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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)

Greenhouse gasses – mainly CO2, but also methane – were involved in most of the climate changes in Earth’s past. When they were reduced, the global climate became colder. When they were increased, the global climate became warmer. When CO2 levels jumped rapidly, the global warming that resulted was highly disruptive and sometimes caused mass extinctions. Humans today are emitting prodigious quantities of CO2, at a rate faster than even the most destructive climate changes in earth's past.

Abrupt vs slow change.

Life flourished in the Eocene, the Cretaceous and other times of high COin the atmosphere because the greenhouse gasses were in balance with the carbon in the oceans and the weathering of rocks. Life, ocean chemistry, and atmospheric gasses had millions of years to adjust to those levels.

Lush Eocene Arctic 50 million years ago

Lush life in the Arctic during the Eocene, 50 million years ago (original art - Stephen C. Quinn, The American Museum of Natural History, N.Y.C)

But there have been several times in Earth’s past when Earth's temperature jumped abruptly, in much the same way as they are doing today. Those times were caused by large and rapid greenhouse gas emissions, just like humans are causing today.

Those abrupt global warming events were almost always highly destructive for life, causing mass extinctions such as at the end of the PermianTriassic, or even mid-Cambrian periods. The symptoms from those events (a big, rapid jump in global temperatures, rising sea levels, and ocean acidification) are all happening today with human-caused climate change.

So yes, the climate has changed before humans, and in most cases scientists know why. In all cases we see the same association between CO2 levels and global temperatures. And past examples of rapid carbon emissions (just like today) were generally highly destructive to life on Earth.

Basic rebuttal written by howardlee

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 6 August 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 851 to 883 out of 883:

  1. MA Rodger @850 ,

    sorry, but your diagram [relative sea level map] is not loading for me.  (And yes, Nigelj, I have tried 3 computers!! )    I will take a stab, and presume the diagram is from Peltier (2002) .

    If I understand you correctly, then relative sea level  (disregarding the "short term" alterations from thermal volume effect and total mass change) during the Holocene interglacial . . . ought to receive a major influence from isostatic rebound of subpolar land regions, as well as from the more general land "rebound" secondary to oceanic siphoning.  And both of these rebounds should produce a late-Holocene fall in relative sea level (i.e. measured at shore-lines).   Perhaps excepting special cases like southern Britain.

    To all of which, we can add in some extra fall secondary to late-Holocene cooling that reduced ocean temperatures and also deposited more ice on land ~  an amount of ice presumably rather less than the 30-meter deposit I mentioned in last paragraph of #849 . . . (a deposit too marginal to really decrease the land rebound).

    I guess the whole question comes back to the nett result on "Mean Sea Level", from all these various factors.  In view of all the difficulties in local measurement of millennial changes, I am not surprised by the scatter of results from different studies on this topic.

    Almost - but not quite - it is enough to feel some sympathy for that remarkable outlier, the (recently) late Dr Moerner.

  2. Correction :-  not Peltier (2002) , but Khan et al. (2015) as per link in #850

  3. Eclectic:

    A direct link to MA Rodger's figure is:

    I notice that it is formatted to display is as a webp object, which might be breaking things for you if that format is not supported in your browser. It seems to be a Google format. YOu might try dropping the "as=webp" part.

    As for isostatic rebound: keep in mind that it is a local effect. The crust is depressed where ice weighs on it, and rebounds when ice is removed. Areas around the ice do the opposite - they flex upwards when the ice pushes downwards, and move back down when the ice is removed. Think of what happens when someone places an object on a waterbed.

    Interpreting sea level changes due to glaciation is rather complex, and evidence is difficult to find.

  4. A long time ago someone at SkS suggested that sea level in the  Pacific ocean peaked about 3-5,000 years ago and then went down about 1.5-2 meters.  This explained how atolls formed and why they have coral rock above current sea level.

    I do not see this change in MARodgers graph at post 843 but it seems consistant with his post at 850.  Is sea level change the current understanding of how atolls came to be above current sea level?  (Most atolls in the Pacific have 1-2 meters of coral stone at their core which cannot form above sea level.  Sea level could have changed more than that.)

  5. Michael - see here perhaps. This is hydro-isotasy at work as opposed to rebound from weight of ice.  Extra weight of water from ice melt deepening the ocean basins, dropping sea level slowly.

  6. It looks like MA Rodgers image in comment 850 does work if you drop the as=webp part of the URL:

  7. Eclectic thanks for pointing that out.  I just noted what looked like quite a marked temperature drop during the last 6000 years pre-spike was accompanied by at least a fairly steady rise in ocean levels.  So the sea level data is questionable. 

      Since the Antarctic ice has stayed relatively stable, it looks like the temperature drop and recent rise has has so far involved only the smaller (less than continent-size) ice sheets.  This would be expected if the smaller sheets were thinner, partly above the sea, and assisted by human  agriculture, deforestation, and industrialization, as these occurred almost exclucively in the N. Hemisphere because of greenhouse gas emissions.  Is an estimate of the proportion of these gasses attributed to the three human activities available?  I know even a rough estimate would be extremely complicated because plants use CO2 and N, as well as sequester them, plus we might know little about the extent of hydrocarbon extraction during the last 200 years, forest utilization, etc., but I bet it could be done.  

    I have read that the entire subarctic zone continues to lift upwards after the glacial ice maximum and subsequent melt.  Are there studies showing the contrubution of each to ocean depths?  

  8. Hal Kantrud @857 ,  the wording of your second sentence is awkward, since it seems to suggest a steady rise in ocean levels during our most recent millennia.  Surely that was not what you meant to say.  As expected, the very modern temperature spike has led to a sea level rise.

    Yet the 4-5000 previous years have been cooling ( roughly 0.7 degreesC) and, as you would expect from basic physics : more cooling overall means more land ice mass and therefore less ocean mass.  Does less ocean mass necessarily mean lower sea level globally?   MA Rodger's post #843 shows an almost flatline sea level for the last 2000 years . . . but as you have seen discussed, there are many factors & difficulties of measurement ~ so the illustrated "flatline" should be viewed as a rough approximation (and we get into comparing different criteria and even definitions of exactly what is meant by "global Mean Sea Level"  and Relative Sea Level).  However, that's all "past history" ~ and the problem we now have to tackle is the current & future sea level rise and associated CO2 problems.

    Bob Loblaw @856 , thank you for that almost psychedelic picture of the world.  I see that it is a model output, so I shouldn't look for granular detail along the coasts!

  9. Scaddenp:

    Thanks for the reference.  Exactly what I wanted to know.  As expected, scientists have figured out a complex evolution of mountains in the ocean.

  10. Eclectic I just drew a horizontal in my eye from 6000 YBP on Rodgers graph to 0 and it looked like about a 2m rise in sea level.  I ignored the more recent plateau.

    Is it accepted by most Earth scientists now that the recent spike in greenhouse gasses is the primary cause of the rise in sea levels, rather than land rebound or other factors?

  11. Hal Kantrud @860 , yes indeed what are we to make of all this !?!?

    On that graph, my eye-crometer shows a MSL uptick of 1m from 3000 BP to 2000 BP, and then zero change 2000 BP to  0 BP (1950.AD).

    I don't know the provenance of the graph - I gather it is displayed on Wikipedia, but I haven't looked to see where Wiki got it from.

    How useful is that thick black line?  You will note how the black line shows no rise in the final century, even though there has been a small measured rise 1850-1950.AD   . . . so the line is "thick" . . . and the strange (i.e. unnatural) angulations over past millennia must be the result of some author "hand-drawing" an approximate fit.  A fit to those many multicolored points scattered near the black line.

    Look at the multicolored data points.  Jamaica / Tahiti / Indonesia / Australia / New Guinea / Rio , and so on.   A great scattering, and showing much vertical spread too.  In other words, the black line is a rough approximation ~ and I advise not getting out your magnifying glass in order to fish for precise detail.

    We know that there has been a huge ( over 100m ) MSL rise from the depths of the glacial 20,000 years ago until the Holocene Optimum [strange name!].  Search, and you will find different studies/graphs from different parts of the world - and depending on the local level of land uplift etcetera, you will find at least some indicating a small fall of sea level in the last few thousand years . . . a fall which you would expect as a result of the 0.7 degreesC cooling [0.7 is a rough figure too, but fairly well established].

    Nevertheless, there are post-glacial local rebounds and ocean-floor depressions ["siphoning"] to be considered with every site measured.  Basically, for practical purposes, I wouldn't be too fussed about it all.  Simply stand back and look at the bigger picture.

  12. Hal Kantrud @860  (continued)

    The rise in sea level during the past 150 years or so is the result of oceanic "swelling" -  roughly half from thermal expansion and half from increased mass (the contribution from melting of land ice).  Land rebound is too small to make much difference over 150 years.  Likewise the "oceanic siphoning" as the ocean floor slowly sags a bit, under the increased weight of total ocean.

    In short, it's a matter of the observed global warming.  If you are questioning the connection between the recent spike in Greenhouse Gasses [ GHG's ] and the recent spike in global warming . . . then you've come to the right place, here at SkS website.  Loads of info available via the Home Page.

  13. For more detail on "sea-level" budget (the various contributors and estimating their impact), try this paper  from Church et al 2011. There may be more recent updates

  14. Eclectic @861,

    "That graph" of Holocene SLR you reference (referenced also @860 & pasted in-thread @843) is a bold piece of work found within Wikipedia and is based on the following:-

    I say 'bold' as combining the messy Holocene SLR data into a simple grand global average is not entirely scientific although it does prove illustrative. The IPCC AR5 does present a graphic (within Fig 5.15) providing the same sort of image surrounded by a blizzard of other graphical SLR data. This IPCC Fig5.15f  is essentially Fig 14 from Lambeck et al (2010) 'Palaeoenvironmental records, geophysical modelling and reconstruction of sea-leveltrends and variability on centennial and longer time scales'.

  15. MA Rodger @864 , thanks for that info ~ you are a champion.

    The study by Lambeck et al.(2010) was particularly interesting.  I apologize for and withdraw my skepticism about the 0 - 2000.BP  flatness of the sea level as shown on "your graph".

    The lateness of the Holocene highstand seemed (to me) probably explicable by some lag effects of the H. Optimum's warmth.  But if I understand it correctly, Lambeck favors a greater Southern Hemisphere effect lingering on and operating by melting some of the West Antarctica ice sheet up until fairly recent times (despite marginally greater NH cooling).

  16. Since we are discussing sea level rise, recent sea level rise is unprecedented over the past 2,500 years (Kopp et al 2016):

    Kopp 2016

    Anthropogenic forcing dominates global mean sea-level rise since 1970 (Slangen et al 2016):

    "the anthropogenic forcing (primarily a balance between a positive sea-level contribution from GHGs and a partially offsetting component from anthropogenic aerosols) explains only 15 ± 55% of the observations before 1950, but increases to become the dominant contribution to sea-level rise after 1970 (69 ± 31%), reaching 72 ± 39% in 2000 (37 ± 38% over the period 1900–2005)"

    Causes of sea level rise since 1900, from NASA and Frederikse et al 2020:

    Frederikse 2020


    1. Glacier-dominated cryospheric mass loss has caused twice as much sea-level rise as thermal expansion since 1900

    2. The acceleration since the 1970s is caused by the combination of thermal expansion and increased Greenland mass loss

    3. Ocean mass increases from land-based ice losses dominated the early 20th and 21st Century sea level rise record; ocean heating was the dominant component from 1970-2000

    4. The closure of the 20th-century sea-level budget derived here implies that no additional unknown processes, such as large-scale deep ocean thermal expansion or additional mass loss from the Antarctic Ice Sheet are required to explain the observed changes of global sea level

    Additionally, new research (Miller et al 2020) affirms modern sea level rise is linked to human activities, and not to changes in Earth’s orbit:

    "Surprisingly, the Earth had nearly ice-free conditions with carbon dioxide levels not much higher than today and had glacial periods in times previously believed to be ice-free over the last 66 million years, according to a paper published in the journal Science Advances.

    Our team showed that the Earth’s history of glaciation was more complex than previously thought,” said lead author Kenneth G. Miller, a Distinguished Professor in the Department of Earth and Planetary Sciences in the School of Arts and Sciences at Rutgers University-New Brunswick. “Although carbon dioxide levels had an important influence on ice-free periods, minor variations in the Earth’s orbit were the dominant factor in terms of ice volume and sea-level changes — until modern times.”

    Sea-level rise, which has accelerated in recent decades, threatens to permanently inundate densely populated coastal cities and communities, other low-lying lands and costly infrastructure by 2100. It also poses a grave threat to many ecosystems and economies.

    The paper reconstructed the history of sea levels and glaciation since the age of the dinosaurs ended. Scientists compared estimates of the global average sea level, based on deep-sea geochemistry data, with continental margin records. Continental margins, which include the relatively shallow ocean waters over a continental shelf, can extend hundreds of miles from the coast.

    The study showed that periods of nearly ice-free conditions, such as 17 million to 13 million years ago, occurred when the concentration of atmospheric carbon dioxide — a key greenhouse gas driving climate change — was not much higher than today. However, glacial periods occurred when the Earth was previously thought to be ice-free, such as from 48 million to 34 million years ago.

    We demonstrate that although atmospheric carbon dioxide had an important influence on ice-free periods on Earth, ice volume and sea-level changes prior to human influences were linked primarily to minor variations in the Earth’s orbit and distance from the sun,” Miller said.

    The largest sea-level decline took place during the last glacial period about 20,000 years ago, when the water level dropped by about 400 feet. That was followed by a foot per decade rise in sea level — a rapid pace that slowed from 10,000 to 2,000 years ago. Sea-level rise was then at a standstill until around 1900, when rates began rising as human activities began influencing the climate.

    Future work reconstructing the history of sea-level changes before 48 million years ago is needed to determine the times when the Earth was entirely ice-free, the role of atmospheric carbon dioxide in glaciation and the cause of the natural fall in atmospheric carbon dioxide before humans."


    From the source paper, Miller et al 2020:

    "High long-term CO2 caused warm climates and high sea levels, with sea-level variability dominated by periodic Milankovitch cycles.

    Sea level rose in the Early Pliocene ca. 4.7 Ma, reaching highs that had not been consistently seen since the MCO. From a sea-level perspective, the Pliocene is marked by three intervals with sea level well (~10 to 20 m) above modern: 4.6 to 4.1, 3.9 to 3.3, and 3.3 to 2.85 Ma.

    GMSL higher than 12 m above modern requires loss of ice sheets in Greenland, West Antarctica, and sensitive areas of East Antarctica, the Wilkes, and Aurora Basins. This interval is of keen interest, because global temperatures were >2°C warmer than today at times when atmospheric CO2 concentrations were on the order of those in 2020 CE (~400 ppm), and thus, the equilibrium sea-level state is relevant to ice sheet trajectories in coming centuries. The peaks between 3.9 and 3.3 Ma were even higher, reaching a peak of ~30 m during Gi13, and thus requiring some melting of the EAIS.

    The development of a permanent EAIS by 12.8 Ma resulted in a change from responding to precession, tilt, and eccentricity to subdued to absent response to eccentricity and precessional forcing that had previously been strong; the 41-ka tilt cycle dominated ice sheet and sea-level response from ca. 12.8 to 1.0 Ma following the development of a permanent EAIS. During the mid-Pleistocene transition, very large, 100-ka paced LIS were amplified by 100-ka changes in CO2 from ~180 (glacial) to 280 ppm (terminations).

    During the last deglaciation (ca. 19 to 10 ka), GMSL rise exceeded 40 to 45 mm/year, providing an upper limit on known rates of GMSL rise. Rates before radiocarbon ages are less certain, although the sea-level rises exceeded 10 mm/year during terminations. Sea-level rise progressively slowed during the Holocene until the late 19th to early 20th century when rates began to rise from near 0 to 1.2 mm/year in the early 20th century to a late 20th and 21st century rise of 3.1 ± 0.4 mm/year.

    Sea level follows long-term trends of atmospheric CO2, with high sea levels associated with high CO2 and warm climates. CO2 played an important role with high CO2 maintaining warmth in the Eocene (with values >800 to 1000 ppm; associated with largely ice-free conditions and high sea levels. Generally, decreasing CO2 values during Middle Eocene to Oligocene led to cooling and glaciation, although a secondary CO2 increase at ca. 35 to 36 Ma may be associated with the late Late Eocene warming. The cause of the CO2 decrease over the past 50 Ma has been widely discussed and debated but must be due to long-term (107-year) changes in CO2 sources (e.g., higher CO2 associated with inferred higher ocean crust production rates) or more likely the effectiveness of sinks CO2 (e.g., increased weathering associated with uplift of the Himalayas or exposure of basalts in tropical regions).

    Our records that suggest nearly ice-free conditions occurred during the MCO and are thus intriguing if this is an equilibrium state for warming levels that will be attained in this century or the next century under sustained greenhouse gas emissions.

    Our sea-level history constrains cryospheric evolution over the past 66 Ma, with ice-free conditions during most of the Early Eocene, MECO, latest Eocene, and possibly the MCO, with ice sheets (up to 40-m sea-level equivalent) in the Middle to Late Eocene greenhouse and with continental-scale Antarctic ice sheets beginning in the Oligocene.

    From 34 to 13.8 Ma, the EAIS varied from larger than today (sea-level ~35 m below present) to nearly ice-free (~50 m above present) but became permanent during the MMCT ca. 12.8 Ma."

    Miller 2020, Figure 4, rotated once:

    Miller 2020

    And the past 40,000 years, from Miller 2020, Figure 4 above:

    Miller 2020

  17. Daniel baily:

    From your second link:

    "Our records that suggest nearly ice-free conditions occurred during the MCO and are thus intriguing if this is an equilibrium state for warming levels that will be attained in this century or the next century under sustained greenhouse gas emissions."

    Ice free conditions means sea level 60-65 meters higher than current!!  Then the question is how long will it take for all that ice to melt.  At a foot a decade it would not be too long before many cities were in troube.  I live in Florida (at over 40 feet elevation) and parts of South Florida are already hurting from only 1 foot of sea level rise.

    RrealClimate has several good discussions of potential sea level rise.  2013 article  2016 article  In general, the IPCC estimates are at the low end of what expert opinion is.  This is because of the method the IPCC uses to come to a consensus.  The IPCC states what a consensus of experts feel is the lowest range of sea level rise.  The average of experts is then higher than the IPCC reports.  There is a lot of discussion on melting rates.  It seems to me that knowledge of probable melting rates is still being developed.

  18. michael sweet @867,

    The quote you cite from #866 is perhaps a little out of context for your purpose as within Miller et al (2015) it is embedded within a discussion of the relationship between sea level, temperature & CO2. With reference to features in its Fig 1, this passage runs:-

    "Relationships among proxy records of CO2, climate, and sea level are not always clear. For example, large changes in Miocene [23Ma to 5Ma] cryospheric evolution and deep-water temperature occurred with only what appear to be modest changes in CO2. The Miocene Climatic Optimum [17Ma] is an interesting case in point where B isotopes indicate variations from 300 to 500 ppm [of CO2] during this interval of ~4°C warmer global temperatures above present. Our records that suggest nearly ice-free conditions occurred during the MCO ... are thus intriguing if this is an equilibrium state for warming levels that will be attained in this century or the next century under sustained greenhouse gas emissions. Similarly, the long-known mismatch between Eocene [56Ma to 34Ma] to Oligocene [34Ma to 23Ma] CO2 proxies (with the main CO2 drop in the later Oligocene) and the earliest Oligocene glaciation has been perplexing, although we show that decreases in CO2 during the Middle to Late Eocene are associated with bottom water cooling and ice growth. In all cases, we note that errors on pre-ice core CO2 values are large (hundreds of ppm) and could mask closer relationships. We look forward to more refined estimates of Eocene to Miocene CO2 to test these relationships in detail." [Geological dates added]

    The MCO marks the point when global temperatures tipped down towards the Pleistocene era of glaciation/interglacial cycles. Through this time, the layout of the globe has changed enough I think to put into some doubt any direct comparison with the MCO and a +4°C world of today.

  19. @michael sweet

    "Then the question is how long will it take for all that ice to melt"

    Sea level rise from ice sheets continue to track worst-case (High scenario) climate change scenarios (discussion here, source paper here). 

    SLR by 2100


    Which, charitably, means 2.0 meters SLR by 2100, given that the Greenland Ice Sheet has tipped into a negative mass balance stateEarth having lost 28 trillion tons of ice in the past 23 years and that Greenland is expected to exceed Holocene loss rates by 2100.

    Greenland's future

    Image Source

    Typically, when climate scientists try to understand some of the expected future effects of global warming and climate change, they first look to the past. And in looking to the past, we can use the example of the climate transition from the icy depths of the Last Glacial Maximum into our current Holocene Interglacial to guide us. From about 21,000 years Before Present (BP) to about 11,700 years BP, the Earth warmed about 4 degrees C and the oceans rose (with a slight lag after the onset of the warming) about 85 meters.

    However, the sea level response continued to rise another 45 meters, to a total of 130 meters (from its initial level before warming began), reaching its modern level about 3,000 BP.

    This means that, even after temperatures reached their maximum and leveled off, the ice sheets continued to melt for another 8,000 years until they reached an equilibrium with temperatures.

    Stated another way, the ice sheet response to warming continued for 8,000 years after warming had already leveled off, with the meltwater contribution to global sea levels totaling 45 additional meters of SLR.

    Which brings us to our modern era of today: over the past 100 years, global temperatures have risen about 1 degree C…with sea level response to that warming totaling about 150 mm. Recently, accelerations in SLR and in ice sheet mass losses have been detected, which is what you’d expect to happen when the globe warms, based on our understanding of the previous history of the Earth and our understanding of the physics of climate.

    Support for above:

  20. Daniel Bailey and MARodger:

    I have not kept up on sea level rise as much as you have the past few years.  Your data and posts help me a lot with most recent thinking.  I note that every IPCC report they raise their estimates of future rise as more is known.

    I remember about 10 years ago I wondered if I would live long enough to see damage from sea level rise (I am currently 62 years old).  I now see damage from sunny day flooding recorded in many locations around the globe, including serious damage in my home state of Florida and city of Tampa.  I now expect to see many homes in Florida abandoned from sea level rise.

  21. This picture shows sunny day tidal flooding in St. Augustine, Florida this past September.

    Flooding in Florida

    The Guardian article says that nusance flooding affects 60% of historical locations (the economy is based on historical tourism) and with 1.5 feet (about 45 cm) of additional rise all historical locations will be affected.

    How can people live with regular flooding?  You would never build a structure in a location that floods once a month.  People everywhere refuse to accept the fact that these locations no longer are viable locations to live.  Eventually they will have to retreat.  Millions of people in Florida live within 1 meter of sea level.

    In addition to tidal flooding, the increase in the water table underground makes it much harder to get rain to drain away and causes more flooding when it rains.

  22. It's time the scientists admit they really don't know what they are doing.


    [TD] You must be more specific in your comments. Future comments like this one will be deleted without warning.

  23. My favoirite hubris spewing climate denier is at it again.

    He was smarting of to a person in Colorado who was discussing the current drought in the Westen half of the US.

    [What a coincidence.

    The same thing happened in the previous Inter-Glacial Period, and the one before that and the one before that and the other 5 before before that.

    Are you foolishly blind enough not to see the pattern?

    The West always undergoes a drought during Inter-Glacial Periods.

    Tell us why this Inter-Glacial Period should be different.

    Ice sheets and glaciers always melt during Inter-Glacial Periods.

    Tell us why this Inter-Glacial Period should be different.

    Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods.

    Tell us why this Inter-Glacial Period should be different.

    Tell us why the sea level should not rise at least another 4 meters like it did in the other 8 previous Inter-Glacial Periods.

    Global temperatures in the other 8 previous Inter-Glacial Periods were at least 7°F warmer than present.

    Tell us why this Inter-Glacial Period should be different.

    Tell us why this Inter-Glacial Period should be a statistical anomaly and be colder with lower sea levels and no melting and no drought.

    Can you do that?]

    This denier is so hostile and over the top smug and arrogant in his replies it's hard to take them seriously.

    Is it accurate that the west always undergoes drough during interglacials?

    Also I don't think it's correct that Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods.

    How can this denier make this claim? Global temperatures in the other 8 previous Inter-Glacial Periods were at least 7°F warmer than present.

    • Tell us why this Inter-Glacial Period should be different.
    • Tell us why this Inter-Glacial Period should be a statistical anomaly and be colder with lower sea levels and no melting and no drought.

    My response to the above bullets would be that this interglacial is nearning it's end and we should be seeing a global cooling effect, but we are seeing a warming effect due to human activity.  Not sure what more I could add to my response to the bulleted statements made by this denier.

  24. TVC15 @873  ~ yes, it's marvellous  to observe the shameless rhetorical mendacity of some of these deniers.  Hard to say whether they believe their own nonsense, or whether they simply aim to score Debating Club points by playing any arguments (arguments 100% true or 95% false).   But .... it is what it is, with these trolls.

    I love their dismissive style of argument - "It's stupid to worry about a few extra degrees of warming, when only 4 billion years ago the Earth's surface temperature has been 200 degrees hotter."   And so on.

    As you know, TVC15, each glacial cycle is somewhat different from each of the others - but what we have to face up to is to deal with modern problems : not the problems of a million years before humankind arrived.   So why is he himself running away from facing up to the present situation?

    Perhaps your denier friend would be unfazed by having a sea level 3 meters deep in his own house's living room?   Or is it only *other* people's houses he is unworried about.

    . . . 2

  25. . . . 2

    TVC15 , a recent high-resolution sea-shore study [ IIRC - Kulp and Strauss 2019 ] indicates that a 1 meter SLR would displace approx 230 million people from their houses and farmlands.  And factoring saline inundation by storm surge, there would be many millions of refugees displaced well before the "average"  1 meter rise is reached.  So that's likely to be getting underway before 2100 even though the full 1m rise won't come until after 2100.   And doubtless, some of these many refugees will need to settle in the neighbourhood of this denier's great-grandchildren.  No social disruption at all !

    And for a 2m rise in sea level, you can add a few hundred million more refugees . . . all wanting to go to higher ground . . . like Colorado, or wherever this unworried denier has been living.

    Western droughts come and go - and sometimes stay for centuries, judging by the history of the last 2000 years.   ( I'm sure your denier friend will tell the farmers and townsfolk, that they simply need to wait patiently, for a few generations or so.)

    Note that the PAGES12k proxy studies show that world temperature is currently same or slightly higher than the peak of the Holocene - though many deniers still falsely claim that we are "colder than the Medieval Warm Period".    Go figure !

  26. TVC15.

    Your interlocutor seems to be using a combination of "Climate has changed before" (this thread) and "we're coming out of an ice age", which is an odd twist on "we're going into an ice age". Maybe he's confused with "we're coming out of the Little Ice Age".

    There is a post here at SkS that talks about sea level over the past 150,000 years. It includes the following graph of sea level, covering the last glaclal/interglacial cycle:

    Sea level past 150000 years


    I do not see anything on that graph to support his claim that sea level is normally 4 to 14m higher than now during interglacials. The peaks of that graph may be a touch higher, but there sure isn't a steady period of higher sea levels in the last interglacial. I suspect his claim is completely bogus, but you could always try to get him to provide his source of data.

    On very long time periods, ocean basin size/shape changes have dramatic effects on relative sea level. On short time scales (glacial/interglacial cycles), local sea level in areas subject to the weight of ice have serous effects due to isostatic depression and rebound.

    Of course, the answer to "why should this interglacial be different?" is "because we're adding CO2 to higher levels than seen in hundreds of thousands of years, and we're looking at rising temperatures and conditions not seen in past interglacials".

  27. Eclectic @874, 875

    Hi Electric,

    Thanks for the responses!

    I don’t understand why this denier is trying to make it a fact that the Western half of the US always undergoes a drought during Inter-Glacial Periods when I don’t think we really know, or do we?

    They mention the other 11 interglacial periods, during which modern humans did not exist until the Holocene. I’ve also head a new epoch called the Anthropocene, with the start date of this new epoch still in debate. Not sure if this is recognized as the most current epoch.

    Since humans did not occupy most of those interglacial cycles, I don’t understand the deniers argument. There is nothing to connect the dots between past interglacials and our current situation.

    Now instead of saying climate always changes, deniers are claiming that what we are witnessing all over the globe today is typical during an interglacial cycle.

    How can we know the conditions of ancient interglacials? Did they all cause drought in the Western half of the US?


  28. Bob Loblaw@876

    [Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods.]

    Thanks so much for responding to this deiners claim and providing that graph!


  29. TVC15 , from experience, you know how anything & everything is grist for the deniers' mill, in their attempts to minimize and/or deny the climate science.   It's always ABCD ~ Anything But Carbon Dioxide.

    And their excuses come in cycles of excuse ~ first: It's Not Warming . . . then: It's only warming a bit . . . then: Yes, it's warming a lot, but it's Not caused by humans . . . then: Well yes it's half-caused by humans, but the Warming is really very very good for us and is saving us from disastrous cooling.

    Then it's back to: the Warming has Stopped and it's cooling now (for at least 6 years' cooling, says the deluded Monckton) . . . and a Colossal Grand Solar Minimum will have all of Canada under a mile-thick ice sheet by 2050 or somesuch date.

    For millennia, the US Southwest has been arid - the opposite of the Northeast.  And you can find other regions of the world likewise arid.  All it needs is a slight variation in climate, and you've got a mega-drought (a drought defined as >20 years) or a super-mega-drought (for centuries).   Nothing very new about that  ~ except that now our Anthropogenic global warming is exacerbating the droughty tendency.  (The exacerbation being the point your denier wishes to deny.)

    I haven't studied past Interglacials w.r.t. aridity & droughts.  Presumably similar overall conditions in the past have caused rather similar episodes. But that's all rather irrelevant to the current situation, which must be dealt with on its own terms.  And the current droughty tendency doesn't disprove the climate science, nor does it fail to point to more of the same trouble in future, as AGW worsens.

    Of course, as we look back in time, the proxy evidence (in past Interglacials) gets fuzzier & fuzzier ~ so conclusions of any sort get more difficult to make.

    The "Anthropocene" is a semi-humorous label.  Not official.  But the label does get up the deniers' noses.  Still, it does rhetorically emphasize that the Holocene is transitioning into something significantly different from the "natural".

  30. A follow-up to the "sea level was higher in past interglacials" claim.

    Is this person claiming that sea level is indeed rising to dangerous levels, but it's natural?

    Is this person claiming that sea level has risen naturally in the past, so rising CO2 and rising temperatures cannot change this previous pattern?

    The part TVC15 has quoted is the usualy mishmash of vague, unspecific claims. Good luck trying to pin the denier down to a specific claim or statement that can be examined by evidence - my experience is that such individuals often are incapable of expressing themselves clearly enough.

  31. TVC15 @873,
    Your denialist is actually making four bold statements that are patently nonsense with the rather pathetic request that you "Tell us why this Inter-Glacial Period should be different."

    "Ice sheets and glaciers always melt during Inter-Glacial Periods." The melting actually happens in the run-up to the "Inter-Glacial Periods" which is what makes them "Inter-Glacial Periods" so in one respect this is entirely straw man territory. If the bold assertion is that glaciers and ice sheets shrink as they do today throughout an inter-glacial, that is false as sea levels of past millennia demonstrate.

    "Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods." This is not supported by the evidence that
    suggests only two or three of the eight had higher sea levels. (The graphic is from here but originates from this web engine.)
    800,000 years sea level

    "Global temperatures in the other 8 previous Inter-Glacial Periods were at least 7°F warmer than present." Again not supported by the evidence. A google search provides many graphical representations of 800,000y temperatures and globally the present interglacial has been warmer than all but three of them (although AGW may be on course to change that ranking).
    800,000 year temperature

    "The West always undergoes a drought during Inter-Glacial Periods." This is a more specialist assertion. That there has been "a drought" in "the West" through the Holocene is potentially correct. It isn't a place with massive rainfall. But more accurately there are periods of drought and periods when the rain is heavier. What we see to make sense of that is a bit of a Hockey Stick situation with drought conditions becoming more wide-spread. The graphic comes from here an account which does address the question "Will anthropogenic climate change cause the West to get drier or wetter?"
    West drought since 800AD

  32. IIRC, the Last Glacial Maximum saw a greater buildup of land-based ice than in previous glacial phases.  By the time maximum warming in the Holocene had been achieved and a natural cooling initiated, a great deal of ice remained in Greenland and Antarctica that had already been lost by that same point in previous interglacials. 

    Given time to reach equilibrium with the modern forcing, global sea levels will eventually surpass any levels achieved in previous interglacials spanning the previous 800,000 years+, and likely far longer than that.

    From Dutton 2015, Figure 5:

    Dutton 2015 - Figure 5

  33. Many thanks to Eclectic, Bob Loblaw, MA Rodger and Daniel Bailey.  

    I gain so much from each of you every time I post denier claims.

    Thank you all for your time and efforts to help educate us! 

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