One (very important) topic I didn't get to address in this story is: how do we know what the temperature, or the co2 level in the atmosphere was millions of years ago? 1/x
Continuous ice core records from Antarctica & Greenland are great. They contain trapped pockets of air containing ancient CO2, & the ice can be geochemically analyzed to reconstruct temps. But they "only" go back 800,000 years. (efforts are underway to retrieve older ice cores)
Beyond that, you need different proxies. And that's where the most important organism in paleoclimate research comes in: foramenifera.
Forams are single-celled organisms that live in the ocean and build their shells out of calcium carbonate. In doing so, they record the chemistry of the seawater around them.
As it happens, water contains oxygen (H₂O). And oxygen on Earth mostly comes in two varieties, a heavier & lighter isotope: ¹⁸O and ¹⁶O (There's also some ¹⁷O but don't worry about it). The abundances of different oxygen (and hydrogen) isotopes in seawater can tell us a lot.
Water made more out of light isotopes is literally lighter, & so evaporates more easily. Ice sheets, which are made of snow that once evaporated as water from the ocean, are made out of this lighter water.
If more and more of this lighter water gets deposited on the land as ice sheets it leaves behind heavier water in the ocean. Forams record this change. What's amazing is by looking at ratio of ¹⁶O:¹⁸O in foram shells you can reconstruct how much ice there is elsewhere on planet
But the temperature of the seawater (and other ocean chemistry) also affects this ratio. So, using all this info, you can reconstruct the global temperature as well. And luckily, we have deep-sea sediment cores filled with fossil forams going back over a hundred million years
Paleoceanographers using these sediment cores have thus reconstructed the ancient planet. For ex. Lorraine Lisiecki & Maureen Raymo showed rapid swings btw glacial/interglacial climates over the past 5.3M yrs. And James Zachos et al. charted temps over entire 66M yr Cenozoic Era
Additional proxies, like the ratio of magnesium to calcium in foram shells, provide temperature estimate when the presence of ice sheets on the planet complicates this isotope picture.
And biology gives other clues to the global climate. These are pictures I took of the Smithsonian's Scott Wing looking for 53 million year-old fossil leaves from the ancient jungles of Wyoming. He's interested in the leaves because...
Smooth-margined leaves are characteristic of warmer tropical climates, whereas toothed edges on leaves are a physiological adaptation to cooler climates. Reconstruct enough local ancient climates and you get a pretty good idea of what the world was like.…
Ancient fossil lipids, once-upon-a-time synthesized by phytoplankton found in seafloor sediments are another paleo-thermometer. In response to warmer water some organisms produce more saturated fats, when it's colder, more unsaturated fats…
And then there are the animals. Find fossil skeletons of alligators on the northernmost islands of arctic Canada, as you do from 50 million years ago (when continents were in similar positions) & it's pretty good indication that the planet was really warm.
Paleontologist Jaelyn Eberle, who has worked on this ancient fauna, told me that a modeler once asked her if she could "put hair on her arctic alligators" as climate modeling community had long struggled to reproduce world as warm as her fossils suggested
What about CO2? Again the remnants of ancient biology help paleoclimatologists reconstruct the planet's climate over Earth history. Plants reduce the number of pores on their surface to conserve water as CO2 rises. In high CO2 climates fossil leaves, like Gingko, have fewer pores
And from deep-sea sediment cores again, long chain molecules of organic carbon made by ancient life ends up in deep-sea floor muck. Just like oxygen there are lighter & heavier isotopes of C, and photosynthesizers prefer the lighter stuff, but can't be as choosy when CO2 is low.
And the chemistry of elements like boron in seawater responds to the pH of the ocean, which is itself a function of CO2. Once again, forams from deep-sea sediment cores record these ancient ocean chemistry changes.
And the presence of minerals in the rock record that can't form at Earth's surface today can also tell you whether atmospheric CO2 was above some threshold in the ancient past.
There are countless other proxies, and paleoclimatologists are constantly developing new ones, applying them to samples that have been arduously collected by thousands of scientists, over decades of geological fieldwork and deep-sea drilling projects like the @TheJR
No proxy is perfect, and there are complications and uncertainties around each of them but...
Together all these records give us a compelling and, I would argue, cautionary story about the climate's history over tens of millions of years (illustrated beautifully in this recent @leafwax paper…).

When CO2 has been high, it has been VERY warm. End.
Bonus pictures of the Smithsonian NMNH's Wyoming field vehicle, "Dino" the 1970 Suburban
Addendum: Just remembered the wildest temp proxy. Snake size. Titanoboa was *preposterously* large snake that lived 60 million years ago. The paleontologists who discovered it argue that--due to snake physiology--it could only exist on a MUCH warmer world…

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More from @PeterBrannen1

21 Feb
Something I think a lot of Silicon Valley-type space enthusiasts really don’t appreciate is that there is nothing we could do, nuclear war-wise or climate change-wise that would make the Earth more uninhabitable than Mars.
Like, even after an End-Permian-style climate catastrophe, or all-out nuclear war, there would still be oxygen and a magnetic field
I know this isn't some new insight, but I still hear people talking about Mars as an "insurance policy" and it's idiotic
Read 4 tweets
4 Feb
Paleoclimatology, is the study of Earth's ancient climates. Taking the extreme long view it becomes unsettlingly apparent that Earth's climate is "an angry beast," as Columbia climate scientist Wally Broecker used to say, "And we are poking it with sticks"…
Within recorded history, climate changes have been linked with the faltering of the Hongshan & Yangshao cultures, the Akkadian Empire, the Bronze Age, the Roman Empire, the Ptolemaic Empire, Ancestral Puebloans, the Khmer Empire, Classic Maya... But recorded history is nothing.
This is our immediate climate context: In red is the span of time that covers recorded history. Stable. But at the bottom of the slope is the depths of last ice age, when sea level was +400 feet lower, an Antarctica's-worth of ice covered North America & icebergs listed off Miami
Read 31 tweets
7 Jul 19
Things (I think) humanity can Survive v Not Survive

Not Survive:
Sufficiently large Large Igneous Province
Sufficiently large asteroid
Vacuum decay
Burn-it-ALL (12k GtC)

Survive but not very fun:
RCP 8.5
Nuclear war
Yellowstone-style eruption
Gamma-ray burst
Bad AI
Geomag storm
NB: I think "civilization" would collapse in every one of these scenarios with possible exceptions of bad AI and big geomagnetic storm
My reasons for "survive"
-RCP 8.5: Scattered, roving bands of hunter-gatherers living at high latitudes a few centuries from now seems like an adequate adaptation to Bad Climate Change
Read 12 tweets
14 Dec 18
PART 2 Over huge area of Siberia, enough lava erupted in a few thousand years to cover the lower 48 United States A KILOMETER DEEP. But as mindblowing as eruptions were, they only covered part of Russia--so lava itself couldn't have killed almost everything on the planet.
It had to be the volcanic gases that came up out of the earth, especially CARBON DIOXIDE. Most ominously, these volcanoes had the misfortune of burning through one of the largest coal basins in the world, the Tunguska Basin.
By burning through this coal, the eruptions released something like 10,000 to 40,000 gigatons of carbon over thousands of years--a truly mind-boggling amount--and raised global temperatures an estimated 10-12 degrees C, acidified the oceans and starved them of oxygen
Read 28 tweets
14 Dec 18
It seems like people are into MASS EXTINCTIONS these days and I wrote a book on them so here's a 2-Part ⚡️MEGATHREAD⚡️ on the worst things that have ever happened
EXTINCTION 1: The first major mass extinction was 445 million years ago, the End-Ordovician. It happened on a planet that as alien as any in science fiction.
N. America was mostly south of the equator and on its side, eastern New England had just rifted off of a supercontinent straddling the South Pole--and wouldn't crash into N. America for almost 100 million years. The midwest was a shallow ocean.
Read 25 tweets

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