The Strength Of The Seasonal Wiggle In Co2

The seasonal wiggle—the difference between summer and winter CO2 concentra-tion—also shows some variability over time. Seen from the CO2 monitoring stations in some places (e.g., from Mauna Loa in Hawaii and from Barrow in northern Alaska), the strength of this wiggle seems to be increasing.

When it was first noticed, this increase in the seasonal oscillation in CO2 was explained in terms of increasing CO2 fertilization (Chapter 8) allowing more green leaves and other seasonal material to build up in the northern summers. If the CO2 was promoting plant growth, more uptake into photosynthesis would provide a deeper summer dip in CO2 concentration. Then, starting from autumn the increased seasonal biomass would decay, producing a larger burst of CO2 into the atmosphere during the winter and early spring. However, this trend in CO2 seasonality can be explained rather more simply in terms of a trend towards warmer Arctic temperatures (Chapter 3), which likewise encourages plant growth in the summer. In fact, the strength of the CO2 wobble correlates nicely with a fluctuation in air pressure patterns known as the North Atlantic Oscillation, which gives warmer conditions in the far north (Figure 7.18).

Figure 7.18. (a) The strength of the seasonal C02 wiggle is strongly related to the state of the North Atlantic 0scillation, a climatic fluctuation which brings warm temperatures to the high Arctic. (b) Variation in the carbon isotope composition of C02 shows that it is caused by changes in plant activity, which is able to select between the isotopes.

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