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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 51 to 75 out of 302:

  1. #106: Crustal heat contributes relatively negligible amounts to the system. Certainly not enough to significantly influence ocean currents. In today's Icehouse climate they are primarily driven by Thermohaline circulation, which in itself is driven by the creation of cold saline waters in polar regions. I have never heard of any correlation between ENSO and tectonic factors. I'd like to see the data for that. Plus, tectonics doesn't fluctuate on ENSO time scales, nowhere near. I don't even know where to start on the last paragraph - not even wrong comes to mind.
  2. Gincko There is a weak statistical relationship between volcanic activities and ENSO with an enhanced probability of El Nino-like conditions in the winter following a volcanic eruption. Contemporary interest in this idea comes from a study published in Nature a few years back [*]. More recent research [**,***] supports a weak correlation and indicates that it is only the most vigorous volcanic eruptions that result in ejection of volcanic aerosols into the stratosphere that produce significant effects on ocean circulation. Of course this effect has nothing whatsoever to do with the heat from the volcanoes per se! That is just silly. As you say, the geothermal flux is simply too weak, although I believe there is some evidence that undersea crustal heat flow can influence ocean mixing and thus help to draw warmth down to the deeper oceans in a warming world (???). Obviously anyone asserting a contribution of geothermal heating to global warming should provide evidence not that this very weak geothermal flux exists, but that this has increased in line with periods of known global warming (like now!). The advocates of that notion seem reluctant to come up with any such evidence…. The weak relationship between strong volcanic eruptions and ENSO is due to the aerosolic load and its effects on radiative solar forcing.. I have to say I haven’t read the papers in detail, so the exact mechanism isn’t clear to me. If I have time I’ll have a more detailed look… [*]J. B. Adams et al (2003) Proxy evidence for an El Niño-like response to volcanic forcing. Nature 426, 274-278 abstract: Past studies have suggested a statistical connection between explosive volcanic eruptions and subsequent El Niño climate events1, 2. This connection, however, has remained controversial3, 4, 5. Here we present support for a response of the El Niño/Southern Oscillation (ENSO) phenomenon6, 7 to forcing from explosive volcanism by using two different palaeoclimate reconstructions of El Niño activity8, 9 and two independent, proxy-based chronologies of explosive volcanic activity5 from ad 1649 to the present. We demonstrate a significant, multi-year, El Niño-like response to explosive tropical volcanic forcing over the past several centuries. The results imply roughly a doubling of the probability of an El Niño event occurring in the winter following a volcanic eruption. Our empirical findings shed light on how the tropical Pacific ocean–atmosphere system may respond to exogenous (both natural and anthropogenic) radiative forcing. [**] Emile-Geay J et al. (2008) Volcanoes and ENSO over the past millennium. J. Climate 21, 3134-3148. abstract: The controversial claim that El Nino events might be partially caused by radiative forcing due to volcanic aerosols is reassessed. Building on the work of Mann et al., estimates of volcanic forcing over the past millennium and a climate model of intermediate complexity are used to draw a diagram of El Nino likelihood as a function of the intensity of volcanic forcing. It is shown that in the context of this model, only eruptions larger than that of Mt. Pinatubo ( 1991, peak dimming of about 3.7Wm(-2)) can shift the likelihood and amplitude of an El Nino event above the level of the model's internal variability. Explosive volcanism cannot be said to trigger El Nino events per se, but it is found to raise their likelihood by 50% on average, also favoring higher amplitudes. This reconciles, on one hand, the demonstration by Adams et al. of a statistical relationship between explosive volcanism and El Nino and, on the other hand, the ability to predict El Ni no events of the last 148 yr without knowledge of volcanic forcing. The authors then focus on the strongest eruption of the millennium (A. D. 1258), and show that it is likely to have favored the occurrence of a moderate-to-strong El Nino event in the midst of prevailing La Nino-like conditions induced by increased solar activity during the well-documented Medieval Climate Anomaly. Compiling paleoclimate data from a wide array of sources, a number of important hydroclimatic consequences for neighboring areas is documented. The authors propose, in particular, that the event briefly interrupted a solar-induced megadrought in the southwestern United States. Most of the time, however, volcanic eruptions are found to be too small to significantly affect ENSO statistics. [***]Christiansen B (2008) Volcanic eruptions, large-scale modes in the Northern Hemisphere, and the El Nino-southern oscillation. J. Climate 21, 910-922 . abstract: The author analyzes the impact of 13 major stratospheric aerosol producing volcanic eruptions since 1870 on the large-scale variability modes of sea level pressure in the Northern Hemisphere winter. The paper focuses on the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) to address the question about the physical nature of these modes. The hypothesis that the phase of the El Nino-Southern Oscillation (ENSO) may control the geographical extent of the dominant mode in the Northern Hemisphere is also investigated, as well as the related possibility that the impact of the eruptions may be different according to the phase of ENSO. The author finds that both the AO and the NAO are excited in the first winter after the eruptions with statistical significance at the 95% level. Both the signal and the significance are larger for the NAO than for the AO. The excitation of the AO and the NAO is connected with the excitation of a secondary mode, which resembles an augmented Pacific-North American pattern. This mode has opposite polarity in the Atlantic and the Pacific and interferes negatively with the AO in the Pacific and positively in the Atlantic in the first winter after the eruptions, giving the superposition a strong NAO resemblance. Some evidence is found that the correlations between the Atlantic and the Pacific are stronger in the negative ENSO phase than in the positive phase, although this difference is not statistically significant when all data since 1870 are considered. The author does not find any evidence that the impact of the volcanic eruptions is more hemispheric in the negative than in the positive ENSO phase.
  3. Thanks for your response, Chris I don't have an issue with significant changes in volcanic activity influencing climate - the correlation is quite obvious. However, Quietman's #106 post (para. 3) seemed to imply that it is the heat released into water that substantially drives climate by altering ocean currents. There's a difference between the influence on climate that subaerial volcanic gas emissions can have (effectively altering the total amount of solar energy in the system), and that of heat out of the earth's interior. The references you cite only focus on the former, but Quietman is mainly arguing direct and indirect influences from the latter. I've never seen any data on the effect of ocean basin heat on the global ocean current conveyor belt. It is generally accepted that this is mostly driven by Thermohaline circulation in our Icehouse climate (dominated by the polar ice caps), which probably overwhelms most other influences on larger scales. Surface current changes (which can influence upwelling) can be caused by changes in atmospheric circulation, but again, this is not influenced by heat released from the earth. It's interesting to note that during a Greenhouse climate (e.g. Cretaceous) the global circulation tends to be driven by Halothermal circulation (dominated by evaporation at the equator), which is much weaker and can easily lead to stagnant oceans.
  4. ginckgo Sorry, that was posted for Patrick who was up to speed on vulcanism. The Volcanos thread has the argument presented with links to recent articles explaining the background. I am missing one reference about ENSO and I can't find it again to post a link. When I do it will also be posted in the Volcano thread. John wants us to keep the subject relevance when we can. From your statement I think you have some reading to catch up on. :)
  5. ps Volcanos actually causing ENSO is far fetched and yes a bit silly. It's what happens on the ocean floor near the subduction zone where the driver for ENSO resides.
  6. Chris Your examples are results of modern education just like these: "This finding has opened new doors in dinosaur research on this part of the continent: "It established that dinosaurs were nesting at this high latitude," said Miyashita. " Science Daily or Live Science "Some dinosaurs (warm-blooded, perhaps) were surprisingly good at withstanding near-freezing temperatures, they say. Witness the team's latest find, a diverse stash of dinosaur fossils laid down just a few million years before the big impact, along what's now the Kakanaut River of northeastern Russia. Even accounting for continental drift, the dinos lived at more than 70 degrees of latitude north, well above the Arctic Circle. " There is no excuse for this poor education. This started in the late 1960s with the long hairs and just went downwards since then.
  7. Here is something that you should know already: The Paleogene Period of the Cenozoic Era: 65.5 to 23.0 million years ago The Paleocene (“ancient recent life”) epoch marks the beginning of the Paleogene Period and the Cenozoic era. The sea-level fell to expose dry land in much of inland North America, Africa, and Australia. South America however was cut adrift with its own unique evolving “ark” of birds, mammals, and reptiles. The Paleocene Epoch of the Paleogene Period: 65.5 to 55.8 million years ago The rifting of the North Atlantic cut off North America from Europe, and South America lost links with Antarctica. India and Scotland were home to mountain-building episodes. The sea-level rose and seas invaded much of Africa, Australia, and Siberia. Climates were generally warm or mild worldwide. Tropical palms flourished as far north as the London Basin. The unusual mixture of tropical and subtropical elements in the northern latitudes in the Eocene suggests that the mean annual temperature of these regions was not as high as in the present tropics, but that the flora was maintained by a greater rainfall than occurs in these northern latitudes today, with no pronounced seasonality in its distribution, and by the absence of winter frost. The Eocene Epoch of the Paleogene Period: 38-54 million years ago There was an increase in volcanic activity, and plate tectonic movement, as India collided with Asia. The last remnant of the supercontinent of Gondwanaland broke up as Australia and South America both separated from Antarctica The Oligocene also marked the start of a generalized cooling, with glaciers forming in Antarctica for the first time during the Cenozoic. The increase in ice sheets led to a fall in sea level. The tropics diminished, giving way to cooler woodlands and grasslands. Although there was a slight warming period in the late Oligocene, the overall cooling trend was to continue, culminating in the Ice Ages of the Pleistocene. The Oligocene Epoch of the Paleogene Period: 33.9 to 23.0 million years ago Source: Palæos
  8. Burning books, and scientific publications, is necessarily banned due to the carbon emissions this would engender. We are therefore left with Dr. Michael Mann and his statistics to erase the following. And soon! We live today in the Holocene Epoch, or the last 11,500 years since we melted out way out of the Wisconsin ice age. Just like we melted our way out of the previous 7 ice ages dating back to the Mid Pleistocene Transition (where we went from the 100,000 year ice age/interglacial cycle [which matches the eccentricity cycle in our orbit] to the 41,000 year cycle [which matches the obliquity cycle in our orbit]). At 11,500 years old, this places the Holocene at precisely one half of a precessional cycle (23k years). All 6 interglacials dating back to the MPT have lasted roughly one half of a precessional cycles). This is also the interglacial in which all of human civilization has occurred, only cave paintings being found in the "written" record prior to 10k years ago. At the Holocene Climate Optimum, between 7000 and 6000 years ago, sea levels were a mere 6 meters higher than today. This was the time period when the Egyptian civilization erupted onto the scene. According to the National Research Council (NRC, 2002), half of the melting which brought us out of the Wisconsin ice age occurred in less than a decade. Sea level swings between the post MPT ice ages and interglacials result in typical sea level changes of only 400 feet. The previous interglacial, the Eemian or penultimate, also known for the first occurrence of fossils of the species Homo sapiens, ran for about 10,000-10,800 years, or almost half of a precessional cycle. It ended with a LEAP (Late Eemian Arid Pulse). Stable platform sites record at least 3 sea level highstands within the Eemian that were higher than present-day sea level. About 20 meters (~65 feet) is known from the Grand Caymans, 52 meters (~170 feet) is reported from a site in Siberia. Sea levels can and do occur at different relative elevations in different parts of the world at the same time. The Eemian ended in a series of pulses, the LEAP being one of them. It lasted 468 years, with dust storms, aridity, bushfires and a decline of thermophilous trees with the onset of glaciation. The LEAP occurred at a 65o north insolation of 416Wm-2, close to the 2005 value of 428 Wm-2, which may be relevant in terms of understanding present climate variability. The onset of the Leap occurred in less than two decades. It ended roughly 117-118k years ago with the Last Glacial Inception (LGI), the beginning of the Wisconsin ice age (NA nomenclature). During the Wisconsin, 24 Dansgaard-Oeshger events lie well recorded in the Greenland ice cores. These run from 1,000 to 4,000 years long, averaging 1,500 years. They have the same sawtooth shape as the major transitions characterized by on average 8C-10C warmings that occur from years to just a few decades (outliers ranging to 16C), floowed by a very rocky gradual descent to ice age conditions. All distinct pulses that are gradually being verified by sediment cores from ocean deep drilling projects. Evidence for D-O events extends in the sedimentary record as far back as 680 million years. Something, we do not well understand, causes these events. We are left to ponder what anthropogenic effects will, or are, to be overlain on this tapestry. The verdict cannot be said yet given such a AGW signal to the noise of natural, frequent, abrupt and seemingly reliable climate change. But it cannot be discounted quite that easily. Read, and save (in my case), thousands of treatments of this subject from literally all fields of paleoclimatology and what we seem to know for sure is that the shift between the two natural states of climate (cold and warm) seem to be responses far out of proportion to whatever forcing seems the most probable. Meaning that doubling CO2 from less than one tenth of a percent to still less than one-tenth of a percent in s few hundred years could very well release some sort of instability, given climate's real sensitivity. One should not discount the fact that as far as the paleoclimate record is concerned, GHGs may not have triggered these events, ice age cold or nearly instant interglacial. There is strong evidence that they may have amplified, perhaps even dramatically, whatever triggered the sudden increase in temperature. Some orbital models predict that of the next 4,000 years the climate will be in a state of sensitive variability, followed by 55,000 years of interglacial style 65oN insolation. No interglacial for the past 5 million years has lasted near so long. And in the past 3 million years hominid braincases went from an average of 500cc to 2,500cc, which, upon considerable research, you may prize out that frequently, researchers cite significant climate change events as a probable cause. During that 5 million years it is estimated from proxy data, that the earth was in its cold state 90% of the time. If, at the end of this half precessional cycle, the tendency may be towards resumption of our cold state, then GHGs, from paleoclimate studies anyway, may actually be needed, as it does seem that their lingering slows eventual descent into the glacial maximums. On the other hand, we know that for a given known input, climate response can be triggered rather suddenly into its other state. We are therefore left to ponder how best to manage. Given the disparity between 3rd world resource demand, and "American Plush", and the ever-consuming desire to achieve that lifestyle (witness the largest population group aspiring, China), we need to rethink the root of the problem. And that is growth. And not just population growth. Growth itself. Resources on Spaceship Earth are not unlimited. With better extraction, purification and manufacturing technologies, there is only so much naturally occurring and accessible such resources. We may have to not only retool, but our economic models of ever increasing profits may need to be modified. Especially at American Plush resource consumption, growth either entails more consumers of resources at the same rate, or ever greater levels of resources consumption by a static population of consumers. It would be sensible to conclude that neither is really sustainable in the long term. We must therefore work out what is feasible related to reliable, abrupt and seemingly unavoidable natural climate change, which we can reasonably guess to occur, while also paying attention to the fact of outsized forcings from what could be several sources, including GHGs. Pondering this, we must eventually face the connection between resources and population. In order to sustain our current level of civilization will require both resources and energy to sustain even a static population. Perhaps we should spend some time thinking about fusion. It has, afterall, powered the known universe since its beginning, and provided all 92 naturally occurring elements. It has the potential to provide some of the answer. Meanwhile, enjoy the interglacial!
  9. Everyone is talking about GGs and other "causes" of GW. I am a service tech for an electrical company and have learned that you can either fix the problem, or fix the symptoms. From what I have read, there is evidence that GG's are indeed caused by GW, which would make them a symptom, much like a fever to a viral infection. Has any research been done to determine what caused the sudden and dramatic reversals in temperatures in the past? If so, where are we in relation to the trigger(s) today? Are we doing any good what so ever by treating the symptoms, or are we actually doing harm?
    Response: CO2 is both the symptom and the cause. We are emitting CO2 into the atmosphere, which warms the temperature, which causes the warming ocean to outgas more CO2. This is the feedback loop which caused the dramatic warmings in the past that took the planet out of the ice ages into interglacial periods. Eg - warming in the Southern Ocean (due to changes in the Earth's orbit) led to outgassing of CO2 which amplified the warming and spread it through the rest of the planet as the CO2 mixed through the atmosphere. Your question is addressed in detail at the CO2 lags temperature page.
  10. sentient Well said and something to ponder. The more important most ignored point is where the source of CO2 and why is it called a feedback. It's life itself IMO. In the age chest phase of an ice age things die, not an extinction event or might but volume of life takes a nose drive. We see in H4 the extant human species nearly goes extinct. We were able to recover, neandertal did not. Not so much as cold as it is food. The climate decreases the available sources including food. Plants that do not die are just surviving, not producing O2 much but still produce CO2 in the process of just existing. This means no new growth to replace what dies. What not deads goes dormant. Bottom line, no food. No food, life forms not capable of going dormant dies. Populations at the top of the food chain decreases. Bottom line, no production of CO2. When it warms up, plants no longer dormant, produces new growth, and food at the bottom step of the food chain. This follows by fauna and repopulates the earth and we seen the growing of all the GHGs acting as as feedback and helps it to defrost. No rocket science, just common sense.
  11. "When it comes to climate, the early Paleogene period (~65-34 mya), at the start of the Cenozoic Era, had one of the most Eden like climates of the Phanerozoic. As the Cenozoic progressed a cooling trend set in leading up to the formation of permanent ice caps and the Pleistocene Ice Age we are still experiencing. But before the world started to ice up our planet underwent one of the most dramatic bouts of global warming known to science—the Paleocene Eocene Thermal Maximum or PETM. Recently, global warming activists have tried to liken human CO2 emissions to the cause of the PETM, 55 million years ago. Is it true, that our actions may trigger a sudden sharp rise in global temperature? The mid-Cretaceous (~125-85 million years ago) and the early Paleogene are among the best known ancient “greenhouse” climate intervals—times when Earth's average temperature was significantly higher than they are today. During the Cenozoic (the last 65 million years) the global climate has cooled substantially, up to a main cooling step at the Eocene-Oligocene transition (~34 mya), which included the development of the first glaciation at a continental scale in Antarctica. Another main cooling step occurred in middle Miocene (14 mya) and was a significant step in the development of the Antarctic continental glaciation." FROM: Could Human CO2 Emissions Cause Another PETM? Read the rest, It's an interesting view.
  12. Thanks QM; an interesting article. I shall have to go and revisit clathrate physical/chemical properties. From what I recall, they only form under extreme pressure/low temperature conditions ( which is why they are mostly found in very deep water).
  13. Global Warming: Scientists' Best Predictions May Be Wrong ScienceDaily (July 15, 2009) No one knows exactly how much Earth's climate will warm due to carbon emissions, but a new study suggests scientists' best predictions about global warming might be incorrect. ... The study, which appears in Nature Geoscience, found that climate models explain only about half of the heating that occurred during a well-documented period of rapid global warming in Earth's ancient past. The study, which was published online July 13, contains an analysis of published records from a period of rapid climatic warming about 55 million years ago known as the Palaeocene-Eocene thermal maximum, or PETM. "In a nutshell, theoretical models cannot explain what we observe in the geological record," said oceanographer Gerald Dickens, a co-author of the study and professor of Earth science at Rice University. "There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models." Read the rest, Interesting.
  14. I'm curious. Do we know what caused the reversal in past warm periods in the Earth's history? What made it get cool again? Clearly, despite CO2 having a positive feedback loop, we didn't get runaway warming. We're not living on Venus. Even if we're headed for higher temps, rising sea levels, drought, mass extinctions, catastrophic loss of human life, etc. At some point won't it top out an head back to another ice age? What's prevented a runaway greenhouse effect in the past?
  15. Right; there are positive feedbacks, but they are not boundless. Water vapour can only be added into the atmosphere up to saturation point, counterbalanced by a tendency to rain more the more there is. Greenhouse gasses trapped under permafrost, ice, in the ocean and wherever else can only be released once, and there's only so much ice to melt to diminish Earth's reflectivity. One of the things that can counteract an excess of CO2 is excess growth/adaption of plants, and IIRC this is what eventually causes the atmosphere to bounce back and temperatures with it.
  16. Quietman. Even if we ignore the impact of past climate change on animals, look what slower-less extreme-warming events had on human civilizations. Two come quickly to mind-the Anasazi & The Khmer Empire both died out because they failed to adapt to a relatively small warming of the planet during the Medieval Warm Period. Another thing-the feedbacks may not be boundless, but evidence of past climate suggests the situation could get very bad if we let it. Throughout much of the pre-Quaternary period, the planet was a good 4-8 degrees warmer than what it was at any point in the Quaternary period. Is it mere coincidence that this higher temperature was at a time when CO2 levels were 10 times higher than today? I seriously doubt it.
  17. As a total layman I can follow the arguments in my simple way to the point where I see that GHGs amplify warming and cooling periods, so more co2 in the atmosphere increases amplification. But when I look to see what the science says about what initiated pre-industrial inter-glacial warm and cool periods i get a bit stuck in debates about solar activity, a plethora of weather oscillations between negative and positive and now feeback loops and planet earth knowing and adjusting its own temperature because it is conscious of an energy imbalance. Global warming doesn't get any easier to understand.
    Response: Hang tight, I'm at this moment working on a post on the whole ice age/interglacial mechanism, the emphasis on making it easy to understand. Hopefully online within a day or two.
  18. The initial statement says Natural climate change in the past proves that climate is sensitive to an energy imbalance. If the planet accumulates heat, global temperatures will go up." This is false: If the planet accumulates heat then it will be hotter and hence will radiate more heat out until it returns to equilibrium per the gray body Stefan- Boltzmann Law. The planet goes from warming in the morning to cooling in the evening as the amount of absorbed energy increases and decreases. It passes through equilibrium temperature exactly twice every day. The concept that the planet could accumulate heat and be out of equilibrium for any extended length of time is just not possible. The computer model however assume this. They are wrong.
  19. If the forcing is transient then the temperature will return to its original equilibrium, yes. If the forcing persists for thousands of years (like CO2 in the atmosphere) then the planet's temperature will reach a new and higher equilibrium while that forcing persists.
  20. "What does past climate change tell us about global warming?" In case you dont know it, your explanation uses the AGW theory to explain the question when the question is really asking for some proof of the AGW theory. Cheers Roger
  21. Rogerthesurf, you'd do better to explain yourself. Failing that, presumably you won't care if your post is deleted?
  22. He probably just wants to "enlighten" us with a link to his own blog. Anyway, a large part of the discussion is about PETM and AGW (with the link to resilient earth). A lot of it sounds like, if we extrapolate our CO2 emissions it would take forever to reach PETM concentrations, and PETM wasn't even caused by CO2 alone, so PETM cannot be used as an indication for the effect of CO2, so AGW does not exist. All the individual statements above are probably correct, except the conclusion. Think about it, have we seen any projections from IPCC showing a 6 -9 C global temperature rise? No. Oh, BTW it can also be found on that website that models can not fully reproduce PETM even if we use the CO2 concentrations at PETM, so models are WRONG! (It is understood why, because GCMs currently use bandwidth parametrisation instead of line-by-line CO2 absorption schemes (because of limited computer power) and will thus deviate if brought very far from their original (~300 ppm) state. This probably introduces a large error when going to extremely large (2000 ppm) CO2 concentrations. This PETM event can (in "skeptical" reasoning) even be used to say "look, even if we get a 3 C global temperature, it is all natural! Because in the near past of the Earth's history it has been 9 C warmer, and humans were not yet around!" And, using this reasoning and the paleo record, all climate variation will always be natural...
  23. The mistake on the part of environmentalists has been to talk about "saving the planet". This leads directly to the sceptics' argument that the planet has changed in the past, and done just fine. Quite true. But we're not really interested in the planet, we're interested in our own welfare. A dramatic change in the climate would, first and foremost, affect our civilisation. Our cities, farmland and economies are far more vulnerable than the planet itself, which will no doubt continue to adapt. To win the global warming argument politically, we need to underline the threat to ourselves from climate change, rather than talking about ice sheets, polar bears and barrier reefs (important as those things might be).
  24. "71.Rogerthesurf at 12:52 PM on 8 April, 2010 "What does past climate change tell us about global warming?" In case you dont know it, your explanation uses the AGW theory to explain the question when the question is really asking for some proof of the AGW theory. Cheers Roger 72.doug_bostrom at 13:53 PM on 8 April, 2010 Rogerthesurf, you'd do better to explain yourself. Failing that, presumably you won't care if your post is deleted? " Sorry your reply did not show up on my "My Comments" page. Thank you for editing my comment (not) However my point is simple. Correct me if I am wrong, but at no point does your explanation discuss the validity of the "Anthropogenic CO2 causes Global Warming" hypothesis. Instead your host of explanations which are all based on the assumption that it (the above hypothesis) is fact, which actually it is not fact but as yet simply an unproven hypothesis. Therefore all your explanations are no better than this unproven hypothesis. I trust that is clear. Cheers Roger I also always post my comments and replies on my other site where my readers can evaluate my questions and your answers. Check under your url and post title.
  25. Roger, I'm not spotting the tautology. Which part of the physics do you disagree with?

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