Reposted from Dr. Roy Spencer’s Blog
February 5th, 2020 by Roy W. Spencer, Ph. D.
Note: What I present below is scarcely believable to me. I have looked for an error in my analysis, but cannot find one. Nevertheless, extraordinary claims require extraordinary evidence, so let the following be an introduction to a potential issue with current carbon cycle models that might well be easily resolved by others with more experience and insight than I possess.
Summary
Sixty years of Mauna Loa CO2 data compared to yearly estimates of anthropogenic CO2 emissions shows that Mother Nature has been removing 2.3%/year of the “anthropogenic excess” of atmospheric CO2 above a baseline of 295 ppm. When similar calculations are done for the RCP (Representative Concentration Pathway) projections of anthropogenic emissons and CO2 concentrations it is found that the carbon cycle models those projections are based upon remove excess CO2 at only 1/4th the observed rate. If these results are anywhere near accurate, the future RCP projections of CO2, as well as the resulting climate model projection of resulting warming, are probably biased high.
Introduction
My previous post from a few days ago showed the performance of a simple CO2 budget model that, when forced with estimates of yearly anthropogenic emissions, very closely matches the yearly average Mauna Loa CO2 observations during 1959-2019. I assume that a comparable level of agreement is a necessary condition of any model that is relied upon to predict future levels of atmospheric CO2 if it is have any hope of making useful predictions of climate change.
In that post I forced the model with EIA projections of future emissions (0.6%/yr growth until 2050) and compared it to the RCP (Representative Concentration Pathway) scenarios used for forcing the IPCC climate models. I concluded that we might never reach a doubling of atmospheric CO2 (2XCO2).
But what I did not address was the relative influence on those results of (1) assumed future anthropogenic CO2 emissions versus (2) how fast nature removes excess CO2 from the atmosphere. Most critiques of the RCP scenarios address the former, but not the latter. Both are needed to produce an RCP scenario.
I implied that the RCP scenarios from models did not remove CO2 fast enough, but I did not actually demonstrate it. That is the subject of this short article.
What Should the Atmospheric CO2 Removal Rate be Compared To?
The Earth’s surface naturally absorbs from, and emits into, the huge atmospheric reservoir of CO2 through a variety of biological and geochemical processes.
We can make the simple analogy to a giant vat of water (the atmospheric reservoir of CO2), with a faucet pouring water into the vat and a drain letting water out of the vat. Let’s assume those rates of water gain and loss are nearly equal, in which case the level of water in the vat (the CO2 content of the atmosphere) never changes very much. This was supposedly the natural state of CO2 flows in and out of the atmosphere before the Industrial Revolution, and is an assumption I will make for the purposes of this analysis.
Now let’s add another faucet that drips water into the vat very slowly, over many years, analogous to human emissions of CO2. I think you can see that there must be some change in the removal rate from the drain to offset the extra gain of water, otherwise the water level will rise at the same rate that the additional water is dripping into the vat. It is well known that atmospheric CO2 is rising at only about 50% the rate at which we produce CO2, indicating the “drain” is indeed flowing more strongly.
Note that I don’t really care if 5% or 50% of the water in the vat is exchanged every year through the actions of the main faucet and the drain; I want to know how much faster the drain will accomodate the extra water being put into the tank, limiting the rise of water in the vat. This is also why any arguments [and models] based upon atomic bomb C-14 removal rates are, in my opinion, not very relevant. Those are useful for determining the average rate at which carbon cycles through the atmospheric reservoir, but not for determining how fast the extra ‘overburden’ of CO2 will be removed. For that, we need to know how the biological and geochemical processes change in response to more atmospheric CO2 than they have been used to in centuries past.
The CO2 Removal Fraction vs. Emissions Is Not a Useful Metric
For many years I have seen reference to the average equivalent fraction of excess CO2 that is removed by nature, and I have often (incorrectly) said something similar to this: “about 50% of yearly anthropogenic CO2 emissions do not show up in the atmosphere, because they are absorbed.” I believe this was discussed in the very first IPCC report, FAR. I’ve used that 50% removal fraction myself, many times, to describe how nature removes excess CO2 from the atmosphere.
Recently I realized this is not a very useful metric, and as phrased above is factually incorrect and misleading. In fact, it’s not 50% of the yearly anthropogenic emissions that is absorbed; it’s an amount that is equivalent to 50% of emissions. You see, Mother Nature does not know how much CO2 humanity produces every year; all she knows is the total amount in the atmosphere, and that’s what the biosphere and various geochemical processes respond to.
It’s easy to demonstrate that the removal fraction, as is usually stated, is not very useful. Let’s say humanity cut its CO2 emissions by 50% in a single year, from 100 units to 50 units. If nature had previously been removing about 50 units per year (50 removed versus 100 produced is a 50% removal rate), it would continue to remove very close to 50 units because the atmospheric concentration hasn’t really changed in only one year. The result would be that the new removal fraction would shoot up from 50% to 100%.
Clearly, that change to a 100% removal fraction had nothing to do with an enhanced rate of removal of CO2; it’s entirely because we made the removal rate relative to the wrong variable: yearly anthropogenic emissions. It should be referenced instead to how much “extra” CO2 resides in the atmosphere.
The “Atmospheric Excess” CO2 Removal Rate
The CO2 budget model I described here and here removes atmospheric CO2 at a rate proportional to how high the CO2 concentration is above a background level nature is trying to “relax” to, a reasonable physical expectation that is supported by observational data.
Based upon my analysis of the Mauna Loa CO2 data versus the Boden et al. (2017) estimates of global CO2 emissions, that removal rate is 2.3%/yr of the atmospheric excess above 295 ppm. That simple relationship provides an exceedingly close match to the long-term changes in Mauna Loa yearly CO2 observations, 1959-2019 (I also include the average effects of El Nino and La Nina in the CO2 budget model).
So, the question arises, how does this CO2 removal rate compare to the RCP scenarios used as input to the IPCC climate models? The answer is shown in Fig. 1, where I have computed the yearly average CO2 removal rate from Mauna Loa data, and the simple CO2 budget model in the same way as I did from the RCP scenarios. Since the RCP data I obtained from the source has emissions and CO2 concentrations every 5 (or 10) years from 2000 onward, I computed the yearly average removal rates using those bounding years from both observations and from models.
Fig. 1. [fixed] Computed yearly average rate of removal of atmospheric CO2 above a baseline value of 295 ppm from historical emissions estimates compared to Mauna Loa data (red), the RCP scenarios used by the IPCC CMIP6 climate models, and in a simple time-dependent CO2 budget model forced with historical emissions before, and assumed emissions after, 2018 (blue). Note the time intervals change from 5 to 10 years in 2010.
The four RCP scenarios do indeed have an increasing rate of removal as atmospheric CO2 concentrations rise during the century, but their average rates of removal are much too low. Amazingly, there appears to be about a factor of four discrepancy between the CO2 removal rate deduced from the Mauna Loa data (combined with estimates of historical CO2 emissions) versus the removal rate in the carbon cycle models used for the RCP scenarios during their overlap period, 2000-2019.
Such a large discrepancy seems scarcely believable, but I have checked and re-checked my calculations, which are rather simple: they depend only upon the atmospheric CO2 concentrations, and yearly CO2 emissions, in two bounding years. Since I am not well read in this field, if I have overlooked some basic issue or ignored some previous work on this specific subject, I apologize.
Recomputing the RCP Scenarios with the 2.3%/yr CO2 Removal Rate
This raises the question of what the RCP scenarios of future atmospheric CO2 content would look like if their assumed emissions projections were combined with the Manua Loa-corrected excess CO2 removal rate of 2.3%/yr (above an assumed background value of 295 ppm). Those results are shown in Fig. 2.
Fig. 2. Four RCP scenarios of future atmospheric CO2 through 2100 (solid lines), and corrected for the observed rate of excess CO2 removal based upon Mauna Loa data (2.3%/yr of the CO2 excess above 295 ppm, dashed lines).
Now we can see the effect of just the differences in the carbon cycle models on the RCP scenarios: those full-blown models that try to address all of the individual components of the carbon cycle and how it changes as CO2 concentrations rise, versus my simple (but Mauna Loa data-supportive) model that only deals with the empirical observation that nature removes excess CO2 at a rate of 2.3%/yr of the atmospheric excess above 295 ppm.
This is an aspect of the RCP scenario discussion I seldom see mentioned: The realism of the RCP scenarios is not just a matter of what future CO2 emissions they assume, but also of the carbon cycle model which removes excess CO2 from the atmosphere.
Discussion
I will admit to knowing very little about the carbon cycle models used by the IPCC. I’m sure they are very complex (although I dare say not as complex as Mother Nature) and represent the state-of-the-art in trying to describe all of the various processes that control the huge natural flows of CO2 in and out of the atmosphere.
But uncertainties abound in science, especially where life (e.g. photosynthesis) is involved, and these carbon cycle models are built with the same philosophy as the climate models which use the output from the carbon cycle models: These models are built on the assumption that all of the processes (and their many approximations and parameterizations) which produce a reasonably balanced *average* carbon cycle picture (or *average* climate state) will then accurately predict what will happen when that average state changes (increasing CO2 and warming).
That is not a given.
Sometimes it is useful to step back and take a big-picture approach: What are the CO2 observations telling us about how the global average Earth system is responding to more atmospheric CO2? That is what I have done here, and it seems like a model match to such a basic metric (how fast is nature removing excess CO2 from the atmosphere, as the CO2 concentration rises) would be a basic and necessary test of those models.
According to Fig. 1, the carbon cycle models do not match what nature is telling us. And according to Fig. 2, it makes a big difference to the RCP scenarios of future CO2 concentrations in the atmosphere, which will in turn impact future projections of climate change.
Just like we were surprised to see how much CO2 nature has absorbed, we will be equally amazed how much more can be absorb. Mother Nature is amazingly resilient. We are learning new things everyday, and I thank Roy for quantifying and graphing current thinking. While not limitless, we now know nature’s ability to “sink” excess CO2 is huge, and to put it in “alarmist rhetoric” … nature’s CO2 sinks are alarmingly efficient.
With respect to CO2 consumption, is the planetary plant world a linear model? Are greenhouses even linear? I think nature has a few surprises that fossils and geochemistry cannot fully explain.
First off, it is clear the BernSCM v1.0 CO2 model presented in this paper…
“The Bern Simple Climate Model (BernSCM) v1.0: an extensible and fully documented open-source re-implementation of the Bern reduced-form model for global carbon cycle–climate simulations” (by Kuno M. Strassmann and Fortunat Joosat University of Bern, Bern, Switzerland)
https://www.geosci-model-dev.net/11/1887/2018/
is a failure. It is simpler construct of earlier more complex Bern model of carbon cycle.
It was a failure while it was under-review in late 2017 before it was published in 2018. It is utterly wrong. Yet it is what the IPCC and the consensus establishment needs to maintain the climate scam on human FF emissions –> atmospheric CO2 growth (MLO record) –> LWIR forcing growth –> temperature and everything else they claim.
A very simple demonstration of the crux of this paradigm failure can be seen in this NOAA/ESRL data graphic:
Note the Barrow Alaska seasonal dynamic range ~22 ppm, peak to trough. The trough (the summer low pt) is in August, the peak of Arctic heating and near sea ice minimum. Yet with all that “warming exposed” seawater and exposed decaying tundra, CO2 is falling very rapidly across the Arctic.
Another informative graphic to show what is happening is this detrended seasonal cycle and the smoothed curve (red).
This detrended graphic stops at 2012, but it shows in 2000, the seasonal swing was (roughly) 15.5 ppm (red, smoothed line), but by 2011 that seasonal swing had increased to almost 20 ppm. And back to the previous graph, you can see that the 2019 seasonal swing was almost 22 ppm at Barrow.
Conclusions: The high latitude biosphere is being highly productive and not only taking in lots of CO2 , it’s summer time drawdown rate is increasing, even as the Arctic has warmed. Not inspite of the warming, but probably EXACTLY because the Arctic has warmed has this drawdown increased over the last 2 decades. Even though more warming summer open water should, from a purely geophysical standpoint, be outgassing more CO2 with each year, that is not what appears to be happening.
This is the opposite behavior predicted by the Bern Model for a warming high latitudes of the Arctic. The Bern Model is failure but the IPCC’s AR6 WG1 report will most certainly base all its emissions-CO2 growth scenarios on in their CMIP6 model runs. That’s IPCC junk science at work.
This should come as no real surprise given the rapidity and degree of seasonal decline in atmospheric carbon dioxide. It’s good to see my suspicions are justified as the mathematics is well beyond me. Will this finding also be ignored by climate alarmists or is that just another of my suspicions?
Another factor to consider. Photosynthesis reaction rate increases with increasing CO2 concentration
RCP8.5 includes permafrost melting effects and that is one reason for a very high CO2 increase in the atmosphere. The question is if that is it a realistic scenario to be called “Business as usual”. The BAU scenario is much closer to the present CO2 growth rate.
I used Ernst Georg Beck’s data to argue the average CO2 level was 360 ppm in the 1800’s and there were spikes to 550 ppm in 1825 and 1942. One Alarmist argued back that nobody accepted Becks graphs because he couldn’t provide the means by which the CO2 level fell back to normal in such a short period of time. If this article is correct then IMHO that objection to Beck’s work has been answered. I also remember reading another article a few years ago that the global tree inventory had been underestimated fourfold. That makes me suspect the IPCC might not have updated their models to correct for that miscalculation.
The situation described in the post makes excellent sense when the entire biosphere is considered. I recently did a review of the past research on some of the primary crop plants and most common tree species to find out what the optimal atmospheric CO2 concentrations were for these species. The optimal level was consistently between 750 and 1000 ppm. Some phytoplankon species go much higher.
It would appear that current CO2 levels and higher should dramatically increase plant growth, and both balance future emissions and provide food and products for humanity’s needs in the future. Perhaps we should focus on removing pollution and ignore CO2. Labeling CO3 as a pollutant was a very ignorant and ridiculous step.
That shouldn’t surprise anyone, ….. given the fact estimates of yearly anthropogenic emissions were formulated specifically to match with the yearly average Mauna Loa CO2 observations. “DUH”, no one ever estimated yearly anthropogenic emissions until a few years after Keeling provided them a “target” to shoot at.
And it makes no matter anyway because yearly anthropogenic CO2 emissions are surely less than 5% of all CO2 emissions into the atmosphere.
The fact of the matter is, iffen the earth was not “tilted” on its axis ….. there would not be a biyearly (seasonal) cycling of an average 6 ppm atmospheric CO2 as defined by the KC Graph.
“The CO2 budget model I described here and here removes atmospheric CO2 at a rate proportional to how high the CO2 concentration is above a background level nature is trying to “relax” to, a reasonable physical expectation that is supported by observational data.”
I think that is where it goes wrong. It is not a reasonable physical expectation. It says that the 280 ppm of a century ago is somehow enshrined in the physics. Diffusion does not work like that.
The model would be right if the excess CO2 were diffusing through a resistive layer which itself held none of it, into a sink which stayed at the previous equilibrium conditions. But it isn’t like that. At each stage in the ocean the CO2 is diffusing into a region where the CO2 levels have already increased. The concentration gradients are reduced by the accumulation of past diffusion, and so the flux is lowered. Past CO2 influx gets in the way of current influx. And the memory of that past 280 ppm attenuates.
They claim to use infrared absorption. Which is what I would have expected.
Let me make two quick observations.
First, if Pat Frank were around to look at this, he would naturally like to know what accuracy, or even what precision one might expect of future CO2 projections from models. There are a scad of unknowns, from historical releases of CO2, manmade and not, the various means by which Earth sequesters CO2 (biosphere, lithosphere and hydrosphere), and so forth. Start propagating just the known uncertainties in these mechanisms and one might be surprised at the result. There are of course biases in what we think we know and then unknown unknowns.
Second, I have mentioned Ferren MacIntyre before, but I read a most interesting exposition of his, ages ago, in which he showed how a doubling of CO2 in the atmosphere (from 320ppm at the time, 1970) would be handled by the oceans. The surface buffer system works with a short time constant of perhaps 20 years (I once calculated the half-life as 10 years, but the 1/e time is a bit longer), but in the long term the deep oceans get involved. Surface waters which drain into the oceans carry bicarbonate and carbonates. Without some means of recycling CO2 back to the atmosphere the oceans would come to resemble something like the Great Salt Lake (pH varying between 8 and 9.6 depending on locale but generally high) instead of 7.4 to 8.2 typical of ocean and the atmosphere would become deprived of CO2. The return is done in the deep ocean. MacIntyre was writing at a time before we knew much about the ocean hot springs on the mid-ocean ridges, which control a good deal of ocean chemistry, but the idea is just the same. Clay and ocean crust exchange hydrogen ions in the ocean for ions like Na, Ca, Mg and CO2 gets returned to the atmosphere. Adding more CO2 to the surface ocean simply shifts equilibrium a bit, but most of the added CO2 will be buried in the deep ocean by some means (detritus, shells, etc). MacIntyre calculated the doubling 320ppm to 640ppm would result in an eventual level of 390ppm by the time equilibrium was re-established.
I do not know what the time scales for the deep ocean reactions are, but the time scale of overturning is pretty long. Some of the bottom waters I have read, returns to the surface in 1,000 to 5,000 years, but if the deep ocean is becoming involved in handling the input of extra CO2 to the atmosphere, then perhaps there is an explanation for why nature seems to be absorbing more than we thought. The question is, what is the rate that CO2 in some sequestered form settles into the abyss, and how long does it take for some fraction of this to return to the atmosphere?
This approach to the carbon cycle and the evolution of atmospheric CO2 are well discussed by Murry Salby at
https://edberry.com/blog/climate/climate-physics/murry-salby-atmospheric-carbon-18-july-2016/
Dr. Ed Berry addresses the IPCC Bern model and finds it lacking at https://edberry.com/blog/climate/climate-physics/human-co2-has-little-effect-on-the-carbon-cycle/ . Much of what Dr. Spencer states in this post is echoed in the referenced presentations. One assumption he has made I think is wrong is that his model “deals with the empirical observation that nature removes excess CO2 at a rate of 2.3%/yr of the atmospheric excess above 295 ppm.” There is good evidence that sinks are working at lower concentrations and are not responsive to some arbitrary condition. If CO2 goes below 295 the sinks will not become constant and unaffected by the concentration. Further if CO2 goes to 0 there can be no absorption but Spencer’s model implies there will be.
Everyone: I visit this site more than once a day. I seldom post, because I have nothing to add to comments from the obviously intelligent participants.
I’m starting to see just an echo chamber. Virtually none of my otherwise intelligent colleagues, friends, and family will even trouble themselves to visit WUWT, because “The Science is Settled”, and they roll their eyes at me.
I convinced my dentist to have a look, and he thought while interesting, WUWT seemed to be mostly political discourse. It was troublesome, I guess, for him to filter through the hundreds of articles and posts.
I think I can safely say that any dedicated skeptic of AGW with a high public profile is, or has, quit /lost his / her job, or is constantly being assaulted by typically very rude and ignorant people while trying to present the truth. You could all give me a list of the individuals who have fought, and continue to fight to bring reason to the general public, and have suffered for it. I thought Taylor et al’s presentations at COP 25 were brilliant. 18.6K views? Really? Seriously? I don’t believe it.
How do we bring people to their senses? Please don’t respond with jokes. I really want to see a plan.
Some time ago I recorded these numbers for the CO2 content
Oceans – 37,000 GT
Biomass(land) – 3,000 GT
Atmosphere – 700 GT
Anthropogenic – 6 GT
Atmospheric content is a balancing surplus in exchange between two largest accumulators. If the above numbers are anywhere near correct values, than even the smallest of changes (0.016%) in the oceans content variability over the last half a century would be greater than the assumed anthropogenic content of 6 GT.
I doubt that it is possible to say that the ocean’s content can be estimated with accuracy of 1 in 6000 (0.016%).
I did a spreadsheet last year showing that the delta in CO2 from its low to its high has almost doubled since the Mauna Loa data recording started. This shows that if we just take emissions down to 1958 levels, CO2 rise will be eliminated.
How much of this is just increased Biomass growth? Also the removal might be even higher, how much CO2 is outgassed from warming oceans and not part of the human increased outputs? Everytime I see a photo from the 1800s or earlier, I’m amazing at how open most of these areas are and how few trees. Heck just watching “The Curse of Oak Island” last night, they showed a photo the island back in the late 1800s early 1900s and it was barren! Nothing like the dense northern forest that cover it today.
I think the amount of forest and biomas growth since 1900 has been incredibly understated.
Why care about IPCC models or conforming to them? Some facts should dispel any notion that MME (Boden etal) drives ML CO2. Real changes in ML CO2 are followed by MME changes by years of lag:
The assumptions underlying the head post model are wrong, and so is the model. Real adjusted for inflation CO2 is declining naturally, resulting from long-term changes to tropical temperatures, according to my all-natural CO2 model, of which MME is a tiny insignificant fraction:
This indicates CO2 is outgassed in perfect proportion to the Nino region temperature ratio of the first to the second half of year (NIR), following natural and biological cycles and laws.
The lack of similar modern CO2 spikes in core data results from data smoothing via attenuation and inadequate temporal resolution. The modern era data is a blip in time compared to core data sampling.
A couple of questions for cognizant;
1. If the atmospheric concentration of CO2 has been steadily decreasing since the Late Jurassic why are we worried about a small increase over the last few decades that seems to be benefiting life forms around the globe? The CO2 level was low enough during the last glacial period to nearly starve plants, especially those at higher elevations. Why would we be concerned about increasing CO2 when all life appears to benefit!?
2. Since the geologic record shows extended periods where average temperatures were substantially higher (5-10 degrees C) than today, why is the POSSIBLE increase of two degrees anything to be concerned with? Life flourished during these warm periods given the rapid evolution and proliferation of plants, fishes, amphibians and reptiles. Why would humans and other mammals be any less adaptable than other life forms?
I am much more worried about the infinite extent of human greed and stupidity that seems to pervade the Green Blob than I am about a few tenths of a percent of CO2 that may be wholly beneficial and transitory!
In these days the 3,200 gigatonnes of CO2 in the atmosphere is increased by 36 gigatonnes a year. That is 0.5%/yr. 20 gigatonnes disappear to the sea and to plants, and as the concentration goes up, these 20 gigatonnes will also be increased with 0.5%/yr. Calculated as compound interest, how may years would it take to raise those 20 gigatonnes to 36 gigatonnes, and that is 119 years, So in the year 2139 the whole supply will disapper and the concentration will not be increased further, and the increase is approx 800 gigatonnes which is the same as 102 ppm, so around 512 ppm will be the top concentration – but wait, the 36 gigatonnes is not a fixed value, so the calculation is in fact more complicated than this. – up to 700 ppm is my guess
Global greening is big and getting harder to
– deny
– hide
– keep from general public awareness
– keep portraying as universally bad
Authors are emboldened to admit that it could be the most significant – and good – element of human caused climate change.
Piao, S., Wang, X., Park, T. et al. Characteristics, drivers and feedbacks of global greening. Nat Rev Earth Environ 1, 14–27 (2020).
https://phys.org/news/2020-01-planet-greener-global.html
Widespread vegetation greening since the 1980s is one of the most notable characteristics of biosphere change in the Anthropocene. Greening has significantly enhanced the land carbon sink, intensified the hydrologic cycle and cooled the land surface at the global scale. A mechanistic understanding of the underlying drivers shows how anthropogenic forcing has fundamentally altered today’s Earth system through a set of feedback loops. Improved knowledge of greenness changes, together with recent progress in observing technology and modelling capacity, has resulted in major advances in understanding global vegetation dynamics. Nonetheless, we still face many challenges ahead.
Alarmists endlessly write about bad and scary things supposedly happening to plants. And yet the planet is greening, not a bit but a lot. How? Are these all bad plants, some kind of zombie plants?
Reduced stomata opening that plants can now do to reduce water loss, due to high CO2, is changed from being a good thing caused by high CO2 to a bad sign of water stress. Catastrophist sleight of hand, dystopic conjuring. But if CO2 is causing drought everywhere, why is the planet greening? Not a bit, but a lot?
A small correction for the head post: “ … as well as the resulting climate model projection of resulting warming, which remain physically meaningless.“
What “excess CO2” ?
I am with those who quite rightly believe that the earth’s current atmosphere is “starved” of CO2 to a point where all life on earth is in real (not fake) jeopardy. Plants the base of life’s food chain are hobbled. During the last glacial maximum CO2 levels dropped to around 180 ppm just 30 ppm above the “Plant Death Zone”. Analogous to humans starved of oxygen trying to climb mountains above 26,300 feet (8000 meters)
Dr Spencer. From the previous article:
The ‘relaxation’ is not back to what it was 150 or 200 year ago, it is an attempt to balance the pCO2 in the ocean with the partial pressure of CO2 in the air above at any point in time. As the ocean “acidifies” it contains more CO2, so the supposed excess CO2 “anomaly” causes less “forcing” than the same level would have done back in 1750. The basic assumption of the model will lead to an exaggerated estimation of how much is removed.
This agrees with your natural suspicion about this result.
Fitting one monotonic rise to another one is pretty easy to satisfy, it is not a great confirmation that you are correct.
The fit to MLO does not “support” the model, you have tuned it to fit MLO. In view of the illogical deviation for a significant part of the record after Mt P. The degree of confirmation is light. This is same fallacy that most CAGW pseudo-science is based on: matching two monotonic changes and taking this as proof of causation.
The result of where it levels off depends largely on the curves section in the future. Using regression on the nearly straight part, corresponding to the available data is a very uncertain way to predict the curvature of the later section, especially in view of the error in the “relaxation” pointed out above.
I share your natural skepticism about the degree of the result, though the idea of some down turn seems likely.
Possible explanation are that certain high co2 consuming plants, bacteria are limited by amount of co2 in atmosphere. Once the amount of co2 increases their biomass starts to take up a larger percentage of the Total biomass, thus increasing the co2 consumption by total biomass in a nonlinear way. Basically the ratios of types of plants and bacteria can change when a nutrient is increased.