How do scientists determine how much CO2 was in the atmosphere thousands of years ago, and to what level of certainty are they able to perform these calculations?
In: Earth Science
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It happened
My area of specialty!!!
I see lots of ice cores mentioned here. Definitely a method deemed reliable.
There are other methods, such as tree cores! Uses very similar measurements and all that. It has to do with molecular processes and enzymatic favoring of different isotopes (C, O, others). Those isotopic signatures remain in the molecular structure and can be quantified, along with other environmental factors, to build a fairly accurate representation of a “moment (grow season) in time”
They use core samples from glaciers which are big rivers of ice that have been around for long periods of time.
But what is a core sample? You may wonder.
Well it’s easy to imagine if you know how it’s made. They use hollow drill pipe where the cutting edges are around the outside of the pipe. So as they cut/drill deeper and deeper, it creates this big long tube of rock inside the drill pipe. When they pull out the drill pipe the inside core sample comes with it and then they push that out of the pipe.
What they’re left with is a long column of ice that they can then analyze. It will show the many different layers of the ice and generally speaking, the deeper they drill, the further back in time they’re going.
However their works isn’t done because you can’t just count the separate layers and think “Ok so 10 layers = 1,000 years” or anything like that. So they have to constantly analyze the core sample to figure out what period of time each layer represents and there can be disagreements about that.
Then once they’ve figured out the time frame for the different layers then they can use what we know about our atmosphered today and compare it with the upper most layers of the ice. Then they can look at 10, 20 or even 50 years ago and what we knew about the atmosphere then and see what’s in the ice.
This way they’re correlating what we know about recent history to what has been captured in the ice.
So ice cores aren’t going to tell you exactly what’s going on but they give you an indication and when you compare it to the data from today, it gives an estimate of what it was like back during some time period.
However you can’t just use one core sample and make a determination. You need to analyze core samples from all over the world before making these determinations. Because CO2 levels may have been higher in the northern hemisphere than the southern for a period of time and that makes a difference in estimates. Same with east and west continents and core samples.
So the more cores you analyze, from different parts of the world, the more confident you can be in your conclusions.
What I want you to do is imagine you’re a giraffe.
Now that you’ve done that completely irrelevant thing, think about snow. Specifically, think about what happens when you walk on snow – you sink. This is because snow is a bunch of tiny ice crystals layered gently on top of each other with lots of air between them. When you stand on it, the air gets pushed out and the ice crystals get compacted. The reason this happens is simply because you are heavy – any other heavy thing put on the snow will also cause this to happen, including more snow.
When you put a bunch of snow on top of snow, the snow at the bottom gets compressed. Some of the air gets forced out of it, and the ice fuses into one big sheet of ice. Some air gets trapped inside the sheet when this happens. In some places, like Antarctica, it snows pretty often, so you keep getting new layers of snow deposited on top of older layers, resulting in the snow at the bottom becoming compressed. This process happens regularly over hundreds of years, creating a thick layer of ice called a glacier or ice sheet. The ice at the bottom of this sheet is older than the ice at the top because of the way snow always falls down from the sky, and not up from under the ground.
You could take a segment out of this ice sheet, a long thin one from top to bottom, and if you did the ice at the top would be very recent and the ice at the bottom very old. All of this ice has air trapped in it, which was trapped when the ice turned from snow to ice, so it’s essentially a time capsule of what the atmosphere was like when that ice was formed. If you open up one of those air bubbles, you can measure the CO2 in it, and you can estimate the age of the ice based on various things like how much snow gets deposited in a given length of time.
Ice cores. Where you drill a big hole in the ice and pull out samples.
When Ice freezes, it traps tons of little air bubbles in it. These little air bubbles have the same composition of the overall atmosphere when that sections of ice froze.
So, if you were to go somewhere like Antarctica, and drill down hundreds of feet, you would get to ice that formed a long long time ago, thousands and even hundreds of thousands of years for the deepest cores.
You now have ice that froze a long time ago, and trapped a sample of the atmosphere from that time in it. You have a sample literally frozen in time.
You can then take that to a lab and do an analysis of how much CO2, Methane, O2, and other gasses are present and compare that to levels we see today and from other younger or older ice cores.
Edit: as for exactly how accurate they are I cannot say what the margin of error is. Each scientific study should report their own margin of error. But when you take an overall view over hundreds of studies and thousands of samples taken, this data is accurate enough and the best option we have to make the models and predictions we need.
One method is to analyze ice. They drill deep holes in very thick glaciers. So the ice from down below is very old, even thousand of years and hast small bubbles of gas included. This gas is very old air which can then be analyzed.
Almost a million years ago, the ratio of heavy and light isotopes of oxygen (O 16/ O 18, H / D or hydrogen to deuterium) provides us with information about temperature. It is crucial that in the process of evaporation of the ocean water, i.e. the phase transition from liquid to gaseous, the isotopes of the constituents of the molecule water (H2O), i.e. O 18 or D compared to the “normal” atoms O 16 or H, depending on the the temperature of the ocean water changes into the gaseous phase at different speeds. Isotope physicists speak of temperature-dependent defraction
It depends on the age of the time frame under discussion. Ice cores preserve bubbles of air that existed at the time of capture, so are good witnesses of air composition, but ice has a limited age (rare to get ice more than a couple hundred thousand years old although there are a few locations where older ice has been recovered). Precision is relatively high as far as such things go. The measurements can be pretty precise, but is the air bubble actually the same now as it was at the time the bubble formed? uncertainty enters from that.
Source material itself (the air) is not homogeneous so you need a good number of analyses to define a statistical population or you cannot really say anything about how well a single measurement is a representation of the real world (works for all analyses not just bubbles). The more data you have, the more confidence you can have.
Other methods vary from stable isotopes (carbon, oxygen, hydrogen, sulfur isotopic variations all provide clues about the nature of ocean-atmosphere chemistry), global carbonate sediment production, weathering rates of rock (CO2 is a weak acid and main source of rock weathering), and lots of other chemical or physical methods of questionable certainty but still decently useful for more ballpark understanding. Obviously, when you get into larger scale processes covering wider time frames, precision is lost. I even knew one line of thought that relied on examination of paleosoils going back well into the Proterozoic. How much should we accept the conclusions? Hard to say. Generally speaking, the changes with/over time are clear and repeatable, but how they translate into actual numbers is a harder thing to declare. High CO2 periods over low CO2 periods can be identified. Is a “high” period at 1000 ppm, 2000 ppm? or what? That is part of the interpretation and it is open to discussion, if one is honest. Trends are fairly easy to identify, but putting hard numbers to the different points in trends is a lot more challenging.
The BIG HEADS like Robert Berner and Dick Holland (both long departed from this world) were the ones who got into this sort of thing in a big way back when I was a student in the 80s. Don’t keep up so don’t know who is doing all the hot work now. Mandatory reading back then though.
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It is a swag. A scientific wild ass guess. There are some here that will tell you different and that I am wrong, but their methods are easy to disprove. Take the very first one I saw when i opened this up, ice cores. The “layers” are NOT indicative of the age of core. There are more than one layet per year. Google about the P-38 that was lost on the ice in Greenland in WWII. Look at how deep it was in the ice and how many layers there were when they finally found it, dug down to it, and recovered it. Plants preserved somehow from long ago? Nope. First problem is knowing EXACTLY when that plant lived. Unless someone was there and left a note next to it and said note is recovered, then that is only a guess. Age of tnis plant when it died? Also a guess, but can be somewhat accurate with trees. However, we still don’t know when it lived, just about how long it lived. Rock layers? Nope. They are dated by the plants that are found in them. However, tje plants are dated differently, by the age of the rocks they were found in. I’m 55 years old. My shoes are 10 years old, you cannot tell the age of one based on the other and then reverse it and derive the age of the first based on the age of rhe second. Is my foot then 10 years old or my shoe 55? Are you beginning to see tje problems here? Carbon 14? Totally inaccurate because it is based on how much Carbon 14 exists today, which is more than a year ago. It would be a somewhat accurate way except for one major problem. It doesn’t get produced in the cells of living organisms, but in the atmosphere. That is a huge problem because of how we get it in our bodies, just like every other living thing, we absorb it. It can be absorbed from the atmosphere, or when we consume something that has Carbon 14 in it. The issue then becomes has the organism being tested reached saturation/equilibrium. That means is it at the point where it cannot absorb any more and it is losing it at the same rate as it is gaining it. Think of a 55 gallon barrel with a line of holes up the side of it. You put a hose in the barrel and start to fill it with water. As the water rises it starts to come out of more of the holes. When it gets to the point where the hose output id equal to the amount escaping from the holes the level no longer rises, nor doea it fall. That is the point of saturation/equilibrium. Since we cannot know if the organism being tested has reached that point then any measurement of how much Carbon 14 remains is useless as a way to determine how long ago it lived. So, there you have it, there is no way to accurately tell.
Ice cores is a common one, but there are many others.
For example, a friend of mine did her PHD on using sheep (or one very specific kind of sheep) teeth to figure out climate at the time the sheep was growing their teeth. Based on the chemical makeup of the teeth (the ratio of different isotopes), she was able to tell what the chemical makeup of f the atmosphere was like, and from that tell what the climate was like.
Doing so with these teeth was very useful because they did not decay quickly over time, and locked in the climate data like ice does, but without needing it to stay frozen. So you could get climate data from places other than the Arctic.
My knowledge of this is limited, as it’s her science not mine, but it was super cool.
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