Saturday, January 9, 2010

1. Are we responsible for the CO2 in the atmosphere? (updated)

There is no doubt that the concentration of atmospheric carbon dioxide has increased since the 19th century. The concern about global warming is based on the assumption that all of this increase is due to human activities. While nothing is certain in science, the best evidence we have indicates that this is incorrect: much of the growth is from natural sources. Let me explain…

The red curve in the figure shows the cumulative mass of anthropogenic CO2 that has been added to the atmosphere since 1890, according to data maintained by the Department of Energy. The total amount through 2008 is 1250 GT (i.e., gigatons, or billions of metric tons). Annual emissions due to human activities are now around 27 GT.

The blue curve shows the actual increase in the mass of CO2 in the whole atmosphere, totaling about 700 GT. The data prior to 1959 were obtained from measurements in tiny bubbles trapped in an ice core taken at Law Dome, Antarctica, and the values after 1959, showing seasonal cycling, are a continuous record by NOAA of the ambient concentration atop Mauna Loa on Hawaii. The present load in the whole atmosphere, including this increase, is now about 3000 GT.

The Intergovernmental Panel on Climate Change (IPCC) believes, without any real evidence, that the blue curve is entirely due to anthropogenic emissions. In other words, some 55% of the CO2 we have emitted remains in the atmosphere. The rest has vanished, presumably absorbed by biomass and the oceans. The IPCC's calculations then indicate that the residence time in the atmosphere of a given CO2 molecule, before it is absorbed by some sink, is at least a century.

Fortunately, we have a direct check of this assumption, as follows:

The radioactive isotope carbon-14 has a half-life of 5700 years. The only reason there is any left on Earth is that it is replenished at a low level by cosmic ray neutrons, which transmute atmospheric nitrogen-14 to carbon-14 (plus a proton).

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.

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.

Fortunately for present purposes, the results of the tests were extensively monitored because of concern about fallout The vertical black lines in this figure show the range of dates for these tests. The red and blue curves show records of the subsequent atmospheric concentration of carbon-14, as measured in Austria and in New Zealand.

The decline in concentration was not due to radioactive decay, because the half-life of the isotope is 5700 years, much longer than the time scale shown.

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.

By 1967, however, the figure shows that the concentrations measured in both places were approximately equal, indicating that the atmosphere was now well mixed. The subsequent decline in both places must then have been due to absorption by natural sinks (primarily biomass and the oceans). Both locations showed  an exponential decline, as indicated by the heavy red and blue trend curves, with a time constant of 16.4 ±0.7 years.

This result proves beyond any reasonable doubt that the residence time of CO­2 in the atmosphere is much shorter than assumed by the IPCC. This has profound implications for the theory of anthropogenic global warming (AGW), as it indicates that most of the CO2 emitted due to human activities has been absorbed long ago. Calculation shows that the quantity remaining in the atmosphere is less than 300 GT, or 40% of the measured increase in the atmospheric load.

The inescapable conclusion is that some 60% of the observed increase is from natural sources. The most probable cause is warming of the surface layer of the oceans (which reduces the solubility of CO2).

If this is true, it means that the IPCC has it backward: instead of the temperature rise being due to the increase in CO2, the increase in CO2 is due to the temperature rise. The warming during the last century must then be due to some external cause, such as a recovery from the Little Ice Age or changes in solar activity.

If we inject a slug of CO2  into the atmosphere, the residence time obtained from the nuclear airbursts means that it will reach equilibrium with the oceans within a couple of decades. In equilibrium, the partial pressure of CO2 above the sea is determined by the sea surface temperature. Adding CO2 to the atmosphere will not change the partial pressure, but will simply reduce the amount released from the sea. In other words, if sea surface warming is the source of most of the observed increase in CO2, the long-term (decadal) changes in concentration are unaffected by whatever we may emit.

If this is correct, it is of course a death blow to the whole idea of AGW.

The IPCC is struggling desperately to find plausible reasons why the CO2 residence time in the atmosphere must be a century, so far without notable success. If this were truly a scientific organization, they would be following the evidence wherever it leads, instead of trying to fabricate support for a preconceived theory. At the very least, we must postpone any expensive commitment until this issue is resolved by objective research.

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