The C02 concentration in the northern hemisphere fluctuates with the seasons: it goes up during the winter when decay dominates (releasing C02) and decreases during the summer when photosynthesis is taking up C02 and temporarily building it into leaves (Chapter 7). When autumn comes, most leaves in the mid-latitude forests and also the tundra are shed and they decay releasing C02.
The seasonal fluctuation in C02 in certain places in the northern latitudes has been increasing over the past several decades. This trend towards wider seasonal swings is strongest in northern Alaska, at a C02-measuring station located at Barrow. A weaker trend towards more seasonal fluctuation is also found at the Mauna Loa measuring station in Hawaii that ultimately gets a lot of air coming down from the Arctic. However, the trend is absent from other stations around the world.
What is one to make of this trend in the seasonal wiggle in the far north? The first explanation put forward when it was discovered was that it was due to increasing C02 fertilization. More C02 might be giving greater summer leaf mass in shrubs and herbaceous plants in the far north: more leaves sucked in more C02 each growing season, and then this was released by decay after the leaves were dropped at the end of summer. This picture seemed to be reinforced by satellite data showing that the Arctic latitudes had become increasingly greener over the last 10 years. Perhaps this is evidence of an increasingly strong C02 fertilization effect?
However, if the trend in the seasonal wiggle is due to C02 fertilization, why is it only noticeable in one part of the world? After all, vegetation everywhere should have at least some potential to respond to C02 fertilization. And, from what little experimental work has been done on exploring direct C02 responses in tundra, it seems to be particularly unresponsive after a few years due to severe nutrient limitation. Also, there are other straightforward explanations as to why the seasonal amplitude of C02 in the high latitudes might be increasing. Plants are known to respond strongly to temperature, and greater warmth in the north (where summer temperatures tend to limit the amount of growth that plants can put on) could be allowing the plants to carry more summer leaf mass. Climate records show that indeed there has been a strong warming trend in the Arctic, especially in the parts of Alaska and northeastern Siberia that also show the strongest trend in both C02 seasonal fluctuation and in greenness measured from satellites. This neatly explains why the trend in C02 seasonality has only occurred in that general part of the world, and there seems no particular need to invoke the poorly understood role of direct C02 fertilization. Why temperatures are increasing is another question altogether, and it could be due to C02 and other greenhouse gases acting upon climate (Chapter 1).
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