This 600-year period of global cooling provides another “analogy” for estimating climate sensitivity.

• Cooling from Medieval Warm Period to Little Ice Age ~ 0.8 K

• Solar forcing changed by ~ 1.0 W m-2(somewhat complicated by volcanic forcing)

Implied total climate sensitivity

(including feedback) 0.8 K per (W m-2) 1826

The “Second millenium analog”….

The “Volcanic analog”….

• Cooling following eruption of Mount Pinatubo in 1991 ~ 0.5 K

• Volcanic forcing ~ 0.7 W m-2

Implied total climate sensitivity (including feedback)

0.7 K per (W m-2)

The “Volcanic analog”….

• Many lines of evidence from the past suggest about 0.8 °C of warming per (W m-2)

• Remember each doubling of CO2 adds 3.7 Watts per square meter

• So expect about (3.7 W m-2) x (0.8 °C (W m-2)-1) = 3 °C per doubling of CO2

Climate sensitivity

Distribu(onsandrangesforclimatesensi(vityfromdifferentlinesofevidence.Thecircleindicatesthemostlikelyvalue.Thethincoloredbarsindicateverylikelyvalue(morethan90%probability).Thethickercoloredbars

indicatelikelyvalues(morethan66%probability).Dashedlinesindicatenorobustconstraintonanupperbound.TheIPCClikelyrange(2to4.5°C)andmostlikelyvalue(3°C)areindicatedbythever(calgreybarandblackline,

respec(vely(Source:KnuN&Hegerl,Nature,2008)

Climate sensitivity

Current and future climate change in the context of the past 20,000 years.

• Note different scales

• Modern changes comparable in magnitude to postglacial, but much faster!

Greenhouse Gas Radiative Forcing

Deglaciation

Holocene“Optimum”

Little Ice Age

(when my son is old)

Temperature history since the ice age.

Modern Record

Next time: Climates of the deep past (geologic time)