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.