Carbon dioxide levels in the atmosphere rise by about 2-3 parts per million every year and the rate is slowly increasing. As well as this upward trend, there is an annual cycle: carbon dioxide levels fall in the northern hemisphere summer and rise strongly in the winter. The reason is that most of the vegetated land area is in the northern hemisphere and during the northern summer plants and trees absorb CO2.One effect of increasing spring and autumn temperatures has been to increase the length of what is loosely called ‘the growing season’. Plant growth can start earlier in spring and can continue until later. It might be thought that this would help vegetation take up more CO2, acting as a counterweight to increased fossil fuel use.
Research published in Nature in early January very strongly suggests that this is not happening. Warmer autumns are associated with a bringing forward of the date at which plants start losing CO2, not the reverse. Higher spring and autumn temperatures are tending to decrease the length of the period each year in which northern hemisphere plants are taking up carbon. If this research is confirmed, this is yet another potential positive feedback because higher temperatures might diminish the ability of biomass to take up carbon.
The article in Nature has an excellent summary of the issue:
The carbon balance of terrestrial ecosystems is particularly sensitive to climatic changes in autumn and spring, with spring and autumn temperatures over northern latitudes having risen by about 1.1 °C and 0.8 °C, respectively, over the past two decades. A simultaneous greening trend has also been observed, characterized by a longer growing season and greater photosynthetic activity. These observations have led to speculation that spring and autumn warming could enhance carbon sequestration and extend the period of net carbon uptake in the future. We find that atmospheric records from the past 20 years show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period.
The research looked at records of carbon dioxide concentrations at a number of different stations around the northern hemisphere. The work showed that when autumn temperatures were particularly high, the date at which the carbon dioxide concentration started rising after its spring/summer contraction was considerably earlier than when temperatures were abnormally low. In a disturbing finding, the researchers showed that the date at which CO2 levels started to rise each year is typically advancing over 0.3 days per year. If continued, this would mean that over the course of the next century the average date at which carbon dioxide levels start to rise will move forward one month. In spring CO2 uptake starts earlier, but the advance is not as great as that in autumn. The length of the period of carbon uptake is decreasing.
Previous research has tended to suggest that global biomass uptake of CO2 should rise until about 2050 and then stabilize. This research suggests that this conclusion is not robust and net carbon uptake may peak much earlier than this.
Footnote  Shilong Piao and others, ‘Net Carbon Dioxide Losses of Northern Ecosystems in Response to Autumn Warming’, Nature, 451 (3 January 2008), 49-52; http://www.nature.com/nature/journal/v451/n7174/full/nature06444.html [accessed 14 January 2008; subscription required for full article].