Where Did All The CO2 Go?

The subject of human carbon dioxide emissions and their build up in Earth's atmosphere is at the center of the anthropogenic global warming controversy. It cannot be denied that humans produce CO2 in large amounts, both from burning fossil fuels and from land use changes. This has led to much gnashing of teeth and renting of garments by excitable ecological doomsayers, but there is something they do not mention: at the same time humanity is spewing forth carbon, nature is busily sucking up that carbon and storing it away. A new analysis of the carbon cycle has produced an unexpected result—not only is the absorption of carbon continuing unabated it has actually expanded. The latest scientific tally indicates that since 1959, approximately 350 billion tons of carbon have been emitted by humans to the atmosphere, of which about 55% has been reabsorbed by the land and oceans.

That Earth's ecology absorbs CO2 has long been known; it is, after all, plant food. Living organisms in ocean waters and on the land absorb carbon from carbon dioxide and sequester it for periods of time ranging from decades to millions of years. The various pathways of this absorption are called sinks. If all the sources of carbon into the environment are matched by the carbon sinks removing carbon then the carbon cycle would be perfectly balanced. But contrary to statements by unscientific simpletons, nature is never in a static balance—it is always in a constant state of tension, of flux, as various processes contend.

Knowing that the sinks are not static has caused many climate change alarmists to issue dire warnings that the sinks would somehow fill up and stop absorbing mankind's carbon effluent. This prediction is interesting, since science does not even know all the mechanisms behind carbon absorption. In fact, for years researchers have been working to identify what has been labeled the “missing sink.” Quoting from chapter 7 of The Resilient Earth:

For more than three decades, the attention of biologists and ecologists studying the global carbon cycle has focused on an apparent imbalance in the carbon budget. The so-called “missing sink” is a result of the following equation:

Atmospheric CO2 Increase = Human Emissions + Land Use – Ocean Uptake

This equation is simple enough: the amount of carbon produced by humans plus the carbon produced by other living things, less the amount absorbed by the oceans, must end up as atmospheric CO2. But, if actual numbers are used, the equation does not balance.

The average annual emissions of 8.5 Gt during the 1990s, 6.3 Gt from fossil fuels and 2.2 Gt from land use, are greater than the sum of the annual buildup of carbon in the atmosphere (3.2 Gt) and the annual uptake by the oceans (2.4 Gt). Here, land use includes carbon from decaying dead vegetation, soil organic matter, and wood products less the uptake by regrowing ecosystems. An additional sink of 2.9 Gt is required to balance the carbon budget. Though this is a small amount, over time, it adds up, 115 Gt of missing carbon over the period 1850-2000.

As we reported in TRE, despite the best efforts of scientists to account for the “missing” carbon, no good answer has been found. There has been a lot of work in this area yet no one knowledgeable in the field would say that every sink has been identified. The balance between sources and sinks of carbon is shown in the illustration below, taken from our book.


Carbon flux showing the missing sink, 1850-2000.

This, of course, has not stopped the argument over continued absorption, with most of the climate catastrophists arguing that the rate must decline or, at best, struggle to remain steady. A recent study in Nature by A. P. Ballantyne and colleagues has shed new light on the short term carbon cycle with a painstaking survey of carbon sources and atmospheric CO2 levels over the past 50 years. The main result of the study can be gleaned from the letter's title, “Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years.” More detail is available from the paper abstract:

One of the greatest sources of uncertainty for future climate predictions is the response of the global carbon cycle to climate change. Although approximately one-half of total CO2 emissions is at present taken up by combined land and ocean carbon reservoirs, models predict a decline in future carbon uptake by these reservoirs, resulting in a positive carbon–climate feedback. Several recent studies suggest that rates of carbon uptake by the land and ocean have remained constant or declined in recent decades. Other work, however, has called into question the reported decline. Here we use global-scale atmospheric CO2 measurements, CO2 emission inventories and their full range of uncertainties to calculate changes in global CO2 sources and sinks during the past 50 years. Our mass balance analysis shows that net global carbon uptake has increased significantly by about 0.05 billion tonnes of carbon per year and that global carbon uptake doubled, from 2.4 ± 0.8 to 5.0 ± 0.9 billion tonnes per year, between 1960 and 2010. Therefore, it is very unlikely that both land and ocean carbon sinks have decreased on a global scale. Since 1959, approximately 350 billion tonnes of carbon have been emitted by humans to the atmosphere, of which about 55 per cent has moved into the land and oceans. Thus, identifying the mechanisms and locations responsible for increasing global carbon uptake remains a critical challenge in constraining the modern global carbon budget and predicting future carbon–climate interactions.

Bottom line—carbon uptake is not diminishing; in fact, it has actually doubled over the last five decades. Without that doubling humanity would have added more than twice the amount of CO2 to the atmosphere than we did. While the abstract nicely captures the overall message of the paper, there are more interesting details contained in the report's body. A better overall picture of the trends found by the study can be obtained from the figure shown below, taken from the report.


Accumulation of carbon emissions in the atmosphere, on land and in the oceans.

As can clearly be seen from the graph, as human CO2 emissions have risen so has the natural uptake of carbon by land and sea. Whether this expansion of carbon uptake is because there is more of it about or because of other changes in the environment. Here are a few more cogent points made by Ballantyne et al.:

  • A commonly used diagnostic for detecting changes in the relative C sink efficiency is the airborne fraction, AF. Our results show that when land-use emissions are included, there is no detectable change in AF over the last 50 yr.

  • From a global mass balance perspective, net uptake of atmospheric CO2 has continued to increase during the past 50 yr and seems to remain strong.

  • Although present predictions indicate diminished C uptake by the land and oceans in the coming century, with potentially serious consequences for the global climate, as of 2010 there is no empirical evidence that C uptake has started to diminish on the global scale.

So the current state of the short term carbon cycle is strong and may be getting stronger. As humans have emitted more carbon dioxide Earth has stepped up its carbon capture. Whether this is relatively short-term sequestration as new forest growth or longer-term storage in the deep ocean and in what proportion can not be stated with confidence (see “The Ocean Plays A Deeper Game” and “Ocean CO2 Storage Revised”). The plain truth is that we still do not know where all the CO2 goes.

In any case, it would appear that there is a negative feedback working to mitigate atmospheric CO2 buildup. That raises a number of questions: if CO2 emissions were to decline would nature return atmospheric carbon dioxide levels to lower, even preindustrial levels relatively quickly? Is the expanded response to growing CO2 levels an indication that Earth's ecosystem is actively working to maintain current conditions? Is all the talk of “irreversible change” so much hand wringing bombast? Science currently cannot answer these questions, but all will become clear in the fullness of time. In the meantime, we should remain calm and take comfort in our resilient Earth.

Be safe, enjoy the interglacial and stay skeptical.

Carbon Dioxide Sink - Unknown

Is the carbon sink by photosynthesis a product of computer modeling? If the model does not realize the reaction rate to be portional to the square of the CO2 concentration this would account for the discrepancy. Photosynthesis is a second order reaction.

CO2 missing volumes

Any analysis of "missing" CO2 assumes we have a good understanding of what is going into the atmosphere. If the anthropogenic CO2 has been overestimated because alarmist a) wish to dramatize the "problem", b) wish to support carbon-capture projects or carbon credit programmes by demonstrating the size of the prize, or c) use a precautionary principle to guess higher when the choice is given, the end result will be to say there is more CO2 going into the atmosphere than can be accounted for.

When the BP blowout happened, it was said that 100,000 barrels per day of oil was coming out of a pipe that spewed a curious white mixture that looked like an oil, gas and water emulsion. BP and the Feds would like to say there is a huge reserve under the sea, and Greenpeace/WWF would like to say that there is a huge amount of oil coming out to foul the oceans. It was in nobody's agenda-driven interests to suggest that BP had a blowout in a small pool. Even when the blowout rate dropped rapidly, no one suggested that the pool was far smaller than first said. Even when, months later, they couldn't find 50% of the oil that supposedly came out of the wellbore, they did not say that the proposed volumes were too high: no, they said that the microbes must have eaten them. No sludge on the bottom, no oil on the top, so IT WAS THERE BUT NOW IT IS GONE (and aren't we lucky?).

I see the BP blowout story here with CO2. The amount the IPCC and everyone says we are creating is too high. We can't find it because it was never there. Researchers can get grants to look for something missing, but not for something never present.

Man's CO2, like BP's Deep Horizon oil, exists to a significant extent in the minds of the ignorant but excited.

CO2 Sinks

If the oceans were storing more CO2, it would be seen from ocean measurements. It's not seen in the upper portions of the oceans & it can't get to the deeper ocean without going through the upper ocean. (Duh!) Therefore, the "excess" CO2 is being absorbed by plant life. QED, respectfully submitted.

Or, our esteemed schiesterists don't understand the "Carbon Cycle."

Arguing from a false premise

I have four words for you: North Atlantic Deep Circulation. Read “The Ocean Plays A Deeper Game” and “Ocean CO2 Storage Revised,” then reconsider your "proof".

Release of CO2 from ocean.

I have read good skeptics often stating that some 95% of the increase of CO2 in the atmosphere is released from the ocean and 5% from humans. Why is this not in the equations?

Release of CO2 from ocean.

The statement of 95% of CO2 coming from the ocean is flat out wrong. Yes, the natural environment is constantly spewing forth well on toward 200 GtC a year from the oceans and biosphere. This, however, is balanced by about that same amount of CO2 uptake in a given year, at 200 GtC. While minor imbalances can exist over years to centuries (leading to the small variations observed in the Mauna Loa and Paleoclimatic records), it is no where near enough to account for the dramatic rise seen since 1850. If one divided human emissions from the natural, then it would be 9/200, or about 4.5%.

As to this paper, it is being misused to argue that the whole of carbon saturation theory is wrong. Sorry to inform you, but the basic chemistry and physiology of the oceanic and biological systems shows that increased warming will result in less CO2 uptake over time. The oceans can saturate for long periods after fast CO2 build-up because the carbonate ions get seriously depleted, after which the erosion of soils and rocks on the continents must slowly refill them, but only over thousands to millions of years. The biosphere only maintains its status as an effective CO2 sink so long as its other many needs are met. Climate change resulting from the heavily CO2 induced energy imbalance will disrupt many of those other systems supplying those needs, such as temperatures, precipitation, storms, nutrient cycling, species range shifts and the attendent ecological disorder. Thus, as warming continues (as it must in the face of continued energy imbalance), the rate of CO2 uptake is sure to slow. Just because it hasn't yet shown in the data is a rather poor excuse for inaction, given we have only warmed 1 deg. C or so over the last 150 years. If the warming goes on as projected by current modeling, then the non-atmospheric carbon reservoirs are sure to saturate.

Depends on the interpretation of the question

The percentage of atmospheric CO2 attributable to humanity is highly dependent on the how the question is framed. During any one year human activity releases ~7 G tons of carbon equivalent. According to the IPCC AR4 the ocean releases 88 Gt and the terrestrial biome 119 Gt, so human emissions are only 3.27% of the total amount of carbon in play. In terms of total resulting static levels we are responsible for ~100ppm out of ~400ppm or 25%. What this report says is that nature has always sequestered more than its own emissions, conveniently absorbing some of man's emissions as well. Not only that, but the amount of human emissions being absorbed has been expanding, not diminishing as one would expect if the sinks were becoming saturated.

While it is true that cold water can hold more dissolved CO2 than warm the ocean is not uniformly warming—there are large volumes of very cold water in the ocean depths that function as a significant carbon sink. Only now is science starting to understand how the ocean works in this regard. But there are other mechanisms in the ocean that need to be considered, ones that will never become saturated. This is the absorption of carbon by oceanic lifeforms like forams and salps (see “New "Jelly Pump" Rewrites Carbon Cycle” for a discussion of the carbon cycle and various sinks). They absorb carbon and then carry it to the bottom where it becomes interred in sediment. Such carbon may not be recycled for hundreds of millions of years.

Is the theory of carbon saturation wrong? Yes it is, in that it does not accurately describe the processes that actually exist. If it was correct absorption would be slowing, not ramping up. But don't feel bad, every scientific theory is wrong, just that some are more wrong than others. After all, science is really just a long running argument, a never ending struggle to correct our own false ideas. To quote Carl Sagan, “A central lesson of science is that to understand complex issues (or even simple ones), we must try to free our minds of dogma.”

If the proportions remain the same

After the ocean uptake, then if the proportions remain the same, 97.73% of the twentieth century warming was due to release from the biosphere and only 3.27% due to anthropogenic CO2. Why would the proportions not remain the same?

Trends in the balance of CO2

a. If the out-gassing is balanced by uptake each year, then if the trend from year to year is to warm, should not the out-gassing be relatively greater than uptake?
b. Would not the dramatic rise since 1850 be most likely caused by a natural warming trend releasing it from the ocean? After all, the recent warming is far from unprecedented.
c. If warming precedes CO2 increase in the atmosphere then would not the likely source be the oceans? To accept the new study must we reject this idea?
d. If the oceans uptake is about equal to that of the out-gassing of the whole biosphere, then would not the proportions of the sources of the increase remain the same? That is 95% natural to 5% human?

Again, that depends

a) When I said that we were responsible for 25% of the CO2 present in the atmosphere that was making the (possibly incorrect) assumption that the rise from 300ppm to 400ppm was due only to humans. As many have noted, warming leads to higher CO2 levels so, if Earth started warming naturally after the end of the little ice age, there may well have been a natural rise in atmospheric CO2 levels.

b) Yes, see a above.

c) Yes, again see a. No, we do not have to reject this study, which merely seeks to quantify the total amount of CO2 released into the atmosphere vs how much atmospheric levels have risen over the same period of time. It neither identifies nor precludes any source or sink.

d) Actually, the absorption by both land and sea are greater than their respective rates of emission (ie the missing sink). Playing devil's advocate, one could predict that a sudden cessation in human emissions could cause a precipitous drop in atmospheric CO2 and hasten a new glacial period.

Physics and Chemistry

"... if Earth started warming naturally after the end of the little ice age, there may well have been a natural rise in atmospheric CO2 levels."

You are too careful here. In fact, there must have been an increase in CO2 out-gassing following the LIA due to warming of the oceans. This conclusion is consistent with both laboratory results (warming water discharges gas as it warms), and the geological record over the Pleistocene, where the lag between planetary warming and atmospheric CO2 concentration is pretty well documented. There is debate over the actual lag period, but the mass of the oceans should be expected to warm much less rapidly than the atmosphere or the land. At present, there are in fact two possible events consistent with alternative estimates of lag period. If the original 800-1,000 year lag is accepted, the current increase in CO2 is largely due to the Medieval Warm Period. If instead you accept more recent arguments that the lag period is shorter - e.g. not more than two centuries, then the cause of the present rise is the warming following the LIA.