Mysterious CO2 activity in New Zealand shows Phytoplankton at work
Tom Quirk both finds a mystery and solves it.
Carbon dioxide is a “well mixed gas” yet CO2 levels over New Zealand start rising there each year in March — a whole month before we see it CO2 start to rise over Tasmania. Air over Cape Grim in Tasmania will be blown by the prevailing wind over to New Zealand about five days later. So these two stations should be showing similar numbers throughout the year. Instead some process in NZ is pushing up CO2 early. Levels also peak earlier in New Zealand, and by September, in early spring, some process around NZ is pulling the CO2 out of the sky. Both NZ and Tasmania share large forested areas, so that wouldn’t explain the difference.
Quirk wondered if it had something do with phytoplankton, so he searched for satellite data that measures chlorophyll in the ocean and shows, voila, that there is major activity right around the Baring Head station at the same time as CO2 levels are falling. Indeed, the station is smack in the middle of a mass phytoplankton bloom.
He calculates that each year about one quarter of a ppm is removed by phytoplankton around Baring Head in NZ. Put into context, the total rise of CO2 each year is around six times that in NZ which makes the effect of plankton seem modest — but the bloom in NZ is but a tiny part of the global rises and falls of phytoplankton all around the world. So the most important question remains unanswered — just how much of the yearly rise and fall of CO2 globally is driven by plankton?
Previously Quirk found a huge 2.5Gt carbon spike in 1990 (which is 9Gt of CO2) — as if three extra China’s were suddenly emitting CO2 that year. The best explanation was that changes in wind patterns and ocean currents meant it was a bad year for phytoplankton. When phytoplankton struggle, they don’t draw down the usual CO2, hence the spike.
Another study (Martiny, 2013) found that phytoplankton might be drawing up twice as much carbon as modelers thought. While Guidi et al 2015 looked at viruses and discovered that only 10 out of 5,000 were predictive of CO2 levels and these were viruses that infected plankton.
Humans put out only 4% of global CO2, so in terms of whether humans can outcompete cyanobacteria et al, the answer appears to be “no”.
What this means is 1/ that there is a lot we don’t know about the CO2 cycle, and 2/ there are very big players out there that have nothing to do with our airconditioners and cars. It means 3/ that there is another large force that can’t be managed with carbon taxes.
There are still mysteries: Quirk notes that something else was going on around Baring Head in the 70s and 80s when there was an even larger difference between the two stations. Perhaps this was due to ocean currents shifting? Until we understand the global carbon cycle why are we even pretending to control it?
A comparison of atmospheric CO2 measurements at Cape Grim and Baring Head
Guest Post by Tom Quirk
Cape Grim 41 S, 145 E and Baring Head 41 S, 175 E have provided long running measurements of atmospheric CO2 made by the CSIRO and the Scripps Institute for Oceanography (SIO). Cape Grim is on the north-west tip of Tasmania and some 2,500 km from Baring Head on the south-east tip of the North Island of New Zealand.
Seasonal variations are easily extracted from the SIO data as monthly measurements and monthly seasonally adjusted values are provided. The CSIRO data is not as helpful but it is possible by smoothing the monthly measurements to get seasonally adjusted values. The difference on a monthly basis gives the seasonal variations.
The seasonal variations of atmospheric CO2 for Cape Grim agree with the observed seasonal variations at the South Pole 90 S but differ from the seasonal variations at Baring Head by of order one month. This can be seen in Figure 1 where the mid-year rise in CO2 comes at Baring Head before Cape Grim. The westerly winds in the “roaring forties” are 20 to 40 km per hour so CO2 at Cape Grim would arrive at Baring Head some 5 days later but this is not seen in the measurements.
There may be an explanation for this as Baring Head is surrounded by oceans that experience chlorophyll-blooms as phytoplankton numbers increase and remove CO2 from the ocean which then rebalances CO2 with the atmosphere through exchange. The phytoplankton bloom is shown in Figure 2 from a fascinating paper by Murphy et al using satellite measurements to detect the ocean surface changes in chlorophyll
The variations in chlorophyll through the year are shown for 9 months in Figure 3 for September 1997 to August 1998. There are seasonal variations and regional variations around the North and South islands of New Zealand.
The composite average chlorophyll-a concentrations for regions around New Zealand are shown in Figure 4. The regions are the Central Tasman Sea (CTS) and the subtropical water east of North Island (STE) to cover east and west of the North Island while the South Island is covered by Subtropical Front regions east (SFE) and west (SFW).
There are chlorophyll concentration peaks in early and late months of the year. So if the phytoplankton are pulling CO2 from the oceans then this would also be seen in the atmospheric CO2. The test is to adjust the Baring Head seasonal values for the months of April to July to match the Cape Grim values and then look at the monthly differences as an indication of the seasonal effect of the phytoplankton. The adjustment is shown in Figure 5 with the alignment of values for May to July.
The amount of CO2 removed by the phytoplankton is shown in Figure 6 as the difference of the Cape Grim monthly values from the adjusted Baring Head values. The maxima appear in September – November and in January – February reflecting the variations of chlorophyll concentrations seen in Figure 4. Phytoplankton are expected to bloom in spring and autumn and there is a strong spring CO2 effect but much less is seen in autumn. So for a year 0.28 +/- 0.5 ppm of CO2 is removed by phytoplankton.
The probable explanation for the Cape Grim Baring Head seasonal variation differences is the strong local effects of phytoplankton in the oceans around New Zealand.
Annual atmospheric CO2 concentrations
The next issue is whether the local phytoplankton have modified the annual CO2 concentrations. This can be done by comparing differences for the annual atmospheric CO2 measurements at Cape Grim and Baring Head less the South Pole measurements. There is are differences as shown in Figure 7 of up to 1.2 ppm.
There are substantial differences in annual CO2 values for the period 1978 to 1989 However there is no evidence that shows any changes in the seasonal variations from the years 1978 to 1989 to the years 1995 to 2015 (Figure 8).
So there are differences in the 1978 to 1989 period that are not driven by the source of the seasonal differences and may be from an ocean source given the latitude difference of 30 degrees…
But for the period 1995 to 2017 the mean differences from the South Pole measurements are 0.05 +/- 0.03 ppm for Cape Grim and -0.17 +/- 0.05 ppm for Baring Head. So the annual difference of Cape Grim from Baring Head is 0.22 +/- 0.06 ppm that is not significantly different to the seasonal value of 0.28 +/- 0.5 ppm.
So the presence of phytoplankton in the ocean near Baring Head may be the cause of the atmospheric CO2 concentration difference from Cape Grim. In addition there may be some Southern Ocean changes.
The conclusion from this analysis is that measurements of atmospheric CO2 concentrations are complicated by local modification and by longitudinal differences.
A further conclusion is that if changes in phytoplankton productivity impact atmospheric CO2 levels as seen at Baring Head, then the peaking of atmospheric CO2 during an El Nino is partly due to well-measured reductions in phytoplankton productivity. This is not conventional wisdom.
 Keeling C. D, Piper S. C., Bacastow R. B., Wahlen M Whorf., T. P., Heimann M., and Meijer H. A., Exchanges of atmospheric CO2 and 13CO2 with the terrestrial biosphere and oceans from 1978 to 2000.I. Global aspects, SIO Reference Series, No. 01-06, Scripps Institution of Oceanography, San Diego, 88 pages, 2001
 R. J. Murphy , M. H. Pinkerton , K. M. Richardson , J. M. Bradford‐Grieve & P. W. Boyd Phytoplankton distributions around New Zealand derived from SeaWiFS remotely‐sensed ocean colour data. New Zealand Journal of Marine and Freshwater Research, 2001, Vol. 35: 343-362 0028-8330/01/3502-0343
 Michael J. Behrenfeld, Robert T. O’Malley, David A. Siegel, Charles R. McClain, Jorge L. Sarmiento, Gene C. Feldman, Allen J. Milligan, Paul G. Falkowski, Ricardo M. Letelier & Emmanuel S. Boss. Climate-driven trends in contemporary ocean productivity, Nature Vol 444 (7 December 2006)