The Science of Everything Podcast - Episode 141: Natural Climate Change

Episode Date: February 29, 2024

An overview of the natural mechanisms by which climate changes over time. Beginning with an introduction to the concepts of radiative forcing and climate sensitivity, we then discuss solar forcing, orbital changes, volcanic eruptions, and silicate weathering, covering how each process operates and the effects it has on Earth's climate over varying periods of time. Recommended pre-listening is Episode 140: A History of Earth's Climate. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to the Science of Everything podcast episode 141, natural climate change. I'm your host, James Fodor. So here we continue on from the previous episode, and we talk about some of the mechanisms by which climate changes over time without human influence. So this builds on the prerequisite, episode 140, a history of Earth's climate, whereas in that episode, we gave a kind of a narrative overview of changes in the Earth's climate over long and shorter periods of time. In this episode, we're going to try to explain some of the reasons for those changes, focusing on the natural forcing mechanisms.

So in particular, we'll talk about the, we'll talk about changes in the radiation emissions from the sun, we'll talk about the effect of volcanic activity, we'll talk about the effect of the changes in Earth's orbits, Malankovic cycles, and we'll discuss the effect of the silicate carbonate weathering processes and the carbon cycle in changing the quantity of carbon dioxide that is in the atmosphere over very long periods of time. So I've already said they're prerequisite is episode 140, so please give that a listen before this one, because it continues directly on. And without further ado, let's get started. So far, I've been describing changes in Earth's climate, and I've mentioned some of the reasons for them. I've mentioned, for example, the change

changes owing to changes in the Earth's orbit. But I haven't really given much of a detailed explanation. So that's what we're now going to turn to. And this will also form an important backdrop against which we can understand the greenhouse effect and the impact of human emission of greenhouse gases. So first of all, let's start by talking about the concept of radiative forcing. Radiative forcing is a change in the energy passed to the atmosphere, or the term is energy flux. So a change in the energy flow to the atmosphere. which is caused by either natural or man-made processes. And energy flux or radiative forcing is measured in watts per meter squared.

So it's energy per unit of surface area. So you can imagine a surface of atmosphere surrounding the Earth, roughly spherical in shape, and you can think about the area of that surface. The amount of energy going through each square meter of that surface is the energy flux. And radiated forcing is a change in that energy flux. Of course, we know that the energy that Earth receives comes basically all from the sun. So there's a certain amount of energy that comes that is absorbed by the Earth from the sun. There's also various sources of energy that are emitted from the Earth and travel back into space.

We'll talk about some of those in a moment. But any changes in these energy fluxes, whether they're from outside in or inside out, so to speak, from space coming in or from the surface of Earth out to space. These are called radiative forcings, so changes in the flux. These are external drivers to climate. So radiative forcing is important because they represent external changes to Earth's climate. A simple example would be if the sun warms up and emits more radiation, Earth receives more of that radiation.

That represents a radiative forcing that's going to warm up the temperature of Earth. Like that's a very simple example, right? It's important to have a concept of these radiative forcings because we need to distinguish between an external driver of the climate from internal climate feedbacks and variability. So there are many mechanisms that serve to either increase or decrease the effect of radiative forcings. So these are positive and negative feedbacks you may have heard about. There are many of those. We'll talk about those later. But by themselves, they don't generate initially.

They don't initiate these sorts of changes. A good example that we already mentioned from before would be the ice albedo feedback. So when you have more ice, that reflects more solar radiation, that causes the planet to cool down, which generates more ice, and so the feedback goes. Now, this is a feedback mechanism, but it can't explain why the temperature changed originally. Why did the initial process get started in the first place? That would have to be explained by some radiative forcing. So something outside of the system, outside of this climate system, had to precipitate that initial change in temperatures, which then brought about that ice albedo feedback cycle, right? And so this is what we mean when we're talking about radiative forcing.

It's these kind of external forces that cause changes in the climate system. This also gives rise to what's called a sensitivity parameter. The sensitivity parameter is the ratio of the change in temperature to the change in the forcing. So basically, because we measure forcings in watts per meter squared, we can ask the question, how much does Earth's average temperature change if I change the energy flux by one watt per meter squared? That's what the sensitivity parameter tells you. So if the sensitivity parameter is one, it means that a one watt per square meter change

in radiated forcing gives rise to a 1 degree Celsius change in the temperature. We'll talk more about this sensitivity parameter in a moment because it's extremely important for understanding any type of radiative forcing on the climate, including that of greenhouse gas emissions. Now, the most obvious source of radiative forcing is changes in solar insulation, so emissions from the sun. But there are other factors as well, so changes in the surface albedo also give rise to radiated forcing. So more ice means a higher albedo, less ice means a lower albedo. There are other things as well, but ice is a major contributor to albedo. Another factor is atmospheric concentration of greenhouse gases. So greenhouse gases absorb outgoing radiation from the Earth,

thereby affecting the radiative forcing at the top of the atmosphere. We'll go through this in more detail in a bit. But the important point is that it's not just radiation coming in, it's also radiation going out that can act as radiative forcing. A volcano exploding and putting a large amount of particulate matter into the atmosphere, that would also be another example of radiative forcing. So it's really anything that can either change the amount of light that comes in from the sun, change the reflection of light from the sun, or block radiation from leaving the Earth.

Any of these things can be a radiative forcing. And it kind of makes sense, right, because the name indicates radiations, right? So anything that can affect the radiation kind of directly, not a feedback mechanism, but like a direct change in radiation coming in or radiation going out. These are all forcings. And they're all measured in watts per meter squared. Now, I mentioned this concept of the sensitivity. parameter. So this is the change in temperature that's brought about by a change in radiative

forcings. Now, this is quite a difficult parameter to estimate accurately, and that's been one of the large focuses of climate research over the last few decades, the IPCC, so the intergovernmental panel on climate change, which is an international consortium of researchers. You probably heard that they produce reports every few years and provide the state of the evidence about climate change and modelling of Earth's climate. They produced a number of estimates of this over the years, and unfortunately, the estimates are still fairly imprecise, although they've gotten a bit more precise in the last few reports.

But there are fairly simple methods that we can use to make at least a crude estimate of the climate sensitivity parameter, and I wanted to talk a bit about that so that we can understand at least roughly what the connection is between radiative forcings and actual temperature changes, because the focus of this episode in particular is understanding the mechanisms of changes in the temperature. And so we need to understand the linkage between radiative forcings, which is about energy and temperature, which is, you know, it's about

temperature. A simple way of trying to estimate the climate sensitivity is just by focusing on the blackbody emissions of Earth at the top of the atmosphere, using the Stefan Boltzmann law. So as I've discussed in previous episodes, everybody in the universe that's above zero Kelvin radiates energy over a particular distribution of frequencies or wavelengths. The high the temperature you are, the high the frequency at which you emit radiation. The Stefan Boltzmann law is an equation which describes the relationship between temperature and the total amount of emitted radiation. This is very important because it contributes significantly to the magnitude of the climate sensitivity.

And so I'll mention this a few times, but the important thing that we need to understand is that the Stefan Boltzmann Law has a power of four. So it's a high magnitude polynomial. In other words, as the temperature increases, as the temperature of anybody increases, the amount of energy that it radiates in a given amount of time, increases proportional to a factor of four with the changes in temperature. That's a very rapid increase in the amount of energy with respect to temperature. That's the important point. It doesn't increase linearly, not even quadratically. It's to the power of four, which is a very, very rapid increase. Now, we'll see why that's important in a moment. So what we're going to do is just understand the process by which we can make a crude estimate of the climate sensitivity using this fact, using this fact that we can imagine that the Earth has an average temperature at the top of the atmosphere, which is where most of the radiation happens from. And we can say, well, how much energy is radiated out at the top of the atmosphere? And that obviously depends on the temperature by a surface. Stefan Boltzmann Law. The Stefan Boltzmann Law is so useful because it

already connects the two things we want to know. We want to know what the connection is between changing temperatures and changing energy fluxes. And that's more or less what the Stefan Boltzmann Law tells us. So we can do some calculus, take the derivative and do some other algebra, and then plug in a few measured parameters such as the temperature of Earth and a few physical constants, and get an estimate of the climate sensitivity. And that gives us an estimate of about 0.25 Kelvin per watts per meter squared. So in other words, if we get a radiative forcing of an additional watt per meter squared, the temperature will go up by 0.25

Kelvin. Now as it turns out, that's not too bad given how simple the method that we used to do that it was, but it was too simple and that value is too low. We need to incorporate other factors beyond simply the fact that Earth is cooling off at the top of the apps. which is all we just considered. There are other factors that are relevant as well to to the sensitivity value. In particular we need to consider the surface of the earth is much hotter than the top of the atmosphere. And because of that, there is a transfer of energy from the surface to the top of the atmosphere through a number of processes. The most important of these processes are radiation, so that's emission of energy and absorption of energy. And convection,

which is basically large parcels of air moving around in the atmosphere, you know, air physically rising up through the atmosphere and expanding and cooling. Now, this requires more complicated modeling, which I won't get into here, but suffice to say, when we combine estimates from these radiative convective models, which estimate heat transfers in the atmosphere as a result of radiation and convection, and we combine that with the radiation using the Stefan Boltzmann law, we get a combined sensitivity of about double what we had before, so about 0.5 Kelvin's per what per meter squared. The main reason for that increase, by the way, is actually a feedback mechanism from water vapor,

because as the temperature increases and as the energy is distributed throughout the atmosphere through radiation and convection, as I just discussed, more water is absorbed and held in the atmosphere, and water acts as a greenhouse gas, so that actually contributes further to a warming effect. But anyway, we'll discuss that in a bit more detail later, but the important point is when you factor that and you get a sensitivity of maybe 0.5 Kelvin. Now that's still very crude because there are other feedback mechanisms that we haven't considered. And more complex analyses using more complex models give a mean estimate for the climate sensitivity

parameter of about 0.8. Although the most recent IPCC report, the sixth report, which just came out a couple of years ago I think, slightly higher values of around 1 have become a bit more likely. But many sources that I look at still use the value of about 0.8. But anyway, the precise value is not extremely important. The point that I wanted to make here is that even using very crude methods of just considering the blackbody emissions at the top of the atmosphere, we got a value of 0.25. And using somewhat more complex methods, but still relatively crude, by incorporating radiation

and convection, which then allows for the feedback of water vapor, we get a value of about 0.5 Kelvin. That's not too far off, the mean estimate from very much more complex methods of about 0.8. So even using quite simple methods, we can get a relatively good approximation of how much warming is expected given certain radiative forcings. This is very useful because it allows us to have a very fairly good-eyed understanding of the key mechanisms that are driving changes in temperatures. And an idea about how large these are likely to be quantitatively. Whether the exact value for the sensitivity is 0.5 or 0.8 or something, that does certainly make a difference, but the overall picture is not going to be dramatically changed as a result of a change in that parameter. So even quite simple methods can give us reasonably accurate results here.

Now, bearing all that in mind, what we're going to do here is look at some of the major natural mechanisms that are operative in accounting for changes in the Earth's climate over time, and also talk about the magnitude of these effects. So how large a change they can elicit and over what time periods they change, both of which are relevant. So let's start with the obvious. The sun changes in solar insulation leading to changes in radiator forcing are called solar forcing. So whenever the sun changes its output, this leads to solar forcing. It could be up or it could be down.

It's all solar forcing, right? We know that the sun does change its output over time. In fact, over very long periods of time, over hundreds of millions of years, the sun has actually been warming quite considerably. And it's still a bit unclear as to exactly why it was that the Earth was as high. as it was, you know, 500 million, a billion, 2 billion years ago, when the sun was actually quite a bit fainter. This is an open problem called the Young Faint Sun Paradox, if anyone's interested in looking at that. So over many millions of years, the sun has become progressively hotter, or it progressively emitted more and more radiation.

But that's only a significant effect over many millions of years. It doesn't explain any changes that have happened over the time span of a hundred or even a few thousand years. There are changes in solar irradiation, which do happen over shorter periods of time. The most well known is the 11-year sunspot cycle. Solar output changes by about one to two parts in a thousand over this 11-year period. A sunspot is a relatively cool region, often kind of circular or kind of spotty in shape, on the sun's surface. And there's a cycle of 11 years that these go through, from having many sunspots to having none. It's thought that this occurs as a result of fluctuations in the sun's magnetic field as it rotates,

but we don't need to worry about the exact reasons for that here. The important point is that it's been observed that the magnitude of solar output varies, reliably by one to two parts in a thousand over the sunspot cycle. This corresponds to a radiative forcing of about one watt per meter squared. Now that's a relatively large amount, not huge, but relatively large. in order to compute the estimated temperature change that would be resulted from this one watt per square meter solar forcing we first need to divide that number by four because this one watt per meter squared is spread out across the whole surface of the earth

solar radiation is only absorbed by the side of the earth that faces the sun and and also the earth kind of curves away from the sun so if you think it through you'll realize that the total amount of energy that's absorbed is actually the same as that of the shadow of the earth, the circular shadow of the earth. So a circle with a radius the same as the radius of the earth. And because of the way geometry works, if you take a circle with a certain radius and compute its area, that area will always be exactly one-fourth of the area of a sphere with the same radius as that circle. So in other words, if you take the shadow of the earth and work at the area of that shadow, that area will be one-fourth of the surface area of the earth.

You then have to multiply by 0.7 to account for the fact that 30% of the radiation is reflected back into space by Earth's albedo. So when you multiply those two factors together, the actual forcing that we get from that 1 watt per square meter change in solar radiance is only about 0.2 watts per meter squared. This leads to between 0.1 and 0.2 degrees of warming, depending on exactly what sensitivity figure we use. That's a small amount of warming, but not entirely trivial. Remember that the difference between the little ice age and the period of time that went before it was less than half a degree, probably only about 0.3 or 0.4 degrees Celsius. So a 0.1 to 0.2 degrees Celsius change is, you know, it's not huge, but that is noticeable on a global scale. However, the sunspot cycle is far too short to generate any equilibrium changes,

because remember that the sunspot cycle is an 11-year cycle, and long-term change. in climate require you looking at at least decade long time spans anyway. So the changes in the total solar irradiation are just happening too rapidly for them to be, to account for any changes that we've observed over the course of the 20th century in global temperatures. In addition to the sunspot cycle, there have also been other changes in the sun's output that are smaller, perhaps half a watt per meter squared rather than one watt per meter squared, so maybe half of the magnitude of the, of the, of the, of the, of the, of the, of the, the sunspot cycle, we've actually seen in the past 50 years or so a very slight reduction

in solar irradiance. I don't know that there's any known reason for this. The sun just fluctuates a little bit. This is only, this is less than one part in a thousand, so it's not a very large effect, and it would be expected to generate less than 0.1 degree of cooling on Earth. But if anything, over the second half of the 20th century, the sun has actually slightly reduced its irradiation. So if anything, the sun in recent decades has actually been contributing slightly to global cooling, although the magnitude of that is, as I said, very small, less than 0.1 degrees. So to summarize, the sunspot cycle is both too small in magnitude, but also too short in time to account for any changes in average global temperatures over the past century.

The sun is gradually increasing its radiation output quite significantly, but that's happening over a period of time of hundreds of millions of years, and so that's far too slow to account for any changes over the past century. There has been more rapid average changes in the sun's irradiation, but only very small. And so solar forcing simply cannot explain the rapid increase in temperatures we've observed over the past hundred years or so. But these are an important factor to consider when we're looking at paleo climates and the long trajectory of changes in past climates. Though in general it's thought that they don't account for many of the fluctuations precisely because the sunspot cycle is too short and the very long changes of the sun's radiation

take place over hundreds of millions of years and represent a progressive increase in warming and should contribute to progressive warming. And that doesn't really explain the ups and downs that we've seen over over geologic time. So the sun matters, but it doesn't really account for most of the variation, either they'd have been seen in historic times or even or even in prehistoric times. Let's now talk about something that I mentioned a number of times that haven't really explained, and that is orbital changes. These are often named after the, I think, Serbian who initially proposed them,

Malankovic. So they're called Malankovic cycles. We all know that the Earth orbits the Sun, and the Earth's orbit about the Sun is approximately circular. However, it's not exactly circular, and there's a few other idiosyncrasies as well, that in combination can give rise to these and Malankovic cycles. So there are three that I'll talk about here.

Eccentricity, obliquity, and procession. The eccentricity of Earth's orbit refers to how elliptical it is. So basically, is it a perfect circle or is it more oval in shape or elliptical in shape? This is relevant for changes in temperature because the amount of solar radiation that's received by Earth,

obviously depends on the... distance between the Earth and the Sun. When the Earth's orbit is perfectly circular, or basically perfectly circular, there's no variation in the distance between Earth and the Sun, it's always the same. But when the orbit becomes more elliptical, the Earth gets a bit closer to the Sun during some times of the year and a bit further away during other times in the year. Currently, the variation in the amount of solar radiation that's received is about 7% between the time when the Earth is closest to the Sun versus furthest away. So it's not a huge difference. I mean 7% is

still is quite significant. It's much more than the sunspot variation, for example, which was like a one part in a thousand. Seven percent is quite large, but that vary, but the seven percent varies over the scale of one year. So obviously, again, that can't explain changes that we've seen recently. But it's still important. When the eccentricity of Earth's orbit is at maximum, the variation, the annual variation in the amount of solar radiation goes from 7% to 23%. So about three times as much when we have maximum ex-intricity compared to now. Ex-intricity affects when the earth is furthest away versus closer to the sun at different times of year, and therefore it has an impact on how severe summer versus winter is at different places on Earth, although,

of course, this will be the opposite for different hemispheres. So, for example, if the Earth is closest to the sun during the northern hemisphere summer, that tends to make the north. Northern Hemisphere summer stronger, but it will also make the southern hemisphere summer weaker because that will occur at six months offset. So the overall change in global insulation is not much affected by the eccentricity cycle. However, the timing of when this occurs is affected, and that can have implications on global temperatures depending on the location of Earth's landmasses and other complex complicated factors. Moving now to Obliquity.

So obliquity refers to the tilt of the Earth's rotation with respect to the ecliptic. The ecliptic is sort of an imaginary sphere around the equator of the sun, which is where most of the planets orbit about. But this rotation is rotated by about 22 to 25 degrees with respect to the ecliptic. And this rotational tilt fluctuates between those values over a cycle of about 41,000 years. The higher the tilt is, the more extreme the seasons tend to be. And it also tends to increase solar insulation at higher latitudes. Because the axial tilt of the earth kind of rotates the higher latitude region, so near the top of the earth, near the poles,

it tends to rotate them relatively towards the sun during their summer and relatively away during their winter. That means that an increased axial tilt tends to have an overall increase of solar insulation at higher latitudes, particularly during their summer, because now they're kind of tilted towards the sun, instead of pointing more away, as they typically do, right? If there were zero eccentricity, the poles would point perpendicular to the ecliptic, and so would get very little sun at all, whereas when there's axial tilt, they point more towards the sun during their summer tend to get more solar radiation. Because changes in the ice, the quantity of ice at high latitudes

is largely determined by the amount of melt that occurs during summer. Having higher temperatures and more insulation during the summer can tend to increase snow melt at the poles, which promotes warmer temperatures. And obviously you're going to have that ice albedo feedback that we talked about before. Finally, we'll talk about axial procession. So axial procession is the change in the direction of the axis of Earth's rotation relative to fixed stars. So bearing in mind that, as we've just said, the axis of Earth's rotation does not point perpendicular to the ecliptic. It's slightly tilted by, you know, about 24 degrees or so.

And the direction of this tilt changes over time. It's a bit hard to explain, but it's similar to the procession of a top. You know, when you spin a top, it often is tilted slightly. And the direction of that tilt processes as the top turns. It processes much more slowly than the rotation. of the top, but you can see it happening at the same time. Hopefully you can sort of visualize what I'm talking about there. Much the same thing happens with the rotation of the earth.

The procession of the axis of rotation occurs with the period of about 25,000 years. And the procession of the axis is why the constellations appear to move in the sky over periods of thousands of years. It's because the direction of the axis of Earth's rotation is actually moving relative to the fixed distance. This is important because currently the perihelion, so the closest point that the Earth reaches in its orbit about the sun, currently this occurs during the Southern Hemisphere's summer. So this means that solar radiation due to the axial incline, so the fact that during the Southern Hemisphere summer,

the Southern Hemisphere is sort of pointing relatively towards the sun, that lines up with the fact that in the Southern Hemisphere during the summer, the Earth is a bit closer to the sun, and therefore that's sloth. magnifies the temperature increase. So these two effects are additive, so they add to each other. So that means that currently for the southern hemisphere, there's a more extreme seasonal variation and the irradiation. Conversely, in the northern hemisphere, when they are pointed towards the sun, that happens when the earth is furthest away from the sun. And so those are, that's a subtractive effect. They kind of cancel out. And again, so depending on the location of the

landmasses around the earth and other complex factors, this axial procession will have differing effects on the overall global temperature. So of the different cycles that we just talked about, eccentricity, obliquity and procession, Malankovicch thought that the obliquity cycle would have the biggest effect, and therefore we should expect to see ice age cycles from one glacial maximum to the next of about 41,000 years, which is the same as the obliquity cycle. In fact, what we observe is that there's quite a bit of variation in the period of ice ages, but it seems to have a periodicity of closer to 100,000 then, to 40,000 years. This is called a 100,000 year problem and it doesn't have any widely accepted solution. So we don't know why ice ages seem to occur

with periods of about 100,000 years. Typically, the onset of glacial conditions is relatively slow. It takes tens of thousands of years during which the Earth gets progressively colder and colder and colder. And then within a fairly short span of a few thousand years, there's an exit of those global conditions, of those global glacial conditions into an interglacial period. And we just saw that when we looked at the period of the last 20,000 years, where the global maximum occurred at about 25,000 years ago. And then starting around 18,000 years ago, there was a progressive warming, which lasted for 6 to 8,000 years until we reached roughly the pre-industrial temperature levels. So that that broad cycle of slow onset of glacial conditions occurring over tens of

thousands of years to rapid offset of the glacial conditions going to interglacial conditions within the few thousand years has been observed over a period of many glacial cycles over the last million years or so. And it's not really known why have that pattern nor why we observe the cycles occurring every hundred thousand years instead of every 40,000 years. So that's still a that's still an open problem. However, it is very clear that orbital changes are critically involved in this process and are some of the major forcing factors that lead to this periodicity in the glacial interglacial period. So we've talked about the effect of solar forcing and we've talked about the effect of orbital changes. Orbital changes, as we've shown,

do have a significant effect on Earth's climate, leading to temperature changes of maybe plus or minus six degrees between the glacial and interglacial periods of time, but they only produce that change over periods of thousands of years, thousands to tens of thousands, depending on whether we're talking about going into the glacial or coming out of the glacial. And that's far too slow to account for any of the changes that have occurred over the last century or so. But it does explain why we see that periodicity over the past million years, and particularly the pattern we've seen of coming out of the last glacial period in the past 20,000 years. Now, it would be remiss of me not to mention volcanic eruptions.

I've sort of mentioned them briefly before. Volcanic eruptions, particularly large ones, which expel very large quantities of particulate matter and aerosols into the atmosphere, significantly increase the albedo of Earth, and therefore produce a very large cooling effect. So these can have very significant effects on Earth's climate. However, typically, this only lasts a few years until the particulates disperse and come out of the atmosphere, though this does depend on the size of the particulates and how much is emitted and that depends on the type of volcano. It's thought that certain periods of very high levels of baseline volcanic activity can lead to an increase in the albedo of Earth via essentially having a larger amount of a particulate matter that hangs around in the atmosphere at any given time,

even though individual particulates sort of fall out fairly quickly. If there are new volcanoes all the time, then you can retain a higher kind of average level. So it's thought that variations in volcanic activity, in turn produced by variations in the amount of tectonic plate activity at different points in Earth's history, may explain some of the variations in Earth's climate over millions of years. Though I've not seen any quantifications of how large that effect could be, but it seems like it could be at least a few degrees because it's known that large volcanoes can have a pretty significant effect on climate. For example, the year without a summer in 1816 was caused in large part by the eruption of Mount Tambora in Indonesia. And that resulted in a significant reduction in world temperatures, I think a degree or two. And very noticeable changes in the appearance of the sky and the sun with sunsets and crop failures around the world and so forth. So these can have very large effects.

And changes in tectonic plate activity over millions of years may contribute to overall. all changes in Earth's temperature over like geological time spans. But individual eruptions are not long lasting enough, nor do they typically affect the equilibrium global temperatures, unless there's some other changes that contribute to go alongside those. And so volcanic eruption certainly can't explain the changes that we've seen in the last 100 years or so in Earth's temperatures. Now, the final natural mechanism that I wanted to talk about is a geological process called silicate weathering, or more generally the carbonate silicate geochemical cycle. There are other such cycles, but this is the one that I wanted to focus on, and it kind of

uses an example of some of these other processes as well. It's thought that the silicate, the carbonate silicate cycle is the most important, very long-term process by which Earth's climate changes. And basically, this whole cycle involves moving carbon from the atmosphere to the lithosphere, the lithosphere being like all of the rocks on Earth's crust and in the mantle. There's a series of chemical reactions which bring about this process. The core reaction here involves carbon dioxide that's in the atmosphere. It dissolves in rainwater to form carbonic acid. Carbonic acid then reacts with silicate minerals, reducing one of the oxygen atoms to water. So reducing meaning it gives up a proton to one of the oxygen atoms. Silicate minerals are probably the most

important class of minerals in the Earth's crust. Silicate minerals are those that contain silica, so that's silicon dioxide, and there are many, many different types of silicate minerals. And so the exact chemical reaction will be different depending on precisely what mineral we're talking about, but I'll just talk about a simple example with calcium silicates that most directly integrates with the biological processes in the oceans that I'll describe in a moment. So if we have our carbonic acid, which is basically carbon dioxide with some extra hydrogens into it, because remember an acid is a species that donates protons. So carbonic acid is basically carbon dioxide with extra hydrogens to donate. Technically the formula is H2C-O-3, but we're not going to get too bogged down in

the chemistry here. The point that I want to make is that when carbonic acid interacts with, or reacts with a silicate mineral, what happens is that one of the protons of carbonic acid is donated, so it's an acid, so it donates a proton, and that proton reduces one of the oxygens in silicate. So that means that the oxygen picks up, picks up a hydrogen, and turns into a water molecule. So pick up two hydrogens, right? It needs two molecules to interact with. But the point is that an oxygen is reduced, picks up some hydrogens, turns into a water. And then what we've got left is just silica, silicon dioxide, and then some molecules of the conjugate base of the carbonic acid. So that's HCO3 minus, right? It's donated one of those protons, donated it to the oxygen,

and now it's got one hydrogen less. Now, so far we haven't really done anything, right? All we've done is changed around the structure of some minerals. We haven't actually taken any carbon dioxide out of the atmosphere because it's still in this carbonic acid form, right, which can just go straight back into the atmosphere. But the important step then happens when this, the ion, the conjugate base ion of the carbonic acid, HCO3 minus, plus the calcium ion that was freed from the silicate mineral that we've been just talking about, these ions, because ions are soluble, they're both washed into the ocean, you know, via rivers or groundwater. And once they end up in the ocean, what happens is that marine organisms, marine organisms take these calcium ions and also they

take the conjugate base of the carbonic acid, so the H-C-O-3-1, and they precipitate this into calcium-carbonate. So calcium-carbonate is just one calcium ion and one carbonate ion, so C-A-C-O-3. This is the important step, because now what happens is that this carbonate ion that's now precipitated along with calcium to form a solid substance, which will initially make up the shell or some structure in the body of a marine organism. And then eventually when the organism dies, the calcium carbonate will be deposited on the ocean floor and eventually buried. The carbon that is in that calcium carbonate was originally in the atmosphere.

The carbon dioxide being taken out of the atmosphere, converted into carbonic acid, then converted into the conjugate base of that carbonic acid, and then finally precipitated into calcium carbonate. And that then is precipitated and deposited on the... the ocean floor and is now being removed from the atmosphere. So this whole process is a weathering process by which silicate rocks are weathered, you know, by the atmosphere, by rainfall, by wind and so forth, and dissolved by carbonic acid in the water.

The carbonic acid then carries the carbon and the oxygen that was originally in the atmosphere into the ocean, where it's precipitated by marine organisms to form calcium carbonate that's deposited on the ocean floor. There are some other similar processes as well, but this is the main one that I wanted to focus on. And hopefully you can see how we go from carbon dioxide being in the atmosphere to it being kind of in solution, and then to eventually it's precipitated on the ocean floor and cannot get back into the atmosphere. Well, it can, but it has to go through the reverse process, right? And that takes a very long time.

The reverse process, by which the carbon dioxide gets back into the atmosphere, occurs as a result of plate tectonics, whereby this precipitated calcium carbonate deposits eventually are subducted. So when there's plate tectonic activity, the oceanic plates go under the, well, it could be going under another oceanic plate or a continental plate. But either way, it's pushed underneath. The material is essentially shoved under another plate. That leads to warming and chemical reactions called metamorphism, whereby the minerals are changed, And that process releases carbon dioxide. It's basically the opposite of the process that originally formed the calcium carbonate.

The metamorphic processes produce the initial silicate minerals and carbon dioxide. And that carbon dioxide then accumulates in bubbles and then is released by volcanoes and is therefore returned to the atmosphere. So that's an entirely kind of geological process. So there's the weathering process, which leads to carbon dioxide being transported to the rivers and then combined with calcium ions and precipitated to the ocean floor and buried, thereby withdrawing carbon dioxide from the atmosphere. The reverse process of minimorphism and volcanism returns carbon dioxide to the atmosphere.

Now these two processes are always occurring, right? But the relative rates of the processes vary, and there's many, many different factors that affect this. I don't think they're fully understood. So the rate of volcanism depends upon the rate of tectonic plate activity, which is thought to vary over geologic time through complex factors that I don't think are well understood. In addition, it's also going to be affected by the number of subduction zones and the relative size and positioning of plates and things like that. The rate of weathering, so that's the process that moves the carbon dioxide out of the atmosphere,

that is affected by the amount of, it's typically mountains that are most rapidly weathered or exposed large areas of rock. So the more of these mountains there are and the more exposed they are, the more weathering there tends to be. Also, the warmer the planet is, the more rapidly these chemical processes occur. So the temperature actually acts as a kind of a negative feedback mechanism there, whereby if there's lots of carbon dioxide in the atmosphere, that tends to increase the temperatures through the greenhouse effect, an increased temperature leads to more rapid silicate rock weathering,

which removes carbon dioxide from the atmosphere, thereby making the planet cooler. other factors will also affect the rate of weatherings such as the number and positioning of tectonic plates and whether there are large mountain ranges that arise as a result of continentsal plates crashing into each other that might lead to an increase in the amount of rock weathering because of the exposed rocks and also the positioning of the plates depending on whether these exposed rocks occur at latitudes and in positions where there's lots of rainfall or less rainfall so many complex factors like this are going to combine to change the relative rates of the silicate weathering versus the metamorphosis and volcanism, which returns the carbon dioxide to the atmosphere.

These processes are very slow. They happen over millions of years. And that's why we see a very slow process of change of temperatures over very long geologic time spans, like millions of years, as we've discussed. So it's this process of silicate weathering and then metamorphism and volcanism, the relative balance between those that's thought to be the primary driving process of changes in the temperature of the Earth over very long periods of time, like millions of years. So on top of that, you also have the direct effect of changes in volcanic activity on the

amount of particular matter in the atmosphere, which changes Earth's albedo. On top of that, you also have changes in solar forcing, which operate over a short 11-year time scale, which doesn't have a very large effect on climate, but has a small effect. but also the progressive increase in solar irradiation over very long periods of time, like tens of millions, hundreds of millions of years. That's tended to make Earth warmer over time, although there may be other factors that mitigate against that. And then in addition to that, you've got the Malankovic cycles, the orbital, the changes in Earth's orbit, the eccentricity of liquidian procession,

which have substantial effects on Earth's temperature, but over relatively shorter periods of time, so tens of thousands to hundreds of thousands of years. So all of these factors, plus others that I haven't discussed, combined, explain why the Earth's climate has varied so dramatically over geologic time and also over more recent time over the previous tens of thousands of years. However, as I've explained multiple times, apart from isolated large volcanic eruptions, all of these processes occur very slowly. Silicate weathering and changes in the positioning of the continents and so forth. that happens over many millions of years, even hundreds of millions of years. That can't really explain any changes in the Earth's climate in the past millennia, let alone the past century. Orbital changes affect the Earth's climate over periods of time of thousands to hundreds of thousands of years,

but not over the period of the last hundred years, or even the last couple of thousand years that we have really high quality evidence for. The only one of these natural drivers that we know of that can really explain changes in global, temperatures over periods of time of centuries to millennia would be solar forcings. But as we've seen, the sunspot cycle is too short and too periodic, so it goes up and down but doesn't consistently trend in any direction to really explain any significant changes. And other very small changes that we've seen in the radiation of the sun can explain a very small amount of change, perhaps small fractions of one degree. Then maybe it could explain that a little ice age, or at least partly,

explain that and they can explain partly why there was a reduced change in temperatures between about 1940 and 1970. A little bit of that can be explained by slightly reduced solar activity. But overall, they cannot explain the magnitude of the temperature increases that we've seen, especially in the late 20th century. So to conclude this section, there are many mechanisms by which Earth's climate changes over time, over different durations of time, from many millions of years to thousands of years, even to the 11-year sunspot cycle. But none of these natural mechanisms can explain the very rapid increase in temperatures and consistent increase in temperatures that have been observed over the past 100 years or so. And to explain that, we must look to the

greenhouse effect, which I've mentioned a number of times in which itself is also a natural effect, but particular we have to look at the enhanced greenhouse effect that's been produced as a result of greenhouse gas emissions produced by burning of fossil fuels. And that's what we will look at in the next episode. We'll talk about the greenhouse effect itself, and then we'll talk about how it's been enhanced by greenhouse gas emissions from human activity. So that concludes the episode today. If you enjoy this podcast, consider supporting us by leaving a favorable review on Spotify or iTunes or the aggregator of your choice. You can also make a financial contribution to the podcast by becoming a

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this year. If you'd like to get in touch with me, you can email me at Fods12 at gmail.com. That's FODS12 at gmail.com. I appreciate and respond to all of the listener emails that I get. So that's all from me and I'll talk to you next time