There is quite a bit of buzz surrounding a talk and pending paper from Prof. Murry Salby the Chair of Climate, of Macquarie University. Aussie Jo Nova has excellent commentary, as has Andrew Bolt in his blog. I’m sure others will weigh in soon.
In a nutshell, the issue is rather simple, yet powerful. Salby is arguing that atmospheric CO2 increase that we observe is a product of temperature increase, and not the other way around, meaning it is a product of natural variation. This goes back to the 800 year lead/lag issue related to the paleo temperature and CO2 graphs Al Gore presented in his movie an An Inconvenient Truth, Jo Nova writes:
Over the last two years he has been looking at C12 and C13 ratios and CO2 levels around the world, and has come to the conclusion that man-made emissions have only a small effect on global CO2 levels. It’s not just that man-made emissions don’t control the climate, they don’t even control global CO2 levels.
Salby is no climatic lightweight, which makes this all the more powerful. He has a strong list of publications here. The abstract for his talk is here and also reprinted below.
PROFESSOR MURRY SALBY
Chair of Climate, Macquarie University
Atmospheric Science, Climate Change and Carbon – Some Facts
Carbon dioxide is emitted by human activities as well as a host of natural processes. The satellite record, in concert with instrumental observations, is now long enough to have collected a population of climate perturbations, wherein the Earth-atmosphere system was disturbed from equilibrium. Introduced naturally, those perturbations reveal that net global emission of CO2 (combined from all sources, human and natural) is controlled by properties of the general circulation – properties internal to the climate system that regulate emission from natural sources. The strong dependence on internal properties indicates that emission of CO2 from natural sources, which accounts for 96 per cent of its overall emission, plays a major role in observed changes of CO2. Independent of human emission, this contribution to atmospheric carbon dioxide is only marginally predictable and not controllable.
Professor Murry Salby holds the Climate Chair at Macquarie University and has had a lengthy career as a world-recognised researcher and academic in the field of Atmospheric Physics. He has held positions at leading research institutions, including the US National Center for Atmospheric Research, Princeton University, and the University of Colorado, with invited professorships at universities in Europe and Asia. At Macquarie University, Professor Salby uses satellite data and supercomputing to explore issues surrounding changes of global climate and climate variability over Australia. Professor Salby is the author of Fundamentals of Atmospheric Physics, and Physics of the Atmosphere and Climate due out in 2011. Professor Salby’s latest research makes a timely and highly-relevant contribution to the current discourse on climate.
Salby’s talk was given in June at the International Union of Geodesy and Geophysic meeting in Melbourne Australia. He indicates that a journal paper is in press, with an expectation of publication a few months out. He also hints that some of the results will be in his book Physics of the Atmosphere and Climate which is supposed to be available Sept 30th.
The podcast for his talk“Global Emission of Carbon Dioxide: The Contribution from Natural Sources” is here (MP3 audio format). The podcast length is an hour, split between his formal presentation ~ 30 minutes, and Q&A for the remaining time.
Andrew Bolt says in his Herald Sun blog:
Salby’s argument is that the usual evidence given for the rise in CO2 being man-made is mistaken. It’s usually taken to be the fact that as carbon dioxide concentrations in the atmosphere increase, the 1 per cent of CO2 that’s the heavier carbon isotope ratio c13 declines in proportion. Plants, which produced our coal and oil, prefer the lighter c12 isotope. Hence, it must be our gasses that caused this relative decline.
But that conclusion holds true only if there are no other sources of c12 increases which are not human caused. Salby says there are – the huge increases in carbon dioxide concentrations caused by such things as spells of warming and El Ninos, which cause concentration levels to increase independently of human emissions. He suggests that its warmth which tends to produce more CO2, rather than vice versa – which, incidentally is the story of the past recoveries from ice ages.
Dr. Judith Curry has some strong words of support, and of caution:
I just finished listening to Murry Salby’s podcast on Climate Change and Carbon. Wow.
If Salby’s analysis holds up, this could revolutionize AGW science. Salby and I were both at the University of Colorado-Boulder in the 1990′s, but I don’t know him well personally. He is the author of a popular introductory graduate text Fundamentals of Atmospheric Physics. He is an excellent lecturer and teacher, which comes across in his podcast. He has the reputation of a thorough and careful researcher. While all this is frustratingly preliminary without publication, slides, etc., it is sufficiently important that we should start talking about these issues. I’ll close with this text from Bolt’s article:
He said he had an “involuntary gag reflex” whenever someone said the “science was settled”.
“Anyone who thinks the science of this complex thing is settled is in Fantasia.”
Dr Roy Spencer has suspected something similar, See Atmospheric CO2 Increases: Could the Ocean, Rather Than Mankind, Be the Reason? plus part 2 Spencer Part2: More CO2 Peculiarities – The C13/C12 Isotope Ratio both guest posts at WUWT in 2008. Both of these are well worth your time to re-read as a primer for what will surely be a (ahem) hotly contested issue.
I’m pretty sure Australian bloggers John Cook at Skeptical Science and Tim Lambert at Deltoid are having conniption fits right about now. And, I’m betting that soon, the usual smears of “denier” will be applied to Dr. Salby by those two clowns, followed by the other usual suspects.
Smears of denial and catcalls aside, if it holds up, it may be the Emily Litella moment for climate science and CO2 – “Never mind…”
Dave in Delaware says:
August 7, 2011 at 10:12 pm
And from what I have read, the deep ocean CO2 is enriched in C12 (lower C13, even less C13 than you would get from fossil fuels).
Sorry, but that is wrong. The deep oceans are around zero per mil d13C, the ocean surface is at 1-5 per mil, the atmosphere is at -8 per mil and declining and fossil fuels burning gives CO2 at -24 per mil in average. Thus any release of the (deep) oceans (including a two-way fractionation over the water-air border) would INcrease the 13C/12C ratio of the atmosphere, indeed diluting the fossil fuel signal (which is observed), but in the wrong direction…
See:
http://homepage.mac.com/uriarte/carbon13.html
John Finn:
I write in hope of giving you some help. At August 8, 2011 at 2:04 am you write to P Wilson and conclude by saying;
“Things are a bit more complicted than I’ve just described but that’s the basic concept in a nutshell.”
Many here – including P Wilson and me – are very familiar with “the basic concept” you outline but we observe that empirical data indicates it is wrong.
You convince nobody by merely repeating what you think over and over and over ….
You need to address the points P Wilson and others make which indicate your “basic concept” is wrong. Unless and until you do that then your repetitive posts are a waste of your time and a waste of space in this thread.
Richard
Michael D Smith says:
August 7, 2011 at 2:59 pm
Michael, in you post you say
There is a very slightly rising rate, so maybe that is related to both increasing emissions ….
I’m sorry, mate, but unless I’ve misunderstood you, you seem to be heading completely off track. If you are referring to your graph the “very slight rising rate” you refer to is the rate of the annual growth rate NOT the rate of accumulation in the atmosphere. This is related to the annual increase in emissions – NOT the actual emissions. I’ll try to illustrate with numbers.
Over the first 10 years of the ML record CO2, concentrations were rising at the rate of ~1 ppm per year.
Over the last 10 years of the ML record, CO2 concentrations have been rising at the rate of ~2 ppm per year.
The change in the rate of growth is 2ppm – 1ppm = 1ppm.
This is the change over 50 years so the trend is 0.02ppm per year. This is predominantly determined by the year on year increase in emissions but is probably also moderated by increasing sink capacity.
This, though, is not the same as the accumulation rate of CO2 in the atmosphere which as we’ve already noted has been between ~1ppm and ~2ppm over the past ~50 years. The accumulation rate (1.56ppm/year according to you) has seen CO2 levels rise by ~50 ppm over the period covered by your graph. The 50 ppm number is the integral (area under the curve) of your dCO2 curve. This is the gigantic elephant in the room.
I can’t think of too many more ways of explaining this point – but try this thought experiment: If the MSU temperature anomaly were exactly ZERO for the next 10 years, (a) what would the period between 2011 and 2020 look like on your graph and (b) what would the atmospheric CO2 concentration be in 2020.
Cheers
“Moreover, despite the temperature increase of the oceans, the atmospheric increase was 100 ppmv (80 ppmv since Mauna Loa started), far higher than expected from the oceans temperature increase.”
I think the problem there is that whilst the biosphere is playing catchup with the reduction of oceanic absorption from warmer ocean surfaces (more CO2 accumulating in the air) the biosphere itself ALSO turns into a source rather than a sink adding to the oceanic effect.
All this is happening pretty quickly with no time for increased long term sequestration from the biosphere. So for a while during the biosphere response to more CO2 in the air the energised biosphere is pumping MORE CO2 into the air from more faster growing organisms and from more surface decay of more and larger organisms.
So we don’t need the human emissions to account for more CO2 in the air than we would have otherwise expected from warmer ocean surfaces. The whole biosphere is at it with our contribution lost in the system because of the vast scale difference.
It is an ever changing natural balance of differential flow rates in different parts of the system with human emissions just along for the ride.
It simply isn’t conceivable that a mere 55% of human emissions going back into the system (after deducting the so called airborne fraction) offsets the entire absorption changes caused globally and naturally by warmer ocean surfaces and an energised biosphere leaving just the other 45% of human emissions as an addition to the atmosphere.
The human contribution as part of the total CO2 exchange is far too small yet that is what has previously been proposed and Salby has called out on the issue.
Humans are out of their league in this process.
“However, a NOAA illustration of the Annual Mean Growth of CO2. Note the group by decade-and no relation to human emissions.
http://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_data_mlo_anngr.png “
No, Richard, but there is a good reason for this (Sheeesh!). The mean rate of ~1.5 ppm per year over the past ~50 years is amplified or attenuated by temperature (about the 50th time I’ve written this in one form or another). In ‘warm’ years the rise is more than ‘cool’ years. The increase in1998 (El Nino) was bigger than in 1992 (following Pinatubo)
However, over the past 50 years, there has always been a year on year rise and the mean rate (or trend) does correlate to human emissions.
Richard, Michael Smith, Allen Macrae, Prof Salby (if you’re reading this blog) et al
Try the thought experiment I suggested to Michael in an earlier post, i.e.
1. Take any global temperature data set – any one will do (GISS, UAH, Hadley) – but SST data is probably most appropriate.
2. Plot delta_CO2 (not CO2) and the temperature anomaly (from 1) on the same graph. Michael Smith has already done this using MSU data using 12 month moving averages. (The close correlation completely debunks the notion that current CO2 levels are responding to Delta_T from previous decades or centuries)
3. Next, assume that the temperature anomaly (mean temp in your chosen data set) remains at ZERO for the next 10 years, i.e. there is no change at all in temperature between now and 2020.
QUESTIONS:
1. What will the delta_CO2 curve on your graph look like over the 2011-2020 period.
2. From your graph (or other) estimate the atmospheric CO2 concentrations in 2020.
If you can’t be bothered to plot your own graphs –use Michael’s here:
http://naturalclimate.home.comcast.net/~naturalclimate/CO2_growth_vs_Temp.pdf
Note the LH vertical axis uses increments of ppm/month.
@ur momisugly Ferdinand Engelbee August 8, 2011 at 3:30 am
Interesting that the link you provided is the very information I was thinking of when I wrote my earlier comment. Having re-read that link, I will give you one part, but insist the the core of my comment is still valid.
* I said … deep ocean CO2 would be ‘even less C13 than you would get from fossil fuels’. You stated – “fossil fuels burning gives CO2 at -24 per mil in average”. The article says ” The oceanic organic matter, that is, the soft parts of marine plankton –…– has a mean value for d13C of –23 ‰.” So CO2 from soft oceanic organic matter is about the same C13 as in fossil fuels, not ‘even less’ as I said.
* However, that same link goes on to describe how the deep ocean receives the low C13 organic matter – “In contrast, deep water receives large amount of organic matter that falls from above, poor in 13C. ”
and the article concludes –
“This mass of water, in its route towards the south across the deep, collects organic carbon that falls from the surface level and is low in d13C. In this manner the deep NADW current becomes poorer in d13C, so that the lowest values are reached in the Pacific, at the end of its long transoceanic trajectory.”
That last comment is what I had remembered. My contention is that, when the nutrient rich waters resurface, as they do off the coast of South America, the ‘natural’ CO2 they release would drive C13 ratios in the atmosphere in the same manner as CO2 released from fossil fuels.
The most interesting part for me is during questions from 52:15 to 53:23 mins. Salby doubts the reliability of ice core CO2 concentrations as a proxy for atmospheric CO2 at the time the ice was laid down. He says to believe it is a “pitfall”, that ‘the CO2 is not representative of atmospheric CO2’ and may not even have orginated from the atmosphere. Sort frequency variations in CO2 concentration between layers disappear, the further back in time you go, indicating that diffusion between layers is the overiding influence.
You need to address the points P Wilson and others make which indicate your “basic concept” is wrong.
There is nothing in anything that P.Wilson has posted which shows that ‘my’ basic concept is wrong. Him not believing something doens’t really count, Richard.
If P.Wilson or you could respond to my thought expreriment here John Finn says:August 8, 2011 at 5:06 am then explain the results we might start getting somewhere but I’m afraid your “repetitive posts” claiming that CO2 rise over the past 50-100 years is “a waste of your time and a waste of space in this thread”.
Delta_T since 1958 : ~0.5 deg C
Delta_CO2 since 1958: ~75 ppm
Over the past 50 years Delta_T has been both positive and negative (sometimes strongly so): Delta_CO2 has always been positive.
John Finn:
Re your post at August 8, 2011 at 5:06 am, I refer you to my post which was addressed to you at August 8, 2011 at 3:31 am.
You seem to have doubted that the advice I offered was genuine because you have ignored it. Please be assured that it was and is sincere.
Richard
John Finn:
You are becoming tiresome.
I have no intention of doing your “thought experiment” when I can assess and have assessed the real-world empirical data, especially when you make such plain wrong statements as “Over the past 50 years … Delta_CO2 has always been positive”.
Read what I have written or stop bothering me. I will ignore any more of your nonsense whether or not it is addressed to me.
Richard
I notice that my copy / paste left off the last letter in the name Ferdinand Engelbeen above. My apology, the error was not intentional.
Dave in Delaware says:
August 8, 2011 at 5:15 am
Unfortunately, the d13C level for deep ocean water is not in one of his figures, but rather deep in the text stands:
In contrast, deep water receives large amount of organic matter that falls from above, poor in 13C. So, after remineralizing the d13C of the DIC is reduced and reaches a null, or slightly positive, value in the deep of the oceans.
DIC is “dissolved inorganic matter”, the sum of CO2+bi+carbonates.
Thus while organic matter from the surface sinks into the deep oceans and lowers the d13C level there, the water still is around zero d13C. Thus diluting the fossils burning signal when upwelling to the surface.
oakwood says:
August 8, 2011 at 5:57 am
The most interesting part for me is during questions from 52:15 to 53:23 mins. Salby doubts the reliability of ice core CO2 concentrations as a proxy for atmospheric CO2 at the time the ice was laid down. He says to believe it is a “pitfall”, that ‘the CO2 is not representative of atmospheric CO2′ and may not even have orginated from the atmosphere. Sort frequency variations in CO2 concentration between layers disappear, the further back in time you go, indicating that diffusion between layers is the overiding influence.
I am very interested to hear where Salby did find that ” knowledge”.
1. That the CO2 is representative for the atmosphere was proven by Etheridge e.a. (1996!) with three cores at Law Dome: He measured CO2 as well as in firn as in ice bubbles at closing depth: the same levels were found. Two cores had an about decade resolution and a 20 year overlap with the South Pole direct measurements:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/law_dome_overlap.jpg
2. The frequency variations are smaller for ice cores that show longer time frames, simply because that are the cores with the smallest precipitation and therefore the longest closing time (~600 years), but the longest record. But there is no measurable migration of CO2 even in the longest (and coldest) ice cores (Vostok and Dome C, resp 420,000 and 800,000 years history): The ratio between CO2 in the gas bubbles and the SH temperature proxy in the ice layer (d18O and dD) stays the same over interglacials and glacials (8 ppmv/degrC), while any CO2 migration should reduce the ratio between these two for each interglacial period, 100,000 years back in time.
Stephen Wilde says:
August 8, 2011 at 3:51 am
I think the problem there is that whilst the biosphere is playing catchup with the reduction of oceanic absorption from warmer ocean surfaces (more CO2 accumulating in the air) the biosphere itself ALSO turns into a source rather than a sink adding to the oceanic effect.
The biosphere is an active sink since at least 1990, including the 1998 El Nino. That can be calculated from the oxygen balance: less oxygen is used than calculated from fossil fuel burning. Thus the biosphere was a net producer of oxygen, thus a net sink for CO2 and preferably of 12CO2…
The human contribution as part of the total CO2 exchange is far too small yet that is what has previously been proposed and Salby has called out on the issue.
Have a look at “Le Chateliers Principle”. You have a lot of huge exchanges, which net result is app. in balance. Two possibilities to change the balance: process conditions like temperature, which have a rapid (4 ppmv/degr.C) and a slow (8 ppmv/degr.C) but limited action on CO2. Add a disturbance, even a small one, like human emissions (currently at 4 ppmv per year) and guess what will happen.
Ferdinand Engelbeen says:
August 8, 2011 at 7:06 am
The question we need to answer: is the long term fossil fuel signal detectable when deluted by a long term cycle of upwelling? Also:What are the natural cycle lengths for upwelling?
@ferdinand meeus Engelbeen
I take the ‘remineralization’ statement as in reference to the formation of inorganic sediments.
In the figure and supporting text, the author clearly differentiates the inorganic from organic –
“In inorganic sediment carbonates in the sea, the d13C is 1‰ and for organic sediment carbon it is -23‰.”
The author has proposed using deep ocean sediments as proxy for paleo reconstructions –
“Comparison of the d13C value of planktonic fossils and benthic fossils (inhabitants of surface and deep waters, respectively) can provide indication of the intensity of oceanic productivity in the past.”
and later
“Currently, the value of d13C of shell carbon from benthic foraminifera in the Atlantic is larger than in the Pacific.”
and finally
“This mass of water, in its route towards the south across the deep, collects organic carbon that falls from the surface level and is low in d13C. In this manner the deep NADW current becomes poorer in d13C, so that the lowest values are reached in the Pacific, at the end of its long transoceanic trajectory.
Although not specifically stated, the author seems to be saying that the deep ocean water mass is more like the organic sediment, which then has an affect on the sediments laid down in the Atlantic (upstream) and Pacific (downstream) sediment formations. However, the author is clear in the statement that the deep current becomes poorer in C13.
Richard S Courtney says:
August 8, 2011 at 3:18 am
The data clearly proves that Henry’s Law is not applicable in this case. Please read what I wrote.
The fact is that the change in atmospheric CO2 concentration at times of ENSO changes proves that Henry’s Law does not apply.
Sorry Richard, but I never heard of one case where Henry’s Law is not applicable. Only that it may take some time to reach an equilibrium, but Henry’s Law still holds for all situations. Including ENSO changes.
That the data don’t follow Henry’s Law only proves that temperature is not the driving force of the increase in CO2…
But there was NOT “a steady increasing increase of CO2 in the atmosphere over all three periods”, and your assertion ignores what I have written.
I said:
” Over the longer term, we see that the period 1945-1975 was slightly cooling”
No matter that the yearly SST was going up and down, the average temperature in that period was cooling, over the whole period. Thus if temperature was the driving force, that should cause decreasing CO2 levels measured over the whole period (including ups and downs at the year by year scale), but we see an overall rise of CO2 in that period. Thus temperature is not the main driving force..
Ferdinand Engelbeen says:
August 8, 2011 at 7:42 am
“Add a disturbance, even a small one, like human emissions (currently at 4 ppmv per year) and guess what will happen.”
Okay, I’d like to play the guessing game with you. My guess is longer growing seasons in higher latitudes, increased primary productivity in the food chain, and lower fresh water requirement per unit of green plant mass.
What’s your guess?
Dave in Delaware says:
August 8, 2011 at 8:06 am
In this manner the deep NADW current becomes poorer in d13C, so that the lowest values are reached in the Pacific, at the end of its long transoceanic trajectory.
That only indicates that there is a measurable influence of organic matter on the d13C value while the NADW travels from the NE Atlantic to the Pacific equator, but doesn’t specify what the “lowest” value is. But there are other sources which make that clear:
http://ethomas.web.wesleyan.edu/ees123/caiso.htm
John Finn says:
August 8, 2011 at 2:45 am
“That is not – and never was – in dispute. If there were no human CO2 emissions CO2 levels would not remain in exact equilibrium at ~280 ppm.”
That’s a nice hypothesis. What experiment was performed to verify or falsify it?
I’m sorry if conducting science by the scientific method is inconvenient for you but I must insist upon it. Otherwise I’m just going to shrug off such claims as the above as “just-so stories”.
PS to Finn:
See Knorr 2009 figure 2 which shows ~5ppm increase in atmospheric CO2. Mauna Loa gives the same year at 3ppm. I believe the difference is Knorr isn’t using a single source for the data as you appear to be doing. Using a single source when more than one source is available isn’t usually considered good form and in less charitable terms is called “cherry picking”.
http://radioviceonline.com/wp-content/uploads/2009/11/knorr2009_co2_sequestration.pdf
Ferdinand:
You say to me:
“Sorry Richard, but I never heard of one case where Henry’s Law is not applicable. Only that it may take some time to reach an equilibrium, but Henry’s Law still holds for all situations. Including ENSO changes”
No! Henry’s Law is ONLY applicable to chemical interactions. It says nothing whatsoever about biological responses. The biosphere is a major part of the carbon cycle on land and in the oceans.
For example, the ocean surface layer is full of life forms. So, the rise of dissolved CO2 in the ocean could be a response to altered biological activity in the ocean surface layer (I don’t think it is but we do not know sufficient to say it is not) and – if so – then that could be the reason for increased atmospheric cO2 concentration.
You continue to think of the system as being static. It is not. It changes in response to conditions because biota do that.
Richard
Dave Springer says:
August 8, 2011 at 9:14 am
PS to Finn:
See Knorr 2009 figure 2 which shows ~5ppm increase in atmospheric CO2. Mauna Loa gives the same year at 3ppm.
Dave
As Ferdinand and I both pointed out the actual rise is not terribly relevant. It’s perfectly plausible that in a strong El Nino-affected year more CO2 could be emitted and less absorbed than normal. I would tend to go for the Mauna Loa figure but if the Knorr figure was correct it wouldn’t change anything.
That’s a nice hypothesis. What experiment was performed to verify or falsify it?
Not an experiment but actual data, i.e. the Law Dome ice core record. Although there is ~8 year smoothing but you can clearly see a drop of ~10ppm from MWP to LIA. But Micahel Smith’s graph (and others like it) is the real clincher.
http://naturalclimate.home.comcast.net/~naturalclimate/CO2_growth_vs_Temp.pdf
See how Delta_CO2 tracks temperature. Now then, Dave, here a question for you. Let’s just suppose that the MSU temperature anomaly over the next 10 years remained at exactly ZERO i.e. there was no warming or cooling.
What would you expect atmospheric CO2 levels to be in 2020? (They’re at ~390 ppm at the moment)
According to a chart in IPCC 4 global sea surface temperature has increased pretty much on a straight line by 0.8C. While difficult to estimate the CO2 outgassing according to Henry’s Law it estimated that a 1C rise in ocean surface temperature will raise atmospheric CO2 by 150ppm.
So we should expect that ocean outgassing in the past 100 years would produce an atmospheric CO2 rise of 120ppm. The actual rise in that 100 years is actually a bit less than 120ppm.
Clearly something doesn’t add up when we factor in anthropogenic emissions of nearly 240ppm (if it was all retained) during that 100 years. Dig it. Starting from a base amount of 280ppm combined anthropogenic emission and ocean outgassing should have added another 360ppm to bring the total up to 620ppm. Yet it’s not quite 400ppm in actuality.
Just as clearly, the carbon cycle is not sufficiently well understood for “just-so stories” about how much CO2 rise is due to anthropogenic contributions. Evidently there are carbon sinks at work which are far from adequately characterized.
This whole debate is so riddled with poor data and just-so stories that science, like Elvis, has left the building. I am thus forced to fall back on facts – atmospheric CO2 has been rising for at least 50 years and this has a great positive effect for green plants which are the primary producers in the food chain. What’s good for the primary producers is good for everything farther up the food chain. Sure there will be displacements but the winners vastly outnumber the losers. CO2 is rising and the earth is getting greener as result. Deal with it.
Richard S Courtney says:
August 8, 2011 at 6:09 am
John Finn:
You are becoming tiresome.
I have no intention of doing your “thought experiment” when I can assess and have assessed the real-world empirical data,….
We’re looking at real world data. We’re using Michael Smith’s graph. I beginning to have serious doubts that you’re able to interpret what the graph is saying. It’ s a simple question rather than a thought experiment, i.e.
Using the graph what would you expect the CO2 level to be in 2020 if temperatures remained constant for the next 10 years?
It’s just interpretation of the graph. If you can’t do it – just say you can’t do it.