Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 274

2010

Spring ZC

1959

Month

Fig. 6.1 The1959cycleofnorthernhemispherecarbondioxidesuperimposedonthe2010cycle,drawnbytheauthor

usingdatawithpermissionfromtheEarthScienceResearchLaboratoryoftheNationalOceanicand
AtmosphericAdministration.SpringZC,springzerocrossingpoint.Thehorizontaldashedlinerepresents
theoreticalzeroyear-on-yeargrowthinaverageannualcarbondioxide.Theupward-slopingdashedline
representsthelong-termincreaseinannualcarbondioxideandiscalculatedasbeingtheaverageannual
increaseinCO 2 in1959andalso2010.

in the warmer 2010 sea surface. So, the increased drawdown more accurately reflects
increased annual global gross primary production. The second thing to note is that
the first half of the 2010 curve leads that for 1959. The upward slope (the sloping
dashed line on the figure) closely represents the long-term average growth in carbon
dioxide (it being the mean increase for the two years 1959 and 2010). Note that in
Figure 6.1 the spring zero crossing point (ZC) for 2010 is ahead of that for 1959.
It reflects spring arriving earlier and an extension of the TGS. The position for the
autumn is less clear. Mathematically, the June/July so-called spring ZC point is
easier to establish: from a biological perspective, it is the point at which new growth
in April and May takes atmospheric carbon dioxide below the average long-term
trend in CO 2 growth. The annual peaks in the northern hemisphere carbon dioxide
cycle also are important. The difference between these superimposed peaks (as well
as the troughs) is greater than the difference in ZC points.
In 2008, a 20-year data analysis of northern hemisphere carbon dioxide concentra-
tions was carried out by a European and North American team led by Shilong Piao,
Philippe Ciais, Pierre Friedlingstein and colleagues, combining satellite chlorophyll
observations and a terrestrial biosphere model. Perplexingly, it concluded that the
autumn rise in carbon dioxide was happening earlier: intuition suggests with a longer
TGS that autumn would come later. This autumnal rise was not so much due to
autumn happening earlier per se, but in the warmer autumn there was greater res-
piration by detritovores (decomposers of biotic material), and this dominated the
extended autumnal photosynthesis (see also Miller, 2008, for a summary review of
this work).