There appears to be a very interesting fine structure to great Southern Ocean (SO) atmospheric CO2 levels if one calculates residuals relative to the ‘official’ NOAA global average. This also applies to individual Northern Hemisphere (NH) and Southern Hemisphere (SH) monitoring stations such as Mauna Loa (MLO) and Easter Island (EIC) respectively, sited in the Northeastern and Southwestern Pacific Gyres.
The purpose behind calculating % residuals relative to the (smoothly rising) global average is that this maximizes factoring out the net effects of (temporal) trends in anthropogenic emissions or oceanic up welling and down welling across the planet. The following graph illustrates this point. The graph below was obtained by analysing all NOAA monthly near surface CO2 data from 1982 to 2007 to compute the annual average for all global stations and then computing annual residuals relative to that global average for:
(a) the Mauna Loa (MLO) station only;
(b) the Easter Island (EIC) station only; and
(c) the (unweighted) pooled average of all SO stations from below 30 S to the South Pole.
Please note that residuals above the x = 0 axis are negative (meaning SO or EIC total CO2 levels are below the global mean) and residuals below the x = 0 axis are positive (meaning MLO total CO2 levels are above the global mean). Note also that the residuals of the annual average CO2 at all SO stations are shown with appropriate one standard deviation error bars (on the mean for all stations). These have reduced in magnitude over the years as the number of SO CO2 monitoring stations has risen from only 3 in 1982 to a contemporary maximum of 9 stations. Only data was used where a full (monthly) annual record was accredited by NOAA so that at stations where a full 12 month record was sometimes not available e.g. due to equipment problems, any estimations of monthly CO2 levels obtained by extrapolation could be totally avoided. In other words, this graph contains no addition â€˜data massagingâ€™ whatsoever.
In this graph we may clearly see that over the period 1982 â€“ 2007 CO2 levels at MLO were always greater than the global average. For MLO 1998 was an obvious peak in exceedance of the global CO2 average but despite the fading of the large 1998 El Nino, at least until 2007 the trend for MLO seemed to be for an increasing margin above the global average.
In this graph we may also clearly see that over the period 1982 â€“ 2007 CO2 levels over the SO were always lower than the global average. For the SO 1998 was not a special year with respect to the negative residual for SO CO2 level below the global average.
However, it can be clearly seen that over the period 1982 â€“ 2007 the (negative) residual of CO2 levels over the SO relative to the global mean has trended towards greater values. In other words over 1982 â€“ 2007 CO2 levels over the entire SO have slowly lagged increasingly below the (rising) global average CO2, falling from about 0.35% below the global average to about 0.55% in recent years â€“ a trend of about 0.1%/decade against the (always rising) global average CO2 level.
Additionally, it can also be seen that the CO2 residual below the global average over the SO has been approaching the long term average residual at the Easter Island Station (EIC), which has always typically lagged about 0.65% below the global average CO2 level since records commenced in 1994.
Following my identification of the above residual and trends in residuals about global CO2 levels I embarked on a spare time investigation to try to understand what may be going on with respect to CO2 dynamics in the SO. My investigations focussed on looking at the important role of cyanobacteria (formerly known as blue-green algae) in the oceans.
Cyanobacteria are very important organisms in the global biosphere because they comprise about 48% of the global living biomass, live in the top 50 m or so of the oceans and are photosynthetic. They absorb carbon in the form of dissolved CO2 and bicarbonate from seawater and respire (emit) oxygen. They are of course the micro-organisms which more or less gave us our 21% oxygen atmosphere following their evolution about 3 Gy ago. After land plants (which evolved from cyanobacteria and no make up about 52% of the worlds living biomass) they are the most important photo-synthesizers on the plant. After simple water temperature effects on CO2 solubility the biotic cycle of uptake of CO2 by cyanobacteria is the next most important mechanism why may affect atmospheric CO2 levels over the oceans.
The following text attempts to summarize the outcomes of my investigations thus far. It is not intended to be a definitive or dogmatic statement but is submitted simply to try to raise interest in the very important issue of global cyanobacterial productivity and its relationship to the global carbon cycle and make a few speculative comments that readers may wish to comment-on and take further.
Below is a two component graph showing average monthly daytime Chlorophyll a (black) over all oceans over the last ten and half years and average monthly daytime sea surface temperatures (SSTs; in green) over the last five and half years for the latitude band 0 (Equator) to 30 N (i.e. Sub-Equatorial NH). Chlorophyll a is a (satellite sea surface colour-sensed) measure of cyanobacterial density â€˜productivityâ€™) at the sea surface.
Note the pronounced 1998 El Nino sea surface temperature (SST) effect on cyanobacterial productivity. Furthermore, please especially note the presence of a bimodal population of cyanobacteria in each annual cycle i.e. a â€˜Consortium Sâ€™ which blooms more-or-less in summer and a â€˜Consortium Wâ€™ which blooms more-or-less in winter. Note also the peaks and troughs in annual SSTs.
Note also the increased strength of the winter 2006 and winter 2008 Consortium W blooms.
Now here below is the equivalent graph for the Equatorial oceanic latitude band along the Equator i.e. 15 N – 15 S.
Note again the presence of a bimodal pattern of populations of cyanobacteria in each annual cycle i.e. a â€˜Consortium Sâ€™ which blooms in summer and a much weaker â€˜Consortium Wâ€™ which blooms in winter.
Similarly, here below is the equivalent graph for the Sub-Equatorial SH latitude band just below the Equator i.e. 0 – 30 S. Note the almost complete absence of a 1998 El Nino SST effect.
Note also the almost complete absence of a bimodal pattern of population of cyanobacteria in each annual cycle i.e. Consortium W dominates completely – unlike the situation with the Sub-Equatorial NH oceans.
Now here below is the equivalent plot for the mid-NH latitudes 30 N – 60 N. Note again the almost complete absence of a 1998 El Nino SST effect. Note well the now marked and very consistent presence of â€˜Consortium Sâ€™ and a shift of the (no stronger) â€˜Consortium Wâ€™ to warmer waters later in each year, relative to more equatorial waters. Note also how the weaker â€˜Consortium Sâ€™ has however increased in activity from a peak Chlorophyll a level of about 0.6 mg/m3 in 1997 to approx. 0.7 mg/m3 in 2006 -7.
The final graph shows the equivalent SH plot for the mid-latitudes of 30 S – 60 S. Note the weak but still-evident 1998 El Nino SST effect. However, most importantly, note the complete absence of â€˜Consortium Sâ€™ unlike the equivalent oceanic band of the NH (30 N – 60N).
There is also a shift of the (now solitary) â€˜Consortium Wâ€™ to warmer waters later in each year (relative to more equatorial waters).
These graphs are sufficient to demonstrate that the behaviour of two vast crops of oceanic cyanobacteria which I have completely arbitrarily labelled ‘Consortium Sâ€™ and â€˜Consortium Wâ€™ depending upon in which part of the year they approximately bloom, in the NH Equatorial and Mid-Latitude oceans, is markedly different to the SO below 30 S (which has only a ‘Consortium W’ type population).
As far as I know this observation does not appear anywhere in the modern scientific literature on the ocean.
Why there should be two distinct NH oceanic cyanobacterial consortia producing two annual phases of blooming in the NH oceans (i.e. blooming over two fairly distinct water temperature ranges) is an interesting and as yet unresolved question.
Is it a modern adaptation to the hemisphere where most anthropogenic CO2 has been increasingly generated over the last 200 or so years?
Or is it (say) a past-evolved consequence of the timing of (say) iron and silica export in dusts (noting these are limiting nutrients for cyanobacterial growth) from the (proportionately larger) NH continents e.g. Sahara, Gobi etc?
It has long been known that cyanobacterial productivity tends to be higher in NH oceans because of the higher iron and nitrogen nutrient levels in those ocean by comparison with SH oceans. This is a consequence of the greater proportion of land in the NH and the consequential observed higher nutrient levels in the surface layers of NH oceans.
My personal inclination is to infer that NH mixed populations of oceanic cyanobacteria might well be adapting already to the more rapidly increasing NH atmospheric CO2 levels, the higher SSTs there and probably the larger anthropogenic fixed nitrogen pollution of the NH by adaptation to establish a stronger ‘Consortium S’ population designed to consume those elevated CO2 and fixed nitrogen nutrient levels.
Yet despite the lack of evidence for two annual consortia in the great SO increasing negative deviations of CO2 levels over the great Southern Ocean from the global mean CO2 level (the ‘residuals’ â€“ refer my first graph above) still strongly suggest that this â€˜CO2 fertilization effectâ€™ is occurring in the SH too.
Possibly we simply can’t discern it in the Northern Hemisphere from looking just at annual CO2 residuals relative to the global mean because the Northern Hemisphere is where the CO2 flux to the atmospheric (both through land and sea-based aerobic decay of natural organic matter and through anthropogenic emissions) is much greater.
Are these data a modern example of evolution in action? They certainly appear to indicate evolution of the vast crop of oceanic cyanobacteria in the direction of increasing adaptation-to and attenuation-of elevated atmospheric CO2 (from whatever source) and increasing SSTs (regardless of their cause).
There is therefore clear evidence that, on a regional basis the modern capacity of the oceans for CO2 removal is both regionally variable and in some regions is likely increasing.
In my view, this is a result of the effects on cyanobacterial primary productivity of increasing CO2 fertilization, perhaps delayed fertilization from iron and silicon fallout/washout from volcanos like Pinatubo, Chaiten etc, but perhaps most importantly, the massive and rapidly increasing input to the coastal shelves of anthropogenic fixed nitrogen.
It is not often appreciated that the total export of fixed forms of nitrogen into coastal shelf waters by mankind is massive and approximately equal to the sum of all natural exports.
In fact the anthropogenic fraction of total nitrogen emitted from the land to the oceans and the atmosphere is much greater than for the anthropogenic fraction of total carbon emitted from the land to the oceans and the atmosphere. Somehow, in the midst of all the hysteria about anthropogenic carbon emissions we rarely get to consider this extremely significant fact.
Cyanobacterial primary productivity has a negative feedback effect on SST which in turn increases CO2 solubility etc.
This arises through:
â€¢ increased sea surface reflectivity (albedo) induced by the blooming of biogenic calcite-secreting cyanobacteria (‘coccolithophores’);
â€¢ increased sea surface reflectivity reduced sea surface evaporation rates caused by mono- and multi-layers of lipids formed on the sea surface during cyanobacterial blooming (via zooplankton predation and the action of cyanobacteriophages); and
â€¢ enhanced cloudiness and increased reflectivity of low level clouds by cyanobacteria-emitted dimethylsulfide (DMS) and isoprene-based aerosol Cloud Condensation Nuclei (CCN).
Finally, in the Australian context, I note (a little mischievously) that the reduced SSTs which Cai and Cowan (2006), recently noted for the seas to the north of Australia might be induced by increasing cyanobacterial primary productivity in the coastal shelf zones of South East Asia and the Indonesian archipelago, itself driven by the known increasing anthropogenic nutrient pollution of those shallow seas.
If that were the case then this effect could well be a subtle but key driver of the increasing dryness of the Murray Darling Basin.