FROM THIS WEBPAGE:
Carefully selected quotes from
a website page I believe is from
the mid-2000s, but is still valuable.
by Ye Editor
"In climate skeptics circles, there is rather much confusion about historical and present CO2 measurements. This is in part based on the fact that rather accurate historical direct measurements of CO2 in the atmosphere by chemical methods show much higher values in certain periods of time (especially around 1942), than the around 280 ppmv which is measured in Antarctic ice cores.
... current climate models underestimate the role of the sun and other natural variations like ocean oscillations on climate and overestimate the role of greenhouse gases and aerosols. But I am as sure that the increase of CO2 in the atmosphere since the start of the industrial revolution is mainly from the use of fossil fuels.
There are several reasons why the hypothesis of large non-human CO2 variations in recent history is wrong and that most of the recent increase in CO2 in the atmosphere indeed is mainly man-made, but that needs a step-by-step explanation.
... in any year of the past over 50 years, the emissions are larger than the increase in the atmosphere. That means that the total mass balance of all natural variables (temperature, ocean pH, vegetation) which influence CO2 levels, is always towards more sink than source over any year.
The natural seasonal exchange between vegetation and oceans at one side and the atmosphere at the other side is estimated at about 150 GtC/yr. But that is not of interest for what the change is over a year, as most of the natural releases are absorbed within the same year. The difference after a year is not more than +/- 2 GtC, mainly caused by temperature changes (El Niño, Pinatubo eruption).
Thus the natural variations over a year are smaller than the emissions. No matter how high the natural seasonal turnover might be, in all years over the previous near 50 years, the natural CO2 sinks were larger than the natural CO2 sources...
Thus it is near impossible that natural sources were responsible for (a substantial part of) the increase of CO2 in the past 50 years. Except that - pure theoretically - a similar, but enormous increase in natural emissions and sinks that parallels the human emissions may give the same result.
- any extra exchange from oceans or vegetation would leave a fingerprint in the 13C/12C rate of change caused by human emissions and in the 14C/12C rate of change from the 1950-1960's atomic bomb spike. But that is not the case.
- there is no known temperature related natural physical process that may increase the CO2 releases without at least a partial negative feedback from increased pressure related uptake by the oceans and vegetation.
This proves that human emissions are the main cause of the increase of CO2, at least over the past near 50 years. But there are more indications for that...
The 13C/12C ratio:
The carbon of CO2 is composed of different isotopes. Most is of the lighter type: 12C (that has 6 protons and 6 neutrons in the kernel), and about 1.1% is heavier: 13C (has 6 protons and 7 neutrons in the kernel).
There also is some 14C (6 protons and 8 neutrons in the kernel), which is continuously formed in the upper stratosphere from the collisions of nitrogen with cosmic rays particles. This type of carbon (also formed by above-ground atomic bomb experiments in the 1950's) is radio-active and can be used to determine the age of fossils up to about 60,000 years.
One can measure the 13C/12C ratio and compare it to a standard. In the past, the standard was some type of carbonate rock, called Pee Dee Belemnite (PDB). When the standard rock was exhausted, this was replaced by a zero definition in a Vienna conference, therefore the new standard is called the VPDB (Vienna PDB). Every carbon containing part of any subject can be measured for its 13C/12C ratio. The comparison with the standard is expressed as per mil δ13C:
Where the standard is defined as 0.0112372 parts of 13C to 1 part of total carbon. Thus positive values have more 13C, negative values have less 13C. Now, the interesting point is that vegetation growth in general uses by preference 12C, thus if you measure δ13C in vegetation, you will see that it has quite low δ13C values.
As almost all fossil fuels were formed from vegetation (or methanogenic bacteria, with similar preferences), these have low δ13C values too. Most other carbon sources (oceans, carbonate rock wearing, volcanic out gassing,...) have higher δ13C values. For a nice introduction of the isotope cycle in nature, see the e-book of Anton Uriarte Cantolla [5].
This is an interesting feature, as we can determine if CO2 levels in the atmosphere (which is currently below -8 per mil VPDB) were influenced by vegetation decay or fossil fuel burning (both about -24 per mil) towards the negative side or by ocean degassing (0 to +4 per mil) towards the positive side as largest possible sources.
From different CO2 base stations, we not only have CO2 measurements, but also δ13C measurements. Although only over a period of about 25 years, the trend is clear and indicates an extra source of low δ13C in the atmosphere.
d13C trends
Trends in δ13C from direct measurements of ambient air at 10 base stations. Data from [6].
ALT=Alert; BRW=Barrow; MLO=Mauna Loa; KUM=Cape Kumukahi; SMO=Samoa; SPO=South Pole.
... Vegetation produces O2, by incorporating more CO2 than is formed by decaying vegetation (which uses oxygen). This means that more 12C is incorporated, and thus more 13C is left behind in the atmosphere. Vegetation is thus relative depleting the atmosphere of 12C vs. 13C and thus not the cause of decreasing 13C/12C ratio's.
... What we can see, is that the δ13C levels as well as in the atmosphere as in the upper oceans start to decrease from 1850 on, that is at the start of the industrial revolution. In the 400 years before, there is only a small variation, probably caused by the temperature drop in the Little Ice Age.
Longer term measurements of the δ13C ratio in CO2 from ice cores show that over the whole Holocene, the variations were not more than +/- 0.2 per mil. Even the change from a glacial to an interglacial period did not give more than 0.2 per mil δ13C change.
... this is a good indication of the influence of fossil fuel burning...
The 14C/12C ratio:
14C is a carbon isotope that is made in the atmosphere by the impact of cosmic rays. It is an unstable (radioactive) isotope and breaks down with a half-life time of about 6,000 years. 14C is used for radiocarbon dating of not too old fossils (maximum 60,000 years). The amount of 14C in the atmosphere is variable (depends of the sun's activity), but despite that, it allows to have a reasonable dating method. Until humans started to burn fossil fuels...
The amounts of 14C in the atmosphere and in vegetation is more or less in equilibrium (as is the case for 13C: a slight depletion, due to 12C preference of the biological processes). But about halve of it returns to the atmosphere within a year, by the decay of leaves. Other parts need more time, but a lot is going back into the atmosphere within a few decades.
For the oceans, the lag between 14C going into the oceans (at the North Atlantic sink place of the great conveyor belt) and its release around the equator is 500-1500 years, which gives a slight depletion of 14C, together with some very old carbonate going into solution which is completely 14C depleted. In pre-industrial times, there was an equilibrium between cosmogenic 14C production and oceanic depletion.
Fossil fuels at the moment of formation (either wood for coal or plankton for oil) incorporated some 14C, but as these are millions of years old, there is no measurable 14C anymore left. Just as is the case for 13C, the amount of CO2 released from fossil fuel burning diluted the 14C content of the atmosphere.
This caused problems for carbon dating from about 1890 on. Therefore a correction table is used to correct samples of after 1890. In the 1950's another human intervention caused trouble for carbon dating: nuclear bomb testing induced a lot of radiation, which nearly doubled the atmospheric 14C content. Since then, the amount is fast reducing, as the oceans replace it with "normal" 14C levels. The half life time is about 14 years.
Again, this adds to the evidence that fossil fuel burning is the main cause of the increase of CO2 in the atmosphere..
1.6 The ocean's pH and pCO2:
If CO2 is increasing in the atmosphere with 50-55% of the accumulated emissions, a part is absorbed by vegetation (see chapter 1.4), another part is absorbed by the oceans. When CO2 is absorbed by the oceans, this is partially in solution in its original form, but some of it reacts with available carbonate ions to form bicarbonate. Between 1751 and 1994 the average surface ocean pH is estimated to have decreased from approximately 8.179 to 8.104 (a change of -0.075), based on recent and older oceanic surveys)/
Although the ocean pCO2 data are scattered in time and covered area, the trends are clear that the average (increasing) flow of CO2 is from the atmosphere into the oceans and not reverse.
This adds to the overall evidence that human emissions are the main cause of the increase of CO2 in the atmosphere.
Conclusion
From the available evidence it is quite clear that human emissions are the main cause of the increase of CO2 in the atmosphere. There is a small influence of temperature on this increase, as warmer oceans emit some CO2 (but warmer land absorbs more CO2 in vegetation!).
The influence of temperature is limited: based on the variability of the CO2 increase around the trend, the short-term (seasons to 2-3 years) ratio is 4-5 ppmv/ºC (based on the seasonal and opposite temperature related 1992 Pinatubo and 1998 El Niño events).
The very long term influence of temperature on CO2 levels (Vostok ice core) is about 8 ppmv/ºC. Thus at maximum, the influence of temperature on the current increase since the LIA is 0.8 ºC x 8 ppmv/ºC = 6.4 ppmv of the over 100 ppmv increase since the start of the industrial revolution.
There are only two fast main sources of CO2 to the atmosphere, besides the burning of fossil fuels: oceans and vegetation. Vegetation is not a source of CO2, as the oxygen deficiency (see chapter 1.5) showed. Neither are the oceans, as the δ13C trend (see chapter 1.3) and the pCO2/pH trends (see chapter 1.6) shows. This is more than sufficient to be sure that human emissions are the cause of most of the increase of CO2 in the atmosphere over the past 1.6 century.
Thus we may conclude:
All observed evidence from measurements all over the earth show with overwhelming evidence that humans are causing the bulk of the recent increase of CO2 into the atmosphere.
But... That humans are the cause of the recent increase of CO2 doesn't tell anything about the influence of increased CO2 on temperature!
Extra: how much human CO2 is in the atmosphere?
A lot of people is confused about this point:
Only a few percent of the atmosphere is currently from human origin. That is because every year about 150 GtC of CO2 (somewhat less than 20% of the total CO2 content) is exchanged between the atmosphere and the oceans/vegetation.
That means that every single CO2 molecule from human or natural origin has a 20% chance per year to be incorporated in vegetation or dissolved into the oceans. This makes that the half life time (the "residence" time) of human CO2 in the atmosphere is only about 5 years.
This was confirmed by the fate of 14C, increased due to atomic bomb testing, after the tests stopped. Thus if humans emit 8 GtC in a given year, next year some 6.5 GtC is still of human origin, the rest was exchanged with CO2 from the oceans and vegetation. The second year, this still is 5.3 GtC, then 4.3 GtC, etc...
This is not completely accurate, as some of the "human" CO2 comes back next year(s), especially from vegetation, as much of vegetation is one-year old leaves, which rotting returns a high part of CO2 incorporated in previous years. This is less the case for the oceans, where more of the absorbed CO2 disappears into the deep oceans, where it isn't directly traceable anymore.
There are techniques to follow human CO2 even there, where they use other recent human-made gases like CFC's and the extra 14CO2 spike from the atomic bomb tests 1945-1960 to track the past emissions. Anyway the "half life", that is the time period in which half of the human induced individual CO2 molecules disappears, is around 5 years.
... Some conclude from this that humans are only responsible for 5% of the CO2 increase and thus, as far as that influences temperature, also only for 5% of the temperature increase. But that is a wrong assumption...
The previous paragraphs are about how much human induced CO2 still is in the atmosphere. That is about the origin and fate of individual CO2 molecules, which atmospheric lifetime is governed by the seasonal turnover (back and forth flows) of about 150 GtC in/out the atmosphere from/to oceans and vegetation, and has nothing to do with the fate of the extra amount of CO2 (as mass) that humans emit, neither with the increase of total amount of CO2 in the atmosphere as result of that.
The latter is governed by the net amounts which year by year are incorporated into oceans and vegetation. That is only 1-7 GtC/year (variable due to temperature variability) or in average 50-55% of the emissions. The half life time of this extra CO2 (as mass) is much longer than the half life time of an individual CO2 molecule: around 40 years [14].
Thus if we should stop all CO2 emissions today, then the increase of 100 ppmv since the start of the industrial revolution would be reduced to 50 ppmv after some 40 years, further to 25 ppmv after 80 years and 12.5 ppmv after 120 years...
The IPCC comes with much longer half life times, according to the Bern model. This is a combination of relative fast (upper oceans), slower (deep oceans and more permanent storage in the biosphere) and very slow (rock weathering) sinks for the extra CO2.
They assume that the first, relative fast, sinks of CO2 will reduce in capacity over the years. That is only true for the ocean surface layer, which follows the atmosphere quite rapidly (1-3 years), but is saturated at 10% of the change in the atmosphere, due to the buffer/Revelle factor.
Some media talk about hundreds to thousands of years that the extra CO2 will reside in the atmosphere. That is true for the last part of the curve, as the smaller amounts of CO2 are getting slower and slower into the sinks. But the bulk (87.5 %) of the extra CO2 will disappear within 120 years as there is no sign of a slowdown of the sink capacity of the deep oceans and vegetation.
From several discussions, I know that it is quite difficult to understand the two different mechanisms which govern the fate of human CO2 in the atmosphere: the fate of individual molecules, governed by exchange rates ("turnover") and the fate of an increase in total CO2, governed by absorption rates (sink capacity).
... there is little doubt that humans are fully responsible for most of the increase of CO2 in the past (at least halve) century, that means that - as far as there is an influence of CO2 on temperature - that humans may be responsible for (a part of) the temperature increase. How much, that is an entirely different question, as that mainly depends of the (positive and negative) feedback's that follows any increase of temperature"