Arguments
What does past climate change tell us about global warming?
What the science says...
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Basic
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Intermediate
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Greenhouse gasses, principally CO2, have controlled most ancient climate changes. This time around humans are the cause, mainly by our CO2 emissions. |
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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
too @550... Pretty much every point you've stated here is fundamentally incorrect.
1) Many solutions, in addition to wind and solar, are discussed to address climate change. Nuclear, CCS, efficiency, tidal, geothermal, hydro are all solutions which are being actively worked on and are being actively implemented.
2) Researchers provide a wide variety of temperature reconstructions for past temperature. There are numberous local and regional records, and there are a great many multiproxy reconstructions as well. Just because something is "inferred" does not mean that it is wrong. That would be a classic case of throwing the baby out with the bathwater. Just because you don't have absolute precisions doesn't mean you can't trust the information you do have.
3) What we do know about CO2 concentrations is, they are rising. There is no question about that fact. We know how much we extract and burn in terms of fossil fuels. We know concentrations are rising due to human contributions. There is very high confidence on this fact.
4) If you actually were to spend any time reading scientific research or listen to what scientists actually say, you would realize that uncertainties are core to the message that they present.
I suspect that your entire comment is posted here mostly as a means to drive traffic to your personal climate denial blog.
About a month ago, I took the Pages-2k graph (last 2000 years of global temperature using tree ring proxies) and roughly calculated the temperature change per century, last 20 centuries, and the standard deviation in this metric for this 2000 year period. I then took 5-century intervals from the Marcott graph (last 11,000 years, ocean sediment data), calculated the average temperature change per century (over 5 centuries), and imposed the standard deviation I'd gotten from Pages-2k to each of these to calculate my best estimate of the temperature change per century for the 100 centuries prior to Christs birth. I then applied this same technique to the Shakun graph (last 20,000 years). However, in that case I used 10-century intervals to get the average temperature change per century and imposed the Pages-2k standard deviation upon that average to get 10 data points representing the likely variance over them. At the end of all this activity, I had 219 data points representing the likely temperature change per century for the 220 centuries (22,000 years) before the 20th century. The average was 0.014 C/century, the standard deviation was 0.077 C/century, so the 3-sigma point is 0.24 C/century. Warming in the 20th century was 0.78 C/century. To me this proves, statistically, that modern warming is nothing like anything that has occurred in the previous 22,000 years. Its about 3 times what would be considered extremely unusual from the natural record. And warming in the last 25 years, if it continues, is about 3 times that again (2.2 C/century). My question is: does anybody know where this kind of analysis has been performed in the Science record? I'm sure it has, and to a much greater degree. I just want to know where to find it so I can refer to it whenever somebody claims 'Its all natural'.
Excellent article new at RealClimate: "The climate has always changed. What do you conclude?"
nijelj
In looking for something else, I just saw this reply of yours on one of the blogs:
"Your understanding or information is wrong. Sea level rose from AD800 to around 1500 then fell until about 1900, then started rising as in the link below. This correlates reasonably well with burning of fossil fuels so all or nearly all this sea level rise can be attributed to fossil fuels."
Just curious but does this sea level rise and drop pre 1900 correlate quite well with the theory of the MWP and the Little Ice Age?
We now have the Chinese study which also seems to support both the MWP and Little Ice Age. If you need I cite for the Chinese study I can get it for you.
Again this does not prove anything about the existing warming but the denial of the MWP and the Little Ice Age is part of the "Hockeystick" theory suggesting that this present warming is anomalous over the last 2000 years.
[JH] Sloganeering snipped.
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NorrisM:
I am not a moderator here, but I've been around a while. In my interpretation, "sloganeering" is the repeated posting of unsupported assertions, covering the same material, while ignoring comments that have pointed out information that contradicts the assertions.
You appear to be simply ignoring a lot of comments. SkS also has a rule against "dog-piling", where one commenter is facing comments from a large number of opponents. Regulars here do try to avoid that by restricting comments, but you can help by selected a very small number of posts to comment on, and sticking to those issues until more-or-less resolved, before going on to other topics.
[JH] "Sloganeering" is explicitly defined in the SKS Comments Policy.
Moderator. I would be happy to avoid "sloganeering" if I knew what it was. I was not using slogans in the part snipped above. Can you define this term? You have my email address. I would be happy to have the definition offline. There are numerous contributors to this website who present political comment who are not chastised when they make reference to matters which are not factual based. For example, the discussion of who is and who is not a "denier" and what various subsets there should be of this classification. Have requested a definition of "sloganeering" before. Could you provide one? Thanks
[JH] "Sloganeering" is explicitly defined in the SKS Comments Policy. You have been advised more than once to read the Comments Policy and to adhere to it.
NorrisM,
For this instance, the data very clearly shows that there was not a global MWP or little ice age. Since they never existed, they cannot be denied and your comment using the word denied appears to be deliberately offensive.
You have previously made statements about the MWP and LIA and have been referred to citations that show they were local events and the global temperature is shown by the Hockey Stick of Mann et.al. Since you have been shown data to support Mann et al and have provided no data (because it does not exist) to support your claim Mann was incorrect you are sloganeering by repeating an unsupported claim.
Many scientists have reproduced Mann's Hocky Stick using a variety of methods and data. It is completely accepted by anyone informed about AGW. Use the search button to find SkS references to educate yourself. If you have a question we are happy to help you understand, but claiming Mann is incorrect repeatedly makes it appear that you are not reading (or reading and ignoring) the answers people give you.
Michael @557 ,
please correct me if I have gained the wrong impression : that impression being --
(A) There was no global MWP, as the numerous "warm patches" of approximately 1,000 years ago were Northern Hemispheric and were minor and not contemporary. In other words, the so-called MWP is nowadays a Eurocentric "beat-up" from denialists who are using outdated ideas and who are being very economical with the truth.
(B) The so-called Little Ice Age actually "was a thing" : as it involved some cooling of both hemispheres (IIRC, caused by two Solar Grand Minima, helped along by a number of above-average volcanic eruptions). Also IIRC : the Little Ice Age was a rather minor affair, constituting a global temperature drop of only about 0.3 or 0.4 degrees below the natural long-term (multi-millennial) slow decline of global temperature [until the modern rapid "Hockey Stick" rise caused by AGW, of course!].
I often see denialists claim that the temperature rise of the 1800's and 1900's was nothing more than a "rebound effect" from the LIA. That argument seems [to me] to be a complete nonsense, since recent temperatures are higher than the extrapolated pre-LIA levels (and the Holocene temperature is on a natural down-curve). And those denialists are supposing that any changes in global climate simply occur for no physical reason. Perhaps they subconsciously think of global climate as being a sort of inner-spring mattress!
Eclectic:
Another weakness of the "recovery from the Little Ice Age" meme is that the Little Ice Age ended a long time ago (in weather terms). Such a "recovery" should be fast in the early stages, and proceed more slowly as it appraoches the state it is recovering to. It wil not, without some magical physics, proceed slowly at first and continue to "recover" at faster and faster rates later on. If the current warming is "recovery from the Little Ice Age", why didn't it recover two hundred years ago?
Tha magical physics of the "Recovery" meme requires that whatever caused the Little Ice Age must proceed very quickly to create the Little Ice Age, but removing tha cause must proceed very slowly to get us back out of the Little Ice Age.
Such physics is not impossible - c.f., adding CO2 to the atmosphere by burning fossil fuels causes a rapid rise, whereas the atmospheric CO2 will not drop rapidly if we stop burning fossil fuels. We have a good physical explanation for that behaviour, though. The "recovery from the Little Ice Age" meme has no such physcial explanation, however, as you point out.
Eclectic,
Bob Loblaw's comments are always thoughtful.
Your understanding of the MWP and mine are much the same. There were various warm and cold periods worldwide but they were not all at the same time.
According to Mann's Hocky Stick from this SkS reference:
Where is the drop in temperature for the LIA? I do not see any noticable drop between 1600 and 1900. There is an overall drop over the entire graph (the Hockey handle) that is probably the descent into the next Ice Age that was stopped by CO2 emissions. As you would expect, there is some noise.
Since I see no special "LIA" I conclude that it never existed.
I see that the above graph confirms Mann's original Hockey stick as very close to the best data available ten years later.
Moderator
Thanks, I have now read your Policy Statement. I would have thought that my references to nigelj's comments re ocean temperatures (on another stream) somewhat corresponding to the periods "alleged" to be the MWP and the Little Ice Age would be considered "additional information" and not just repetition of earlier positions. Surely a matching of ocean temperatures tells you something about atmospheric temperatures at the same time.
Furthermore, my reference to the recent Chinese Academy of Science recent paper analyzing temperatures over the last 1,000 years in China was also something which shows that this was not just a period isolated to parts of the Northern Hemisphere. Does anyone have any comment on this? I have read somewhere else (and I do not have a citation) that during the MWP the tree line of the Rocky Mountains was much higher than at other times. This would just about "connect the dots" for all of the Northern Hemisphere since we have Northern Europe, Greenland, Newfoundland and China already.
The above graph has posted the Michael Mann 1998 hockey stick. My understanding is that sometime around 2007 he revised his graph. Could someone post his most recent one? For some reason, I thought it was not so "flat". Perhaps because it does reach back further than 1400 to cover the period from 800 AD. If this is totally "out of the blue" with reference to the Rocky Mountain treeline, I will find my source for this statement or acknowledge that I cannot find it. However, my guess is that you have heard this before.
I will again reiterate that even if there was an MWP, it does not prove anything about today's temperatures but only goes to show that this has occurred before when it was not caused by man.
[JH] Please provide a link to the Chinese Academy of Science paper that you have cited.
Norrism,
This is the graph for the Northern Hemisphere from Mann et al 2008 :
There might be a more recent paper that I did not find. Mann has done the analysis with no tree ring data and it is the same as with the tree ring data.
It is usually possible to get free copies of papers if you Google them (I found this paper using Google). Apparently it is only for the Northern Hemisphere (as was the other graph I posted). More data is available for the Northern Hemisphere so Mann only did the Northern Hemisphere in 1998.
Here is a global analysis by Marcott et al (SkS article about Marcott)
Marcott is the red line. The small bump up is around 1000 years ago and is too early for the MWP. Current temperature is about 1.0 on this graph.
Keep in mind that we expect the temperature to decline after the Holocine maximum to a new ice age. This is the decline from 5000 bp to 150 bp. AGW then kicks in in earnest. AGW might have slowed the decline in temperature from 5000bp on from early farming releasing CO2.
I see no indication of a MWP in any of this data.
NorrisM @554,
You sepcifically ask about the reference made in a comment on another thread which concerned data from Kemp et al (2011) and a Sea Level reconstruction used to make a comparison with the good-old hockey stick. The actual graphic shown was::-
The fit with the hockey stick only works back to AD1000. Simplistically, the SLR found by Kemp et al is saying that (if the SLR is the result of global temperature changes) the temperature was in balance with sea level up to AD1000 after which average global temperatures rose by about 0.15ºC, a level maintained for a period of 400 years, then dropped back for a period of 500 years, after which we have the instrument record showing a global rise of 0.5ºC to 1950 and a similar rise since. So any MWP & LIA are globally very tiny features relative to post-AD1900 warming.
An alternative is that globally temperature was much flatter with the SLR resulting from regional temperature variation in places that SLR would be sensitive to; places like Greenland perhaps.
NorrisM & Michael Sweet:
Please take any further discussion of the Medieval Warm Period to the thread of the SkS Rebuttal aricle, How does the Medieval Warm Period compare to current global temperatures?
If I may beg JH's indulgence for a moment, to make a general point about both the Little Ice Age and the MWP issue :-
I hope that readers [including NorrisM] can see, in total Holocene perspective, that the LIA & MWP (if discernible at all) are very tiny ripples in global temperature compared with the huge rise which is AGW. And that neither MWP and/or LIA supply any valid argument against the size/severity & anthropogenic causation of the recent global warming.
Moderator
Here is the cite requested:
Advances in Atmospheric Sciences, August 2017, Volume 34, Issue 8, pp 941–951
Quansheng Ge et al., Chinese Academy of Sciences
Abstract
This paper presents new high-resolution proxies and paleoclimatic reconstructions for studying climate changes in China for the past 2000 years. Multi-proxy synthesized reconstructions show that temperature variation in China has exhibited significant 50–70-yr, 100–120-yr, and 200–250-yr cycles. Results also show that the amplitudes of decadal and centennial temperature variation were 1.3°C and 0.7°C, respectively, with the latter significantly correlated with long-term changes in solar radiation, especially cold periods, which correspond approximately to sunspot minima. The most rapid warming in China occurred over AD 1870–2000, at a rate of 0.56° ± 0.42°C (100 yr)−1; however, temperatures recorded in the 20th century may not be unprecedented for the last 2000 years, as data show records for the periods AD 981–1100 and AD 1201–70 are comparable to the present. The ensemble means of dryness/wetness spatial patterns in eastern China across all centennial warm periods illustrate a tripole pattern: dry south of 25°N, wet from 25°–30°N, and dry to the north of 30°N. However, for all centennial cold periods, this spatial pattern also exhibits a meridional distribution. The increase in precipitation over the monsoonal regions of China associated with the 20th century warming can primarily be attributed to a mega El Ni˜no–Southern Oscillation and the Atlantic Multidecadal Oscillation. In addition, a significant association between increasing numbers of locusts and dry/cold conditions is found in eastern China. Plague intensity also generally increases in concert with wetness in northern China, while more precipitation is likely to have a negative effect in southern China.
NorrisM @566 , thank you for pointing out the Chinese multi-proxy historical data study, published only 5 weeks ago (and receiving a mention last month in WattsUpWithThat & other websites too).
I haven't accessed the full paper, but the Abstract & Conclusion & a temperature chart (per the Chinese Academy of Sciences).
There were two noteworthy temperature rises shown in the China region, peaking at around year 1100 CE and the second at around 1250 CE. The peaks were (respectively) 0.4 degrees and 0.5 degrees above the 1851-1950 mean. In other words they were quite low relative to current world temperatures. And judging per my eyecrometer, those and the other minor peaks/troughs show only small excursion (from the mean) and showed rather poor temporal correlation with the Mann et al (2008) peaks/troughs — but of course I would be happy to bow to a more rigorous mathematical analysis of that point.
All in all, there seems to be nothing very startling (or upsetting to mainstream climate science) in the new Chinese study. Even the "quick to trumpet incompatibilities" WUWT has had little to say about it; and nor was there anything insightful/intelligent/relevant to be found in the attached WUWT comments column [ but no surprise there ;-) ].
As I mentioned in post #565, neither the LIA or MWP "ripples" can detract from the AGW tsunami.
NorrisM, Electric & Michael Sweet:
Please take any further discussion of the Medieval Warm Period to the thread of the SkS Rebuttal aricle, How does the Medieval Warm Period compare to current global temperatures?
Please answer this hypothetical for me. If we stop all human CO2 emmisions tomorrow would the temp at some point warm the earth to the critical stages it has done in the past? Am I right to understand that the polar caps have for the most part melted in the past?
Scott, the first important thing to understand is that climate is a reflection of the energy balance. If that changes for any reason, climate changes. Climate does not change by itself. CO2 was much much higher in the past, but the sun was fainter. We only got polar caps when CO2 dropped below a level for it to get cold enough. In the Pleistocene, CO2 dropped to point where the slow orbital cycles (Milankovich cycles) could drive an ice-age cycle due to variations in insolation happening at around 65N. Before that (the last time we had CO2 above 400ppm), you had icecaps but not ice ages.
Could they melt again? They will. The sun as a mainline star, is very slowly increasing its output and has been doing so since formation. At some point, a billion or so years into future, it will be too hot for liquid water. Eventually, as its fuel is exhausted, the sun will expand and probably consume all the inner planets - 5 billion years into future from memory.
A volcanic eruption on the scale of the Deccan traps could also push so much CO2 out that it warms the climate though that might be the least of our worries.
Scaddenp, so what you are saying is if we completely eliminate human CO2 emissions we could send ourselves into another ice age? Or am I oversimplifying the issue? If there is one immutable truth it would be that human intervention on mother nature is usually dangerous.
Scott0119 @571 , clearly you need to do a lot more reading to educate yourself on climate matters. Undertake some "science 101" basic education. Examples : read the climate science summaries on the websites of the U.S. National Academy of Sciences; of the U.K. Royal Society; of NASA; of NOAA; of the American Geophysical Union; of the Pontifical Academy of Sciences; or of other Societies of eminent scientists. If you prefer a more piecemeal approach, then select a few of the Climate Myths (see via the top/left corner of the SkepticalScience home page) — and read a number of them that interest you particularly [like with delving into a dictionary, you will soon find "nearby items" which will also interest you].
The "truth" which you quote may be less "immutable" than you think : but you are certainly correct that "human intervention on mother nature" [by injecting 100's of Gigatons of fossil-origin CO2 into our atmosphere] is definitely proving to be dangerous. It is high time that we all woke up to that !
If we completely eliminate CO2 emissions, CO2 concentrations and temperature will continue to rise for some time, because once temperature starts to rise by any cause, all the natural feedbacks that amplify the ice-age cycle still cut in. Detail on our commitments here though carbon-cycle feedbacks are still major research topic. Because they are slow (hundreds of years), they dont affect our immediate future.
CO2 that we have already emitted stays there. There are very slow processes (millions of years) that gradually pull CO2 from the atmosphere. If we every needed to warm the planet, then very easy to make extremely powerful GHGs that would warm the planet again far more efficiently than CO2, but on those long time scales, you have to remember the changes in the sun too. Too many generations away for me to be much concerned.
Couldnt agree more about danger of humans interfering with mother nature. Our unplanned change in atmosphere concentration is biting us.
@ eclectic@572. "clearly" that is why I am here. I know these questions may seem uneducated but if memory serves from my School years if you don't know something ask.
@ scaddenp @573 thank you for your answer. You seem to have a grasp on teaching.
Scott0119 @574 etcetera : Quite so. There's no such thing as a stupid question per se, yet a poorly-worded question may give the impression that the questioner is being disingenuous or subpontine in intent. Especially so, in the context of making contemporary statements in other threads, giving a similar impression.
Cast your mind back to your School days, Scott, and you will recall that you were expected to do your basic homework. And picture what your Science teacher's opinion of you would have been, if you had skipped doing your homework, and you had then declared that you "were not yet convinced that the globe is round". Or if you had expressed similar baseless doubts about similar well-established facts.
Do your basic homework first, and then you will be able to ask intelligent questions. That makes for the most efficient use of your time. SkepticalScience is an excellent (indeed, award-winning) repository of scientific information. Read. Think. And you will find there are very few questions that you need to ask. Any clarifications needed — and excellent teachers such as Scaddenp are happy to help answer a genuine question arising from your "homework".