It has become a popular belief among climate sceptics that nuclear bomb test 14CO2 data falsifies the Bern model [1, 2010; 2, 2013]. The Bern model is used to link atmospheric rise in CO2 to manmade emissions and lies at the heart of IPCC projections for the future trajectory of global CO2. What could be more important?
In this post I present a simple ocean surface water mixing model that explains why 14C cannot be used to predict the sequestration rate of CO2. Each year the ocean inhales about 92Gt of carbon from the atmosphere that is tagged with 14C. This inhaled CO2 mixes with the 1020 Gt carbon in surface ocean water before about 90 Gt is exhaled. The CO2 exhaled is not the same CO2 that was inhaled and is depleted in 14C. If the 14C tracer were to work, it would be necessary to assume that the CO2 exhaled had the exact same 14C ratio as that inhaled, the portion of 14C removed from the atmosphere residing in the 2Gt sequestered C. The CO2 exhaled is depleted in 14C and this gives an artificial false picture of rapid CO2 sequestration rates.
The dilution of 14C in the atmosphere by burning fossil fuel that contains zero 14C is a further process that gives rise to artificial rapid decline in the 14C curve that is not related to sequestration rates . The IPCC also recognises that 14C cannot easily be used to describe CO2 sequestration processes and on this occasion I agree with them.
Figure 1 Comparison of 14C decline from atomic bomb tests (red) with the Bern model (blue). The unlabelled Y-axis is 14C and the unlabelled X-axis is years since bomb test (1962). This post explains why 14CO2 cannot be used to model the sequestration rate of CO2 from the atmosphere and hence it cannot be used to falsify Bern. This does not mean that Bern is correct. Chart from WUWT .
In late 1962 the Earth was subjected to a gigantic experiment. Retired NASA Astronaut Phil Chapman writes :
Between August 2 and Christmas in 1962, just before the atmospheric test ban went into effect, the Soviet Union detonated no fewer than 36 nuclear weapons at their test site on Novaya Zemlya in the high Arctic, with an incredible total yield of 141.5 megatons of TNT equivalent. The series included, on Christmas Eve, the second-largest bomb ever tested, with a yield of 24.2 megatons.
When a nuclear weapon is detonated in the atmosphere, the neutrons emitted from the blast cause a sudden and quite substantial increase in the 14C content of the atmosphere. The excited carbon immediately combines with oxygen, so the effect of an airburst is to inject a slug of CO2 into the atmosphere that is tagged so that we can watch what happens to it.
14C is naturally radioactive with a half life of 5,700 years. It is formed on Earth by nature by the action of galactic cosmic rays on nitrogen and this provides the foundation for radiocarbon dating and for tracing the impact of galactic cosmic rays on Earth’s climate over time. In 1962, the atomic bomb tests injected a huge slug of additional 14C that spread around the globe and was monitored at a number of sites. This clearly provides a means of tracking the mixing time of the atmosphere but there are problems using this data to define sequestration rate of CO2 from the atmosphere into biosphere and ocean sinks as described below.
Phil Chapman makes some key observations :
The prevailing winds presumably took the cloud of 14CO2 right around the world to the site in Austria, which was 2,000 miles southwest of Novaya Zemlya. It took about two years more to reach New Zealand, in the Southern Hemisphere. At the time of the peak in New Zealand, the concentration was still higher in Austria, indicating that the carbon-14 was not yet evenly distributed in the whole atmosphere. Thus some of the initial decrease in Austria was apparently due to dilution of the cloud as it spread.
Figure 2 Atmospheric 14C following atomic bomb tests in Russia in 1962 . The data are useful for examining atmospheric mixing times but can the exponential decline be used to model natural CO2 sequestration rates?
Some bomb 14C arrived in new Zealand within a year, it took 2 years to reach a peak in New Zealand and about 10 years to fully homogenise between monitoring sites in Austria and New Zealand (Figure 2). This is all valuable insight to atmospheric mixing rates.
The bomb 14C data has been used to model sequestration rates of CO2 from the atmosphere. The exponential decline of bomb 14C from 1969 onwards is not due to radioactive decay since the half life is over 5000 years (Figures 1, 2, 3). It was therefore hypothesised that the decline was due to sequestration of CO2 from the atmosphere by fast sinks and the data could be used to model CO2 sequestration rates.
In Figure 3 I fit an exponential decline to a set of bomb 14C data  and deduce very roughly that the decline rate is of the order 7% per annum. This is much more rapid than the decline rates deduced by Roger Andrews and myself as discussed in recent posts [4, 5] and is much more rapid than the mean decline of the Bern Model that is not exponential and is dominated by slow processes (Figure 1). The consequence of the rapid 7% decline deduced from bomb data is shown in Figure 4. Such rapid decline does not match atmospheric observations and the gap between emissions and atmosphere needs to be filled by a natural CO2 flux.
Figure 3 Background image of bomb 14C from WUWT . The red line represents a 7% per annum exponential decline, equivalent to a half-life ~ 10 years for CO2.
Figure 4 Comparing actual emissions that are declined at 7% per annum with actual atmosphere there is gap that if the bomb 14C decline was correct would need to be filled by a natural CO2 flux.
When I first saw information on the bomb sequestration rates I thought the conclusions had to be bomb proof. The ramifications were profound. Natural sequestration rates were very fast and would pump away manmade emissions within a few decades and a major component of the rise in atmospheric CO2 was natural in origin.
We had a really good discussion on Roger’s post a few weeks ago  attempting to reconcile the various lines of evidence and Roger said:
What’s out of whack here is the 8-year bomb test residence time. It’s telling us only part of the story.
And that got me thinking. What processes may be in action that might lead to the bomb 14C data giving a false result? I used to make my living offering isotope analyses and data interpretations to the oil industry much of this based on understanding the distribution of isotope ratios in seawater and formation water systems. It didn’t take long to come up with the answer which is in fact the answer articulated by the IPCC.
The starting point is to have some understanding of the carbon cycle. For the purpose of this illustration I will focus on the ocean carbon cycle (Figure 5) although the argument may be applied equally to the land biomass side of the story.
Figure 5 The ocean carbon cycle according to IPCC Grid Arendal.
Every year the oceans exchange approximately 90 Gt C with the atmosphere. 92 Gt go in and 90 Gt comes out again. Surface ocean waters contain about 1020 Gt C and so what happens is that 92 Gt goes in, mixes with 1000 Gt and what comes out again is not the same CO2 molecules that went in. What we are trying to measure using the bomb 14C data is the rate at which that notional 2 Gt difference is sequestered. The bomb 14C data can only be used to measure that if the CO2 exhaled had the exact same 14C composition (14C/12C) as that inhaled and this will clearly not be the case.
For those who require convincing I have created a model to illustrate the process (Figure 6). At t1 the atmosphere is greatly enriched in 14C relative to the ocean. The ocean does contain some natural radiogenic 14C. The ocean inhales CO2 with the 14C signature of the atmosphere (t1.4). This then mixes with the carbon resident in the upper ocean layer (t1.6). And then finally the ocean exhales CO2 that will have a very different d14C ratio than the CO2 that was inhaled. In this simple model the ocean exhales the exact same amount of CO2 that is inhaled but the mixing and 14C dilution process that takes place in the shallow ocean results in a significant reduction in the amount of 14C in the atmosphere without any CO2 being sequestered. It should therefore be obvious that 14C in the atmosphere cannot be used to measure the rate of CO2 sequestration. 14C gets pumped down at an artificial high rate giving the false impression of rapid CO2 sequestration. Roger was right.
Figure 6 14C in red and 12C in yellow. It is not necessary to show 13C for the purpose of this exercise. In this simple model the amount of CO2 in the atmosphere and ocean at t2 is the same as at t1. No Co2 is sequestered but the concentration of 14C in the atmosphere is reduced from 50% at t1 to 41% at t2. Clearly 14CO2 cannot be used to measure CO2 sequestration rate.
d13C compositions will be affected by the same effect and should not be used to model atmosphere processes without taking this shallow ocean mixing process into account.
I recognise that many readers of Energy Matters may prefer energy based posts and so we will try to get back on that theme for a couple of weeks. “What’s up with the weathering sink” to follow in a few weeks.
 Phil Chapman 2010: Are we Responsible for the CO2 in the Atmosphere?
 WUWT Gösta Pettersson 2013: The bombtest curve and its implications for atmospheric carbon dioxide residency time
 WotsUp 2013: Watt about the bombtest?
 Energy Matters Euan Mearns 2014: What’s up with the Bern Model?
 Energy Matters Roger Andrews 2014: The residence time of CO2 in the atmosphere is …. 33 years?