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you

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[Music]

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well everyone I hope you're having a

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great conference I'm glad to be here

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into session so I'm going to talk about

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some work on the habitable zone which I

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hope by now you've heard about and this

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is work I've done with Ravi Natasha

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sunny and Jim casting so to start with

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what do we mean by habitable zone we

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mean really a liquid water habitable

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zone this is important because it has

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two biologists we like to be imaginative

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and we don't want to say that the

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habitable zone is the only place that

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could possibly ever be habitable but

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this is an observational tool that helps

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us constrain exoplanet observations it's

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based on the orbital distance of a star

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the orbital distance of a planet from a

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star it's the region where you could

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sustain liquid water on the surface well

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what do you really need for that you

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need a rocky planet this doesn't really

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apply to gas giants you need an

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atmosphere with you know n2 water vapor

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and co2 this is least the assumptions

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that go into our model you can do this

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with some other greenhouse gases and

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there's plenty of papers that do that

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but but for calculating just the

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essential outer and inner edge of the

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habitable zone we're really interested

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in water vapor co2 is greenhouse gases

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and nitrogen to buffer the atmosphere

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add some pressure

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you also need some method for recycling

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volatile plate tectonics is a really

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good way of doing this we don't really

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have any other great examples of other

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ways of recycling volatiles on earth but

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there's there's some see reticle ideas

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out there you know stagnant lid or

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something like that but you need to be

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able to recycle the volatile so this

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plot I hope many of you have seen I will

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go through it very briefly though this

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is the the habitable zone as drawn by

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sunny we have the effective temperature

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of the star here or you can just think

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of this as stellar type to F g/km dwarfs

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down here and this is the effective

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stellar flux you could also think of

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this in terms of distance if you want so

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this is closer to the star this is

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further away from the star earth is

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right here we're situated in this nice

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habitable region where we can have

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liquid water on the surface thankfully

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but if earth were to be pushed a little

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further in toward the Sun you would

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start to increase the rate of

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evaporation of oceans and you would

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actually start to lose your water vapor

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would probably first happen is a moist

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greenhouse where the water actually gets

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photolyze out at the top of the

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stratosphere but if you push further in

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or if conditions are slightly different

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you may even enter a runaway greenhouse

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where

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very rapidly so the inner edge I'm not

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going to focus much on today you might

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hear about that later in the session

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though the outer edge of the habitable

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zone is where we're concerned for this

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problem the outer edge is defined by the

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maximum amount of warming you can get

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out of co2 so if you pull the planet

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away from the Sun and I'm going to show

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you in the next slide why this is but we

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expect that a planet like Earth that

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tectonically active would build up

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carbon dioxide in its atmosphere as you

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move away from the Sun as it gets colder

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the the threshold then where you have

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which says maximum greenhouse that's the

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point at which the warming you get from

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the greenhouse effect of co2 is actually

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outweighed by the cooling that you get

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from Raleigh scattering because you have

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a very dense co2 atmosphere so this

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defines the outer edge of the habitable

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zone but beyond that your planet is

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basically an ice ball so what's going on

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with this outer edge of the habitable

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zone why do I say that you're going to

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build up co2 what's going on is it's the

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long term carbon cycle the carbonate

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silicate cycle refer to it as starts

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with volcanoes volcanoes put co2 in the

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atmosphere the co2 stays in the

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atmosphere where it's a greenhouse gas

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but you've got water on the planet

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you've a hydrological cycle so some of

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the co2 is going to get dissolved in

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rainwater where it rains out onto land

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here when when carbonic acid dissolved

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in rainwater hits calcium silicate rocks

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you get weathering which breaks down

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into ions that will then run off the

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rivers into the ocean where they then

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saturate the ocean with these ions if

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you have life on your planet you know

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life might make shells and things out of

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these if you don't have life that that's

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okay this process still works you just

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saturate your ocean until precipitation

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occurs anyway and so then you've got

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these calcium carbonates on the ocean

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floor which then subduct and

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metamorphism changes this into casi o3

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and releases the co2 which then goes

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back into volcanoes so this is a carbon

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cycle it's not going to help us with

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with contemporary climate change it

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operates on a half a million year

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timescale or so but this is this

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weathering stuff is temperature

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dependent

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and so as the planet cools as you move

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earth away from the sign in a you know

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thought experiment

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you're slowing down the rate of

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weathering which means you're going to

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increase the amount of co2 in the

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atmosphere and this is what gives us the

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a dreaded habitable zone well what was

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pointed out and we didn't actually

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realize that this was pointed out by a

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by a Tajik and also Kristin Manoa

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pointed 2020 15 the weathering process

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is not just temperature dependent it's

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also dependent on how much co2 you have

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and maybe this actually seems kind of

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obvious in retrospect if you have more

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co2 you're going to whether you have

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more weathering and you're going to end

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up with an overall you know different

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concentration of co2 in the atmosphere

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than you would otherwise have so this is

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important because this actually gives us

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some very different behaviors that are

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transient some some time dependent

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climate states that we didn't get

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otherwise when we only considered the

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temperature dependence not the co2

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dependence so this is what we call it my

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title has this phrase a limit cycle and

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so this is a climate limit cycle which

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means you see you're alternating between

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a glacial state and a warm state driven

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by these oscillations in co2 so what I'm

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doing these are calculations with a one

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dimensional energy balance climate model

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I start planet this is a planet around a

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G star I'm looking at like an early

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Earth case where the solar constants

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about 70% and in the tenth of the

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volcanic outgassing rate as today and

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I'll explain why it shows that in a

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minute so imagine that you start with a

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cold planet that's an ice ball and it

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completely frozen but and with a an

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active carbon silicon carbonate silicate

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cycle so what happens is your frozen

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planet so you've got volcanoes

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outgassing into the atmosphere adding

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greenhouse warming so this Green Line

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shows co2 here's the access for co2

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partial pressure black line is

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temperature so as we march forward in

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time you're building up co2 slowly from

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volcanoes now the fruit the surface is

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frozen so you're not weathering any of

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it down you're just building co2 to the

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atmosphere so we continue building co2

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in the atmosphere until you reach this

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critical point where you've got enough

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warming

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that your ice-melt you Daglish d

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glaciated lanit and you very rapidly

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warm so now you're warm you're above for

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you thing you're a habitable planet in

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the sense of being able to have liquid

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water on its surface but what happens

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now weathering turns on you you start

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drawing down all the co2 from the

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atmosphere and we we go we get this

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sawtooth co2 goes down because

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weathering turns on well now you've lost

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your greenhouse effect so temperature

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goes down as well you plummet back into

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a glacial state and so this is what we

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call a limit cycle is long periods of

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glaciation followed by punctuated

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periods of warmth repeated over time

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this limit cycle phenomenon doesn't

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behave the same around all stellar

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spectral types it's actually a function

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of stellar type so here's what the plot

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I just showed you exactly the same

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that's a rounded G star so here I've

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kept the parameters the same still 70%

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of solar constant same volcanic

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outgassing rate we're just around an F

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star now so a hotter star but it's not

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just hotter it actually puts out more

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energy in the blue end of the spectrum

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and less energy in the red end of the

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spectrum so what that does if you get in

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hot enhanced ice albedo feedback this

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has been discussed by I think Bob

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Heverly omoi shields and others that you

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would expect this so you get enhanced

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ice albedo feedback around an F F dwarf

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stars and so that actually just

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increases the frequency that the timing

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of these events so you build up co2 here

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you warm but notice how sharp this peak

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is you have a very very narrow range of

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time when your planet is warm most of

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the time you're in this cold phase where

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you're just building up co2 in the

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atmosphere but not D glaciated so this

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this suggests that if a planet is in is

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called in this limit cycle perhaps the

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g-type stars have a slightly more

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likelihood for for sustaining longer

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periods of warmth than the F dwarfs now

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I've mentioned volcanic outgassing read

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a couple of times the volcanic

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outgassing rate as you may imagine is a

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very critical parameter for this because

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that determines how quickly do you build

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up co2 in your atmosphere and so I've

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shown that here right here this is the

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volcanic outgassing rate

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we're 10 to the 0 this is just based on

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its scaled to present-day earth the

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other important factor to think about

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though is how much co2 should be

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dissolved or sequestered in soil now

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it's a tricky question and I'd love to

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talk to you more about this later I

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don't want to get bogged down the

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details but on earth today

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co2 sequestration is very much driven by

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life and if we had a planet that was in

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limit cycles you may think that your

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soils not going to be fully inhabited by

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microbes you might you may reach a very

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different equilibrium soil co2

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concentration and so what we've shown

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here is this is the range of parameter

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space where co2 partial pressure vs.

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outgassing rate in this shaded region

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you would expect limit cycles whereas in

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this gray region you would not expect

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limit cycles there's just illustrator

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for a G star around for an early Earth

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case really the point of this plot is

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these two things are not really well

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constrained for exoplanets we got a good

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idea of these values for Earth today but

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you know they're not easily constrained

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with observations but I think we need to

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think more about what would we expect

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out guessing and soil co2 to be on

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exoplanet so when we do this here's the

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new version of the habitable zone that

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you get it's a function of the volcanic

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outgassing rate as I just mentioned so

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we've drawn a couple lines here this

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line is if the volcanic outgassing rate

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on the planet is 1/10 of Earth today and

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this is if it's half today if it's equal

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to earth today then this line is out

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here and the planet doesn't have limit

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cycles at all so again it's definitely a

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function of volcanic outgassing rate so

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you notice here around F dwarfs this

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this habitable region this blue region

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there's no limit cycles then in this

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region here there are limit cycles so

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the F Dwarfs this this range of stable

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climate is very small compared to G Dorf

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whereas K and M dwarfs actually remain

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stable without limit cycles at all

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because they have reduced ice albedo

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feedback so the last thing I want to

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mention is that we also looked at

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applying this not just to the outer edge

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of the habitable zone in general but

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specifically did the problem of early

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Mars could the floovio features on early

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Mars be telling us that Mars was caught

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in this limit cycle early on in

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history and this just is a slightly

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different way of plotting an average

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surface temperature here co2 partial

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pressure and what we're showing here is

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this red line gets above this weathering

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curve which means you're going to cycle

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sorry this plot is a little confusing

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but the basic message is if you have in

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these calculations if you have an 80%

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co2 atmosphere with 20% hydrogen you get

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cycling whereas if we have you know only

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5% hydrogen we don't get cycling

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however Robin Wordsworth recently did

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some calculations where he showed that

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in a mixture of co2 and h2 you actually

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get collision induced warming from the

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hydrogen that actually is much more

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efficient than this so we're actually

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redoing some of these calculations now

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and we're pretty sure that you're going

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to be able to show that you can get

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cycling on Mars with with much less

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hydrogen than we've required and so I'm

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at the end of my time I'll just stop

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here and take some questions if you're

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able to please go up to the microphone

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to ask your questions hi Jacob calling

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Goldblatt University of Victoria I'd

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like to add to your introduction a

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little bit which is the co2 dependence

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of westburg was proposed by a paper by

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burner in the early 80s but and then

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there were limit cycles in the earth

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science literature especially from

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Tajikistan the 8th late 90s and early

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2000s and I know that Jim didn't realize

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that so I'll blame your post your PhD is

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like this one we do cite those in our

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paper ok but ya know that's that's a

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very good point actually before people

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thought about limit cycles for

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exoplanets it was thinking about does it

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explain some features of early Earth and

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I think you even are on a paper where

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you look at the snowball earth episodes

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it's yes correct and they leave sort of

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types of see yes so this may have

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occurred on earth as well for other

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reasons yes the most substantive

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question is why does it matter if you're

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have if you have sub warm periods let's

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just slice then and set down spores

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which can last for millions of years

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between those limit cycles and we're

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just a sonically habitable

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yeah I'm glad you asked that question

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and oh my slide disappeared that's okay

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I had a statement that said earth-like

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planets with complex life may be less

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prevalent than those with simple or no

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life I put a question mark there and I

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was actually thinking about you when I

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put that question mark there because

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you're right does what does this mean if

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you have 80 million years of glaciation

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followed by ten million years of warmth

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we wouldn't like that our type of

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complex life would not survive I would

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not make the strong statement that all

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potential life in the universe cannot

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survive this so this is a new question

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00:14:10,650 --> 00:14:08,190
to be had I think I have an easier time

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imagining microbial life surviving that

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type of transition than something

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resembling complex animal life but we

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should have this conversation thanks

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00:14:21,570 --> 00:14:19,000
Jacob we have time for one quick

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question in the carbonate silicates

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cycle do you have any idea how efficient

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00:14:30,090 --> 00:14:26,560
the I guess the pulling up of that co2

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during volcanism is do is it 90% goes

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00:14:33,600 --> 00:14:31,930
back up in the atmosphere and 10% keeps

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on going down or you have any idea about

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00:14:37,290 --> 00:14:35,650
the relative percentages there I don't

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00:14:48,100 --> 00:14:37,300
know that number did Robbie or Colin do

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but for the cycle that you're invoking

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here you're assuming a hundred percent

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00:14:55,250 --> 00:14:53,730
that's right we just prescribe a

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volcanic outgassing rate that's how much

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00:14:59,300 --> 00:14:56,790
and we also get solar constant equal to

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00:15:01,640 --> 00:14:59,310
0.7 you're assuming that you have 30%

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00:15:04,220 --> 00:15:01,650
land on the ocean on the earth rather

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than the almost 0% land covering that

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most earth scientists who study

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00:15:12,410 --> 00:15:10,460
continental growth will tell you we had

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00:15:13,790 --> 00:15:12,420
what I can't remember what land

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00:15:15,740 --> 00:15:13,800
distribution we did with it I think we

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00:15:16,970 --> 00:15:15,750
did aqua planets actually we'll have to

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00:15:18,950 --> 00:15:16,980
we'll have to cut it out there okay

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00:15:21,340 --> 00:15:18,960
we'll get out Martha all right let's uh