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 Image
But we merely live in the latest interglacial of an ice age world. Everything from previous graph is now in red, but if you go back 120,000 years the earth has emerged from the ice again, temps were similar to today (though warmer at the poles) & sea level was 20-30 feet higher Image
There are hippos & water buffaloes splashing in the Rhine, lions hunt forest elephants near Leipzig, & giant tortoises march through an Illinois unknown to humans. The Florida Keys is built from fossil coral reefs & shoals, & along w/ the lower 1/3 of Florida, is under the waves
In past 2.6 million yrs, planet has swung dozens of times between such warm, brief interglacial springtimes & deep glaciations. Swings are paced by wobbles of planet which bring more/less sunlight to North hemisphere summers, either stifling ice sheets or allowing them to swell
But waxing and waning of sunlight in the N hemisphere follows the gradual slope of a sine wave. The Ice Ages of the Pleistocene, on the other hand, end with all the jaggedness of an EKG. This is where carbon cycle feedbacks & the nonlinear dynamics of ice sheets come in.
Here's the last 2 million years, as the planet has swung between ~180 and ~280 ppm of CO2 and minor shifts in sunlight. The planet is extremely dynamic. Image
But when CO2 is high enough you can escape this glacial/interglacial world. Today we are outside of the range in which we evolved. To find an analog in Earth's past for our current atmosphere we have to go back over 3 million years to the Pliocene Image
This is much diff world. N and S America are separated by narrow straits where Panama should be & animals now iconic to S America, like big cats, llamas and tapirs, haven’t yet migrated from N America where they first evolved. Around now there are red pandas & rhinos in Tennessee
In the middle Pliocene CO2 was ~400 ppm, the planet as a whole was 2-4°C warmer, but the Arctic was an astonishing 10-15° warmer, with beaver-filled evergreen forests on the coast of the Arctic Ocean. Sea level was 70 feet higher than today.
If we keep CO2 at 400 ppm indefinitely, we will eventually get there… Image
But with modern CO2 over 410 and rising, we've already passed the middle Pliocene, so even more ancient analogs are required for our strange world. We now heroically jump more than ten million years into the past.
Everything that has come before is in red, & we're now 16 million years deep into earth history. CO2 is ~500ppm and the planet is 4-8° warmer than today. There are swamp forests in Iceland, parrots in Siberia, and conifers on the coast of Antarctica. Sea level is 150 feet higher. Image
The warm spell was likely kicked off by volcanic CO2 from the Columbia River Basalts, rocks that now span a vast region in the northwest US, but were once overflowing rivers of lava. They were of a rare, catastrophic class of volcanic eruptions called Large Igneous Provinces Image
Though the topography of Antarctica, and ocean circulation in Southern Ocean were subtly different, the ice sheet in the Miocene seemed responsive to small changes in CO2--losing what would be 30-80% of the modern Antarctic ice sheet over thousands of years.
A controversial process called Marine Ice Cliff Instability (MICI) might have been at work, wherein Antarctica wasn't slowly shedding ice in response to warmth, but collapsing like a stack of dominoes. It's uncertain whether MICI could be in our future
With much less ice on planet & deserts mostly replaced by forests & vegetation, planet was darker and absorbed more heat. In the very long-term our planet may similarly change color. Changes in geography/topography account for some Miocene warmth too but most warming was from CO2
But in the worst-case CO2 emissions scenarios for our future (a very avoidable path, and one that we have no reason to go down) even the Miocene isn't an extreme enough analog. Now it's time for a tremendous leap 40 million years further into the the geologic past.
Everything described so far (16 million years of climate change) is now crowded to the right side of the graph. As CO2 climbs and the planet warms we end finally in the Early Eocene 50 million years ago--one of the hottest periods in the history of animal life. CO2 is 600-1400ppm Image
A small woodland creature that would become ancestor to all whales prances through forests of a Pakistani archipelago. India is adrift in the Indian Ocean. And there are no ice sheets on the planet.
Antarctica instead hosts monkey puzzle trees & marsupials. There are rainforests in the Arctic.
Climate modelers have long struggled to reproduce a world this hot. Historically models have had to ramp CO2 to over 4,000 ppm to reproduce this feverish world. But the models are catching up.
It could be that the Earth grows increasingly sensitive to CO2 as it warms…
It could be that some clouds begin to break apart after some CO2 threshold, inviting dramatically more sunlight to hit and warm the ocean.…
Perhaps other powerful greenhouse gases, like methane and nitrous oxide awoke from the worldwide swamps, and warm, moldering soils of this greenhouse world.
Taking the whole sweep of the age of mammals into view it now becomes apparent just how radical the ongoing, modern experiment on the climate really is. The vertical lines at the end of the graph are the full range of IPCC warming scenarios to 2300 Image
The rate of potential future warming is nearly unsurpassed in Earth history, even in many of major mass extinctions--extinctions caused largely by volcanic CO2. We don't need to go down this path, but will require stopping--even reversing all carbon emissions in next few decades.
This is the last line of my story and I deeply believe it Image
Graphs adapted from Burke et al., 2018… and Westerhold et al., 2020… which was released with this wonderful and alarming graphic, making all of the same points above Image
Lots of great paleoclimate writing out there from @HowardYLAPE @alexwitze @voooos @themadstone @sarahkaplan48 & others. For the Holocene this @Annaleen piece (& new book!) has much more subtle & up-to-date discussion of how climate has influenced societies…
Last note to say this story not possible w/o Scripps Fellowship from @CU_CEJ which let me spend year thinking only abt this topic & auditing classes (like @TomMarchitto's paleoclimate courses). If you're a science journalist & can make the move I can't recommend it highly enough.

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More from @PeterBrannen1

25 Feb
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.
Read 24 tweets
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
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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