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Carbon Dioxide the Dominant Control on Global Temperature and Sea Level Over the Last 40 Million Years

Posted on 5 March 2013 by Rob Painting

Key Points:

  • Because the water contained in land-based ice sheets is ultimately derived from the ocean, over long (geological) timescales global sea level is largely determined by global temperature and, consequently, the temperature-dependent volume of ice stored on land.
  • Since the concentration of carbon dioxide in the atmosphere (The Greenhouse Effect) exerts such a powerful influence on global and polar temperature, it therefore follows that it should also exhibit a strong relationship with global sea level over geologic intervals of time.
  • Foster & Rohling (2013) examined time slices of paleo data covering the last 40 million years to uncover the details of this carbon dioxide-sea level relationship. Surprisingly, they found a consistent and robust relationship between carbon dioxide and sea level irrespective of other contributing factors.
  • Based on the concentration of carbon dioxide in the atmosphere as of 2011, the authors estimated that future sea level is committed to rise  24 metres (+7/-15 m) above present-day once the land-based ice sheets have fully responded to the warming and the Earth is once more in equilibrium. 
  • The authors estimated that this sea level rise will likely take place over many centuries, if not several thousand years, but it nevertheless represents the long-term consequences of human industrial activity, and is  further evidence that CO2 is the Earth's "main control knob" for global temperature. 

Figure 1(a) - Relationship between atmospheric CO2 (in parts per million) and global sea level (in metres) over the duration of the ice core record. The dashed horizontal line depicts the pre-industrial state for comparison (b) Shows the cross-plot of CO2 versus sea level rise over this period. CO2/Co on the horizontal axis is the CO2 ratio relative to the pre-industrial where 0.0 respresents the pre-industrial, with negative values (below) and postive values (above) relative to pre-industrial. From Foster & Rohling (2013) 

Background Context: Greenhouse Gases and Planetary Warming

Carbon dioxide is the most significant of the greenhouses gases, gases that trap heat in Earth's atmosphere and reduce the rate of heat loss to space in the upper layers of the atmosphere. As a result the temperature of the planet's surface and ocean is largely dependent upon the concentration of greenhouse gases in the atmosphere. Generally-speaking; increase the concentration of greenhouse gases and the planet traps more heat, reduce them and the planet cools. Because of this behaviour, renowned glaciologist, Richard Alley, has dubbed carbon dioxide Earth's biggest temperature control knob

The oceans are the Earth's largest heat reservoir and, in comparison to the atmosphere, they respond very slowly to warming because of their immense mass, vast heat capacity, and the length of time it takes for heat to be transported down to the deep ocean. The long-lived nature of CO2 therefore means that the oceans (and the atmosphere) will continue to warm, albeit at a comparatively much slower rate, even long after humans have ceased pumping planet-warming greenhouse gases into the atmosphere (Meehl [2012]). In other words, we have yet to see the land-based ice fully respond to human intervention in the Earth's  climate.

How High Will The Water be Mama?....

In examining ancient sea level it would, of course, be preferable to be able to match sea level at any given point in Earth's history with global temperature, especially as further warming is in the pipeline. However, a number of problematic assumptions have to be made in order to transform various paleo data into global temperature estimates over seperate geological periods. Foster & Rohling (2013), therefore, quantified the relationship between two entities that could be measured more directly - land ice volume/sea level and carbon dioxide levels. So, based on the carbon dioxide/sea level relationship, and the greenhouse gases humans have already emitted, how high is this sea level rise we are already committed to? The authors used high quality data from the Earth's geological past to answer this question.

Sea Level & the Ice Cores

Ice core records retrieved from drilling expeditions on the Antarctic ice sheet date back 800,000 years (Petit [1999]Luthi [2008]) and provide a high fidelity record of the Earth's atmospheric carbon dioxide concentration over this interval. This is due to the fact that the ice contains bubbles of air which became trapped in snowfall and were eventually sealed off from the atmosphere over time. Each air bubble, therefore, contains a tiny sample of carbon dioxide from the atmosphere at the time it was sealed off, and each successive layer of ice deposited enables a reconstruction of atmospheric carbon dioxide going back in time - as shown in Figure 1.

During the last few million years (the Pleistocene), the Earth has been in the grip of the ice ages - long cold intervals (glacials) interspersed with shorter warm periods such as today (interglacials)- which were driven by orbital forces, and reinforced by carbon dioxide acting as a feedback (although the details are sketchy). Even though the ice sheets only provide us with 800,000 years of this period, it is most probable that sea level rarely exceeded the pre-industrial level. This can be seen in Figure 1 for the last 550,000 years.

So, although useful in establishing the overall carbon dioxide/sea level relationship, the overwhelming majority of the ice core record is not greatly informative of the future because sea level and atmospheric carbon dioxide were generally much lower than present-day, and sea level change involved the growth or disentegration of the gigantic Laurentide and Fennoscandian ice sheets in the Northern Hemisphere - ice sheets which do not exist today. Furthermore, the  relationship of steadily increasing CO2 with rising planetary temperature  is not a linear one. So we can't simply extrapolate the past CO2/sea level relationship into the future with any confidence.

Seeing Further Back in Time - Alkenones and Boron Isotopes

Throughout the last 40 million years the Earth has cooled in fits and starts from a very warm state with virtually no land-based ice, to the current glacial/interglacial period where monstrous ice sheets are capable of growing upon the land masses of the Northern Hemisphere. Given that Earth is headed toward a future where the polar ice sheets are greatly reduced, warmer and cooler periods  of ancient Earth might yield some insight. To examine these intervals it was necessary for Foster & Rohling (2013) to turn to other paleo data, such as alkenone and boron isotopes. These methods rely on the knowledge that carbon dioxide dissolved into the ocean, and therefore ocean pH, are dependent on the partial pressure of carbon dioxide in the atmosphere.

Alkenones are organic (natural) compounds produced by certain types of ocean surface-dwelling phytoplankton and, like other paleoclimate proxies, provide information about the state of Earth's climate at the time they were formed. Carbon dioxide from seawater is incorporated into living tissue through the process of photosynthesis, and alkenones are one of the by-products. As a result, alkenones contain information about the amount of carbon dioxide dissolved in seawater and therefore atmospheric CO2 concentrations - since they are derived from the well-mixed sea surface layers.  These organic compounds are extremely durable and are highly resistant (although not immune) to long-term degradation (diagenesis). Over the course of time the bodily remains of alkenone-bearing phytoplankton settle on the ocean floor and build up in  sediments. By examining sediments taken from cores drilled into the seafloor it is possible to use our knowledge of chemistry and biology to work backwards and reconstruct atmospheric CO2 at the time the alkenones were formed. See Pagani (2002) for a review of alkenones as proxies. 

Boron isotopes are another sediment-based method employed to determine ancient carbon dioxide concentrations. This technique differs somewhat from alkenones in that the chemical signatures are stored within the fossilized shells of foraminifera (forams), tiny marine life which live near the sea surface (planktic). Like alkenones, this surface-dwelling characteristic is important because the surface ocean layers are well-mixed with the atmosphere, and therefore ocean pH, once reconstructed, allows atmospheric CO2 to be calculated (albeit with some degree of uncertainty). Minerals such as boron dissolved in seawater are incorporated into the shells of forams and, because the boron isotope ratio is predominately a function of the ocean pH, they are recorders of ocean pH at the time when the shells were formed. See Foster (2008) for details. 

The S-Shaped Nature of the Sea Level/Carbon Dioxide Relationship

Foster and Rohling (2013) examined 5 time slices over the last 40 million years -the ice core record dating back 550,000 years, 2.7-3.1 million years ago during the Pliocene, 11-17 million years ago during the Miocene, 33-35 million years ago during the Eocene-Oligicene boundary, and 20-40 million years ago during Eocene-Oligicene. Sea level for the ice core record was based upon the Red Sea reconstruction (discussed in this SkS post). For the other time slices  estimates are based upon the oxygen isotope ratio (global ice volume signal) in fossilized forams (Waelbroeck [2002]), the magnesium/calcium content of foram fossils to isolate the oxygen isotope/global ice volume signal (Barker [2005]). And finally, analysis of the character of depositional features (lithofacies) and physical structure of ocean floor sediments (backstripping), which can be used to estimate global sea level volume.      


Figure 2(a) - cross-plot estimates of atmospheric CO2 and sea level. The data are labelled according to the time period and reconstruction technique employed. The dashed vertical and horizontal lines denote the pre-industrial CO2 and sea level respectively. The vertical and horizontal bars on each data point represents the uncertainty of the sea level and CO2 estimates respectively. 3(b) Probabilistic analysis that fully accounts for uncertainty in both sea level and CO2 data, with the bold line indicating the probability maximum . From Foster & Rohling (2013).  

Interestingly, data and subsequent atmospheric CO2 and sea level reconstructions, show very good agreement in spite of the various methods used. This can be seen especially over the ice core record, where data from the Miocene and Pliocene are consistent with the Red Sea reconstruction. This is surprising given the different continental configurations, oceanic pathways, and  mountain-building (orography) over this vast stretch of time might have been expected to yield varied estimates. This general agreement does, however, give some degree of confidence that the results are robust.    

The most notable feature of the reconstruction, however, is its s-shaped (sigmoidal) nature. Despite global temperature decreasing between 650 to 400 parts per million (ppm) of atmospheric CO2, the sea level response appears to taper off. It is obvious that this interval is very light on data points, but given the characteristic shape of all the data, it seems unlikely that the overall complexion will change once the interval is populated with more data. It should be noted, however, that this data (above 400ppm) is generally from intervals when land-based ice was growing and sea level falling as a consequence. In contrast, data from 180-400 ppm comes from both cooling and warming periods, which suggests that ice sheet growth and disintegration, between these CO2 concentrations, exhibit similar behaviour. 

That may not be the case with rising sea level at values above 400ppm, the system may exhibit hysteresis. In other words, above 400 ppm warming and sea level rise may not behave in the same manner as it did when the planet was cooling and sea level was falling. The ultimate fate of land-based ice above atmospheric CO2 concentrations of 400 ppm could be path-dependent. 

........24 Metres High and Rising? 

Atmospheric CO2 concentration as of 2011 was around 392 ppm, and based upon that and the carbon dioxide/sea level relationship revealed in their research, Foster & Rohling (2013) calculated that long-term sea level rise will reach 24 metres (+7/-15 metres at 68% confidence) above present-day sea level once the planet has fully responded to the warming. This is likely to be achieved through extensive disintegration of the West Antarctic and Greenland ice sheets, and a substantial part of the coastal sector of the East Antarctic ice sheet.

The existing land-based ice is equivalent to 60-70 metres, so the loss of 24 metres of sea level worth of ice suggests that over a third of the ice sheet volume may already be committed to disintegration. As noted in the introduction, this dramatic response represents the long-term consequences of human industrial activity and reinforces the concept that atmospheric CO2 is Earth's main temperature control knob.

For more information about this type of research go to:

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Comments 51 to 91 out of 91:

  1. James Hansen, Makiko Sato, Gary Russell and Pushker Kharecha have a draft paper Climate Sensitivity, Sea Level, and Atmospheric CO2 which contains the following succinct description of the geological carbon cycle and temperature change over the Cenozoic (65Ma - present):

    Carbon dioxide is involved in climate change throughout the Cenozoic era, both as a climate forcing and as a climate feedback. Long-term Cenozoic temperature trends, the warming up to about 50 Myr BP (before present) and subsequent long-term cooling, must be primarily a result of the changing natural source of atmospheric CO2, which is volcanic emissions that occur mainly at continental margins due to plate tectonics (popularly "continental drift"). This CO2 source grew from 60 to 50 My BP as India subducted carbonate-rich ocean crust while moving through the present Indian Ocean prior to its collision with Asia about 50 Myr BP (Kent and Muttoni, 2008), causing atmospheric CO2 to reach levels of the order of 1000 ppm (parts per million) at 50 Myr BP (Beerling & Royer, 2011). Since then, atmospheric CO2 declined as the Indian and Atlantic Oceans have been major depocenters for carbonate and organic sediments while subduction of carbonate-rich crust has been limited mainly to small regions near Indonesia and Central America (Edmond and Huh, 2003), thus allowing CO2 to decline to levels as low as 170 ppm during recent glacial periods (Petit et al., 1999). Climate forcing due to CO2 change from 1000 ppm to 170 ppm is more than 10 W/m2, which compares with forcings of the order of 1 W/m2 for competing climate forcings during the Cenozoic (Hansen et al., 2008), specifically long-term change of solar irradiance and change of planetary albedo (reflectance) due to the overall minor displacement of continents in that era.

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  2. Rob Honeycutt@8, "biggest" versus "main" is quibbling. I know "biggest" was used in the title of Ally's lecture. I also know he wasn't addressing the general public at the fall 2009 AGU meeting. But Ш used "main" above because that was the word being used here. My point is the greenhouse gases do serve as a thermostat for the climate in the sense that ordinary people understand in their daily lives, and CO2 is the main greenhouse gas because it has a very long residency. For example, many cooking recipes begin "Preheat the oven to..." Why? Because there is a noticeable time delay between setting the desired temperature and the oven finally reaching that temperature. Using the term "thermostat" or "main thermostat" for the atmospheric CO2 concentration would make it easier for non-scientists to grasp the fact that the current increased global temperature DOES NOT correspond to the "setting" on the global thermostat any more than the temperature in your oven one or two minutes after you turn it on corresponds to the setting of its thermostat. Depending on the oven and what temperature you set, it might take ten or twenty minutes or even longer to reach the equilibrium temperature. One estimate of the relaxation time for the earth temperature-CO2 equilibrium is about 700 years (roughly half the turnover time of the thermohaline circulation).

    Kevin@38, "removal of CO2 from the atmosphere is a worthwhile endeavor because the decline in temp is the same as the incline has been." Yes and no. If we reduced today's 394 ppm to 350 ppm CO2, then the temperature would continue to increase for several hundred years because we are still far from the equilibrium temperature for 350 ppm. If you look at the graph, 350 ppm is about 15% higher than the highest peak in the last 800 thousand years. Getting the CO2 down to 300 ppm might stop the temperature increase. Might. Getting it down to 250 ppm would most likely start a slow cooling. So, yes, reducing the CO2 level would be good. No, it won't cause a temperature decline UNLESS the reduction in CO2 is sufficiently large.

    In summary, greenhouse gases are the thermostat for the temperature of the earth climate. No greenhouse gases, which is impossible for our planet because there would undoubtedly be a small amount of water vapor in the atmosphere, means a snowball earth with little or no life as we know it. Life, as we know it, has a rather narrow range of tolerable temperatures. In the past different natural factors changed the thermostat setting, rapidly in some cases, where rapidly typically means a time scale of a century or millenium, and slowly in other cases, where slowly typically means a time scale of ten or a hundred millenia or even a million years. If you don't like atmospheric CO2 "controlling" the temperature of the earth's climate, then you are free to think that whatever it is at a given time that changes the setting of the CO2 thermostat is what is really controlling the the temperature (via the CO2 concentration). Currently, humans are cranking the thermostat up and up and up. So, you can say, "No, CO2 is not 'causing' the climate to heat up. People are 'causing' the climate to heat up by putting more CO2 in the atmosphere."

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  3. Just a quick point on assymmetry. I dont see that symmetry should be expected. At least two processes are not. Firstly ice sheet/shelf/glacier development versus break up is assymetrical largely because of calving. eg a glacier retreats much faster than it advances. Also affecting albedo is vegetation loss/recovery. The second one is methane (which becomes CO2) from swamps and possibly clathrate though isotope studies suggest swamps are most important. Accumulation is slow process, release very fast.

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  4. Kevin,

    I have also asked your question of many, and so far have not received a satisfactory answer either.  It is said that the Milankovitch cycle is not "strong" enough to force an ice age or an interglacial, yet it sure seems to be able to reverse a warming trend just as CO2 is higher than at any time in that cycle and continuing to rise.  If that is the case, the Milankovitch forcing must be stronger than the forcing from the CO2 at its peak.  Otherwise, the system would continue to rise and run away.  And it hasn't.

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  5. Can anybody explain where all the water went during the low sea levels in the Miocene? Figure 2a shows Miocene sea levels (green squares) that, mostly, are less or much less than pre-industrial sea levels, some of them comparable to the low-stands during Pleistocene glacial maxima. Miocene climates are generally thought to be warmer than today, with no major northern hemisphere ice sheets and Antarctic ice volumes that fluctuated but were less than today's. RealClimate article on the Miocene

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    Moderator Response: (Rob P) Most likely into ephemeral ice sheets in the Northern Hemisphere. The early Miocene period was very warm and is the interval under discussion at Real Climate. The ice sheets most likely formed in the latter part of the Miocene. See below from Zachos(2001):

    See also the site linked to in the green box at the bottom of the post. Still lots of research to be to be done to sort out some of the disparities.
  6. jzk. On a global scale milankovich forcing vary solar by 1/m2 but at high latitudes the forcing very much higher. (40 I think??) It easily can force high latitude change to albedo. However, albedo feedback alone is insufficient to create the global change. (Ie what you see is a change in forcing in NH affecting climate in SH where the forcing is the in the opposite way). However, there is no problem explaining the global change when you factor in the changes to GHG that are forced by the milankovich changes eg see Hansen et al 2012 and see how closely albedo + GHG does at reproducing the ice age cycle, assymetry and all. As always, not a bad idea to read the appropriate chapter in IPCC WG1 (chp 6).

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  7. jzk...   Actually, the forcing from Milankovitch cycles and the direct radiative effects of atmospheric are very well understood and calculated.  Not even the Richard Lindzen's and Roy Spencer's of the world dispute these figures.

    I would suggest that if you're not getting a satisfactory answer, then maybe you aren't reading or comprehending the research.

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  8. scaddenp,

    During the last 400,000 years, when temperature levels were highest, CO2 levels were also at their highest (of the cycle so far) and the reusultant forcing from the CO2 was at its highest.  Yet, that, very powerful forcing, according to AGW theory, is not enough to overcome the forcing from all else because temperature starts dropping and CO2 levels continue to rise.  If CO2 forcing is stronger than everything else, how can that be? 

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  9. BillEverett @52...  If I understand Dr Alley's work (specifically his "Biggest Control Knob" lecture), the CO2 rock weathering thermostat idea comes from Dana Royer's work.  And then there is also Andrew Lacis' work on the CO2 thermostat that describes the mechanism on shorter time scales.

    I'm quibbling over the "main" vs "biggest" exactly because of the semantics issue that Kevin seems to be misunderstanding.  The problem seems to arise from how the thermostat changes.  

    Kevin seems to be saying, if the temp goes up before the thermostat (CO2), then CO2 can't be the thermostat.  Where he's getting it wrong is, he's not comprehending the forcing and feedback aspects of the thermostat, and I think that's where quibbling over phrasing becomes important.

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  10. jzk...  As has been pointed out in this thread many times now, the most recent research suggests that there may not be a lag.  And even if there is a lag, that doesn't in any way disprove the relationship between CO2 levels and global temperature.

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  11. jzk...   What you also don't seem to yet understand is that it's not the radiative effects of CO2 alone that are what makes CO2 the biggest control knob.  The direct radiative forcing of CO2 is not that much greater than Milankovitch cycles (AFAIK).  But it's the other properties of CO2 that make it important.  The fact that it's a long lived, well-mixed, non-condensing atmospheric gas is what makes it the control knob that it is.

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  12. Rob Honeycutt,

    Actually when presented with that very issue, that CO2 is necessary to drive interglacials and glacials, Richard Lindzen said "I don't think there is any case to be made for that."  He has further cited Roe's "In Defense of Milankovitch" which states "Thus, the relatively small amplitude of the CO2 radiative forcing and the absence of a lead over dV/dt both suggest that CO2 variations play a relatively weak role in driving changes in global ice volume compared to insolation variations."

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  13. jzk...  Except that Lindzen stands virtually alone in such statements regarding glacial-interglacial cycles.  

    And you also managed to completely dodge my point that the direct radiative effects of CO2 are well accepted.  Please read my following post @61 about the properties of CO2.

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  14. Since my comment #16, there's still confusion of timescales in the discussion of feedbacks and "control knobs."  There's also some misunderstandings of Richard's AGU presentation.  He said himself that CO2 does not explain "everything" and he was looking at the broadest view of climate in a geologic perspective, and telling it as a historian.  That was his choice for giving a ~60 minute presentation on over 4 billion years of climate history, and I think it was a good choice, but obviously it's not fully inclusive.  Rather clearly, CO2 is not the principle driver behind abrupt climate changes, ENSO dynamics, regional climate variability, and it only tells part of the story over glacial-interglacial cycles (a CO2-focused perspective is valuable in the tropics, not so much at higher latitudes where the orbit and cryospheric feedbacks dominate and have a different structure than projected global warming changes).

    On geologic timescales, the weathering feedback being discussed (which by the way goes back at least to Walker and Kasting's paper in 1981) has classically been invoked to be important for long-term CO2 evolution.  This comes back in discussions of the Ordovician climate, the Neoproterozoic snowball deglaciation, the evolution of climate in the last 60 million years, etc...for a recent discussion of the Cretaceous to Cenozoic problem, see e.g.,  this paper.  This mechanism has also worked its way into the planetary science community as a viable mechanism to expand the limits of the habitable zone in the search for extrasolar life, and could conceivably work on any planet with plate tectonics and liquid water.  Keep in mind that this mechanism works on timescales longer than orbital, so you can still get considerable temperature fluctuations on sub-million year timescales, or can have positive carbon cycle feedbacks, operating primarily from the ocean or biosphere (as is evidently the case for the glacial-interglacial problem in the last several million years).

    The 'control knob' concept on geologic timescales comes not only from the fact that CO2 is a big player in climate evolution over geologic time, but also because CO2 can interact with the climate through a variety of negative carbon cycle feedbacks (on million year timescales), and positive carbon cycle feedbacks (on glacial-interglacial timescales), and thus inserts its role in climate evolution almost everywhere we can look.  It's only on rather short (decadal) timescales that we usually don't need to think about boundary condition changes from CO2.  For the timescales in which carbon cycle feedbacks tend to be positive, it's not a 'thermostat' anymore, but still a 'control knob.'  And in the case of glacial-interglacial changes, that control knob is very good at communicating the orbitally-paced fluctuations to the equator and globally.

    Other mechanisms do not work as a thermostat or control knob because they don't interact with the climate.  The sun, for example, changes independently of the climate. Moreover, solar evolution over geologic time is a one-way street, gradually brightening at ~7-8% per billion years.  On shorter timescales, the changes are usually too small for a lot of purposes.  Volcanic activity displays no to weak dependence on the mean climate state (there might be issues with the ground temperature or ice sheet loading on the crust, but these are probably small).  These things are very important for understanding climate evolution on all timescales.  Volcanic forcing, for example, is very important for the last millennium and there's still some uncertainty in these forcing reconstructions.  But volcanoes don't get up and organize themselves to blow up every year and cause an ice age. 

    In the Andrew Lacis worldview, the 'control knob' concept revolves moreso around the fact that CO2 is the most important non-condensing greenhouse in Earth's atmosphere, is very long-lived, and is most capable of changing on anthropogenic warming timescales.  The CO2 in the atmosphere is required to maintain a significant water vapor greenhouse, and any CO2 modifications in the future (likely to be the most dominant forcing over the next century) will set the stage for any water vapor or cloud responses.  Of course, other forcings could offset this tendency, but they tend to either be too small (solar variations), too slow (Milankovitch), or too short-lived (volcanoes, ENSO) to get in the way of making confident projections of late-century climate.  In this case, it's the uncertainty in climate response (sensitivity) and emission scenarios that dominate, rather than minor issues in how forcings are specified or what initial conditions you need to deal with (as in the decadal-projection problem).

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  15. Not sure if anyone's answered Kevin's question about what triggers the descent back into a glacial?

    Basically the glacial state is the equilibrium for icehouse earth. The Milakovitch peaks concentrate enough of the (almost constant) solar radiation in the latittude band where most of the land ice is which temporarily shifts the equilibrium, and the feedbacks take over to do the rest. Once the Milankovitch peak is over the feedback alone is not sufficient to maintain the interglacial state, causing a gradual descent back into a glacial.

    That's also why interglacials tend to be shorter than glacials.

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  16. jzk - please read what I actually wrote (and preferrably some of the scientific literature pointed to in Chp6). Globally averaged, the CO2 forcing are around 3W/m2 but note that this applied to all of globe since GHG are well mixed in the atmosphere. Locally (60N), the milankovitch forcing is much higher. Easily enough to locally overwhelm GHG forcing. Ice sheets changes also change sealevel, albedo,  - and GHG. Got an alternative model where the number add up for a synchronous SH glaciation to NH forcing. 

    Now what Roe actually says is "This implies only a secondary role for CO2– variations in which produce a weaker radiative forcing than the orbitally-induced changes in summertime insolation – in driving changes in global ice volume"

    Your quote does not from Roe. Roe is quite correct - CO2 is secondary to NH summertime insolation which does the driving - but its an important feedback that makes it global. You might want to look at Denton et al 2010 as well as my earlier Hansen cite.

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  17. scaddenp,

    My quote is not from Roe?  I suggest you read it again.  "relatively weak" is how CO2 forcing is described as it relates to driving global ice volumes.  You admit that CO2 is secondary, but this article alleges it is the "Dominant control."  Is it the Dominant, or Secondary?

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  18. jzk...  You're talking about two different things.  The article here is saying dominant control over the past 40 my.  Roe is in reference to glacial-interglacial cycles.

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  19. It also looks to me like Roe is very specifically discussing global ice volume as opposed to global temperature.

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  20. And that jibes very well with what scaddenp is saying relative to the radiative forcing being much higher at latitudes above 60N lat.  

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  21. Rob says: "It also looks to me like Roe is very specifically discussing global ice volume as opposed to global temperature."


    What causes global ice volumes to change?

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  22. Kevin#1: I disagree with your analysis of the semantics re "big control knob". I offer the analogy of open microphone / speaker +ve feedback. You cough in the mike or tap it and a fierce whistle occurs due to +ve feedback. Stopped by a limiting circuit or loss of power. If no sound had entered the mike there would have been no whistle of increasing amplitude but nobody would suggest that your cough or you yanking the power plug out or a fuse blowing were a "big control knob". In fact, the amplifier might have a control knob you can adjust for more or less feedback.

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  23. jzk

    Please see 37 and links. Your questions answered.

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  24. Rob Honeycutt@59, "I'm quibbling over the 'main' vs 'biggest' exactly because of the semantics issue that Kevin seems to be misunderstanding."

    Okay. For me, the difference between "biggest" and "main" is rather insignificant. I used "main" in my initial comment because that was the term in Rob Painting's fifth key point and it was used in other comments preceding mine.

    I prefer the more specific term "thermostat" to the general term "control knob" when using an analogy to explain the situation to other non-scientists because (1) temperature is what the knob controls and (2) it makes it easier to understand time delays between changes in CO2 and in temperature.

    For those having difficulty understanding the role of greenhouse gases in our climate, I recommend the segment from 4:20 to 6:36 in a very good half-hour lecture on YouTube.

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  25. As a note on this, and in regards to the (mis)conceptions exhibited by Kevin and others:

    CO2 feedbacks to a forcing act as an amplifier of those forcings. A temperature rise reduces oceanic CO2 solubility, CO2 increases, temperature rises more. The gain is less than 1.0, so the overall effect is stable, but the end temperature swing is much higher with that positive feedback than it would be with just the initial forcing. 

    Note the important point - CO2 increases due to a temperature rise, increasing total radiative imbalance, temperatures rise more. 

    But a temperature rise and ocean solubility is not the only possible cause of a CO2 rise. For example (ahem) we have been burning fossil fuels at a tremendous rate, and have increased CO2 to levels not seen in the last million years or so. So - temperatures will rise


    From an electronics point of view, with CO2 as the amplifier (which might make more sense to those with engineering outlooks) - rather than increasing the input to the temperature dependent amplifier (Milankovitch forcing, with inherent time delays), we've been directly and very quickly raising the amplifier offset. With the same end result - temperatures are rising. We're turning it up to 11.

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  26. jzk...  It looks like you got the answer to your question @71 from BBD back at comment #37.

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  27. BillEverett...  I actually don't disagree with you.  I think the difference is minor (if any difference at all).  I guess I get frustrated with how "skeptics" can so easily misinterpret the meaning to be something completely wrong.  Thus I want to come up with iron clad phrasing.

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  28. KR, you should have bolded "but the end temperature swing is much higher with that positive feedback than it would be with just the initial forcing."  That seems to be the stumbling block.

    The thermostat, when left alone, brings the room to the designated equilibrium.  The thermostat has several control knobs.  The CO2 knob (amongst others, e.g. CH4) moves when the Solar Variation knob moves, but the CO2 knob can also be moved independently.  When the Solar Variation knob moves, the CO2 knob moves at several times the rate of SV movement. Indeed, it has the greatest range of movement of any of the knobs (if we tie SV's range to its geo-historical likely range).  Water vapor has a knob, but it's tied to all the other knobs, and it has a powerful spring that kicks it back to its pre-existing state almost immediately. 

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  29. KR,

    There are no misconceptions on my part.  The title of this article claims that CO2 is the dominant control of sea level over the past 40 million years.  That is quite a claim.  It is interesting to note that everyone here has noted the role the Milankovich cycles has played in starting and stopping the ice ages.  However,

    This strongly supports the dominant role of CO2 in determining Earth’s climate on these timescales and suggests that other variables that influence long-term global climate (e.g., topography, ocean circulation) play a secondary role.

    this is the quote from the abstract of the paper.  Note the absense of any mention of Milankovich cycles or any other astronomical occurance.  It is also interesting to note that the reference sources listed for the paper do not list any regarding astrological events.  Did the author compare CO2's role to that of Milankovich cycles?  Don't know.But if no comparison was made, then clearly the author can't claim that CO2 is the dominant force. 

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  30. Kevin...  I sure hope that "astrological events" is a typo on your part.  I don't think you're going to find anything in any of the published literature about how the phases of scorpio or cancer will influence global temperature.

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  31. Kevin

    Please see # 47 previous page and # 51 above and links. Too much argumentation, not enough reading.

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  32. Kevin - if the by far largest part of the scale of temperature swings due to forcing changes is indeed long term CO2 levels (as is evident), then it is indeed dominant, and other factors are secondary

    And if that dominant factor, CO2 level, changes independently from (for example) temperature-dependent ocean solubility with temperature, the biological pump, and silicate weathering, then we should expect to see temperature changes. As observed.

    If you have any evidence (paper links would be appropriate, continued semantic quibbling would not) demonstrating that CO2 is not the largest factor, then by all means present it. So far, I have seen nothing of the sort from you. 


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  33. Kevin

    Also see Chris Colose # 64

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  34. Andy Skuce @ 55

    I thought yours an excellent question too. I have been in touch with Dr Gavin Foster, who has very kindly given permission for his response to be posted in comments here:


    "That is a good question and well spotted.  Our interpretation
    here (built on early work by us - Foster et al. 2012 attached) is 
    that actually there must have been some northern hemisphere
    ice growth during the middle miocene. This is in line with a
    number of other indicators (e.g. ice rafted debris in the north 
    atlantic). This does however conflict with a straightforward 
    reading of the Zachos' benthic foram d18O curve.  
    Its important to note though that this is not global temperature - 
    rather its actually a mix of ice volume and temperature but 
    the temperature of deep sea, not a global average surface.  
    Deep water forms at high latitude so this is telling us about 
    high latitude temperature predominantly. A recent paper by
    Herold et al. (2011; Paleoceanography) compiled a number 
    of independent deep water temperature estimates for the 
    Miocene and showed that deep water was around 6 oC warmer than 
    today - that will account for a large chunk of the d18O 
    change from Miocene to recent (~1 permil).  We dont know 
    why this is right now but clearly the Zachos d18O curve is not as 
    straightforward as it first appears.The best support for 
    our interpretation is we know from several sources (using 
    different techniques from geochemical to stratigraphic) that 
    there was ~70m of sea level fall during the middle 
    miocene climate transition (the cooling that terminated the 
    middle miocene warm period).  In the absence of northern 
    hemisphere ice this would have to be all Antarctica based ice, 
    yet we know from ice proximal drilling that the Antarctic ice 
    sheet was very much in existence during the Miocene (and 
    indeed since the early Oligocene, 34 million years ago), 
    though it may have been reduced slightly.  This raises lots of 
    more questions of course and we dont know all the answers 
    yet but we are working on it!"

    [Foster et al. (2012) abstract]
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    Moderator Response: [RH] Fixing page formatting (I hope). ...Okay. Mostly fixed. :-)
  35. RH - Erm, sorry about that. Not sure what happened.

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    Moderator Response: [RH] No problem. I'm not quite sure what happened either.
  36. And just a further point on CO2 and glacials. While Milankovitch forcing pace the glacial cycles, there wouldnt be any glacial cycle if CO2 concentration was higher. The milankovitch forcings were still there before the Quaternary. Its just that CO2 was too high before that for them to significantly affect planetary albedo.

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  37. Kevin @79

    From the abstract for the paper: "On 103- to 106-year timescales, global sea level is determined largely by the volume of ice stored on land, ....Here we use observations from five well-studied time slices covering the last 40 My"

    They are looking at a much longer time scale that the ice age cycles. Milankovitch orbital changes are concerned with what happens within a single glacial cycle over time scales of 50,000 to 100,000 years. They are looking at a 40 million year span. At that timescale Milankovitch cycles just cancel each other out.

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  38. Bill Everett @22, 52, and 74.

    I am 100% with you on your persistent advocacy for use of the 'thermostat' analogy (rather than 'control knob').  As you have pointed out, promoting the view that atmospheric CO2 acts as a thermostat reinforces the reality that there is a great deal of inertia in the climate system.  Without this, 0.8 Celsius for 40% increase would imply 2.5 C for double. However, since there is inertia in the system, we are heading for 3 to 6 C for double pre-Industrial CO2 levels.  Therefore, given the disruption being caused by 0.8 C, I really do not understand how anyone can remain complacent about where humanity is taking this planet...  To mark Earth Day 2013, Michael Mann has posted an excellent extract from his book, The Hockey Stick and the Climate Wars, on the Weather Underground website today:

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  39. Speaking of CO2, any chance of someone serious doing a critical review of:
    Lightfoot and Mamer, 2017

    "Back Radiation versus CO2 as the cause of climate change"

    No tricks zone (July 31, 2017) has a glowing write up - comments point out some flaws but it would be good to have someone give it a serious closer look and critique of the tricks applied by Doug Lightfoot in the paper.

    (1) Robust scientific evidence shows the sun angle controls water vapour content of the atmosphere, the main component of back radiation, as it cycles annually.

    (2) Water vapour content measured as the ratio of the number of water molecules to CO2 molecules varies from 1:1 near the Poles to 97:1 in the Tropics.

    (3) The effect of back radiation [water vapour] on Earth’s atmosphere is up to 200 times larger than that of CO2 and works in the opposite direction.

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  40. citizenschallenge @89,

    I haven't read it yet, but the full Lightfoot & Mamer (2017) paper is available here. It's findings as you set them out @89 appear highly implausable but I'm sure a read of the full paper will inform us all a lot better.

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  41. citizenschallenge @89.

    Having now read Lightfoot & Mamer (2017), I can report that it is total nonsense. It is not the first nonsense from these authors which include Lightfoot (2010) 'Nomenclature, Radiative Forcing and Temperature Projections in IPCC Climate Change 2007: The Physical Science Basis (AR4)' [ABRTRACT] and Lightfoot & Mamer (2014) 'Calculation of Atmospheric Radiative Forcing (Warming Effect) of Carbon Dioxide at Any Concentration ' [PDF], this last setting out much of the argument now presented in Lightfoot & Mamer (2017) (although strangely this ealrier work is unmentioned in the later). Yet the bold and revolutionary assertions on AGW within this earlier work have not set the world alight since publication, a telling result. Instead it has gone un-noticed into the oblivion of nonsense-filled literature.

    And Lightfoot & Mamer (2017) will follow. It says nothing other than there is on average for any location and month much more H2O at the bottom of the atmsphere than there is CO2, and that the hotter the location/month the greater the disparity. They also arrive at the astounding finding that it is hotter in the tropics and in summer months than it is in the polar regions and winter. Further, they identify a general correlation (which they fail to actually calculate) between temperature and the angle of the sun up in the sky. (I recall noting in prevoius days that the sun is not static in the sky but appears to vary in angle through the day. Thinks - would this Lightfoot&Mamer correlation still hold for time-of-day?).

    Lightfoot&Mamer fail to comprehend the concept Radiative Forcing (RF). They would greatly benefit from a quick read of UN IPCC AR5 Chapter 8 section 8.1 (which they do not cite in their paper) or a proper read of UN IPCC TAR Chapter 6 (which they do cite but somehow fail to understand). Not the least of this ignorance is their use of surface back-radiation as though it were RF when by definition RF concerns the imbalance at the tropopause (with adjustment for stratospheric influences) and has nothing to do with surface back-radiation.

    "The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropo-spheric temperatures and state held fixed at the unperturbed values." UN IPCC TAR (2001) Section 6.1.1

    Their fraught calculations of the H2O/CO2 ratio do not apply to the tropopause. Their discussion concerns the properties of back-radiation which result from surface air temperature (SAT) but they rather overlook the physical mechanisms that maintain the SAT which are all to do with the atmosphere above, all the way up to the tropopause.

    Whichever way you cut it, Lightfoot&Mamer(2017) is a rich vein of total nonsense.

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