The Science of Everything Podcast - Episode 140: A History of Earth's Climate

Episode Date: January 31, 2024

A journey through the history of Earth's changing climate, in which we explore how Earth's climate has fluctuated over the past billion years. After reviewing the various temperature proxies used to estimate past climactic conditions, we discuss the varying hothouse and icehouse periods of Earth's history, the onset of the Quarternay Ice Age, the current interglacial period, the anomoly of the Younger Dryas, the Medieval Warm Period and Little Ice Ages, and the rapid rise in temperatures during the twentieth century. Recommended pre-listening is Episode 90: Climate Systems. 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 140, A History of Earth's Climate. I'm your host, James Fodor. So in this long-awaited episode, we're going to start talking about climate change. So in this episode, we're going to discuss the history of Earth's climate. So we're going to go back right to the beginning of the Earth's climate. Well, not quite to the beginning. We'll start about a billion years ago and work forwards looking at what's been happening to the the Earth's climate, particularly focusing on temperatures as well as CO2 concentrations over the past billion years, and then we'll look at increasingly higher resolution at the last million years, at the last 20,000 years, the last 2,000 years, and the last 200 years. We'll look at major trends,

as well as the ways that we are able to estimate climactic changes over such long periods of time. And this is all important for getting a better understanding both of the operation of Earth's climate system, but as well as to get a background baseline of the variability of the climate prior to human influences. Recommended pre-listening is episode 90 climate systems, which will provide some important background information. So let's make a start and get into talking about the history of Earth's climate. But before we do that, let's make sure we understand what we're talking about and give a definition of climate. So I've talked about this before, but climate is the long-term patterns of weather in a region. In this case, we're talking about global climate, so long-term weather

patterns across the globe. And when we talk about weather patterns, you can think of that as the values of certain meteorological variables over long periods of time, so years to decades. So this would be things like rainfall, temperature, humidity, and so forth. And obviously, this has implication for things like storms, sea levels, crop growth, and other things that of importance to humans and the natural world in general. Now, in contemporary jargon, climate change and global warming are often used sort of synonymously. I think people tend to use climate change a bit more often these days. Global warming, however, refers to a very specific phenomenon, the ongoing increase in global

average temperature. So if you like, global warming is a specific type or instantiation of climate change. and in order to understand how and why global warming is happening, we need to understand the background of the various factors that affect Earth's climate over long periods of time. So that's what we're going to focus on in today's episode. And before we go further, it's probably useful to talk a little bit about how we know about paleo-climates,

that is the past climates, about how we can get information about what the temperatures were 200, 2,000 or 20,000 years ago. For information about what the climate was like, two, ten or two hundred million years ago, the only real information we have is geologic in nature. So ocean sediments, rock formations, fossils, things like that. That can give us a lot of information about what chemical processes were occurring at the time, which tells us something about the temperature.

It could also tell us about things like sea levels, what species lived at certain times, which can tell us about plausible temperatures as well. And so based on these sort of inferences, people have constructed approximate series of temperatures going back 500 million years. Now, much more accurate information is available over the last million years or so, and much of this is derived from information from glaciers. People probably know that one of the major sources of information about Plast climates is derived from ice, and particularly ice cores.

So what they do is they drill a big long shaft, essentially, down into the ice sheets. or glaciers, particularly in the Antarctic or Greenland, where they have the thickest ice and hence the longest series of data. Because there is usually very noticeable annual variations between summer and winter, where much of the snow falls during the winter, and gets compacted over time, you can count the layers and see at an annual resolution, certainly for the more recent data, it gets a bit compressed near the bottom, but certainly for the last few thousand years, you can see very clear annual records of the volume of snowfall. But not only that, when snow falls and compacts, it preserves small air bubbles,

which actually contained the air from that region at that particular time. So we actually have preserved samples of air from past climates, going back tens of thousands, even hundreds of thousands of years. So that can be analyzed, for example, for its carbon dioxide content. Another thing that you can do, both from the air but also from the water itself, is look at the oxygen molecules. So there are two main isotopes of oxygen that are found in water molecules, oxygen 18 and oxygen 16. Oxygen 16 is the standard by far most abundant isotope. Oxygen 18 is a heavier isotope with two additional neutrons.

Because it's slightly heavier, water molecules containing oxygen 18, if you're not, at a slightly higher temperature than water molecules containing oxygen 16, so the regular ones. As such, the ratio of oxygen 18 to oxygen 16 will be higher as the temperature increases, effectively because the higher the temperature is, the more oxygen 18 you're going to have evaporating and therefore being present in the snowfall. So we can use this ratio of oxygen 18 to oxygen 16 as a proxy for the temperature in that area at least at the time that that snow fell and then became compacted. So this is one very useful proxy that's used in order to estimate the temperatures going back tens, even hundreds of thousands of years, as I said, particularly from the very

long time series data that they're able to get from the deep ice sheets in Antarctica and in Greenland. Similar methods, however, can be used in other parts of the world in order to calibrate because you only get measures from one site so ideally you want to make take measures from many different places around the world and calibrate those against each other to produce a more accurate global average of temperatures and that's why that's one of the reasons why we're able to get more accurate estimates for more recent time periods there are other proxies that are used as well for example tree rings trees respond to changes in climactic variables

like ice does trees grow faster typically when it's warmer and this is often reflected in a change in the thickness of growth rings. So many trees have growth rings which show you how much they've grown in a given year. By looking at old living trees, as well as fossilized trees, we can get information about the temperature at the time that tree lived and at different years in the lifetime of the tree depending on the width of the growth rings. This evidence from tree rings goes back several thousand years and is another method that we can calibrate the, that used to calibrate and augment the ice data. So using a combination of these proxies, as well as a range of methods for calibration and

interpolation and different assumptions about exactly how to fit the pieces together, you can construct long time span series of temperature estimates. We can construct more accurate and higher precision estimates of temperature for more recent years because we have more proxies and the quality of the evidence is better. So the highest quality estimates go back for about 2,000 years. Then there's still fairly good but not as good evidence, mostly from the ice core data going back 20,000 years, slightly less quality again, going back maybe a million years,

those are from mainland ice core data. And then anything beyond that uses mostly sedimentary evidence and geologic evidence, which is much cruder, but still provide you with some reasonable methods of estimating, temperatures out to at least 500 million years or so. So now that we understand a little bit about how past temperatures are estimated, let's talk in more detail about the history of the Earth's climate, and in particular how temperatures have changed over time.

So let's start by talking about a very high-level view of the last billion or so years. Unfortunately, we can't go back as far as we would like. Earth has existed for about 4.5 billion years, but information about the climate doesn't, isn't preserve very readily. and so we don't have very detailed information that goes back more than about a billion years, and really only the last 500 million can we see anything very specific. One thing that we do know, however, is that throughout Earth's history, its climate has fluctuated between two sort of main different states.

These are called greenhouse and ice house states. So an ice house state is a period of time when Earth has at least one region of, of year-round ice at one of the polar caps. Note that there's a bit of inconsistency and language here. I find some sources talking about cool house versus ice-house states. I'm going to use the term ice house just so I can contrast that with greenhouse. I think it's a little bit clearer. And the way I'm using the term is just to refer to an in a period of somewhere there's at least one year-round large region of ice at least one of the Earth's poles. The contrasting state a greenhouse state is a period of time in which there are no year-round regions of ice at either of the poles.

And obviously that happens when the Earth is much warmer. So throughout Earth's history, most of the time the Earth has been in a greenhouse state. That is, it's been relatively warm such that there are no large year-round regions of ice at the poles or elsewhere. At the moment, hopefully you are aware that we are living in fact in an ice house. period. In fact, we're living in a fairly cold period of Earth's history called an ice age. So an ice age distinct from an ice house period is when there are year-round large regions of ice at both of the Earth's poles. So this is a more intense version of an ice house period. As I said, some sources use the language a bit differently, but I'll talk about ice house as the

general cooler period and ice age as the very colder part of that. So throughout most of Earth's history, Earth has been relatively warm and in the greenhouse phase or period, but currently we're actually in an ice age. That might come as a surprise to some people because of all to talk about global warming, but we'll come back to that distinction in a little bit. So in terms of the overall history of Earth, it's thought that a greenhouse period lasted from roughly the beginning of Earth until about two and a half billion years ago, so the first half-ish of the Earth's history. And then about two and a half billion years ago, there was the first glaciation. period, which was an ice house period that lasted for about 300 million years. This was called the Hironian glaciation period. And it's thought that this was, that this reduction in global temperatures and beginning of the first ice house period may have been triggered by the evolution

of oxygenic photosynthesis, that is, photosynthesis, which produces oxygen. This would deplete the atmosphere of greenhouse gas, of the greenhouse gas carbon dioxide and introduce free oxygen into the atmosphere. Because carbon dioxide is a greenhouse gas, it warms up the planet, and thereby depletion of carbon dioxide through oxygenic photosynthesis would have the potential to substantially reduce the temperatures on Earth. Now, not much information survives about that long ago, so I don't know that that's definitively known whether that was the cause of this first glaciation, but it's at least a plausible candidate. Obviously, we'll go on to explain more later about how and why greenhouse gases like carbon dioxide warm the planet, but for the moment we'll just

take that as a given as we go through the history. So after the planet warmed up again, after 300 million years or so of being relatively cold, there was another long greenhouse period that lasted until about 720 million years ago. And that marks the beginning of a period called the cryogenian, very well names, because it was a very cold period of an ice house earth, which ran from about 720 to about 635 million years ago. For reference, some of you may be familiar with Cambrian explosion, which is when many large forms of animal life developed. That began about 550 million years ago.

So the cryogenian was a little bit before the Cambrian. So the cryogenian is an interesting time period, because although the whole period was an ice house period, there were certain parts of that when the earth was colder than others. So remember the distinction I'm drawing here between ice house versus ice. age. So the whole period was an ice house period of over 100 million years long, but certain parts of it were especially cold. Indeed, it's thought that the earth got so cold at certain times that not only were there permanent ice caps at the poles, which is what constitutes an ice age,

but it's thought that these ice caps and glaciers actually spread so far that they actually reached the equator and kind of joined, such that the whole surface of the earth was covered in ice. This is sometimes called a snowball earth. Again, because not much information survives from this period, it's not definitively known whether this happened or not. It's hypothesized, and at least from what I've read about it, it seems plausible that this has happened at least on a few occasions. The reason this is possible is because of a feedback,

a positive feedback mechanism called ice albedo feedback. Essentially, ice and snow is quite reflective. It reflects a lot of incident radiation from this. sun, and that operates to cool the earth. So the more ice and snow there is on earth, the cooler it gets because of all the energy that's reflected. So basically, if ice begins to form, for some other reason, that reflects more light, increases the albedo of Earth, the reflectivity, essentially. This then cools the Earth further, which leads to more ice forming, which cools the earth further, which leads to even more ice forming, and so forth. And eventually, so the theory

goes. If the conditions are just right, you can have a situation where the whole Earth is covered in ice, and the entire planet is basically like Antarctica or the North Pole are today. Now, this didn't happen throughout the entire Cryogenian period, only if it did happen, it would have been for relatively short periods, perhaps millions or tens of millions of years, though I don't know exactly how long it's thought to have lasted. There are also variations of this theory, which says that perhaps there were small regions around the equator where there's most intense solar radiation, and so they're the warmest year round, where it wasn't completely frozen over, but there were regions of open ocean, or perhaps small islands here and there that weren't

completely covered in snow. And this is called a slush-ball earth variation of the theory, because it's thought that it was mostly frozen over, but not completely. It thought that this might have been important for the survival and evolution of very early forms of multicellular life, bearing in mind that the Cambrian explosion is less than 100 million years after the end of the cryogenes, and so there's early forms of life that are already developing, and they needed somewhere to survive. So if the entire Earth was covered in ice, that makes it a little bit difficult. Life can still survive in the oceans, but there's a question about how photosynthesis is possible if ice covers everywhere. So there's various hypotheses and

different discussions about how to make sense of that. But that gets a bit far outside of what I to discuss here. Anyway, so far we've covered two of these ice house periods, the very old Horonian glaciation about two and a half billion years ago, and then the cryogenian one, about 700 million years ago. After the end of the cryogenian, there was another greenhouse period which lasted until about 450 million years ago. This relatively short ice house glaciation period of about 30 million years, so much shorter than the previous ones, may have been caused by the evolution of land plants pulling a lot of carbon dioxide out of the atmosphere. So just as the Heronian glaciation may have been caused by the evolution of oxygenic photosynthesis,

in this case by microorganisms, there would have been bacteria at that time, because eukaryotes didn't develop until a little bit after that. This third glaciation called the Andean Saharan Glaciation may have been caused by land plants evolving and thereby dramatically increasing the amount of photosynthesis and pulling a lot more carbon dioxide out of the atmosphere. So the conclusion of the Andean Saharan Glaciation, there was another greenhouse period which ran until about 360 million years ago when there was yet another ice house period called the late Paleozoic Ice Age. So this occurred at the end of the Paleozoic era. So for those who are familiar, the Paleozoic era is that which preceded the Mesozoic era and the Mesozoic era was the time

when the dinosaurs lived. So the late Paleozoic Ice Age was just before the start of the dinosaurs essentially, or the emergence of the dinosaurs. It's thought that this ISAS period may have been related to the formation of the supercontinent Pangaea, supercontinent being a grouping together of all of the continents, or nearly all of them, in one very large landmass. And that occurred during the Paleozoic era when that supercontinent pangea was forming. It's thought that this may have been related to the reduction in greenhouse gas in the atmosphere as a result of silicate weathering processes, which I'll describe.

a little bit later. So we have this fourth ice house period which lasts basically right up until the start of the Triassic, so just ending just at the beginning of the time of the dinosaurs. And we then have another greenhouse period lasting right throughout the age of the dinosaurs and until about 34 million years ago. About 34 million years ago marks the beginning of the late Cenozoic ice house period. We are currently still in this ice house period. We are currently still in this ice house period. In fact, where in a particularly cold patch of it, sometimes called the Quartinary Ice Age. The way I'm using the terms here, an Ice Age is a particularly severe period of an ice house period when there are permanent ice caps on both of the poles. So the beginning of the late

Cenozoic Ice House period is marked by the emergence of a permanent ice cap in the southern hemisphere in Antarctica. Only with the development of a permanent ice cap in the northern hemisphere around the Arctic, do we enter into an ice age proper? And that occurred with a further reduction in average temperatures, which occurred about two and a half million years ago. So two and a half million years ago marks the beginning of the current ice age in the broader late Cenozoic ice house period. So to summarize, the history of Earth over the last two and a half billion years has been one of alternating between these greenhouse and ice house periods, spending most of the time in greenhouse periods, but notable periods in ice house as well.

And currently we're in actually an ice age, which began about two and a half million years ago. Now, I mentioned that I would explain a bit about this, because most people don't realize that we're currently in an ice age, especially with all the talk about global warming. So currently, the average global temperatures are quite low by world historical standards. throughout the vast majority, probably at least 90% of the past 500 or so million years, for which we have reasonable data, the average Earth temperatures have been substantially hotter than they currently are. So we're starting from a baseline of being quite cold.

Humans evolved about 100,000 years ago or so, but modern agriculture only began about 10,000 or so years ago. And that time when modern agriculture began, or around then, was a period of time when we were, when the Earth was just leaving or just exiting a glacial period, which is basically the coldest part of an ice age. Currently we are in what's called an interglacial period, so that's the time between the glacial periods, right? Hopefully that makes sense. The reason these occur is because of changes in the Earth's orbit about the Sun, which I'll discuss in a little bit later.

But basically, even when you're in an ice age, there are times where it's relatively colder and times when it's relatively warmer. I mean, they're all quite cold by comparison with Earth's history, but there's times when it's warmer and times when it's, you know, there's times when it's really cold and there's times when it's, you know, slightly less cold. And because we're in an interglacial period, we're in one of these slightly less cold periods. Now, that happened, as I said, about 10,000 years ago when we emerged from the last glacial period and into an interglacial period where it's a little bit warmer, but still in an ice age. However, because of anthropogenic global warming, the planet is quickly warming up.

And if we reach about two to three degrees of warming relative to the pre-industrial temperatures, then we will essentially reach temperatures that haven't been seen since about three million years ago, that is before the current ice age. So two to three, probably more like three degrees of warming relative to pre-industrial temperatures would put us at temperatures that are actually higher

than the ice age level temperatures. Still ice house level temperatures, right? So it's still relatively cool, but much higher than it's being. seen recently. To give a sense of the magnitudes that we're talking about here, relative to pre-industrial temperatures, the hottest that the Earth has been, at least in the last billion years or so, is probably about 15 degrees hotter. Here we're talking about average temperatures at the surface of the Earth. As for the coldest the Earth has been, it's

harder to find this information because that would have occurred probably during the cryogenian period, and there's not much information about that. But as far as I've been able to tell average global temperatures would likely have had to have been sub-zero, which puts at least 15 degrees lower than current temperatures. The current average global temperature is about 15 degrees Celsius, so 15 degrees less than that would put you at about zero. So you can think of it, for simplicity, we can imagine that currently the Earth is about 15 degrees Celsius. Again, this is on average over all of the surface of the Earth over the entire year. The hottest the Earth has been, you know, at least in sort of the last billion years or so, maybe has been

30 degrees Celsius on average, and the coldest have been it's maybe zero degrees or probably a bit below zero during the cryogenient. Now, that may not sound like a huge variation because you think, well, you know, a hot day in summer could easily be much hotter than 30 degrees. It could be 35 or 40 degrees, depending on where you live, whereas a cold day in winter could go sub-zero easily, right? So a zero to 30 degree range doesn't sound like that much. But remember that that's a faulty comparison because we're talking about these temperatures that I've been giving are averaged over the entire Earth over the entire year. And so even a one degree difference is actually quite a lot. So the total warming of the Earth caused by human emission of greenhouse gases in the past two centuries amounts to about 1.2 degrees Celsius.

And already because of that, relatively modest, or at least apparently modest amount of warming, we're seeing temperature record. records being broken pretty much every year. You can look up graphs and see just how much hotter it gets, like in summer and all of the other changes that are produced as a result of this, you know, a seemingly small change in average temperatures of a single degrees Celsius. Imagine then what difference it would make between pre-industrial temperatures and going up to 15 degrees Celsius hotter. That would be absolutely crazy by today's standards. Sure, there would still be some parts of the world, which would be relatively cool, or some seasons or certain days that would be relatively cool,

but there would also be those that were unbearably hot, and we probably couldn't survive in without modern technology to help us. And conversely, if the average temperature on Earth was about zero degrees Celsius, then there would be unbearably cold temperatures at many places and many times as well. So the point to be made here is that overall, the variation in Earth's temperatures, over the history of the climate is very large. And it's even large relative to the fairly substantial changes that have occurred in the past century or so

due to anthropogenic global warming. However, there's an important caveat here, which is that although the temperature of variation across Earth's history has been very large, it's also been very, very slow. So these greenhouse versus ice house periods that I've been talking about typically last hundreds of millions of years,

some of them even over a billion years, for some of the greenhouse periods. And even some of the shorter fluctuations that occur within an ice house or within a greenhouse period of time where the temperature might change by plus or minus four or five degrees, even those take millions of years. For example, just looking at the past 100 million years or so, the temperatures that occurred during the early Eocene period

about 50 million years ago of about plus 14 degrees relative to pre-industrial, temperatures, so extremely hot by today's standards, those gradually decreased over the subsequent few tens of millions of years until the Earth entered an ice house period about 30 million years ago. That change of a reduction in temperatures of perhaps 10 degrees Celsius took about 20 million years. Temperatures continue to decline over time until we entered the current ice age period about 2.5 million years ago. So that additional reduction of maybe 5 or 6 more degrees, took another 30 or so million years. Now, there were times in history where changes have been relatively faster than that.

But looking back at Earth's climactic history, at one degree change in average temperatures over 100 years, exceptionally rapid, almost unheard of. I say almost because there are certain conditions that can lead to very rapid changes in temperatures, such as asteroid collisions, which can produce a global winter that lasts a long period of time because of all the ash and dust thrown up into the atmosphere.

But those are usually quite transitory. They're because of very specific phenomena like volcanic eruptions or asteroid collisions that produce a small transient response, but then the climate goes back to normal within perhaps a few years or decades at most. That's not true with the anthropogenic global warming that's been happening over the past century or so, which will take thousands of years for the climate to adjust to. So it's not a transitory response in the same way. So just to recap a bit about where we're at, we've talked about the variation in Earth's temperatures between about plus 15 at the high end.

during the peak of the greenhouse periods and minus 15 at the low end relative to pre-industrial temperatures at the coldest parts of the ice ages during the ice house periods. We talked about how in the last 30 million years or so we've been in an ice house period and in the last two and a half million years we've been in an actual ice age with permanent ice caps at both poles. And we've also talked about how during this ice age period it alternates between even colder periods called the glacial periods where the glaciers expand and it gets colder and interglacial periods where the glaciers retreat and it gets a little bit warmer and these variations um on scales of maybe 50 to 100 000 years are the result of changes in the earth's orbit which i'll discuss in a moment

currently we're in an interglacial period since about 10 000 years ago or so and it's a little bit warmer relative to what it was previously but still quite cold overall relative to earth's history However, that's very rapidly changing as a result of anthropogenic global warming. Now, before we get on to talking about the mechanisms of climate change, I want to zoom in a little bit and look at the last 20,000 years, and then the last 2,000 years and the last 200 years. So we can understand the variation on a bit of shorter time span than we have been talking about, which is grand scale of hundreds of millions of years. So looking at the last 20,000 years or so, this is interesting because it represents the end,

of the last glacial period and the move into the interglacial period, which happened over a period of several thousand years. So the last glacial period, sometimes called the last ice age, although, as I said, that's misleading, because really we're still in an ice age, it's just we're in an interglacial period of an ice age. It started about 100,000 years ago, and ended about 12,000 years ago. But the ending is not sudden. I mean, it's relatively fast in geological terms, but it still takes a few thousand years. The last glacial maximum, when glaciers were at their largest and the earth was at its coldest, occurred at about 25,000 years ago.

At that time, Canada and large parts of the northern United States were almost completely covered in ice, as well as were large parts of the British Isles, Germany, Poland, and Russia, covered by the Scandinavian ice sheet and other parts of the world as well, also had very significant accumulations of ice, especially around mountainous areas. At the time of the last glacial maximum, the Earth's temperatures were perhaps six or even seven degrees colder than they were for the advent of anthropogenic global warming. Now, since that time, the Earth has progressively warmed as we came out of that glacial period and moved into the interglacial period. So over a period of perhaps six or eight thousand years, Earth's temperatures warmed by about five or six degrees. which is quite fast, as I've said in geological terms, although bearing in mind that this is not really a geological process, it's a fairly transient process that happens every 50 to 100,000 years as a result of variations in Earth's orbit.

But even so, we're talking about maybe a degree every thousand years. Now there was a very interesting period, which occurred about 13,000 years ago, called the Younger Dryas. this was a return or partial return to glacial conditions so it represented a kind of a deviation the earth had been warming up for a few thousand years moving away from the glacial period but during the younger dryas over a period of several hundred years the earth kind of reverted backwards temporarily

and became a bit colder again the name of the period is a little bit odd it's called the younger dryas because there have been a number of these periods that have been described and it's the most recent one so it's called the youngest, and Dryas is the name of a genus of flowers that gives us name to certain types of glacial sediments. But anyway, the name is not super important. What's interesting is that the start and end of this period were relatively rapid, maybe a few decades, and it affected temperatures across the globe, so it wasn't localized. However, some people have pointed to this as an instance

of very rapid natural climate change. It's important to understand that although the younger dry ice was a global phenomenon, so it did affect global temperatures, and it was fairly rapid in terms of its onset and offset, only a few decades. The actual change in average temperatures was quite small. This is sometimes confusing because people sometimes show graphs of air temperatures over Greenland, which did change very rapidly, up to 10 or 15 degrees within a single century, so very, very rapid changes in temperatures. But these have been extracted from ice core data that was collected from Greenland and refer only to the temperatures of the air over Greenland. That is not reflective of overall global temperatures, which, although they were quite variable

by regions, some were affected in different ways during the younger dryness. On average, warming was only a fraction of a degree, perhaps 0.2 or 0.3 degrees. So this is much smaller than the rising temperatures observed in the late 20th century, which is more than 1 degree. and this is about the best case we have, most rapid example we have in the last 20,000 years, of rapid changes in global temperatures driven entirely by natural processes. So although it is a very interesting period, it still doesn't really provide an example when temperatures have changed as rapidly as they have been over the last century or so.

There's been much discussion about what the cause of this was, so why did we have this partial reversion back to glacial conditions for, a few hundred years before the earth started warming up again. Originally, it was thought to have been caused by a shutdown in the North Atlantic circulation of warm tropical waters up north, which helped to keep north western Europe warmer than it would otherwise be. And this shutdown of the circulation in turn was thought to have been triggered by a massive deglaciation which occurred, resulting in a sudden influx of fresh water into the Caribbean,

thereby shutting down the warm circulation of tropical waters. by cooling the region of northwestern Europe in particular and around Greenland, which then had global effects. You know, it's kind of this nice story. However, in recent years, there's been a lot of pushback against this hypothesis because of a lack of any geological evidence for this catastrophic event happening. Also, the younger dryest onset was relatively rapid in terms of most climactic changes. You know, it occurred over a couple of decades, but it still seems a bit slower than would be expected by this sort of catastrophic deglaciation hypothesis. And so this hypothesis has fallen out of favor these days, although you may still see it

in all the references. As far as I can tell, there's not a lot of agreement on what caused the Ungadryas. People have proposed other events such as a meteor impact or volcanic eruption, but there doesn't seem to be a very clear answer as to what caused the Unger Dryas. Nevertheless, it does represent a very important natural fluctuation in the Earth's climate, and probably the best case of rapid, naturally driven changes in the past 20,000 years. But as we've seen, the overall global temperature change over a couple of decades was only a small fraction of one degree. And so it's not comparable to the magnitude of the change we've seen in the past century or so due to anthropogenic global warming. So, just to recap again,

25,000 years ago, we have the glacial maximum, and then a few thousand years after that, we see the progressive transition from the glacial to interglacial periods, so between about 18,000 years ago and 10,000 years ago, we see a gradual warming of the Earth by about maybe 6 or 7 degrees Celsius, so quite rapid in geological terms, but still fairly slow by contemporary terms in terms of their anthropogenic level warming. From about 10,000 years ago, with the start of the Holocene period, the beginning of agriculture, we reach roughly modern temperatures,

modern meaning pre-industrial of course, global temperatures stayed roughly constant from about 10,000 years ago up until about the Industrial Revolution. We can zoom in a bit on the very end of that period of time, so roughly the last 2,000 years where we have higher quality data. This is important because it provides us a baseline of natural variation

over a fairly short period of time, but a very relevant period of time because it represents much of the historical period and provides the immediate historical context for the recent changes which have occurred as a result of greenhouse gas emissions starting from the Industrial Revolution. Average global temperature over the past 2,000 years can be summarized with a couple of important events or periods across this time. In particular, the period of time from around 900 to 1,200, or some people say a bit earlier than that, but let's say 900 to 1,200, is sometimes called the Mediore. evil warm period. This was a time of relatively warm climate in the North Atlantic regions, especially around Europe, where there was unusually warm weather and particularly

good crop yields in many places. We know this not only from the climate reconstructions, but also from historical documentation from these time periods. Decades ago, some people argued that the medieval warm period may have actually been about as warm as the late 20th century, and therefore recent changes in temperatures are not actually unusual from a recent historic. standpoint. It's now known that this is not true. So the medieval warm period not only was very much colder than the late 20th century, early 21st century, but the medieval warm period, as shown by the most recent reconstructions, which use more representative global data, not relying so much on just northwestern Europe, has actually shown that the medieval warm

period was not a global phenomenon. While it was true that northwestern Europe and the Atlantic region more generally showed abnormally high temperatures, this was not true for the globe as a whole. So there was no actual increasing global average temperatures in this period really at all. So it was not a global phenomenon. Now following the medieval warm period, or a little bit afterwards, depending on exactly how you date things, perhaps from about 1500 to about 1800, was a period of time called the Little Ice Age. It's not very well named because remember we're still in an Ice Age and the Little Ice Age was not a return to. to the glacial period that we exited about 10,000 years ago, it's just a bit of a nickname.

And what it refers to is a reduction in temperatures. Again, this was originally noticed in the Atlantic area, so Europe and North America, where there were particularly many examples of very cold winters encroaching glaciers and frozen over ponds and rivers of areas that didn't typically freeze over. Crop practices throughout Europe had to be altered and because of the shorter and less reliable growing season and there were many periods of scarcity and famine. And unlike the medieval warm period, which turned out not to be a global phenomenon, but was more of a local, regional phenomenon, the Little Ice Age is a global phenomenon. That is, if we look at global average temperatures

over this sort of roughly 15 to 1800 period, we do see that they are slightly lower than temperatures before and temperatures afterwards. So the Little Ice Age represented a reduction in global temperatures of perhaps half a degree Celsius, probably. a bit less than that. The many of a warm period in the Atlantic probably reflected a similar increase in temperatures, but remember this was only regional, not a global increase in temperatures. Now this is interesting, remember, because when we're looking at the magnitude of temperature variations, the little ice age only represented a maybe half a degree Celsius reduction in average temperatures, but even that was enough to have very substantial effects on people

at the time in terms of the harvest, in terms of trade, in terms of rivers and lakes freezing over in terms of just people noticing that the weather was colder on average than it had been in previous centuries. And that's only half a degree Celsius. Remember that the recent anthropogenic induced global warming is more than one degree Celsius and that's over a shorter period. In the last 15 to 20 years many such reconstructions have been published using slightly different assumptions and methodology and you do see a bit of variation between them. But in substance they all show very much the same picture. Relatively constant global temperatures over the past 2,000

years, a slight downturn around the little ice age, maybe 15 to 1,800, and then a very rapid increase in the 20th century. There's really not a lot of doubt at this point about what the trajectory of temperatures looked like in the past 2,000 years, and that there really hasn't been any variation, even close to the magnitude that we've seen in the past 100 years or so. Now let's turn to the last period that we're going to look at, which is the past 200 years. The past 200 years is important because this is the period of time when we have access to instrumental measurements of global temperatures. Not quite 200 years, maybe 170, but close enough. So global instrumental temperature measurements date back to the 1850s. Of course in the

earliest few decades the thermometers and the coverage was not as good, so those aren't quite as reliable. The series has become more reliable as time goes on, but it's still accurate enough to be useful. These days, these measurements are collected from thousands of meteorological stations, boys, ships, weather balloons, satellites, and aircraft, all spread across the globe. Sometimes people attempt to argue that there are various biases or inaccuracies in these methods of data collection. For example, people have argued that a disproportionate number of the meteorological stations are located in urban areas, which are subject to an urban heat effect, which it is alleged has biased the temperature measurements upwards.

These effects are already well known about and have been corrected for, but also it doesn't really explain the trends that we observe because these measurements have been collected by a vast range of different methods, as I've just articulated, many of which are not subject to the urban heat effect at all. So all of these sources of data show a very consistent warming trend, which began in the early 20th century. As I said, that a little ice age lasted until maybe the early 19th century, and then there was a little bit of warming that happened in the 19th century, just to kind of bring the planet up a little bit out of the little ice age. But it's really since about 1900,

we've seen consistent rapid warming. And that warming's accelerated dramatically since the 1980s. As of the recording of this episode, so 2022, the total temperature increase relative to pre-industrial levels, so this is often fixed at 1750, although if you fixed it at 1850, it wouldn't make too much of a difference because there wasn't very much temperature change between then. A total temperature increase between pre-industrial levels and the present is about 1.2 degrees Celsius. Now something that's often pointed out is that there was a period between about 1940 and 1970 when global mean temperatures were roughly static or maybe even falling a tiny amount about 0.1 degrees Celsius. And there was a lot of comment from scientists that,

at the time as to why this might have been the case. Now we know that this was due to a combination of factors, including some natural forcing, but also a major factor was increasing emission of aerosols. So air pollutants, which introduced particulate matter into the atmosphere, which block out light, actually contributed a significant cooling effect in the atmosphere during that period of time. Now, since the 1970s, greenhouse gas emissions, especially CO2, have substantially increased. So greenhouse gas emissions were still significant in the early to mid-20s. century, but they become dramatically more significant in the late 20th and early 21st century.

At the same time, improved emission standards and other controls introduced to reduce the emission of aerosols have led to a reduction in overall aerosol levels. So whereas in the mid-20th century, say 1940 to 1970 or so, loosely speaking, there was a rough parity between emission of aerosols and emission of greenhouse gases. Greenhouse gases tend to make the earth warmer, aerosols block out or reflect the light and tend to make the Earth a bit cooler, those were in rough parity. So importantly, the greenhouse effect was still occurring. It's just that it was masked by this other effect. But since the 1970s or so, aerosol emissions have decreased significantly, at least relative to greenhouse gas emissions, which have increased dramatically. And so

instead of being balanced, the greenhouse gas emissions are now dramatically swamping the much small aerosol emissions, as well as natural forcings. And so that's why we've seen in the past 40 or 50 years, very dramatic increases in global temperatures. It's also important to understand that there's quite a lot of year-to-year variation in temperature change. So you can't really interpret temperature changes from one year to the next, or even over a small period of time. If you look at small periods of time, even up to five or ten years, you may not see any significant warming trend. That's because there is a lot of variation, both in human-induced factors and also natural forcings,

which lead to changes in year-to-year average global temperatures. In order to really understand the effect of anthropogenic global warming, you have to look over multiple decades. And then the trend is very, very clear. But some people focus on small variations one year to the next, or noise at the level of a few years. For example, people often focus on the year 1998, which featured a very large Al-Minio effect, which caused a spike in global temperatures, which was rather an outlier.

The years following that saw a return to more typical conditions, which are relatively cooler. And so if you look at the trend from 1998 till like 10 or so years after that, you might not see a very significant warming trend. But that's only by cherry-picking particular years and not looking at the overall trend, where it's very clear that 1998, as well as some other years, were outliers, and the broader warming trend is very evident. Other evidence as well supports the significant warming, particularly in the latter half of the 20th century.

So, for example, we have records of the extent of glaciers since the late 1940s from across the world, and what we find is that the vast majority of glaciers show a total loss in mass over that period of time. Obviously, there's change from year to year, and there are a few years on record. when there's an overall gain in mass, mostly earlier in the period. But overall what we see is that most years since 1950, in fact almost all years since 1950, on average glaciers around the world have lost mass, and as far as I know, every single year since 1990, that has been the case.

And furthermore, over that period of time, almost every glacier has lost mass. A few have gained mass due to local idiosyncratic differences in their particular climactic conditions, but the vast majority have lost mass. Now, we've talked extensively about the history of the Earth's climate, the degree of variation, and how that's changed and the rates of change, and we've talked about the methods that we use to reconstruct past climates, and we've talked at different levels of granularity about the changes in Earth's climate over millions of years, over tens of thousands of years, over thousands of years, and over the last 200 years.

The broad picture, to sum that up, is that currently we are in an interglation period of an ice age, average Earth's temperatures are significantly warmer than they were during the time of the glacial maximum, perhaps five or six degrees warmer than then, but still quite a lot colder than compared to most of Earth's history. However, when we look over the past 2000 years or so, we said that there was relatively little change in Earth's overall temperatures, perhaps a reduction of less than half a degree in the little ice age period of the 15th to 90s. 19th centuries roughly. But since around 1900, so at the start of the 20th century, we see a

very dramatic warming, with total warming since the pre-industrial period of about 1.2 degrees Celsius. The picture here, therefore, is of great variability in Earth's climate and particular average temperatures, but that variability mostly occurring over very long periods of time. Millions of years in case of the very large fluctuations across geological time, and thousands of years in the case of coming out of the glacial period. into the interglacial period. Temperature changes of on the order of one degree per century, which is what we've seen over the 20th century, are really unprecedented, at least in the last 20,000 years, and probably in the last million years. If current trends continue and we reach between

two and three degrees of warming, it's likely we will achieve global temperatures not seen before the beginning of the current ice age, so that is about three million years ago. So that concludes today's episode. Hopefully you found it interesting. If you enjoy this point, and would like to spread the word. I would appreciate a positive review on the aggregator or podcast app of your choice. You can also make a financial contribution to the podcast, and at the moment any financial contributions will go to support my editors who are helping to curate images that are combined with the audio podcast to create YouTube videos for the uploading of past episodes to YouTube. So we've already released a few episodes there. You can help to support

the effort of creating more of these videos by making a contribution through PayPal or becoming a Patreon supporter. I very much appreciate all of the generous contributions that have been made in the past, and they have certainly helped a lot to this of releasing YouTube-based content. You can also support that effort by just going to the YouTube channel and liking any of the videos or subscribing to the channel. You should be able to find that just by searching the Science of Everything podcast on YouTube. Finally, if you'd like to get in touch, you can send me email, my address is FOD12 at gm.com. That's FODDS12 at gmail.com. Thanks very much again for listening and I'll talk to you next time.