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[Music]

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[Applause]

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good morning everyone and thank you to

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the conveners for the invitation today

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I'm going to be speaking about something

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that I don't think many people are

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thinking about and that is the role of

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ocean salinity in Earth's climate system

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I'm going to develop this story in the

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context of the faint young Sun paradox

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and Archaean habitability but I'd like

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to note that these relationships are

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relevant across the entirety of the

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geologic time scale and represent a

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major challenge when it comes to

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modeling the climates of potentially

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habitable exoplanets and so even if the

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faint young Sun paradox isn't keeping

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you up at night I hope you'll agree that

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these are really important results

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nonetheless so we've already heard about

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the phantom Sun paradox a couple of

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times today but just to briefly recap

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that issue the problem is that the

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luminosity of the Sun has been steadily

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increasing throughout Earth's history

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with the implication that early Earth

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might have been much colder than today

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and so here I'm just tracking a

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potential surface temperatures assuming

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a current green house versus no

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greenhouse and the ANA take-home here is

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that Earth's surface temperature was

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potentially below the freezing point of

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water for most of Earth's history unless

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we had an additional source of one so

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conventional solutions to the faint

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young Sun paradox involve higher levels

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of carbon dioxide and or methane like

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we've already heard about today and so

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on the left here i'm just showing GCM

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results for a planet that has a modern

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atmospheric composition but Archaean

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insulation and the result is of course

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snowball glaciation that's not a

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profound result that is just simply the

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definition of the fan son paradox then

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on the right here I am showing a planet

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that has a little bit extra co2 and a

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little bit extra methane within the

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range of uncertainty allowed by the by

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the rock record and other models and

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problem solved it's no longer a snowball

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and so some people are willing to put

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the

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young Sun paradox to rest and accept

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this as a as a solution it's not my goal

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to challenge the importance of co2 our

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methane and resolving this climate

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problem but instead today I just like to

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point out that the composition of our

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atmosphere has not been changing in

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isolation and the composition of our

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ocean has been changing as well and in

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ways that we don't understand

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particularly well and so for example our

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ki and salinity is extremely poorly

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constrained the the theorists have

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speculated that well if you take all of

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the evaporates on land and you just put

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them back in the ocean the the

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inevitable result is that our key in

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salinity should have been much higher

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than today

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potentially by a factor of two

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geochemical proxies for salinity are

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limited and fairly ambiguous and the the

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geochemists have had a hard time

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demonstrating the likelihood of higher

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salinity and in fact have even suggested

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that well maybe our ki and salinity was

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lower than today and so the bottom line

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here is we have no idea what our kina

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salinity was and this matters a lot the

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reason is that the salinity of the ocean

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interacts with the climate system in a

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number of ways so for example salinity

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determines the temperature at which

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water sea water has its maximum density

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and so it'll affect the density

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stratification of the ocean and ocean

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circulation patterns in ocean heat

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transport so that's a relatively direct

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impact on planetary climate but it turns

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out that that's only important for

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really big changes in salinity and so

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I'm not going to talk about that further

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today but that's something we should be

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thinking about for for exoplanets for

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which we have no constraints on salinity

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in the context of the potential range of

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our key in salinity though turns out

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that freezing point suppression with

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increasing salinity could potentially be

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so the the difference is in freezing

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point as you change salinity are small

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we're talking about a degree or so in

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either direction but it's nonetheless is

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important because if you change the

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amount of CI is just a little bit by

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changing the freezing point of water

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when you actually do is you increase

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absorption of incident radiation and

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that actually yields warming so this is

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a positive feedback that salinity

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directly interacts with and so the

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specific hypothesis that I set out to

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test was that small changes in salinity

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could actually yield big changes and

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climate due to the interaction with this

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positive positive feedback and so that

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is exactly what I found

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so now I'm going to walk through several

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GCM experiments all simulating an

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Archaean like aqua planet I assumed the

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equivalent of 500 ppm methane and all of

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my simulations I assumed Archaean like

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insulation of 1106 watts per meter

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squared and I assumed in our teen day

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length of just under 17 hours and so all

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the simulations I'm going to show you

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today just simply differ in their

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salinity and in the co2 content of their

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atmospheres and so as we move across the

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rows here I'm changing salinity from

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somewhat less then modern to somewhat

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more than modern relatively small

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changes and as we move down the columns

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I'm changing co2 levels in the

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atmosphere and so these three panels

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here really highlight the the headline

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result of my study and that is that

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salinity has a big impact on ice cover

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in these simulations for a burn ocean

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with 20 PSU practical salinity units I

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am generating a climate that has a

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stable ice margin all the way down to 15

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degrees north and south of the equator

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and if I increase salinity slightly up

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to 50 PSU

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the result is a climate that is

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effectively ice-free and is in fact

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warmer than present-day earth so this is

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this is a huge difference and as we look

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at much lower co2 levels only 20 times

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the pre-industrial level you see that

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the difference between the salinity

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scenario is it's the difference between

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being a snowball and not being a

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snowball and so visually this is not as

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dramatic as the top row but biologically

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this is a big deal and so these these

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results here demonstrate that a salinity

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really is an important component of our

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Earth's climate system now if we look at

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an intermediate co2 scenario though we

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see that the extent of ice cover isn't

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actually that sensitive to salinity

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within this region of parameter space

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now this is not a counter example to the

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importance of salinity and this is

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actually itself an interesting result

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because the emergence of this stable

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climate state is somewhat unexpected so

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this is referred to as the your Malinda

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climate state and it's characterized by

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the stability of very low latitude ice

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in this case down all the way to five

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degrees and north and south of the

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equator and this is somewhat unexpected

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because the ice Obito feedback tends to

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destabilize a low latitude ice and favor

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runaway glaciation - a snowball state

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and so the fact that this is not a

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snowball his itself interesting so

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what's going on here has everything to

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do with atmospheric circulation and

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specifically the descending branch of

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the Hadley cell these latitudes are very

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dry and have low cloud cover and very

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limited precipitation with the result

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that sea ice at these latitudes is bare

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sea ice rather than snow-covered sea ice

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and bare sea ice is much less reflective

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than snowy sea ice

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and so the result is that this bear C is

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plus low cloud covers low albedo and

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this compensates against the ice albedo

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feedback I should also note that I I

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sped up the rotation rate of the planet

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to simulate an arc and a Lang and I

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reduced atmospheric pressure to half a

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bar compared to 1 bar today and both of

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those changes actually limit meridian

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all heat transport away from the equator

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with the the added benefit of making the

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equator just a little bit harder to

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glaciation so both of those changes in

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combination with the albedo

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relationships favor the emergence of

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this normal and climate state and so the

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next thing that I was interested and

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thinking about was what it would take to

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push the system into snowball glaciation

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or whether or not we might expect that

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the normal gun is more likely for an

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arcane or a paleo Poros oeq glaciation

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than a snowball and so what I did is I

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started with each of the three distinct

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climate states that I simulated for the

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100 times co2 scenarios corresponding to

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the three different salinities so I have

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the twenty PSU scenario and orange and

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the 50 PSU scenario in gray so you can

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see that over at the hundred times co2

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the the 50 PSU scenario is basically

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ice-free whereas the twenty PSU scenario

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is heavily glaciated and then from those

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steady states I progressively decreased

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co2 until all of my simulations achieved

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snowball status and so the take-home

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message here is that within the

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intermediate ranges of co2 all all three

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salinities are producing a normal gun to

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climate state that is not different in

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its total ice cover but the critical co2

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thresholds in which you enter the your

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mogan state and in which global

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glaciation becomes inevitable are very

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different and

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so even though salinity doesn't always

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manifest as differences in sea ice cover

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having higher salinity does impart

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resiliency against global glaciation and

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so salinity is a bit of a master

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variable with regard to the climate

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system okay and so that brings me to the

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end of my talk and I will just summarize

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by saying I hope I've convinced you that

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salinity is a really important

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consideration and salt may in fact be an

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essential ingredient to the habitability

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of archaea you Stephanie questions yeah

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the connection between salinity and sea

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ice goes I'm wondering about the

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connection with clouds because clouds

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are not randomly distributed over the

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earth and I'm wondering if if you have a

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polar cap that's white

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do you preferentially you don't have

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clouds there or because if you had more

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clouds there then the effect you're

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talking about would go away so so can

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you talk a little bit about the

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correlation or anti correlation with

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cloud cover and the ice so I have not

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specifically looked at how cloud cover

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differs between my my simulations but

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you are you are right to point out that

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it should matter in particular the

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emergence of the normal gun state is

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directly tied to snow and so I would

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expect that there could be differences

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here since the threshold for initiating

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that differs so dramatically but you

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would agree that ice cover doesn't

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matter if you just cloud covered above

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it oh right yeah yeah and so that's why

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the descending branch of the Hadley cell

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is really controlling

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yep hi john toter from university of

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washington so if you form ice on on a

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snowball earth you're gonna concentrate

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the residual sea water and so you're

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gonna have a pea pod effect there have

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you have you looked at that I have not

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specifically looked at that but rocky 3d

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the GCM that I use does

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for serve mass and so assault is

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concentrating as as glaciation is

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progressing and so my gut instinct is

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that that feedback does not have a big

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impact

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okay so that was included in this mom

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yeah I didn't specifically set out to

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study that but it is implicitly included

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yeah uh uh hi Eli more Rowan University

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um so the presence of ice cover could

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potentially really reduce weathering

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although you know it's night it's hard

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to say what continents were like early

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Earth but without with reduced

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weathering you'd have even reduced

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salinity so do you think that could even

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be a further feedback towards glaciation

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hmm well that that really depends on

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what sets the salinity of the ocean um a

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long time skills so first some ions the

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weathering will be really important for

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other ions that is less so and so that's

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certainly something worth worth

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considering that I have that I have not

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done um I actually used an aqua planet