This is Georgie Bites with Oliver Strimpel.
The history of the Earth's climate is archived within the
sedimentary record. That is because there are
certain physical, chemical and biological properties of
sedimentary rocks that can serve as indicators or proxies of the
climate that prevailed when they were deposited.
Over the past few decades, we've discovered more and more
proxies, enabling us to probe more aspects of past
environments such as the temperature and composition of
the atmosphere and of the oceans, sea level, the presence
and extent of ice caps, and even patterns of wind and rainfall.
But now there is a heightened interest in such work, since we
want to understand what the rapid human generated rise in
carbon dioxide means for the planet.
Isabel Montagnez has been reconstructing past global and
regional climates throughout her 30 year research career.
She is a Distinguished Professor of Earth and Planetary Sciences
and Director of the Institute of the Environment at the
University of California, Davis. One climate period in particular
has attracted her attention, the late Paleozoic Ice Age.
That is because in certain key respects, it can serve as an
analogue of the present day. It was the only time in Earth
history when carbon dioxide levels increased abruptly during
a period of recurrent glaciation.
That Ice Age eventually gave way to a much warmer greenhouse,
Earth devoid of ice. Isabel Montanez, welcome to
Geology Bytes. Thank you, Oliver.
It's a pleasure to join you today.
It's impressive that we can reconstruct such apparently
evanescent things as atmospheric temperature and global sea level
through geological time. I said that we use evermore
proxies for this purpose. Can you tell us what these
proxies are? And let's start with the
traditional ones. So some of these proxies that
we've been using for decades now are oxygen isotopes of fossils
of marine shells and corals that formed in the ocean, and equally
as many for minerals that form in ancient soils on land.
And these can give us information like local surface
temperature and the composition of the seawater at the time or
of rainwater in which they formed and the carbon isotopic
composition, which we typically do with the oxygen, sometimes
elemental concentrations of these minerals.
Then when we combine all those together, it provides
information on the local temperatures, humidity, rainfall
patterns. And I think one that is
particularly important is how we reconstruct CO2 in the past.
And there are probably nearly a dozen approaches to
reconstructing CO2 in the past. All of them involve fossils of
living organisms or minerals that are forming in the oceans
or on land. And each of those has their
weaknesses and strengths. So we typically use multiple of
these proxies, like we might use the minerals and soils and the
plants that form in the ecosystems associated with those
soils and they will then together give us pretty good
constraints on what CO2 in the atmosphere was in the past.
Can you pick a few of the most important proxies and explain
the physical mechanisms or chemical mechanisms by which
they managed to record things like CO2 level or temperature?
Yeah. In the oceans, the proxies of
CO2 typically involve the isotopic composition of marine
shells. And why do they do that?
Because these isotopic compositions are very sensitive
to the pH of the sea water. And in turn, the pH is a good
recorder of how much CO2 the ocean is dissolving.
You know, the ocean is like a big sponge and it is always
absorbing CO2. It is limited by how much it can
take by the other carbon of chemicals in the ocean.
So pH is a good indicator of what the balance of those other
carbon molecules are. If we can reconstruct pH and we
can reconstruct another component of the ocean, like how
much one of these dissolved carbon molecules were in there,
then we can reconstruct CO2 in the atmosphere.
And what's really exciting is we've done this mostly on using
these micro fossils called foraminifera and it's worked
very, very well. Some of the best estimates of
CO2 in the past, unfortunately those former menifer have only
existed the last at best couple 100 million years and that means
we have this big void prior to that.
And just literally if a month ago someone has shown how well
this works on fossil brachiopods.
So that has really now expanded our ability to use that type of
CO2 proxy way back to 500 million years on land.
There are several different approaches, but two of the best
are those carbonate minerals, we call them pedogenic or soil
formed carbonates in soils and they are in any soils that
formed in relatively drier conditions.
So the southwest of the US would be a good example, Africa,
Brazil. And these form around roots
because roots respires CO2 at night and that CO2 gets
dissolved in the poor waters of the soil and they get
oversaturated and that comes out as a new mineral carbonate.
The very cool thing about soils is that there is a balance of
how much of the CO2 in the soil is from those roots respiring or
microbes respiring in the soil, and how much is actually
infiltrated from the atmosphere. And the balance of the
atmospheric CO2 versus the soil form CO2 is recorded in the
carbon isotopes of the carbonate mineral as well as the organic
matter in the soil. And that can be then used to
reconstruct CO2 in the atmosphere.
And then the other one that I think it's just so elegant, but
very straightforward and that is that all plants, vascular plants
that have formed since you've 440 million years ago or so,
have breathing pores on their leaves or on their stems, what
we call stomata. They are little holes where
water comes out because plants transpire water, but it's also
where they take in CO2 from the atmosphere to photosynthesize.
And so there is a very simple inverse relationship by how much
CO2 is in the atmosphere and how many of those pores they have on
their leaves or stems. If there isn't a lot of CO2 in
the atmosphere, they need lots of those pores to be sure that
they get enough CO2. And vice versa, if there's a lot
of CO2 in the atmosphere, they don't need as many of them.
And it's to their advantage to not have too many because as I
mentioned, they lose water through these pores and they
don't want to lose water, especially if it's getting drier
in seasonally or with time. And it's a very tight connection
between the amount of CO2 in the atmosphere in those pores that
develop. And we simply use how many pores
per square millimeter on a leaf. We're not talking about proxies
based on isotopes. Why should they be sensitive to
these environmental parameters? Is it something to do with the
reaction kinetics of the different weight of the
different carbons in the carbon dioxide from the different
isotopes? Absolutely.
The proportion of the heavier versus the lighter isotope into
the mineral, for instance, will have a lot to do with
temperature. And if there are other
parameters in the soil or in the water that are affecting the
rate at which those atoms are being brought in, yes, that will
also affect it. It's a lot easier to move or
incorporate the lighter isotope than it is a heavier mass.
For plants, it's a bit more complicated in that it has to do
with their physiological functioning.
How well are they taking in CO2 or photosynthesizing and
processing it? Does any of the material you
analyze for these proxies include the actual original
organic matter from these skeletons, or from these leaves
that survives, or the soil? Or are you just talking about
the replaced material that generally forms a fossil?
Great question. Minerals in soils, very, very
small minerals will often form effectively cages around this
organic matter and we call it mineral associated organic
matter. Now millions of years ago, sure
it's not preserving it perfectly, but it is pretty
impressive how most of the molecular structure that it can
preserve. And another way is these
carbonates that form in the soil actually entomb the organic
matter that was in the soil as they were precipitating for
plants. It isn't in fact, the actual
cellulose or lipids of the actual leaf.
It's that waxy layer, a cuticular layer that protects
the leaf. And there's a layer on the
bottom of the leaf and there's a layer on the top of the leaf.
So effectively, those breathing pores that we measure their
sizes, their depths, and the number of them on the leaf.
They're casts in that waxy cuticular layer and that is
preserved beautifully. I can show you images from a
microscope of something 300 million years old and a ginkgo
from 100 million years and a modern one, and you'd be hard
pressed to know the difference. Wow, that's amazing.
The actual original wax material.
It is and we have even extracted from them some of those lipids,
so you do get things stuck on there that are preserved.
Can you tell us about some of the more recently developed
nontraditional proxies? The turn of the Millennium has
been an explosion of development of new proxies.
I think we can attribute a lot of that to the technology, the
machines that give us the possibility to do isotopes
across the periodic table. It's just amazing.
It's something that 40 years ago we couldn't have even imagined.
I'll give you a couple of examples.
Uranium isotopes in carbonates that form in the ocean, and even
some people are trying this in lakes.
These are now allowing us to look at the oxygenation levels
on the ocean floor where a lot of these shells and life might
be living. Today.
Our deep ocean is quite oxygenated, but it turns out
that when you have global warming, that changes
considerably. So these uranium isotopes that
are now measurable in these shells or even just disseminated
carbonates in old sediments, allow us to see how the
oxygenation is changed, whether anoxic conditions are forming
anywhere on the sea floor, whether it's off a coast or deep
in the ocean. And in turn, it tells us about
how changes in the ocean circulation were happening.
And the very cool thing about uranium isotopes is you can do
those in one location on Earth and it's representative of the
whole ocean because of the well mixed nature and the residence
time of uranium in the oceans. Another example, going back to
the periodic table, we now do the stable isotopic compositions
of strontium, calcium, magnesium, lithium, potassium,
mercury, and these allow us to look at degrees of weathering on
land, which is really important because weathering on land is
tightly coupled to CO2 in the atmosphere and to temperature.
So this gives us an opportunity to again, track what's happening
in changes in greenhouse gases that we're getting from other
proxies and how is the landscape responding.
And that's important because those responses in turn remove
CO2 from the atmosphere. In fact, it's one of the ways
that the Earth has maintained its temperatures through
maintaining its greenhouse gases in these balances of these
processes, Mercury isotopes, fingerprints of passive volcanic
episodes, so we can now tie them directly to the other isotopes.
Was the increase in CO2 or methane in the atmosphere driven
by volcanism? These same isotopes are telling
us something about aridity and seasonality on land in different
places, and they can even tell us about other sources of
greenhouse gases that aren't volcanoes, like melting methane
out of the oceans. So it really opens up a whole
new world for us beyond just temperatures, effective moisture
and maybe seawater and precipitation composition.
And then lastly, I'd be remiss if I didn't say they're all of
these biologic proxies, primarily what we call
biomarkers. Those are biomolecules.
So rather than looking at the organic matter that we extract
as a bulk form, we can actually extract specific biomolecules
like lipids, hope pains and others.
And each of them have their own function.
So we can look at things like the physiological functioning of
the plants, how did they respond to drought, Were they very
resilient to deep freezes. But we can also look at
microbial interactions with all of these other environmental
conditions and their role and temperatures.
They're another independent source of temperatures as well.
So all of this tool bag, no one group will do all of them, but
for any given time period there are just enormous amounts of
information that is being developed.
So how do you try and make sense of all the paleoclimate data
you've been able to reconstruct from all these proxies?
So our approach is to contextualize that data by using
an Earth system model. This model, which was produced
by the National Center for Atmospheric Research, or NCAR,
is our US model for the IPCC's work, but also for predicting
the climate over the next 10 days.
They are the most comprehensive, all inclusive of every process
that is in the ocean, the atmosphere on land that we know
of that influences climate. Climate is effectively the
interaction of the ocean and the atmosphere.
So these models not only allow the ocean and the atmosphere to
interact and be fully in equilibrium, it allows for cloud
differences and development. It allows for aerosols.
It models all of the changes in the circulation of the ocean,
the effects of sea ice on that circulation, continental ice
sheets, how do they influence the temperatures, the air
humidity, sea ice response, and turn back to the ocean?
It has vegetation. Vegetation interacts with the
atmosphere and the soils in terms of moving carbon and
water. So it influences once again the
atmosphere and the ocean and it just iterates so that these
processes are fully interacting and feeding back on one another.
And the proof that these models are really comprehensive and
correct is that they are calibrated to the modern where
we have 100 plus years of excellent data from around the
world and they can reproduce those very, very accurately.
OK, so you've managed to build this model.
How do you make use of it? What kind of questions can you
interrogate it with? So you start out with the
boundary conditions, those baseline boundary conditions,
and then they are run to evaluate the climate processes
and the ecosystem functioning. What is their sensitivity to
this changing CO2 and ice sheet distribution?
You don't usually change much else because you're not
interested in whether mountains are building or weathering down
or whether the ocean basins are changing in configuration.
Those are millions a year time scale things.
But you are interested in what happens if you lose tropical
forests or whether temperatures are changing and rainfall
patterns are changing. So that's what we're after.
We're interested in seeing what is the sensitivity of these.
We're also interested in are there thresholds, are there CO2
thresholds where things just plummet or where the changes
start accelerating? And are there tipping points
where those forests disappear, or where their ice collapses, or
where the monsoon system effectively collapses or
intensifies? I said in my introduction that
you've been especially interested in the late Paleozoic
Ice Age because it can serve as an analogue to the present day.
How far does the similarity go? I want to stress that it is the
process scale we're after here. Continental configuration was
quite different. In fact, it was the opposite of
today. It was a supercontinent.
Today we have our continents at their maximum distribution away
from one another on the globe. Many things are different.
That said, the processes in the Earth system, whether it's in
the climate system, the ecosystems on land and how they
interact with the environmental parameters.
So the atmosphere, the oceans, land, life, these processes have
existed and fundamentally functioned as they do today
since we've had ecosystems in terms of general structure and
configuration similar to what we have today.
So 400 million years or so, certainly the last 300 million
years. So inferring how climate may
change in the future in one particular region from looking
at the deep time, we can't do that because of the continental
configuration differences and other things like that.
But evaluating how these processes in the climate system,
in the ecosystems, ice response, ocean response, evaluating how
they respond to increases in CO2 or vice versa, decreases and the
consequent warming or cooling that goes with that on a global
scale and how those responses interact with one another and
how they may even feedback on the initial warming or cooling,
that is fundamental information that we can glean from looking
at past periods. The main thing that makes a time
period relevant or not is whether the atmospheric CO2
concentrations were within the range of what we are seeing
since we started burning fossil fuels and what we project into
our fairly near future. And that is the case for the
late paleozoric ice house. In fact it's the only time when
CO2 was in the range of what we see since the pre industrial and
what we're going into except for a transient period in the last
several million years. But isn't the fact that there
was ice or recurrent ice on the poles something that also
distinguished that period from others?
Of course this is the last time other than the last 34 million
years where there has been continental scale ice on Earth
all the time. In between there may have been
ephemeral, short lived, maybe small Alpine, maybe continental
glaciers, but nothing that we can find in the rock record nor
do we produce in climate models. So this is it.
This is the last time that we see the ice house as it came on
the onset and its response to short lived warming, essentially
the experiment we're doing now and ultimately it's terminal
demise as it turned into a greenhouse.
I want to come back to the boundary conditions that you
mentioned. So when the configuration of the
continents was completely different and presumably ocean
circulation as a result was completely different, how come
we believe that the sensitivities on a global scale
would still be relevant to the situation we find ourselves in
today? So I'm going to give you an
example of our modeling that gives us detailed seasonal
response to the CO2 changes within that range of the pre
industrial to what we anticipate in the future.
And then how we can say that we have confidence in that modeling
based on the proxies that we've developed.
We have the CO2 proxies, we have the temperature proxies for the
late Pilozoic Ice Age. And now we had these uranium
isotopes and the uranium isotopes were telling us that up
to 25% of the sea floor was going anoxic globe.
Yes, different continental configuration, so clearly
different ocean circulation and with ice sheets and we said this
is during ice house where we have big ice where we are
generally cool. In fact, other studies we've
done have shown that this is the only other time that global
temperatures on in line with what we're humans have been
seeing since they've evolved in today.
Why are we going 25% anoxic of the sea floor?
That's phenomenal. That has huge implications for
the future if correct. So the climate models, we use
them with the right boundary conditions to evaluate how the
presence of ice and sea ice included would change the ocean
circulation. Despite the fact that again we
are looking at very different continental configurations and
ice only in the Southern Hemisphere with the doubling of
CO2 or even 50% increase essentially where we are today
from pre industrial, we can see changes for instance in the
Northern hemisphere polar region, major changes in sea ice
formation, but more importantly in circulation at the mid depth
of the ocean, we can see where deep water formation is forming.
So during the times of low CO2, we have deep water formation at
both poles, very similar to today, even though the
continents in their very different configuration, we take
the CO2, we double it and we lose the Northern hemisphere
deep water formation, very different world.
And we see that then in the climate on land and we see in
the sea surface temperatures, we do a 50% increase in CO2 to make
it more like today, 400 PPM. And what do we discover?
It's a threshold because under that we're not seeing much
change. At 400, all of a sudden we see
these major changes and not much more.
Even when we double go 50% more, and it fits our proxies
beautifully, it tells us that about 18 to 25% of the sea floor
would go anoxic because of this change.
So again, different boundary conditions in terms of
continental configuration has to be different ocean circulation
because you have one huge ocean called Panthelasa and then a
more huge Mediterranean. That's the other part of the
ocean, open to the other one, but still very different.
But what have we learned? It doesn't matter the ocean, it
circulates and it still couples with the atmosphere, and it is
still equally sensitive to changes in CO2.
Where the water may go down deep to circulate in our conveyor
belt may be different, but it's not about where those continents
are. It's about the CO2 and the
warming effect on the ocean and the ice and sea ice behaviour
that seems to be robust regardless of that boundary
conditions of the continent. Wow, That's a pretty dramatic
conclusion then, that it just so happens based on your model that
we've gotten to the tipping point.
You can imagine we put that out there and the default response
by scientists is, oh, come on, it's so different back then.
And our response back is yes, but step back and ask yourself
why the responses are not that different.
It's because there are physics in the system that don't care
where the continents are. They care about the ocean as a
whole and how it's operating. It's always a conveyor belt.
It may slow down, it may accelerate, but it's still a
conveyor belt that is strongly governed by how much sea ice is
in the polar regions, how much continental ice is there, how
warm the ocean is in that region, how stratified it is.
So nuanced differences, yes, hugely.
But as a global system, it's still operating as the systems
as we know it. What can we learn then from your
late Paleozoic Ice Age or ice house model with regard to our
climate future and I presume one of the differences that even
though CO2 was rising rapidly than it was not rising on
anything like the rate that has happened since pre industrial
times? You hit the nail on the head.
I would say two key findings. Just by changing the climate
initially, and if the ecosystem in a given location is changed,
we can actually change other parameters in the Earth system
substantially. For example, we've modeled that
if we changed the ecosystem in the tropics, the wet tropics at
that time, by moving one type of ecosystem and replacing it with
another, based on the fossil record that these shifts really
happened, we could change the outflow from the Amazon basin by
fourfold or decrease it by half. So changing the ecosystem in a
given location can change our runoff.
So flooding it can change the coastal environment because it
can bring a lot of nutrients to coastal waters.
That is not a good thing. It causes eutrophication.
It can change the atmosphere's water because it can put less in
the atmosphere and more in the ground to run off.
So that's a big finding. We're looking at a time period
similar to Dave in terms of ice sheets, where CO2 was at its low
point, precisely in the range of the lows of the last glacials
during the Pleistocene, the last million years.
And it rose within these glacial interglacial cycles, similarly
doubling. But there were also these
intervals of rapid rises in CO2 that were driven by volcanism
and maybe even changes in ocean circulation.
The important thing here is that we are seeing enormous changes
in the climate system, influences on the ecosystems and
land in the ocean, changes in the oxygenation state of the
oceans, changes in ice sheet stability, all in response to
these changes in CO2 within the range of our pre industrial
Pleistocene to our future. And as you said, at rates
minimally an order of magnitude slower.
And that should be eye opening that our Earth system is capable
of responding to changes in CO2 analogous to our world and
future at all. Because we should be thinking,
why would it not be now? Isabel Montanez, thank you very
much. It's very much my pleasure.
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