Nature

January 3, 2008

CARBON CYCLE

Sources, sinks and seasons

John B. Miller

Changes in the phasing of seasonal cycles of carbon dioxide in the atmosphere mark the time when a region becomes a source or a sink of CO2. 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 half being 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 absorption by the land and

ocean has accelerated over time.

Unfortunately, we have no guarantee that the 50% discount will continue, and if it disappears we will feel the full climatic brunt of our unrelenting emission of CO2 from fossil fuels. Indeed, climate models that include descriptions of the carbon cycle predict that terrestrial uptake of carbon will decrease in the next century as climate warms. As they describe elsewhere in this issue, Piao et al have used

observational data to show that rising temperatures may already be decreasing the efficiency of terrestrial carbon uptake in the Northern Hemisphere.

Piao et al. looked at changes in the phasing of seasonal cycles of atmospheric CO2 concentrations at ten sites north of about 20° N. Seasonal cycles of atmospheric CO2 are caused prima-

rily by the terrestrial biosphere moving from being a net source 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 between photosynthesis and respiration. Changes in the phasing therefore reflect changes in the timing 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. Given the trend towards warmer autumn temperatures, they also found that the ZC was occurring an average of 0.4 days earlier per 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. But, most 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 'greenng' trends that showed a lengthening of the growing season in both spring and autumn in the Northern Hemisphere. 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 atmospheric observations into atmospheric debits (photosynthesis) and credits (respiration) that are mechanisti-

cally relevant. The model results 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.

Piao and colleagues' results link temperature and carbon uptake, but using them to predict the future trajectory of carbon uptake is tricky. Even if we know that temperatures will increase, we still need to know temperature trends for spring and autumn. If spring temperatures rise more quickly than those in autumn, sinks could get larger, whereas more rapid increases in autumn temperatures would cause sinks to

diminish. Furthermore, the authors point out that, so far, spring temperatures have been rising faster in Eurasia than in North America, whereas autumn temperatures have been rising

faster in North America, adding a level of geographical complexity to future projections.

Even for now, however, the picture remains incomplete. Just as measures of greenness from space can't determine total carbon balance because they miss the respiratory side of

the equation, so the study by Piao et al. doesn't address carbon balance in the winter and summer. And the annual net carbon balance is what is needed in order to understand whether carbon sinks are disappearing, remaining steady

or getting stronger.

In light of Piao and colleagues' results, and of two recent studies showing diminishing ocean sinks in the critical carbon-uptake areas of the North Atlantic and Southern Ocean, it may

seem odd to consider that carbon sinks might be getting stronger. But this is exactly what the steady airborne fraction of global CO2 is telling us. The global CO2 signal is most significant for two reasons: first, it is the most robust determi-

nation of carbon uptake, because the errors in atmospheric observations and fossil-fuel emissions are very small; and second, the global CO2 signal is the one that is relevant for the radiative balance that drives global climate change.

So, what gives? For every report of a shrinking sink, there should be even more reports of increasing sinks to satisfy the global constraint. It's possible that we are not looking in all the

right places. For example, given the high and increasing amounts of biomass productivity in the tropics, and how poorly observed they are, it would not be surprising if some of the increasing sinks were there. Indeed, some studies show increasing biomass (that is, sinks) in tropical forest plots.

Making more observations in the tropics, and in other poorly observed regions in the ocean and on land, will certainly help us find the sinks necessary to balance the global numbers. But as Piao and colleagues' study has shown, to develop greater mechanistic understanding (and thus predictive power),

there is also a great need to identify observational constraints on photosynthetic and respiratory fluxes. _

John B. Miller is at the University of Colorado and

the NOAA Earth System Research Laboratory,

325 Broadway, Boulder, Colorado 80305, USA.

e-mail: john.b.miller@noaa.gov

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posted to ClimateConcern