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Annual cycles of atmospheric CO2 concentration

At Director Alivisatos’ climate change lecture on Monday, January 31st 2011 there was a question raised concerning the amplitude of the annual cycles of atmospheric CO2 concentration. Specifically, the question was why the amplitude of the annual cycle seemed to be constant when deforestation would lead one to expect that the amplitude would be decreasing with time.

The answer is that the annual signal comes mainly from the growth/decay cycles in the mid-high northern latitudes. Evidence that the annual cycle signal derives from high northern latitudes comes from the observation that the amplitude of the annual signal is latitude dependent. At high latitudes, the growing season is telescoped into 3-4 months, and so the CO2 amplitude is large. The amplitude decreases in the sub-tropics as the growing season lengthens. In the southern hemisphere, the large expanse of oceans yields milder seasons and small amplitudes to the CO2 annual cycle. The mixing time of CO2 within a hemisphere is 3 months. The strong signal generated in the northern hemisphere must propagate by atmospheric mixing to the southern hemisphere, which requires about 1 year. The dispersion accompanying this transport greatly attenuates the annual signal. There is also a slight lag in the CO2 concentration in the southern hemisphere because most of the CO2 emissions are in the northern hemisphere.

pCO2 in the surface oceans are seasonal, but the air exchange is slow. As a result, the ocean imparts little seasonality to atmospheric CO2.

The following figures (and many more) can be found here.

Figure 1

Barrow, Alaska CO2

Barrow Alaska (about 71°N; record obtained by the Scripps Institution of Oceanography network) shows the largest annual signal; amplitude is almost 20 ppm and can be seen to have increased somewhat over the past 30 years.

Figure 2

Mauna Loa, HI CO2

The Mauna Loa (19.5°N) record has smaller amplitude of about 6ppm:

Figure 3

Baring Head, NZ CO2

For stations at high southern latitudes, there is only a very small annual cycle. The above record is from 41.4°S latitude.

There are seasonal variations of CO2 in the tropics. The variations are not regular – as photosynthesis is both light- and water-limited, and some observations show higher photosynthesis during the dry season than the wet season.

Deforestation currently is restricted to the tropics. Deforestation ceased in northern latitude regions some 30 years ago. Currently, the forest biomass at mid-high northern latitudes is increasing. It is not clear what the atmospheric CO2 signature of deforestation may be: the phasing depends on the fate of the detritus. Deforestation by fires would release CO2 to the atmosphere in a pulse, the amount of CO2 (vs CH4) depends on the moisture status of the fuel and the temperature of the fire. The remaining detritus could decay slowly in situ, or be bundled and burned in the dry season, or be exported for conversion to wood products. In any case, it is unlikely that the deforestation impact on the annual cycle could be seen clearly, given not only the variability of the “background” annual cycle, but also because convection mixes the CO2 to the troposphere (large mixing volume) and dilutes the surface signal.

Figure 4

Bukit Kototabang, Indonesia CO2

CO2 at Bukit Kototobang, Indonesia. Figure generated by

Figure 5

Arembepe, Bahia, Brazil CO2

CO2 at Arembepe, Bahia Brazil. Figure generated by

Figure 6

CO2 emission from land use modification

CO2 emission from land use modification. Figure taken from Global Carbon Project 2010. Data source: R.A. Houghton 2010 (personal communication); GFRA (2010).

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Donald J. DePaolo

Donald J. DePaolo

DePaolo's research interests focus on using naturally occurring isotopes to explore a variety of questions. From analyzing the isotopic composition of old ocean sediments (and their implications for climate shifts) to understanding isotopic fractionation of molten materials, and many more applications beyond.

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Inez Fung

Inez Fung

A question driving my research is how will atmospheric CO2 and climate co-evolve, and what we can do about it? My team and I continue to analyze atmospheric, land and ocean observations pertinent to the carbon cycle, and to synthesize them in atmospheric transport models to infer how CO2 sources and sinks have been changing. We are also using the coupled carbon-climate model at NCAR to project how land and ocean carbon sinks will change with accelerating global warming and with human activities.

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