UQx DENIAL101x 3.4.3.1 Daily and yearly cycle

Scientists predict that human-caused global warming should result in certain specific patterns of warming. Because these patterns are consistent with what we expect to happen as a result of the increased greenhouse effect, they’re considered “fingerprints” of the human influence on the Earth’s climate. As far back as 1865, physicist John Tyndall predicted that warming caused by the increased greenhouse effect should cause nights to warm faster than days, and winters to warm faster than summers. He was able to make this prediction by knowing that at night and during the winter, the Earth’s surface cools by radiating heat out to space. Greenhouse gases trap some of this heat, slowing that nighttime and winter cooling. The sun doesn’t shine all the time, the greenhouse effect is at work 24/7. Additionally the moon gives us a good counter-example because it doesn’t have an atmosphere. During the day, there’s nothing between the Sun and the Moon’s surface to block incoming sunlight.

At night, there are no greenhouse gases to trap the outgoing heat from the Moon. As a result, the difference between day and night temperatures is extreme. Daytime temperatures on the moon reach 120˚ Celsius, or 250˚ Fahrenheit. Nighttime temperatures fall below minus 200˚ Celsius, or minus 330˚ Fahrenheit. At the other extreme, Venus has a runaway greenhouse effect, much bigger than the greenhouse effect on Earth. Its temperature is an intense 460˚ Celsius, or 730˚ Fahrenheit. It’s like this day and night, all year long. Venus doesn’t even have seasons because its greenhouse effect is so strong. As these two examples illustrate, the bigger the greenhouse effect, the smaller the difference between daytime and nighttime temperatures. We know humans are increasing the greenhouse effect on Earth by burning more and more fossil fuels. If the greenhouse effect is increasing, then the difference between nighttime and daytime temperatures, and between winter and summer temperatures, should be shrinking. There’s a common myth that global warming is caused by the Sun rather than humans.

That myth fails to account for the available evidence. If the Sun were responsible, we would see an entirely different pattern of global warming. In that scenario, we would expect to see the Earth warming most when sunlight is bombarding the surface the most – during the daytime, and during the summer season. That means that if the Sun were responsible, we would see days warming faster than nights, and summers warming faster than winters. These expected patterns of global warming give scientists a clear test to determine whether the evidence matches the fingerprints of human or solar-caused warming. It took over 130 years before John Tyndall’s prediction was confirmed, but over the last few decades, surface measurements have found nights warming faster than days, and winters warming faster than summers. The difference between nighttime and daytime temperatures, and winter and summer temperatures, is shrinking, just as Tyndall anticipated would happen due to the increased greenhouse effect.

Fingerprints in the Earth’s climate change, like these changes in global warming patterns, clearly point to humans, and not the Sun, as the culprit responsible for global warming over the past century..

Attack on science

Hayhoe: These days, to get attacked, all we have to do is step foot off campus and tell anybody, even a local Kiwanis club, or a local church, or even a group of elementary school kids, that climate change is real, and then the angry letters start to flood in. Mann: Typically the attacks are not really about the science. The attack on the science is a proxy for what is really an effort to discredit science that may prove inconvenient for certain special interests. Oreskes: That’s when I started getting attacked. And that was when life sort of changed, it was a bit going through the looking glass. I started getting hate e-mail. What happened then was I mentioned to a couple of colleagues what was going on, and one of my colleagues at Scripps, at the Scripps Institution of Oceanography, said to me, “You should talk to Ben Santer.

Something sort of similar happened to him.” Santer: I remember sitting in a bar in Madrid with Stephen Schneider, the late Stephen Schneider, immediately after the final sentence had been agreed on in the 1995 report, a sentence that’s forever engraved on my memory. The balance of evidence suggests a discernible human influence on global climate. Here we are at this bar, and Steve says to me, “This changes everything, you know. Your life is going to be changed forever.” I had no idea what he was talking about. I really didn’t. Hayhoe: There is definitely a pattern of what happens: nasty e-mails, complaints to your university, requests for your e-mails, and a lot of attacks online. Mann: Often it takes the form of an attack on individual scientists. It’s part of the strategy of ad hominem attack.

Santer: Go after the scientist. Go after their integrity. Go after their funding. Make life miserable for them. Mann: I have received letters in the mail that in one case contained a while powder that I had to actually report to the FBI. They had to come to my office and investigate this and send this off to a lab to make sure that it wasn’t anthrax or some very dangerous substance that my entire department would have been subject to because of this. Santer: Then there’s the power of the Internet, which really was not available back in 1995, to harness your supporters to go after individual scientists, send them threatening e-mails or worse, and let them know, “We’re watching you. We don’t like you. We don’t like what you do.

” Mann: One of the tactics that you see in climate change denialism is an effort to spin and misrepresent peer reviewed scientific studies. So often studies that say one thing, for example, show that some aspect of climate change is even worse than we thought, will somehow be spun by climate change deniers as if it doesn’t provide evidence for concern. Oreskes: Clearly misrepresenting scientific information, cherry picking scientific data, one egregious example that we talk about in the book is an early work by Jim Hansen that Bill Nierenberg, Bob Jastrow and Fred Seitz take out of context and use it to argue that climate change is caused by the sun when, in fact, if you go back to the original paper, Hansen is arguing exactly the opposite. Santer: I think an additional weapon in the arsenal is Freedom of Information Act requests, which are being used not really to advance understanding or, again, shed light on complex scientific issues but as a tactic to threaten, to intimidate, to throw a spanner in the works to take up your time.

Mann: They will bully editors to try to get them to retract articles that are a threat to their case, their case being that climate change isn’t real, it’s not something to worry Oreskes: The weirdest day of my whole life practically was the day I got a phone call from a reporter in Tulsa, Oklahoma ,who said to me, “Are you aware of the fact that Senator James Imhofe is attacking you?” [laughter] I was like, at that time, I honestly didn’t know who Senator Imhofe was. In fact, I think I had been to Oklahoma maybe once but, I mean, and so I said, “No, I have no idea.” At first I thought he was making a mistake, this was some other, well, I have a very unusual name, so it didn’t seem plausible it was some other Naomi Oreskes. And then he had, he read to me from this speech that Imhofe was making and it was part of what we all are very familiar with now that I was a part of the “global conspiracy,” the scientific conspiracy to bring down global capitalism. And I remember thinking, “Conspiracy?!? Scientists are not that organised.” Santer: hacking e-mails, releasing them, all of these things. The technology has moved on since 1995, but it’s the same playbook: don’t really focus on the science and advancing understanding, contributing, but tear down, destroy.

Hayhoe: I think the best we can do is shield ourselves from the attacks and try not to dwell on them, unless it’s a safety issue, in which case we should take appropriate steps, and try to move on, focusing on what we want to achieve rather than what’s trying to hold us back. Mann: So if you are a prominent scientist, if you participate in the public discourse, as I’ve often said, you better develop a thick skin because you will be attacked personally. Hayhoe: My number one rule of thumb is: do not Google myself. I don’t want to see. My number two rule of thumb is to not read the comments section. I don’t want to know. Oreskes: One of the things that I think is really important us that by writing about these things and by documenting about it in a scholarly way with high standards of documentation, we can explain to our colleagues, our institutions, editors at journal, and the public and the media what this is. Because this is not a scientific debate.

I mean if I have one message that’s what my message has been all along and it still is: this is not a scientific debate; it’s a political debate. But it’s a political debate being made to look like a scientific debate. We now know why people do that. Because it’s a very very effective strategy because if you can make people think it’s a scientific debate then people will think it’s too soon to act. But if people see the truth, if they realise that this is a political debate, that it’s related to people’s ideologies to their values, structures, that gives a whole different cast. So it’s very very important for people to understand the character of what this thing is. Santer: Some things are worth fighting for. That perhaps was the most profound lesson for me back then: that a clear public understanding of the science, doing the kind of thing that you’re doing here, that was truly worth fighting for..

Carbon cycle

House: The carbon cycle is, very simply, it’s about the cycling of carbon through natural systems – through plants, through soils, through the ocean – and back out into the atmosphere. Le Quéré: In the natural carbon cycle, there’s a lot of fluxes of carbon dioxide, so the carbon goes in and out of the ocean, in and out of the terrestrial biosphere every year. House: The carbon is constantly flowing between these different systems and large amounts of carbon moves all the time. Le Quéré: I mean in the terrestrial biosphere, in the trees and the forests, it’s very easy to see. If you live in a place that has a forest area with seasons, you see in the winter the trees they have no leaves, and the spring comes and the leaves build up. This is all good carbon dioxide that goes in the leaves. And in the fall and in the autumn when the leaves fall down then their carbon is emitted back in the atmosphere.

So you have a huge signal there of CO2 going in and out of the atmosphere. House: So the ocean will take up the CO2, it dissolves in the surface of the ocean and also when the ocean will release CO2 to the atmosphere and that depends on the concentration of CO2 in the atmosphere and the concentration of CO2 in the ocean. And they form a balance with each other. There’s a continuous massive exchange of carbon dioxide between the atmosphere on land and the atmosphere on the ocean. That is roughly in balance until we introduce human change. Osborn: The experiment that we’re inadvertently perhaps conducting with the climate system is to move huge volumes of carbon from these stores undergrounds in the form of fossil fuels and bringing them to the surface and burning them and adding this carbon to the atmosphere. Le Quéré: What we’re doing now is putting everything out of balance, so we’re adding carbon to the atmosphere. It’s new carbon. It’s not part of the natural cycle.

It’s one that we’ve dug out of the fossil reservoir where they were stored, and we’ve put them back in the atmosphere. This is new carbon, and it puts the system out of balance. House: Although the human emissions are much smaller than the natural fluxes, the natural fluxes approximately are in balance and so they’re not causing an increase of carbon dioxide in the atmosphere. The human emissions, however are very rapid, and the natural systems don’t have time to respond to them. And so you get a net imbalance of raised carbon dioxide concentrations in the atmosphere. Lunt: It’s unequivocal that the amount of carbon dioxide in the atmosphere is increasing and is increasing fast and is increasing faster than ever. House: Oh the rate of change now is incredibly rapid, and what’s more it’s pushed us outside the bounds of what we’ve seen in terms of atmospheric concentration throughout the Ice Ages. Thompson: We have not had levels of C02 at 400 parts per million by volume in 800,000 years of history. House: In the Earth’s past throughout in and out of the Ice Ages, the concentration of CO2 in the atmosphere ranged between about 180 parts per million to 280 parts per mission.

And it took thousands of year for it to change between those states. The difference is now it’s gone up to 350 and even topping 400 parts per million on a single day basis. And that’s happened over a period of a couple hundred years. Friedlingstein: Every single generation is emitting more than the previous generation because emission of CO2 increased exponentially. We emit it so far, if you start from the beginning, which is like the industrial revolution in 1750 or something, when we start to burn fossil fuel, from that time up until today we emitted something like 2000 gigaton of CO2. More than half of this has been emitted over the last 50 years. Thompson: And we know where that CO2 is coming from because we do the isotopes of the carbon. We know it’s coming from fossil fuels. Le Quéré: So carbon is increasing in the atmosphere, but it doesn’t entirely stay there, so about half of the emission and maybe a bit more than half of the emission that we put in the atmosphere ends up in the natural environment. It ends up in the ocean and in the forest. Friedlingstein: For the carbon cycle today absorbed about half of the emissions we put in the atmosphere, so we emit, as I said, 40 gigaton of CO2 per year, about half of it, 20 gigaton of CO2 are taken back from the atmosphere by the land and by the ocean.

House: There’s a multitude of different processes that remove carbon dioxide from the atmosphere. So for example, CO2 from the atmosphere dissolves in the surface of the ocean and then that’s turned over and taken into the deep ocean. Really for that amount of CO2 to be completely removed from the atmosphere it has to be completely dissolved and go down into the deep ocean. And then we’re talking about geological timescales – so hundreds and thousands of years. Le Quéré: So what happens when we put carbon emissions into the atmosphere, new carbon from burning fossil fuel or from different station, what happens is this takes a long time for this carbon to readjust in the land and ocean. Eventually if we’re prepared to wait long enough, so that’s thousands of years, a lot of this carbon, maybe 70 percent will end up in the ocean, and the reason this takes time is that you have different adjustment times, so the CO2 goes in the surface ocean, it takes about 1 year to dissolve. But how it is transported from the ocean’s surface to the intermediate and to the deep ocean depends on the ocean circulation.

The ocean circulation takes hundreds to a thousand years to mix the entire ocean. That’s the timescale that is really relevant here is taking a molecule of CO2, we’ve put it in the atmosphere, how long is it going to take before it ends in the deep ocean? House: So about 65 to 80 percent of the carbon dioxide pulse that’s put into the atmosphere will be removed within about 2 to 200 years. The rest of it, the remaining 35 percent, will take between 2 and 20 millennia to be completely removed from the atmosphere. So roughly you have to think whatever we’re doing today, whatever CO2 is being emitted, roughly a third of it is going to stick around essentially forever really when you consider it in our lifetime. Pelto: We can’t change the atmosphere, the chemistry, with one of the main constituents carbon dioxide by 25 percent and expect nothing to happen. You change your diet by 25 percent. You decide you’re going to start consuming 25 percent more calories, and you don’t change your exercise or anything else. You can’t realistically expect nothing to happen. And that’s what you have to understand.

If we change fundamentally our atmosphere chemistry, we can’t expect climate to stay the same..

Climate change through geological time

During this course we’ll examine the problem of climate change through geologic time, as revealed in the field of paleoclimate. We’ll look at records like this that span, in this case, the history of the planet. Going back about 4 and 1/2 billion years. On this particular diagram, we see a proxy, shown here by this black curve, for the volume of ice on the planet. Going forward from 4.6 billion years ago to the present here, notice that this time scale is not linear. For example, the first five billion years of Earth’s history is compressed into this region of the diagram. One sees that the volume of ice on the planet has varied greatly through its history. There were some early glaciations, for example, quite a few of them, centered around 550 million years ago here. Some very interesting times, for example, here during which 90 percent of the marine life died, and there was a high volume of ice on the planet. And on the other hand, a very warm period from 80 million years ago, or so, spanning to 40 million years. After which time, the volume of ice on the planet slowly increased.

These fluctuations, you see at the very end of the record, are the fluctuations associated with the great glacial interglacial cycles of the last three million years. Why did the climate vary this way? How come they were glaciations early in the Earth’s history? A period of almost no ice, and then back to an ice climate. The Earth’s climate history abounds with interesting problems and paradoxes. One of the most well known is called the Faint Young Sun Paradox. Basically, a question of why wasn’t the early Earth frozen. Here we see a graph going back to the beginning of the Earth, that shows an estimate of the solar output as a fraction of today’s value. So this ratio is one today, but it was only about, a little bit more than 0.7 in the beginning of the Earth’s history. And what you see here is an estimate from very elementary climate model considerations of the Earth’s surface temperature. Making certain assumptions, like the composition of the atmosphere was constant through this time.

Which is almost, certainly wasn’t. But according to this calculation, before roughly 2 billion years ago, the Earth’s main surface temperature was below freezing. And certainly, three or four billion years ago, the Earth’s surface temperature should have been so cold that there would have been no liquid water on the planet. Yet, geological evidence is very clear on the point that there was plenty of liquid water early in Earth’s history. So why was it that, in spite of this lower solar output, the Earth’s climate was not so cold? There’s evidence that the Earth went through a cycle of extreme climate swings, centered about 550 million years ago. These climate swings may have been so extreme, that there were periods of time when the Earth was essentially a snowball covered with ice, reflecting all incoming sunlight. But alternating between this, and states, were the Earth was completely free of ice. So the snowball Earth landscape might have looked something like this. Whereas, the hothouse Earth might have looked something like this, even at the poles. The Earth’s climate has gone through these extreme swings.

Why did this happen, and why was it focused about 550 million years ago? We’ll also talk about the problem of the Earth’s very warm climates. For example, during the Cretaceous and the Eocene, there’s very little evidence of ice at either of the poles. And the mean temperature in the Arctic could have been as warm as 20 degrees C in the annual average. We have fossil remains of reptiles, like this beast here, in places like Greenland. Where they certainly don’t exist today. What explains the warmth of this period of Earth’s history? We’ll also talk about the use of proxies for making deductions about the Earth’s temperature and other properties, such as, the volume of ice on the planet, going back in time.

This shows a proxy that consists of the ratio of two isotopes of oxygen in deep sea marine organisms, or their fossils, going back 65 million years. This is a good proxy for the volume of ice on the planet and also for the temperature of polar oceans. And shows that the temperature, or inverse ice volume, reached a maximum in the early Eocene period, about 50 million years ago. And broadly speaking, has been on the decline since then. Although, with interruptions. For example, there’s evidence that about 25 million years ago, there was an abrupt thawing of ice in Antarctica. And then it became reglaciated about 15 million years ago. Since that time it’s been a steady downward slide. But with these large oscillations toward the end of the record, which we know as the great glacial interglacial cycles. Why did the climate behave this way? Notice, as well, there is a spike in temperature somewhat more than 55 million years ago, known as the Paleocene-Eocene Thermal Maximum, where the temperatures really got hot, really fast.

What caused this to happen? Can we understand that? Now, if we focus on a much more recent period of the Earth’s history, a tiny sliver of the graph you just saw, we get marvelous records of the Earth’s climate behavior from ice cores in the Arctic and in the Antarctic. Here, for example, is a record of temperature constructed from oxygen isotope proxies going back 450,000 years in Vostok, in Antarctica, as well as a proxy measure of global ice volume. You can see that all three of these records co-vary so that ice volume, and the proxy for temperature co-varied with spikes in temperature occurring roughly every 100,000 years in between which the climate was quite cold. This is when the great ice sheets covered parts of North America and Europe, for example. Whereas right at the moment, we’re living in a uniquely stable and warm period of time, called the Holocene, for the last 10,000 years, or so, there have been other warm interglacials like here, here, and here. Why did the climate vary in this almost cyclical way? And what does that imply about the climate of the future? Here is a reconstruction of what the ice cover might have looked like at the peak of the last glacial period about 18,000 years ago when there were huge sheets of ice covering much of North America and Eurasia and bits of South America and Australia as well.

So New York and Boston were probably under a mile or two of ice 18,000 years ago. 18,000 years is the blink of an eye, geologically. How could this happen? When will it happen again? Accompanying these changes in ice volume are very large changes in sea levels. This is a reconstruction of sea levels around the world, going back 24,000 years. So if we go back then, we see that sea level, relative today, was about 120 meters, almost four hundred feet lower. At the end of the last glacial period, sea levels began to rise as the ice melted. They rose 120 meters, stabilizing at their current values, about 7,000 years ago. You’ll see that the last 7,000 years have been spectacularly stable. Both in sea level, and actually in temperature, as well. It is not an accident that this is the period over which human civilization has thought to have developed. Now if we focus on the last 2,000 years, we get a record that looks like this. What you see on this graph is the temperature, relative to its 20th century average, reconstructed by various different research groups, using different proxies.

So these different colored curves represent different estimates. And, naturally because their proxies, and they are not perfect measurements of temperature, they disagree with each other. Nevertheless, they show a similar pattern. A gradual warming over the first 1,000 years of the record, to a maximum called the Medieval Warm Period. This is the time when Greenland, for example, was briefly settled. Followed by a decline in temperature to a broad minimum, over the period between about 1600 and 1850. This is known as the Little Ice Age. After that, temperatures began to rise quite rapidly, as revealed, both by the proxy records, and by actual measurements with thermometers; the instrumental record. And you can see the temperature here, in 2004, was quite a bit larger than what we have seen over the last 2,000 years according to these proxies. Why did the temperature vary this way? Why the temperature increase for the first 1,000 years, then start decline again, and then suddenly go up towards the end of the record? Here’s a reconstruction over the same period of time of temperatures in the Arctic. Once again, we see reconstructions from various proxies, including ice cores and tree rings in blue.

With uncertainty represented by the gray shading. Where as this red line here, at the end, shows the instrumental record. And we see a similar pattern, although not much evidence of the Medieval Warm Period in this record. But a gradual descent of temperature reaching a minimum around the year 1800. And then rising quite rapidly after that. What explains this temperature record? Now, if we focus on the instrumental record, which dates back only about 150 years. Here’s a graph showing the annual mean of temperature, according to several different groups, relative to its average over the period 1961 to 1990. You can see that despite some small disagreements among these groups estimates of the temperature, they basically show that the temperature was fairly level until about 1921, when it started to go up. Reaching a maximum in the 1940s. But then declining slightly into the ’60s and ’70s, and then rising quite rapidly afterwards. This is an interesting variation of climate, as reflected by temperature.

And we like to know why this is happening. And will it continue to happen? .

Making sense of the slowdown

The Earth’s climate is controlled by the energy balance at the top of the atmosphere. If more heat enters the atmosphere than leaves, then the planet warms. Adding heat trapping gases changes the balance, which in turn causes warming. Ocean heat measurements show that the planet is indeed absorbing heat. Despite this fact, it is often claimed that the global warming has stopped. This claim is inspired by evidence that warming of the atmosphere has been slower over the past one and a half decades. This slowdown is sometimes called the hiatus. However, there are other factors which affect the atmosphere over shorter periods. These can cause faster or slower warming of the atmosphere. To understand the slowdown in warming, we need to understand some of these factors.

If we look at the global surface temperature over the past 3 decades, there are big changes in temperature from year to year. We know the cause of some of these variations. One of the biggest is the El Nino cycle. El Nino is a phenomena in which heat is stored up in the western Pacific Ocean, and then released to the atmosphere in the eastern Pacific. This happens over the course of a few years. El Nino is not predictable, but we can track it in retrospect through sea surface temperature measurements. If we compare past El Nino cycles with temperature changes over the past three decades, we can see that there is a strong relationship between the two. El Nino years tend to be hot years. Recent years have been dominated by the cool phase of the cycle. This is responsible for some of the slowdown in warming. However, El Nino doesn’t explain everything. There are cooler periods in the early eighties and nineties which don’t fit the El Nino cycle.

These were caused by two major volcanic eruptions, El Chichon and Pinatubo. Dust from the volcanoes spread in the upper atmosphere, cooling the surface. Smaller eruptions happen all the time, but can also affect temperatures. There has been an increase in the number of small eruptions over the past few years, offsetting a bit of the greenhouse warming. Another factor is the solar cycle. Satellites tell us that the sun varies in brightness with the sunspot cycle. The last cycle has been particularly weak. A dim sun also offsets a little bit of warming. Yet another factor is pollution. Rapid industrialisation in Asia has led to more particulate pollution in the atmosphere, which also has a cooling effect. The final factor is in the observations themselves. Two of the major temperature data providers, the UK Met Office and NOAA, don’t include the Arctic in their global temperature calculation, because there are no weather stations there.

But the Arctic has been warming faster than anywhere else on the planet. Missing it out leads to an underestimation of the rate of warming. To recap, greenhouse gases have continued to grow over the last one and a half decades. But over the same period, volcanoes, the weak sun and pollution have had a cooling effect, and the rate of warming has been underestimated as well. Two recent studies have put all of these together. If we ignore the short term influences, climate models predict faster warming than we have observed. However, if we use global temperature estimates, and add the influence of El Nino, volcanoes, the weak sun and pollution into the models, then the agreement is good. What can we conclude from this? When we put everything we know into the models, the answers match what we observe. So the slowdown in warming makes sense in retrospect, and doesn’t give us a reason to doubt the models.

However, we couldn’t have predicted it in advance, because we can’t predict volcanoes, pollution or the sun. The slowdown in warming has created a whole family of myths with different levels of sophistication. At one extreme, it is possible to argue that the hiatus should reduce our estimates of climate sensitivity. This is a genuine scientific argument, although the analysis we have just seen suggests that no reduction is required. At the other extreme, it is sometimes claimed that the hiatus disproves the role of CO2 in global warming. They claim that CO2 has increased, but the world hasn’t warmed. This is an example of a strawman, and a complex cause fallacy. Climate science doesn’t claim that CO2 is the only factor which affects temperature. This is why the hiatus is so hard to deal with. The myths may be wrong, but they are simple and convincing. The complex cause fallacy exists because people like things to be simple, but explaining the complex drivers of climate is hard. But in the end, all the hiatus myths revolve around drawing attention away from the big picture. When we look at the big picture, the hiatus does not change our understanding of human caused global warming.

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Human CO2 emissions trump volcanoes’

In the past 150 years, human emissions have put a lot of carbon dioxide in the air. We now measure a concentration of about 400 parts per million. This is about 40% higher than at any time in the past 400,000 years. Of all of the conclusions of modern climate science; this is one of the most reliable. But, despite all of the evidence, some people persist in claiming that the recent rise in carbon dioxide is all natural— for example, they say that instead of it being caused by humans, it all came out of volcanoes. Now, it is quite true that volcanoes emit some carbon dioxide. Over very long periods of geological time those small amounts can add up to make a really significant change to the atmosphere.

However, over a couple of hundred years, the emissions aren’t large enough to make a difference. There are two main classes of volcano: there are the ones that erupt under the ocean and the ones that erupt into the air. Both kinds are linked to the goings-on at the boundaries of the tectonic plates and to the upwelling of hot rock from the Earth’s mantle; the layer below the Earth’s crust. The undersea volcanoes are by far the more numerous, making up about 90% of the world’s volcanoes, although few of us have ever seen them. These volcanic chains are where new ocean crust is produced. But undersea volcanoes don’t produce very much carbon dioxide—only about 100 million tonnes per year—about the same amount as an average US state emits. Humans produce about 350 times as much carbon dioxide as the undersea volcanoes do. Carbon dioxide not only gets produced at the oceanic ridges, it also gets consumed there. What happens is that the newly formed basaltic rock undergoes chemical changes when it contacts seawater. This reaction absorbs carbon dioxide from the water at a rate of about 150 million tonnes per year.

The mid-ocean ridge volcanic processes as a whole, therefore, probably consume more carbon dioxide than they emit. We are much more familiar with the kind of volcanoes that erupt into the air. The biggest chain of these is the so-called “Pacific Ring of Fire”. This is a belt running all the way around the ocean from New Zealand to Japan, then to Alaska and down to the Andes. Old oceanic crust is consumed at these places and they form volcanoes that produce much more carbon dioxide than the ones under the sea. The magma in these volcanoes comes not just from the Earth’s mantle, but also from the melting of the more carbon and water-rich rocks in the crust. One reason these types of volcano tend to be more explosive is because of the larger amount of water vapour and carbon dioxide in their magma. Mount Etna in Sicily is one of the most prolific carbon-dioxide producing volcanoes in the world. It produces about 13 million tonnes per year, but this amount is still only about half as much as what Sicily’s five million people emit from burning fossil fuels.

In addition, dormant volcanoes and volcanic lakes together emit as much carbon dioxide as the actively erupting volcanoes do. Altogether, volcanoes that emit carbon dioxide into the air produce much more than undersea volcanoes: about five times as much. Volcanic rocks on the surface undergo weathering and this chemical process absorbs carbon dioxide out of the air, about 180 million tonnes per year, that’s approximately one-third of the amount put into the air by volcanoes. So if we add up all the sources of volcanic carbon dioxide, we get 640 million tonnes per year. Once we subtract the carbon dioxide that the reactions with volcanic rocks consume, we are left with a net 310 million tonnes per year. This last amount is roughly equal to the human emissions from the country of Turkey, that’s less than one percent of all human emissions. Human emissions for the planet as a whole in 2012 were 60 to 120 times bigger than volcanic emissions. Carbon dioxide emissions from cement-making alone are 3 to 6 times bigger than those from volcanoes.

Not only are volcanic emissions much too small to account for the rising carbon dioxide levels in the air, but, over the past few thousand years, natural emissions and natural sinks must have been in rough balance. The carbon dioxide composition of the air started to change really quickly after the 1950s. We can readily explain this as being due to the greatly increased rate of consumption of fossil fuels after the end of the Second World War. On the other hand, if volcanoes had suddenly started to erupt many times faster in the second half of the twentieth century, we surely would have noticed. After all, volcanoes don’t just silently produce carbon dioxide, they also throw out huge quantities of ash and magma and they often cause havoc for humans living nearby. Only about 40% of the carbon dioxide emitted from any source remains in the air, the rest goes into the oceans and is taken up by plants on land.

If we add up the carbon dioxide emissions and convert them into concentrations in the air, we see that emissions from humans over the past hundred years fit the observations like a glove, but the volcanoes don’t even come close. People who incorrectly blame volcanoes for the change in the air take the fact that volcanoes do indeed produce some carbon dioxide and then they jump to the false conclusion that this amount is enough to explain the increase we have measured. And they haven’t done the basic arithmetic that shows that it isn’t nearly enough to make any real difference at all in such a short time period. We know what caused the recent rise in carbon dioxide concentrations. We did..