Time (years)

CO

2 co

nce

ntr

atio

n (p

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.)

Spring ZCdate

Autumn ZCdate

One annual cycle

Long-termtrend

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 increas-ing fossil-fuel emissions, this remarkably stable ‘airborne fraction’ has meant that the rate of carbon absorption by the land and ocean has accelerated over time1. Unfortu-nately, we have no guarantee that the 50% discount will continue, and if it disappears we will feel the full climatic brunt of our unrelent-ing 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 warms2. As they describe elsewhere in this issue (page 49), Piao et al.3 have used observational data to show that rising temper-atures 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 concentra-tions 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 tempera-tures, they also found that the ZC was occur-ring an average of 0.4 days earlier per year. In addition, they identified a temperature corre-lation 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 1990s4. But, most significantly, Piao et al. found that the advance-ment of the autumn ZC was occurring at nearly the same rate as the advancement of the spring

ZC, meaning that gains of carbon uptake dur-ing spring were being cancelled out by carbon releases in autumn.

The shrinking autumn-uptake signal seems to contradict earlier satellite-derived ‘green-ing’ trends5,6 that showed a lengthening of the growing season in both spring and autumn in the Northern Hemisphere. To better under-stand this apparent conflict, Piao et al.3 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 ris-ing faster in Eurasia than in North America, whereas autumn temperatures have been rising faster in North America, adding a level of geo-graphical complexity to future projections.

Even for now, however, the picture remains incomplete. Just as measures of greenness from space can’t determine total carbon bal-ance 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 sum-mer. And the annual net carbon balance is what is needed in order to understand whether car-bon 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 Atlantic7 and Southern Ocean8, 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 emis-sions 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 shrink-ing 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 increasing9 amounts of biomass productiv-ity 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 plots10.

Making more observations in the trop-ics, 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’ study3 has shown, to develop greater mechanistic understanding (and thus predictive power), there is also a great need to identify obser-vational 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

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.

Figure 1 | Zero-crossing (ZC) dates. These dates, shown by red dots, are defined as the time when the annual cycle of atmospheric CO2 crosses the calculated long-term trend in CO2 concentration. In spring, this occurs as net CO2 uptake is increasing and atmospheric CO2 concentration is falling, and in autumn as net CO2 release is increasing and atmospheric CO2 concentration is rising. The phasing of this cycle is determined by net carbon uptake or release throughout the year, which, in turn, is the balance between respiration and photosynthesis. Because net flux is the relatively small difference between the much larger photosynthetic and respiratory fluxes, small fractional changes in either photosynthesis or respiration can have large impacts on the timing of the CO2 seasonal cycle. Piao et al.3 observed trends (blue arrows, indicating a shift in the cycle from the dashed to solid line earlier each year) in both the spring and autumn ZC date, indicative of a changing balance between photosynthesis and respiration brought on by increasing temperature.

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NATURE|Vol 451|3 January 2008NEWS & VIEWS

Electromagnetic cloaka b

Light rays

Cloak

Electromagnetic tunnel

Electromagnetic wormhole

Imagine walking down a footpath, staring unconcernedly at the clear track in front of you. Suddenly, you stumble over an object. You look down, but there is nothing to be seen on the ground. You step back and try a different angle of view, again without success. But you know something must be there, because you can feel it.

This situation is brought a step closer to real-ity with a device dreamt up by Greenleaf et al.1 and described in Physical Review Letters. The authors propose a way of creating an ‘invisible tunnel’ through which photons, the elementary particles of light, can propagate between two seemingly unconnected points. To an observer standing where they emerge from their tunnel, the photons seem to come from nowhere. To an external viewer, they seem to be teleported from one place to the other. And anything within the tunnel cannot be seen by anyone. In analogy to the infamous ‘wormholes’ — a prediction in general relativity of tun-nels through space-time that connect distant areas of the Universe — the authors call their

brainchild an electromagnetic wormhole.The wormhole works (in theory) by neatly

combining concepts from differential geom-etry, general relativity, electromagnetism and the theory of ‘metamaterials’2. Metamaterials are composite, nanostructured materials with specifically tuned electromagnetic properties. The authors construct the tunnel wall of their wormhole using a metamaterial layer that is designed to bend light waves around it without reflection, much as water waves bend around a tree branch or similar obstruction lying just below the surface of the water. This layer thus renders whatever is inside it invisible. The idea draws on techniques proposed3,4 for the creation of an ‘invisibility cloak’ (Fig. 1a) — a device that has already been constructed and proved viable, at both microwave5 and optical6 wavelengths.

The advance in Greenleaf and colleagues’ scheme1 is that a cloaked object can ‘see’ into the outside world at the end of the tunnel, because photons are also free to propagate through it. The tunnel ‘deceives’ photons into

Figure 1 | It’s behind you. a, The invisibility cloak devised by Pendry et al.3 uses specially structured ‘metamaterials’ to open up a ‘hole’ in photon-space, inside which one can place an object. Photons are naturally redirected around the object, rendering it invisible, at least when viewed with photons at a certain wavelength. b, Greenleaf and colleagues’ electromagnetic wormhole1 is a natural extension of the invisibility idea, with exciting potential applications. For example, in principle two flexible wormholes attached to the frame of a specially designed pair of half-moon spectacles could project the photon-space behind the head to the half-moon area of the lenses, providing a seamless 360° view.

OPTICS

Watch your back Kosmas L. Tsakmakidis and Ortwin Hess

A proposal for transporting photons invisibly between two unconnected points in space seems worthy of a Star Trek plot. But it is in principle wholly realizable, and could open up new vistas — literally.

1. Canadell, J. G. et al. Proc. Natl Acad. Sci. USA 104, 18866–18870 (2007).

2. Friedlingstein, P. et al. J. Clim. 19, 3337–3353 (2006).

3. Piao, S. et al. Nature 451, 49–52 (2008). 4. Keeling, C. D., Chin, J. F. S. & Whorf, T. P. Nature 382,

146 –149 (1996).5. Myneni, R. B. et al. Nature 386, 698–702 (1997).

6. Zhou, L. M. et al. J. Geophys. Res. 106, 20069–20083 (2001).

7. Schuster, U. & Watson, A. J. J. Geophys. Res. 112, doi:10.1029/2006JC003941 (2007).

8. Le Quéré, C. et al. Science 316, 1735–1738 (2007).9. Nemani, R. R. et al. Science 300, 1560–1563 (2003).10. Baker, T. R. et al. Phil. Trans. R. Soc. Lond. B 359, 353–365

(2004).

thinking that remote regions are connected to each other so that they naturally follow the path inside the cylindrical channel. This cylinder is not part of conventional three-dimensional space, but is part of a higher-dimensional space outside it. Its topology is rather like the handle of a coffee cup connecting two areas of the cup’s surface. If the handle is hollow and has two open ends, it presents an alternative route (other than staying on the surface of the cup) for getting from the one place to the other.

The key to the realization of this scheme is that this new photon-space does not natu-rally exist in real space. Rather, using suitable coordinate transformations, the authors tweak Maxwell’s equations — the set of equations that describe the workings of electromagnetic waves — to simulate it. The equations retain their form on passing from the real to an artificial photon-space; the only thing that is required to complete the deception of the photons is to modify the values of the electric permittivity and magnetic permeability (numbers that codify the degree to which a material allows electric and magnetic fields to pass).

Such a concealed communication channel could be deployed for military purposes for the secret transmission of information or stealth technologies. But it might also find its way into civilian applications: rerouting mobile-phone signals around obstacles, for example, or shielding sensitive medical devices from interference by magnetic resonance imag-ing scanners. But the possibilities don’t end there. Two of Greenleaf and colleagues’ invis-ible electromagnetic tunnels, built into the frame of a pair of special half-moon specta-cles, would effectively ‘glue’ the photon-space behind the head to the photon-space in front of the eyes, allowing one literally to watch one’s back (Fig. 1b).

Currently, metamaterial technology allows the construction of invisibility cloaks that work well for only a limited range of frequencies. The electromagnetic wormhole in its present form can also be only short, otherwise the image of an object being transmitted through it becomes noticeably distorted. The true potential of such schemes will become clear in future experimen-tal tests. What is plain now is that innovations are coming thick and fast in this burgeoning world of ‘transformation optics’. ■

Kosmas L. Tsakmakidis and Ortwin Hess are at the Advanced Technology Institute and the Department of Physics, University of Surrey, Guildford GU2 7XH, UK. e-mail: o.hess@surrey.ac.uk

1. Greenleaf, A., Kurylev, Y., Lassas, M. & Uhlmann, G. Phys. Rev. Lett. 99, 183901 (2007).

2. Eleftheriades, G. V. & Balmain, K. G. Negative Refraction Metamaterials: Fundamental Principles and Applications (Wiley-IEEE Press, Hoboken, NJ, 2005).

3. Pendry, J. B., Schurig, D. & Smith, D. R. Science 312, 1780–1782 (2006).

4. Leonhardt, U. Science 312, 1777–1780 (2006).5. Schurig, D. et al. Science 314, 1780–1782 (2006).6. Cai, W., Chettiar, U. K., Kildishev, A. V. & Shalaev, V. M.

Nature Photon. 1, 224–227 (2007).

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NATURE|Vol 451|3 January 2008 NEWS & VIEWS