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[Music]

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okay thank you everyone and as we're in

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the sleepy sloth before lunch in a dark

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room with increasing co2 I'd like to

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take you back to earlier we shared some

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properties with this and talking about

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some work I've been doing with an

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excellent undergraduate students

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Victoria McDonald is now to grad school

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at u Dobbin like great clouds later as

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Kelly McCusker so we're talking about a

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problem that I've worked on quite a lot

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which is the faint young Sun paradox is

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main sequence stars get brighter through

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their lifetime so for a planet hanging

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out in the habitable zone like Earth

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that means we're going to get a

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monotonic increase in the solar constant

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through time and the geologic record on

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earth is well results about the last

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three and a half billion years and it

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looks like earth with mostly warmer that

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it is now we are in a glacial period now

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but evidence of glaciation is large yet

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absent for Earth history and in the

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first 8/9 of Earth history which we call

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the Precambrian there are a few low

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latitude glaciations but largely earth

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that's the introduction for problem ur a

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quantity I'm going to use quite a lot

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advaita de forcing which to first

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approximation is proportional to third

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the temperature change

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if we want to go back to a fashionable

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time period we need about 50 watts per

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square meter of radiative forcing

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remember that number it's going to

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become important if we're going to do

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that with co2 alone and in a 1d model so

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where we took out all the cloud feedback

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we'd need about a hundred thousand ppm

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and that's not consistent with Joe

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chemical forcing there have been some

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indications in fire work and three

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dimensions began to work by Benjamin

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Shan a and Eric wolf that in 3d lower

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co2 is needed and what I

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during this talk is really

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systematically look at the feedbacks

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involved there they will give you a bit

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of background first because no one yet

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has talked too much about evolution of

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terrestrial planet climate so this is a

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cartoon of the evolution of Earth

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atmosphere it had twelve orders of

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magnitude of dynamic range so when

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you're thinking about how might you

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detect an earth-like planet remember

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that it's really weird and there's a lot

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of big changes some places we could

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constrain how little we know other

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things whether than absence of error

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bars that include that implies that I

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don't even know what the error bars are

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and there's lots of stuff back here but

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in the non simplified version I might

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put back in the gist of this is we're

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looking at a high co2 atmosphere co2

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I've sketched in red here without oxygen

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bit of me phone we're not going to be

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using that today that's the kind of

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atmosphere that we're looking at and

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we're going to be looking at clouds and

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here is uh an older paper of mine where

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I did a parabola space exploration of

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clouds in one dimension this is a good

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plot for looking at what clouds have

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ever done for long and short ways that

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is reflecting incoming sunlight and long

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ways other to that is thermal emission

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are the two effects that we would sum to

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get nursing first clouds reflect

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incoming sunlight if the cloud cover the

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bigger faction of the skies if they have

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more water in them

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they reflect more sunlight so they're

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more blue that means they're more of a

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cooling effect and it doesn't really

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matter that much how high we put them

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but for the long wave the thermal effect

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it really does matter how high we put

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them because low clouds if we get a put

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a cloud just hovering around the top of

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you're different from the surface so the

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greenhouse effect come from a

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temperature difference so these low

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clouds won't have it like the

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temperature difference if I got a few

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tall people if we stood have someone on

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Dori and children and put the clouds

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quite high there'd be a lot colder than

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we were down here so that's going to be

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a big greenhouse effect from that pile

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to look big bed that means they've got a

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big warming effect so we sum these

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together high cloud more them's going to

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give the warming no clouds more of them

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is going to give the cooling but if we

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want to resolve the fade young Sun we

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need to take our low cloud this is how

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much we have today and we need to get

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rid of them we need to get them down

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here this talk I'm going to be talking

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mostly about these low clouds and

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getting rid of them using physics on

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earlier this is an unusual talk for me

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because no chemistry or biology will be

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invoked so yes a photo I took where are

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the postdoc at NASA Ames and I was out

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hiking improving counties this isn't

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very high we're looking at low clouds

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offshore of San Francisco these are the

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same clouds and satellite so these are

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really good clouds for getting

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reflection and they have a lousy

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greenhouse effect

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so visualize this visualize a miserable

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day at AG you where you've just been

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totally clouded out so this is the most

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important slide of the talk so pay

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attention we're going to talk about the

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physics here so we're where we get low

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clouds typically in the descending limb

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of the Hadley cell over really it's

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really good if we've got upwelling cold

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water fat loss chance of just go that's

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off Pulu off Namibia that kind of place

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is really good we've got climatological

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descent here but a cold sea surface

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temperature so we might have warmer

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level off this is Sigma W is a constant

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wet bulb potential temperature because

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this is the moist

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a bat and then here we've got a

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temperature inversion at the top of the

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atmospheric boundary layer and we could

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measure the strength of that inversion

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with a potential temperature difference

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which is called

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lower trophic stability or a wet bulb

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potential temperature difference which

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is called estimated inversion strength

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see the terms and the meteorology

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literature so what do we do with our low

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clouds well convection shallow

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convection in the boundary layer is

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driven by net thermal cooling from the

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cloud so these clouds will go to a emit

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black body radiation they're going to

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receive back radiation from the

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atmosphere and then there's a net

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cooling and that net cooling is what

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drives this shallow circulation here

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that's going to help us build the cloud

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it's going to bring air up to the point

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that it condensed here now if we want to

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get rid of these clouds we want to

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entertain dry air from the three

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troposphere remember this is descending

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limb of the Hadley cell so this is nice

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dry air and if we entrained this into

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the boundary layer that's going to dry

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out our cloud so let we're back in this

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room and we're in a dark room not many

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photons high co2 so high co2 is going to

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mean that we've got a stronger

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greenhouse so this back radiation term

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here is going to be bigger so this net

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radiative cooling is going to be weaker

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so that bad for our clouds we're not

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going to form clouds as well now become

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obvious and aside or to time when we've

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got a weaker solar constant that's going

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to be less good at driving convection in

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the tropics we've got less sunlight

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coming in we'll drive less convection

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and it's the moist convection in the

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tropics which sets the temperature in

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the hole aloft in the whole atmosphere

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so we go to cool the free troposphere

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here so we're going to have a weaker

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inversion here

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that we conversion means that we go to

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into a more dry air and we can dry out

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those clouds so no solo concert - OH -

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atmosphere the hypothesis is is that we

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should have less low clouds so let's do

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some model experiment for that and we're

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running this with an off-the-shelf

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version and back to off-the-shelf

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version of the community our system

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model we're using the five ocean cam for

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40 years average last 20 everything

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we're doing modern-day except we're

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tolling solar constant down and turning

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co2 up but we get the same mean surface

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temperature modern continents non ozone

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modern everything else because we just

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want to look at this one feedback and do

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have controlled an experiment of

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possible keeping the model near modern

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day conditions too much as we can so

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that we might actually trust what the

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model does there is a hope then if you

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want to look at a papers where other

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things have been changed like rotation

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rates continent positions all that kind

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of thing that I refer you to paper by a

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12 inch mesh on a look to this kind of

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problem before we also did some ones

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with cam 5 for smaller range of solar

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constant the tendency seems to be the

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more modern the climate model the left

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range of conditions we can run it for

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that is focus so here's the beginning of

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our model climatology all these baths on

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the left-hand side we have our control

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case on the right-hand side so this is

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the right-hand side of the left hand

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plots that is the middle we have point

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eight solar constants and then we have a

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different field here so less L are

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constant higher co2 we have warm up old

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cooler low latitudes will that pick up

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we've got a stronger greenhouse effect

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that really matters at the poles where

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it's low water vapor low solar constant

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well that really matters at the low

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latitude this is also good for avoiding

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glaciation

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we have less laces hayflacks not much

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different to sensible heat flux bit more

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model climatology less free fit that

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goes with less latent heat flux this is

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really let big of us tropical convection

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and then these measurement of how strong

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that inversion at the top of the

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boundary layer it the no drop affects

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the ability that if the potential

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temperature difference well look we see

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that is smaller everywhere we've got a

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weakling version then the fancier

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measure which is the estimated inversion

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strength the difference in wet-bulb

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potential temperature also just the same

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thing that is left over us we've got

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this weaker inversion this is implying

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to us straight away that we're going to

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get less low clouds because these low

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clouds are so well correlated to yes to

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the strength of that inversion that's in

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the metrology literature that's the 1990

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paper from Tony Flynn go another so this

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is our climatology looking in slices

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through the atmosphere temperature we do

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the same potential temperature we are

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cooler aloft as we're expecting we see

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that a wet bulb potential temperature

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here we're cooler

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aloft will also drier here as well so

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here we are low cloud changes so white

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less clouds we can even see it without

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taking a different field here so we have

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annihilated our low clouds here this is

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a good result for us we have completely

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annihilated then we've also made their

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mum thinner there's less water in them

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and then if we look at this short wave

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cloud forcing here I remember that a

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global average hence we want 50 watts

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per square meter to resolve the faint

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young Sun and we've got numbers that are

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strongly positive you know pretty much

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everywhere here so we're getting a

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really kickin shortwave cloud forcing

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from reducing these low clouds so this

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is looking for 110 percent down to 70

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percent solar for think this is a merely

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systemic effect going slow so wish what

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we're showing here is that these

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physical feedbacks on low cloud which

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arise from first-order changes the

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planetary climate are making a really

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big contribution to stabilizing

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planetary climate look at high cloud

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changes just the kicks well what we see

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is that we do have more high clouds but

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they're thinner so the net effect is

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quite small we were if one word lighting

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a paper on high cloud feedback for would

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want to think very carefully about the

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convection scheme in the model I'm not

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writing such a paper so I'm not thinking

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that carefully about the convection

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schemes and what we see is that merely

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not much happens if near net net zero

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here as we go through time so altogether

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now this is looking as we increase L a

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constant from the beginning of our

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history to some billion years in the

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future well this is the gray line here

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of how much co2 would expect from a 1d

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model this is how much co2 we use this

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is less

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consistent with fire work and here is

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the reason here we have our strong short

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wave cloud forcing here so if we look at

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the kind of time we're interested in

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compared to now that net changing that

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if about 20 watts per square meters out

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of the 50 we needed we've got 20 watts

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per square meter out of this low cloud

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forcing as I only have 1 minutes and 38

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seconds I will move to my conclusions

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instead of reading you my conclusion

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that'll say there's an interesting

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recent work so I think of a paper by

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tapioca row in next year science this

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year actually showed these clouds

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changes if you go from a GCM to a large

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Eddy simulation could be been nonlinear

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as well certain the weakness in this

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work in GCM we show these smooth changes

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I might expect to see some even stronger

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changes than the ones we see possibly

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with nonlinearities involved but that

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aside the take-home message here is that

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I would like to reclaim for physics 40

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percent of the feedback necessary to

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resolve stabilizing Earth's climate so a

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[Applause]

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I call them very very interesting as I

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discuss in my recent book on the history

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of planetary climate studies in the 70s

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cloud feedbacks were were noted as being

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the sort of strongest lever available to

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talk the climate back in the past

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there's a very interesting paper in 2001

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by Chen Wang who at Columbia where he

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purports to show how the changing

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balance between low and high cloud could

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completely stabilize the Earth's

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feedback through our geologic time I

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mean it's a 1d model there's some hand

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waving but you're sort of advancing some

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some already well shot and ground I

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think okay thanks how could you say if

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you could send me that lesson that'd be

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great

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hi Caitlin Loftus Harvard I guess kind

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of following up in what you're talking

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about in Tapia's somewhere kind of

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resolving cloud models how much do you

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expect that we can trust the cloud

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parameterizations and cam even we're

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looking at such a radically different

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environment here so we use two different

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versions of the community atmosphere

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model here and in the different versions

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the radiation boundary layer and low

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cloud schemes are all changed so that

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gives me live actually it's not two

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different models it's like what are the

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half different models and the general

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theme of I think less low cloud we've

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also seen and Jeremy maybe could comment

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on this in the LMDC model and I think

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we've got a good physically-based

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explanation for it so I'm feeling fairly

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comfortable at the moment but I always a

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hierarchy of climate models and always

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more climate model would be would be

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you mentioned polar amplification how

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BIG's the clear skies shortwave feedback

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does that help you as well I have let's

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see if it's on here is it on here I I

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have no idea of the answer I would have

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to look that up yeah okay could the

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non-linearity if you included it could

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it like you know turn on the cloud deck

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at some point and then cause the first

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snowball for example like do you think

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that's a reasonable hypothesis Oh

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so let me think about that for a moment

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I would be no I do not believe it could

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do that the reason is if that if you if

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you decreasing co2 will help you get low

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clouds but then the low carbon fund

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really quickly so if you start cooling

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well unless you build low clouds more

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when you cool which I don't think you

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will then those clouds feedback can come

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off or on quicker than you can glaciated

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panet so that wouldn't be my preferred

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way of making if noble my preferred way

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of making phob all is just to bring down

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co2 gotcha just to follow on from a

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previous question another big positive

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feedback potentially is what happens to

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the ocean if you've got a big polar

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amplification you're warming the polar

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regions parts of the inversion is set by

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these cold waters from cold currents if

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the intermediate ocean starts to warm up

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you start removing the conditions

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irrespective of co2 so if you think

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about what the ocean is doing with polar

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amplification that could make this

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effect potentially bigger but you need a

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couple GCM to look at it so I'll be

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difficult yeah thanks manager and we

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could also protect we could do it

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coupled or we could potentially do it

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with just making prescribed queue for

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making

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imaginary queue flux files and one of my

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00:21:00,650 --> 00:20:59,130
colleagues have been trying to tell me I

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00:21:02,390 --> 00:21:00,660
should do that anyway so now you're the

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second person to tell me to do that

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hey so Robin Wordsworth Harvard

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University and thanks : I think it's a

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nice work how well do you really think

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well thanks Robin can we can we cut off

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the video feed yeah you know I kind I

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don't understand the Paleo cell work so

436
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I won't comment on it I think you know

437
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Clara blackbirds work with calcium

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isotopes where really what it's showing

439
00:21:55,640 --> 00:21:53,370
of the DI c2 alkalinity ratio which

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isn't a co2 constraint but it's used as

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one I think that is probably more only

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that for simple-minded people like me I

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can understand that better so I I'm