2008年06月18日

CARBON CYCLE : Sources, sinks and seasons

CARBON CYCLE
Sources, sinks and seasons

Changes in the phasing of seasonal cycles of carbon dioxide in the atmospher mark the time when a region becomes a source or a sink of Autumn warming.

One study of such changes prompts thought-provoking conclusions.



We are currently getting a 50% discount on the climatic impact of our fossil-fuel emissions.

Since 1957, and the beginning of the Mauna Loa record of atmospheric carbon dioxide, only about half of the CO2 emissions from fossil-fuel combustion have remained in the atmosphere, with the other halfbeing taken up by the land and ocean.

In the face of increasing fossil-fuel emissions, this remarkably stable airborne fraction' has meant that the rate of carbon absorotion by the land and ocean has accelerated over time (Canadell, J. G. et al. Proc. Natl Acad. Sci. USA 104,18866-1870/2007).

Unfortunately, we have no guarantee that the 50% discount will continue, and uf it disappears we will full climatic burnt of our unrelenting emission of CO2 from fossil fuels.

Indeed, climate models that include descriptions of the carbone cycle predict that treestrial uptake of carbone will decrease in the next century as crimate warms (Friedlingstein, P. et al. J. Clim. 19,3337-3353/2006).

As they describe elsewhere in this issue (Nture,7174,1-106/2008, Letter p.49), Piao et al. have used observational data to show that rising temoeratures may already be decreasing the efficiency of terrestrial carbon uptake in the Northern Hemisphere (Piao, S et al. Nature 451,49-52/2008).


Piao et al. looked at changes in the phasing of seasonal cycles of atmospheric CO2 concentratin at ten sites north of about 20゜N. Seasonal cycles of tomospheric CO2 are caused primarily by the terrestrial biosphere moving from being a net souece of carbon to the atmosphere (mainly in winter) to becoming a net sink (mainly in summer), where net carbon uptake or release is determined by the balance betweenphotosynthesis and respiration.

Changs in the phasing therefore reflect changes in the riming of when the land is a net sink or source to the atmosphere.


Piao et al. used a metric for the phasing known as the 'zero-crossing date' (the ZC date, which is when the seasonal cycle crosses the line that delineates the calculated long-term trend in CO2 concentration Fig.1).

They found that higher temperatures led to earlier ZC dates and colder temperatures to later ones, Givin the trend towards warmer autumn temparatures, they also found that the ZC was occurring an average of 0.4 days earlier par year.

In addition, they identified a temperature correlation with the ZC dates and a trend towards earlier ZC in the spring that was similar to a trend evident in a previous analysis of data from between the 1970s and 1990s (Keeling, C. D., Chin, J. F. S. & Whorf, T. P. Nature 382,146-149/1996).

But moust significantly, Piao et al. found that the advancement of the autumn ZC was occurring at nearly the same rate as the advancement of the spring ZC, meaning that gains of carbon uptake during spring were being cancelled out by carbon releases in autumn.


The shrinking autumn-uptake signal seems to contradict earlier satellite-derived 'greening' trends that showed a lengthening of the growing season in both spring and autumn in the Northern Hemisphere.(Myneni, R. B. et al. Nature 386,698-702/1997; Zhou, L. M. et al. j. Geophys. Res. 160,20069-20083/2001)

To better understand this apparent conflict, Piao et al. used a computer model of the terrestrial biosphere to help separate the observed 'bottom line' net carbon fluxes of the atmospheroc observations into atmospheric debits (photosynthesis) and credits (respiration) that are mechanistically relevant.

The model reslts suggest that increased autumn respiration (triggered by warmer temperatures) dominated over the autumn photosynthetic gains that were seen by the satellites as a longer green period.

Moreover, the model also shows that the loss of carbon in autumn seems to largely cancel the uptake gains made by earlier, greener springs, just as the atmospheric data did.


(...to be continue...)


--------------------------------------------------------
### DataBace ###
nature Vol 451 | Issue no.7174 | 1-106 | 3 January 2008
Letter p.49 / Net carbon dioxide losses of northern ecosystem in response to autumn warming / CEA-CNRS(仏) S. Piao etal. (update)
News & Views p.26 / CARBON CYCLE : Sources, sinks and seasons / John B. Miller (update)
THIS ISSUE p.xvii / Autumn warming (update)
### related story ###
炭素サイクル:ソースとシンクとシーズン』 - science-de-survivor 2nd
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