Greenhouse Bulletin no. 122, April 1999
Dioxide and Methane Revisited
Dr Vincent Gray
This Bulletin reviews several recent papers dealing with carbon dioxide and methane in the atmosphere
1. Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. by A. Indermühle, T.F. Stocker. F. Joos, H. Fischer, H.J. Smith, M. Wahlen, B. Deck, D. Mastrolanni, J. Tachumi. T. Blunier & B. Stauffer. 1999 Nature , 398, 121-126.
2. Restless carbon Pools. By Phillippe Ciais 1999. Nature , 398 111-112.
These two papers have had much publicity from Fred Singer and others, but, essentially they merely reiterate and amplify what has already been published by Etheridge et al., 1996 J. Geophys Res 103 15,979-15,993, and already reported in Greenhouse Bulletin No 120 in February 1999. These conclusions are:
That the concentration of carbon dioxide in the pre-industrial atmosphere was never in equilibrium, but fluctuated significantly. · That changes in atmospheric carbon dioxide in this period were probably caused by changes in the various carbon pools, brought on by changes in global temperature. It is unlikely that the temperature changes were a result of the changes in carbon dioxide concentration. · That current carbon cycle models do not explain several features of the past record, notably a very slow growth, or even decline, in carbon dioxide concentration between 1935 and 1945.
The ice core results from the Taylor Dome Antarctica (77° 48'S, 158° 43' E) supplement the previous results from Law Dome (66° 44'S 112°E) to give a record of atmospheric carbon dioxide concentration for the past 1000 years which has overcome many of the inaccuracies of previous studies.
It should be realised that air trapped in ice cores has a different age from the ice which surrounds it, because the upper part of the core is firn, snow which has not yet trapped the air within it. Estimation of the time lag between the age of the ice and the age of the air is difficult, and different for each core. The more accurate techniques in these studies estimate an age difference of 30±1, 30±1 and 58±1 years for the three Law Dome cores. Since the measured carbon dioxide concentration in a single age layer is an average over this entire period, the plot of concentration against age is really a moving average over many years rather than a succession of individual age measurements. This is why the period from 1935 to 1945, which shows a slight increase from the ice core measurements, would probably be a fall if individual annual measurements were known.
The reason why the change in carbon dioxide could not have been responsible for a change in temperature is to be seen in the Law Dome results for the Little Ice Age (c 1550 to 1850 AD) when carbon dioxide concentration fell from 284 to 275ppmv when the temperature fell by an estimated 1°C. According to the latest model calculation of the greenhouse effect, such a fall in carbon dioxide could only account for a fall of less than 0.1°C for the IPCC "Best Estimate" climate sensitivity, from the greenhouse effect alone.
The paper by Indermühle et al uses the carbon dioxide fluctuations and carbon 13 isotope changes to calculate changes in carbon sources and sinks for the past 10,000 years.
The introductory article by Phillippe Ciais gives a good summary of the longer paper, but rather spoils things by stating that carbon dioxide "is expected to double from current levels during the next century", a prediction which would need an immediate fivefold increase in the rate of atmospheric accumulation of carbon dioxide to become true. I have pointed this out to him, and he has conceded that he should have said "it may" increase to this degree. I have refrained from asking for a probability estimate, or from placing a bet I will not be around to collect.
3. Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests. by K. J. Nadelhoffer, B.A. Emmett, P. Gundersen, O.J. KjÆnaas, C.J. Koopmans, P. Schleppi, A. Tiétema and R.F. Wright. 1999. Nature 398 145-148
4. `The Mysterious Missing Sink' D. W. Schindler 1998 Nature, 398 106-107.
The paper by Nadelhofer et al records the results of a study of forests in six European countries (UK, Denmark, Norway, Netherlands, Switzerland) and three forests in the USA in which radioactively labelled nitrogen fertiliser was added and its uptake by the trees measured in the leaves, trunk and branches to find whether it was influencing carbon uptake. They concluded that the added nitrogen only made a minor contribution ( less than 20%). This, in spite of their belief that "tree growth in northern temperate climates is typically nitrogen-limited". The tracers were applied to large plots in the forests (103 - 105 m2 ) across one to three year intervals, either directly (without fertilizer, simulating ambient nitrogen inputs, and with chronic fertilizer applications), or in forest canopy throughfall. At the one- to three-year timescale of these tracer studies, most labelled nitrogen inputs to the forests entered the forest floor and soil pools, or under the high rates of nitrogen input, were exported as inorganic nitrogen. The results indicate that soil rather than tree biomass is the primary sink for NOx and NHx inputs to temperate forests. This is consistent with analyses of tree growth which have not yet shown detectable increases in tree biomass accumulation with nitrogen deposition at the 15N-labelled sites. The authors conclude that elevated nitrogen deposition is not the primary contribution to the northern latitude CO2 sink. as suggested by current modelling scenarios which assume that tree biomass accumulates 80% of atmospheric nitrogen input.. Other factors, such as changes in forest use and management, CO2 fertilization, or long term redistribution of nutrients from soils to trees resulting from reforestation or climate warming are more likely to be responsible for the positive carbon sink in northern and mid-latitude forests. One is forced to ask whether a "one to three year" growing period is really sufficient to give such firm conclusions. Also, wherever the nitrogen goes, except that part that is re-converted to nitrogen gas, it must surely fertilise some plant growth, involving absorption of CO2, somewhere along the line, even if., ultimately, in the ocean.
The paper by Schindler is essentially a confession that carbon models are currently all at sea, since they do not know where the carbon emissions are going., or what processes control the absorption of that part which does not enter the atmosphere. He speculates that perhaps the temperate forests already have enough nitrogen from fossil fuel emissions, thus suggesting that their growth rate may decline. However, there should be room for further nitrogen fertilization in boreal (northern) forests.
5. Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability. D.M. Etheridge, L.P. Steele, R.J. Francey and R.J. Langenfelds. 1998, J. Geophysical Research 103, (D13)m 15,979-15993.
This study is an extension of the greatly improved investigation on ice cores reported above with carbon dioxide. Here there are results from 1000 A.D. to the present from three Antarctic ice cores, two Greenland ice cores, the Antarctic firn and archived air from Tasmania..
As with carbon dioxide, the atmospheric methane from 1000 A.D. to 1800 A.D. fluctuated with the global temperature. The fluctuation was about 40 ppb around an average of 695 ppb, with an interpolar difference between 24 and 58ppb. There was a large increase in methane growth rate from 1945 to 1990, peaking at about 17ppb per year in 1981, and falling rapidly after that, and expected to become zero in 2006.with an atmospheric methane concentration of about 1700ppb. They calculate that there was an average total methane source of 250Tg yr-1 from 1000-1800 A.D., reaching near stabilization at about 560Tg yr-1 in the 1980s and 1990s. These results are backed up by measurements on carbon 13 isotopes.
This paper confirms what I have been saying for some time, that methane emissions, and the resulting atmospheric concentrations, have stabilized, and that IPCC scenarios and future projections which assume large future increases in atmospheric methane concentrations are erroneous..
R. Gray, M.A.,Ph.D., F.N.Z.I.C.
75 Silverstream Road
Phone (FAX) (064) (04) 4795939
April 4th 1999
GREENHOUSE BULLETINS are regular commentaries on the greenhouse effect and on climate change which have been published privately by Vincent Gray since 1991. Subscriptions are available from the above address. A list of back issues and individual Bulletins are also available on request.
Review Comments to dr Gray's Paper
Subject: Review paper on Daly's
Date: Fri, 9 Apr 1999 12:54:58 +0100 (BST)
From: firstname.lastname@example.org (COURTNEY)
To: VINCEGRAY@xtra.co.nz CC: email@example.com
Thenkyou for your fine review paper recently published on John Daly's web site that reports papers concerning atmospheric methane and carbon dioxide concentrations.
I write to draw your attention to another and very recent paper (viz. Benitez-Nelson CR & Buesseler KO, Nature, vol.398, pp502-505 (1999) ) that reports the "Variability of inorganic and organic phosphorus turnover rates in the coastal region".
According to Benitez-Nelson & Buesseler organic sequestration of carbon dioxide is limited by availabilty of phosphorus in coastal waters. (This agrees with my own finding in the 1980s that algal blooms in the North Sea are related to availability of inorganic phosphorus).
If Benitez-Nelson & Buesseler are correct, then I assert that some variation in observed atmospheric carbon dioxide concentration probably relates to supply of phosphorus to coastal waters, possibly related to variations in run-off of agricultural phospate fertilisers. The magnitude of this effect is not known and is difficult to quantify, but it could be significant.
This assertion is also pertinent to Jarl Ahlbeck's paper recently posted on Daly's web site.
I hope this is helpful
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