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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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What does past climate change tell us about global warming?

What the science says...

Select a level... Basic Intermediate

Greenhouse gasses, principally CO2, have controlled most ancient climate changes. This time around humans are the cause, mainly by our CO2 emissions.

Climate Myth...

Climate's changed before

Climate is always changing. We have had ice ages and warmer periods when alligators were found in Spitzbergen. Ice ages have occurred in a hundred thousand year cycle for the last 700 thousand years, and there have been previous periods that appear to have been warmer than the present despite CO2 levels being lower than they are now. More recently, we have had the medieval warm period and the little ice age. (Richard Lindzen)

At a glance

Just imagine for a moment. You fancy having a picnic tomorrow, or you're a farmer needing a dry day to harvest a ripe crop. So naturally, you tune in for a weather-forecast. But what you get is:

“Here is the weather forecast. There will be weather today and tomorrow. Good morning.”

That's a fat lot of use, isn't it? The same applies to, “the climate's changed before”. It's a useless statement. Why? Because it omits details. It doesn't tell you what happened.

Climate has indeed changed in the past with various impacts depending on the speed and type of that change. Such results have included everything from slow changes to ecosystems over millions of years - through to sudden mass-extinctions. Rapid climate change, of the type we're causing through our enormous carbon dioxide emissions, falls into the very dangerous camp. That's because the faster the change, the harder it is for nature to cope. We are part of nature so if it goes down, it takes us with it.

So anyone who dismissively tells you, “the climate has always changed”, either does not know what they are talking about or they are deliberately trying to mislead you.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further Details

Past changes in climate, for which hard evidence is preserved throughout the geological record, have had a number of drivers usually acting in combination. Plate tectonics and volcanism, perturbations in Earth's slow carbon cycle and cyclic changes in Earth's orbit have all played their part. The orbital changes, described by the Milankovitch Cycles, are sufficient to initiate the flips from glacials (when ice-sheets spread over much of Northern Europe and the North American continent) to interglacials (conditions like the past few thousand years) and back  – but only with assistance from other climate feedbacks.

The key driver that forces the climate from Hothouse to Icehouse and back is instead the slow carbon cycle. The slow carbon cycle can be regarded as Earth's thermostat. It involves the movement of carbon between vast geological reservoirs and Earth's atmosphere. Reservoirs include the fossil fuels (coal/oil/gas) and limestone (made up of calcium carbonate). They can store the carbon safely over tens of millions of years or more. But such storage systems can be disturbed.

Carbon can be released from such geological reservoirs by a variety of processes. If rocks are uplifted to form mountain ranges, erosion occurs and the rocks are broken down. Metamorphism – changes inflicted on rocks due to high temperatures and pressures – causes some minerals to chemically break down. New minerals are formed but the carbon may be released. Plate tectonic movements are also associated with volcanism that releases carbon from deep inside Earth's mantle. Today it is estimated by the U.S. Geological Survey that the world's volcanoes release between 180 and 440 million tonnes of carbon dioxide per year - as opposed to the ~35 billion tonnes we release.

Epic carbon releases in the geological past

An extreme carbon-releasing mechanism can occur when magma invades a sedimentary basin containing extensive deposits of fossil fuels. Fortunately, this is an infrequent phenomenon. But it has nevertheless happened at times, including an episode 250 million years ago at the end of the Permian Period. In what is now known as Siberia, a vast volcanic plumbing-system became established, within a large sedimentary basin. Strata spanning hundreds of millions of years filled that basin, including many large coal, oil, gas and salt deposits. The copious rising magma encountered these deposits and quite literally cooked them (fig. 1).

Fig. 1: schematic cross section though just a part of the Siberian Traps Large Igneous Province, showing what science has determined was going on back then, at the end of the Permian Period.

Now laden with a heavy payload of gases, boiled out of the fossil fuel deposits, some of the magma carried on up to the surface to be erupted on a massive scale. The eruptions – volcanism on a scale Mankind has never witnessed - produced lavas that cover an area hundreds of kilometres across. Known as the Siberian Traps, because of the distinctive stepped landforms produced by the multiple flows, it has been calculated that the eruptions produced at least three million cubic kilometres of volcanic products. Just for a moment think of Mount St Helens and its cataclysmic May 1980 eruption, captured on film. How many cubic kilometres with that one? Less than ten.

Recently, geologists working in this part of Siberia have found and documented numerous masses of part-combusted coal entrapped in the lavas (Elkins-Tanton et al. 2020; fig. 2). In the same district are abundant mineral deposits formed in large pipes of shattered rock as the boiling waters and gases were driven upwards by the heat from the magma.

Fig. 2: an end-Permian smoking gun? One of countless masses of part-combusted coal enclosed by basalt of the Siberian Traps. Photo: Scott Simper, courtesy of Lindy Elkins-Tanton.

It has been calculated that as a consequence of the Siberian Traps eruptions, between ten trillion and one hundred trillion tons of carbon dioxide were released to the atmosphere over just a few tens of thousands of years. The estimated CO2 emission-rate ranges between 500 and 5000 billion tonnes per century. Pollution from the Siberian Traps eruptions caused rapid global warming and the greatest mass-extinction in the fossil record (Burgess et al, 2017). There are multiple lines of hard geological evidence to support that statement.

We simply break into those ancient carbon reservoirs via opencast or underground mines and oil/gas wells. Through such infrastructure, the ancient carbon is extracted and burned. At what rate? Our current carbon dioxide emissions are not dissimilar to the estimated range for the Siberian Traps eruptions, at more than 3,000 billion tons per century. The warning could not be more clear. Those telling you the climate's changed before are omitting the critical bit – the details. And when you look at the details, it's not always a pretty sight.

Last updated on 14 February 2023 by John Mason. View Archives

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Further reading

RealClimate article published by Prof. Stefan Rahmstorf on July 20, 2017:

The climate has always changed. What do you conclude?


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Comments 26 to 50 out of 420:

  1. David Only the Australian mammals? I was not aware of particularly large mammals in Australia. As for all the continents except Africa, what about the Indian subcontinent?
  2. "I was not aware of particularly large mammals in Australia" - no they became extinct some 25,000 years ago. It was mainly mammals, but did include some giant emu-like birds, and some giant reptiles. India I'm not sure about. There were extinctions in South East Asia though, as well as the Americas of course. The equivalent large animals of Africa (elephant, rhino, giraffe, lion etc) all survived for reasons which are debated. I think it is because Africa straddles the equator and climate change therefore always left some refuge areas.
  3. David Re: "but did include some giant emu-like birds, and some giant reptiles." Yes I am familar with these and I recall a large carniverous "roo" as well. The reptiles are / were varinids and there are claims made to their survival. South East Asia and the Indian subcontinent still have their large species (elephants, bengals and 8 to 10 meter pythons (with claims to larger). North America and Eurasia definately did not fare well, I agree. South America lost some of it's megafauna but they were not truely giants and many survived (again with rumors of still more). So it seems that the most poleward areas were hit hardest, no?
  4. "So it seems that the most poleward areas were hit hardest, no?" Well no, the hardest hit were those where a shift in climate to hotter drier times led to environmental conditions that large species couldn't cope with. And in addition on the continents whose geography precluded the formation of refuge areas - which is where Africa, straddling the Equator, comes in - whichever way the climate zones move in Africa you are always left with areas that can support megafauna. I am really not sure why you think that cooling conditions causes extinctions unless you think that this means global warming is a good thing. If that is the case you are going to be sadly disappointed.
  5. David Climate changes ineither direction causes extinction events. But it is a matter of degree. Warming opens up new environments at the same time as it makes existing ines more harsh. What happens is that life follows the environment. If it gets warmer life shifts poleward and colder it shifts towards the equator. This paper might help: Long-Term Cycles in the History of Life: Periodic Biodiversity in the Paleobiology Database Adrian L. Melott* Department of Physics & Astronomy, University of Kansas, Lawrence, Kansas, United States of America Abstract Time series analysis of fossil biodiversity of marine invertebrates in the Paleobiology Database (PBDB) shows a significant periodicity at approximately 63 My, in agreement with previous analyses based on the Sepkoski database. I discuss how this result did not appear in a previous analysis of the PBDB. The existence of the 63 My periodicity, despite very different treatment of systematic error in both PBDB and Sepkoski databases strongly argues for consideration of its reality in the fossil record. Cross-spectral analysis of the two datasets finds that a 62 My periodicity coincides in phase by 1.6 My, equivalent to better than the errors in either measurement. Consequently, the two data sets not only contain the same strong periodicity, but its peaks and valleys closely correspond in time. Two other spectral peaks appear in the PBDB analysis, but appear to be artifacts associated with detrending and with the increased interval length. Sampling-standardization procedures implemented by the PBDB collaboration suggest that the signal is not an artifact of sampling bias. Further work should focus on finding the cause of the 62 My periodicity.
  6. ps This agrees with earlier work done at Berkeley, from a 2005 article in the San Francisco Chronicle: "Mass extinction comes every 62 million years, UC physicists discover" David Perlman, Chronicle Science Editor Thursday, March 10, 2005
  7. I did not copy the link but I did keep the article: With surprising and mysterious regularity, life on Earth has flourished and vanished in cycles of mass extinction every 62 million years, say two UC Berkeley scientists who discovered the pattern after a painstaking computer study of fossil records going back for more than 500 million years. Their findings are certain to generate a renewed burst of speculation among scientists who study the history and evolution of life. Each period of abundant life and each mass extinction has itself covered at least a few million years — and the trend of biodiversity has been rising steadily ever since the last mass extinction, when dinosaurs and millions of other life forms went extinct about 65 million years ago. The Berkeley researchers are physicists, not biologists or geologists or paleontologists, but they have analyzed the most exhaustive compendium of fossil records that exists — data that cover the first and last known appearances of no fewer than 36,380 separate marine genera, including millions of species that once thrived in the world’s seas, later virtually disappeared, and in many cases returned. Richard Muller and his graduate student, Robert Rohde, are publishing a report on their exhaustive study in the journal Nature today, and in interviews this week, the two men said they have been working on the surprising evidence for about four years. “We’ve tried everything we can think of to find an explanation for these weird cycles of biodiversity and extinction,” Muller said, “and so far, we’ve failed.” But the cycles are so clear that the evidence “simply jumps out of the data,” said James Kirchner, a professor of earth and planetary sciences on the Berkeley campus who was not involved in the research but who has written a commentary on the report that is also appearing in Nature today. “Their discovery is exciting, it’s unexpected and it’s unexplained,” Kirchner said. And it is certain, he added, to send other scientists in many disciplines seeking explanations for the strange cycles. “Everyone and his brother will be proposing an explanation — and eventually, at least one or two will turn out to be right while all the others will be wrong.” Muller and Rohde conceded that they have puzzled through every conceivable phenomenon in nature in search of an explanation: “We’ve had to think about solar system dynamics, about the causes of comet showers, about how the galaxy works, and how volcanoes work, but nothing explains what we’ve discovered,” Muller said. The evidence of strange extinction cycles that first drew Rohde’s attention emerged from an elaborate computer database he developed from the largest compendium of fossil data ever created. It was a 560-page list of marine organisms developed 14 years ago by the late J. John Sepkoski Jr., a famed paleobiologist at the University of Chicago who died at the age of 50 nearly five years ago. Sepkoski himself had suggested that marine life appeared to have its ups and downs in cycles every 26 million years, but to Rohde and Muller, the longer cycle is strikingly more evident, although they have also seen the suggestion of even longer cycles that seem to recur every 140 million years. Sepkoski’s fossil record of marine life extends back for 540 million years to the time of the great “Cambrian Explosion,” when almost all the ancestral forms of multicellular life emerged, and Muller and Rohde built on it for their computer version. Muller has long been known as an unconventional and imaginative physicist on the Berkeley campus and at the Lawrence Berkeley Laboratory. It was he, for example, who suggested more than 20 years ago that an undiscovered faraway dwarf star — which he named “Nemesis” — was orbiting the sun and might have steered a huge asteroid into the collision with Earth that drove the dinosaurs to extinction. “I’ve given up on Nemesis,” Muller said this week, “but then I thought there might be two stars somewhere out there, but I’ve given them both up now.” He and Rohde have considered many other possible causes for the 62- million-year cycles, they said. Perhaps, they suggested, there’s an unknown “Planet X” somewhere far out beyond the solar system that’s disturbing the comets in the distant region called the Oort Cloud — where they exist by the millions — to the point that they shower the Earth and cause extinctions in regular cycles. Daniel Whitmire and John Matese of the University of Louisiana at Lafayette proposed that idea as a cause of major comet showers in 1985, but no one except UFO believers has ever discovered a sign of it. Or perhaps there’s some kind of “natural timetable” deep inside the Earth that triggers cycles of massive volcanism, Rohde has thought. There’s even a bit of evidence: A huge slab of volcanic basalt known as the Deccan Traps in India has been dated to 65 million years ago — just when the dinosaurs died, he noted. And the similar basaltic Siberian Traps were formed by volcanism about 250 million years ago, at the end of the Permian period, when the greatest of all mass extinctions drove more than 70 percent of all the world’s marine life to death, Rohde said. The two scientists proposed more far-out ideas in their report in Nature, but only to indicate the possibilities they considered. Muller’s favorite explanation, he said informally, is that the solar system passes through an exceptionally massive arm of our own spiral Milky Way galaxy every 62 million years, and that that increase in galactic gravity might set off a hugely destructive comet shower that would drive cycles of mass extinction on Earth. Rohde, however, prefers periodic surges of volcanism on Earth as the least implausible explanation for the cycles, he said — although it’s only a tentative one, he conceded. Said Muller: “We’re getting frustrated and we need help. All I can say is that we’re confident the cycles exist, and I cannot come up with any possible explanation that won’t turn out to be fascinating. There’s something going on in the fossil record, and we just don’t know what it is.”
  8. So the point is that it is cyclic, regardless of cppling or heating, there is a causitive agent that does not include mankind and cold is much worse than warm (evidenced by our own near extinction at H4 in the neandertal paper).
  9. Sorry for all the typos, I'm diabetic so my eyesight is blurry after eating.
  10. Oh dear Quietman, you can find any kind of pattern you like, looking back at any kind of record (just as you can see apparently meaningful figures in melted cheese on toast), but that doesn't mean they are real or have a single "cause". The mentions of "Nemesis" and "Planet X" and UFO believers should give you some inkling that this stuff is suspect. Of course there are all kinds of causes of extinctions of the tens of thousands of species that have become extinct since life evolved on the planet. Volcanic eruptions might have played a role in some times and places, impact of an asteroid is possible I suppose, but for the vast bulk of species variations in climate are undoubtedly the cause. And for most water-based life forms hot and dry is more of a challenge than cool and wet. Dinosaurs may be an exception, but I doubt it. And then there are all the ocean life forms that have become extinct - not much prospect of asteroids and volcanoes affecting them (changes in temperature and acidity yes). Please, forget about Neanderthals, they are a red herring. You seem to be searching for something, anything, rather than accept that (a) CO2 concentrations are rapidly (in paleontological scales) increasing; (b) we know the physics and chemistry that causes the changes this will bring; and (c) the changes, not in models, but in the real world of glaciers and ice caps, droughts, heat records, storms, changes in species distribution and behavior, are already evident and rapidly accelerating. Talk about "natural timetables deep inside the Earth" is just whistling in the dark.
  11. David The Nemesis hypothesis, while nothing more than that, has been proposed serveral times in the past. Just because the crazies have picked up on it and made a cult centered around the concept does not invalidate the idea. But it has been largely abandoned by the scientific community. The mention is in the article, not a peer reviewed paper, by the reporter because of it's popular appeal. I don't have access to their paper as I did for the first reference at PLos One. Regardless of knowing the cause of this cyclicity, it does appear to exist.
  12. UW-Milwaukee Study Could Realign Climate Change Theory Scientists Claim Earth Is Undergoing Natural Climate Shift but like the telegraph said "Nobody listens to the real climate change experts".
  13. Anyone still following this thread will be interested in Comment number 94 suggests that it is much too mild and gentle and what is needed is a full blown offensive against denialists.
  14. Sort of a witch hunt? That is how ignorance always reacts to real science.
  15. Nah, witch hunts were against people who were merely imagined to be causing the community harm.
  16. Then it is the skeptics that should have a witch hunt. Ok, everyone, lock and load. :)
  17. One man's sceptic is another man's denialist.
  18. That IS a truism. One man's alarmist is another man's fundamentalist as well. Sorry, I'm agnostic. I see the greens as deniers of nature. CO2 is mans sin, pray for penance from the God Algore.
  19. Ah, why oh why couldn't I see it before. Yes, I have been a denier of nature, of course I have. All that concern about increasing CO2 and the destruction of habitats all around the globe, and the extinction of species, and rising pollution levels in sea and air, all because I, um, deny nature. And those who are happy to see giant corporations wreaking havoc on the planet, and who have absolutely no concern for the consequences, indeed, can see none, other than to mindlessly chant about climate always changing (gee, who knew, what an eye opener it was when I discovered THAT!), they are the what? True conservationists? Humanists? Libertarians? Rhetorical questions, I'm afraid, I really don't want to know how you label the delusions you are working under. Just go away. Learn something about the real world. Come back.
  20. Well Dave, I have been around long enough to have experienced warm winters before. Growing up on Long Island we had snowball fights wearing no more than jeans and t-shirts. You can fool the kids but you can't fool us old timers, sorry.
  21. ps I am retired on a healthy pension, I think I know about the real world, but you still have much to learn. Read my comments and links in the volcanos thread.
  22. Patrick These maps show the progression of the 3rd Ice Age (Carboniferous-Permian) beginning before it's onset in the Devonian through the End of the Permian. Devonian Early Carboniferous Late Carboniferous Permian These two maps show the 4th Ice Age (current) and the modern world (current interglacial): Current Ice Agea> Current Interglacial I already posted the Eocene and Miocene in the "It's the Sun" thread so I wont repeat them here. Compare the Ice Age maps to the earlier Ice Age and you can see easily how different the earth was and how unlikely another glacation actually is at this point. Think about it. What do you think caused the long extreme cycles in the late part of the this age?
  23. The changes in climate from changes in the Earth are not immediately obvious unless you realize that ocean currents have a much stronger effect than air currents. This is because the atmosphere is not efficient as a buffer, ie. GHGs are not as efficient as the ocean (or any body of water) at storing energy including heat. What is key to understanding how oceans drive climate is the convection currents (upwelling and downwelling currents). We know that ocean temperatures are not globally uniform. Some bodies of water are warmer or cooler than others. But what is more important is that the depths are different temperatures and the colder water sinks while the hotter water rises. Simple. Next comes the more complex part.
  24. The temperature of the ocean floor is variable. The depths have a thinner crust in general so transfer more heat and the volcanic active areas transfer heat from the magma directly to the water. The proximate temps change rapidly simply because the the amount of cold water available to sink this heat. But just like applying heat to the bottom of a pot of water, convection currents are created in both up and down directions. Some of these are fairly constant coming from the ocean depths but some are highly variable where volcanic activity exists and this can be at any depth along the ridges. At times of eruptions or even just increased plate movement the heat released into the water increases and either creates a new current or strengthens the existing current. As explained in the volcano thread, the ENSO is a real time example of this very driver. The subduction of the pacific plate under the Andes slows and speeds up in an irregular cycle. It's irregular because of the continental crust being uneven densiity and shape and made of materials of various ores with different melting points.
  25. This is why we get a warning when El Nino is beginning by volcanic eruptions in Chile. It's not the volcano that causes El Nino, it's just a symptom of the tectonic activity and it's accompanying vulcanism. And I have to stop for now since my grandson is having a tamtrum and I can't think straight with the noise.

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