Over the course of time CO2 emitted to the atmosphere is sequestered in carbon sinks. There are two places it can go:
* Into the ocean sink, or
* Into the terrestrial biosphere sink (vegetation, soils etc.)
How much goes into each? I’m still looking into that. In the meantime I’m presenting some observational data which suggest that the terrestrial biosphere sink is growing rapidly and may be making a larger contribution than is generally supposed.
We begin with the monthly CO2 record for Point Barrow, Alaska. It shows an increase of about ~55 ppm CO2 between 1975 and 2008, which is within a few ppm of what CO2 records elsewhere in the world show, and the pronounced seasonal CO2 cycle characteristic of the Arctic. At first glance there’s nothing unusual about it. (Note: the unadjusted Scripps monthly CO2 data I used to construct the Figures in this post have now been overwritten by seasonally-adjusted monthly data, although daily data are still available):
Figure 1: Point Barrow CO2 record
But when we detrend the data and isolate the seasonal CO2 cycle, this is what we see:
Figure 2: Point Barrow seasonal CO2 cycle and seasonal cycle amplitude
Except for the outlier point in 1980 the amplitude of the seasonal CO2 cycle shows a straight-line increase from 14.7 ppm CO2 in 1976 to 17.6 ppm CO2 in 2006, a 20% increase over 30 years.
Do other records show the same thing? Figure 3 plots the seasonal cycle and amplitude data for the ten longer-term stations in the Scripps CO2 data set. The five Northern Hemisphere records (Alert, Barrow, La Jolla, Mauna Loa and Cape Kumukahi) all show appreciable seasonal CO2 ranges and straight-line increases in seasonal cycle amplitude (although interestingly Mauna Loa, the most widely-used, gives the most erratic results, most likely because of elevation. The record from Cape Kumukahi, which is located a short distance away at sea level, is much better-behaved). Christmas Island on the Equator shows a much smaller seasonal cycle and the four records in the Southern Hemisphere show hardly any seasonal cycle at all (all plots have the same vertical scale). American Samoa is the only record in the Southern Hemisphere that shows a detectable increase in seasonal cycle amplitude:
Figure 3: Seasonal CO2 cycles & amplitudes, all Scripps records
Station locations are shown on the map below:
How large are the increases when considered on a global scale? Figure 4 shows a global CO2 time series I constructed by averaging five records at latitudes that average out close to the Equator – Barrow, La Jolla, Christmas Island, Baring Head and South Pole, all of which have data between 1978 and 2006. The records track each other closely in terms of absolute CO2, with the Northern Hemisphere records showing a few more ppm than the Southern Hemisphere records and much larger seasonal cycles that are in antiphase to those in the Southern Hemisphere. The global mean plot, however, is dominated by Northern Hemisphere seasonal cycle.
Figure 4: Global CO2 series
Figure 5 shows the detrended global series. Over the 26-year period the amplitude of the global seasonal cycle increased by 22% from 4.9 to 6.0 ppm:
Figure 5: Global seasonal CO2 cycle and seasonal cycle amplitude
Figure 6 plots the Figure 5 seasonal cycle CO2 amplitudes against mean annual atmospheric CO2 concentration. The two are strongly correlated (R squared = 0.95, increasing to 0.98 when the outlier point at the bottom left – the 1980 Barrow data again – is excluded):
Figure 6: Seasonal CO2 cycle amplitude vs. atmospheric CO2 concentration
These results all point in one direction – increased atmospheric CO2 is causing the terrestrial biosphere sink, the bulk of which is in the Northern Hemisphere, to expand at a rate which is closely linked to the concentration of CO2 in the atmosphere. In other words, more CO2 is causing the terrestrial biosphere carbon sink to grow. The question is, how fast?
Well, what could have caused the amplitude of the global CO2 seasonal cycle to have increased by 22% between 1978 and 2006? The simplest explanation is that the terrestrial biosphere was 22% larger in 2006 than it was in 1978. How much does this represent in terms of added carbon? The world’s vegetation is estimated to weigh around 600 gigatonnes, and 22% of that is 132 GtC, which over 28 years works out to an average increase in sink size of 4.7 GtC/year. Not peanuts.
Is this number realistic? Right now I have no idea, but based on my ongoing review of the annual global carbon balance data it may be the best one we are going to get.