carbon cycle: fickle trends in the ocean

2
CARBON CYCLE Fickle trends in the ocean Nicolas Gruber A model analysis of the uptake of carbon dioxide in the North Atlantic carries with it a cautionary reminder about interpreting what may be short-term trends as signals of long-term climate change. Several observational studies have suggested that the carbon sink in the North Atlantic Ocean has decreased in recent decades, possibly reflecting the impact of long-term climate change. Writing in Global Biogeochem- ical Cycles, Thomas et al. 1 propose that this latter interpretation needs to be viewed with great caution. From their modelling work, they argue that the observed trends instead reflect fluctuations on a decadal timescale that are a response to climate variability in the North Atlantic region. An underlying question in this line of research is whether the global carbon cycle has already been subject to fundamental alteration as a result of anthropogenic climate change. This question is often asked in the context of concern that the ocean’s carbon sinks, and those on land, will not continue to mitigate anthro- pogenic carbon dioxide emissions as they have in the past 2 . One modelling study 3 , for example, has suggested that the oceanic sink, which has removed about 30% of the global anthropo- genic emissions over the past 250 years or so 4 , might be stalling. Some of the best observa- tional evidence comes from the North Atlantic Ocean, where long-term measurements of the surface ocean’s partial pressure of CO 2 (pCO 2 ) indicate that its carbon uptake from the atmos- phere has decreased in recent decades, perhaps owing to climate change 5,6 . Thomas et al. 1 challenge this interpretation. The North Atlantic is the largest ocean sink for atmospheric CO 2 in the Northern Hemisphere, with half of the flux in the North Atlantic being driven by the uptake of anthropogenic CO 2 (ref. 7). The detection of long-term changes in this sink is challenging, however, because the sink varies substantially from year to year. That variation is largely associated with the North Atlantic Oscilla- tion (NAO) 8 , which is the dominant mode of climate variability in this region. The NAO is a large-scale seesaw in atmos- pheric mass between a subtropical high- pressure system, typically near the Azores, and a subpolar low near Iceland. A positive phase of the NAO — that is, a stronger pressure gradi- ent between these two systems — is associated with more and stronger winter storms cross- ing the North Atlantic on a more northerly route, causing major anomalies in sea surface temperature, currents and convective activity throughout the North Atlantic (Fig. 1). The NAO is characterized by substantial decadal trends, starting with a period of negative values in the 1960s, a long-term trend towards very positive values that culminated in the early 1990s, and a decreasing trend since. Using a global model of the ocean carbon cycle, forced with observed atmospheric con- ditions, Thomas et al. 1 investigated how these variations and trends in the NAO have affected the North Atlantic carbon sink since 1979. They find substantial year-to-year changes in the surface-ocean carbon cycle, with an overall tendency for increased uptake in the temperate and subpolar North Atlantic during positive phases of the NAO. This response is driven primarily by large-scale reorganizations of the North Atlantic circulation (Fig. 1). Thomas and colleagues’ comparison of the model-simulated variations in pCO 2 with observational data suggests that the model captures the main characteristics of the oceanic response to NAO variability. This permits them to put analyses of relatively short-term obser- vational trends 5,6 into a longer-term context. They point out that most of the observations were made after the early 1990s, a period dur- ing which the NAO changed from very positive to near normal values. Their model indicates that normal to negative phases of the NAO are associated with a lower oceanic uptake of CO 2 ; so the trend towards lower NAO states since the early 1990s could explain the tendency of the North Atlantic to take up less CO 2 than expected during this period. Thomas et al. also speculate that the North Atlantic carbon sink will probably rebound in the coming years, once the present NAO trend reverses. The message that analyses of trends occur- ring over short periods need to be interpreted with great caution is clearly an important one — not least because scientists are often tempted to claim the detection of a climate-change signal in the absence of a full characterization of the variability. That said, are their conclu- sions robust? Or is it nevertheless possible that the carbon cycle in the North Atlantic has already undergone a fundamental shift? It is clearly too early to draw final conclu- sions. But it is of interest that an extended analysis 9 of long-term observations indicates that the carbon sink in several regions of the North Atlantic could have been decreasing for several decades already, implying that Thomas and colleagues’ analysis of NAO- driven variability is only part of the puzzle. What about carbon sinks in other parts of the global ocean? With a few notable exceptions 9 , Figure 1 | Impact of a positive phase of the North Atlantic Oscillation. In a positive phase, a stronger Azores high and stronger Icelandic low produce more and stronger winter storms on a more northerly track. As a consequence, the subtropical gyre extends northwards, and the North Atlantic Current accelerates, transporting increased amounts of warm, saline waters with low carbon concentrations northeastwards. This causes an intensified sink (minus sign) in the eastern subpolar North Atlantic, because these waters have the potential to take up a large amount of CO 2 from the atmosphere when they are cooled along their northward journey. At the same time, the Labrador Current intensifies, bringing fresher, colder waters with high carbon concentrations from the Arctic into the subpolar gyre, creating a diminished sink (plus sign) near the coast of Canada. In the subtropical gyre, warm conditions and reduced convective activity also lead to reduced carbon uptake 8 . (Graphic based on the modelling results of Thomas et al. 1 .) Northward extension of subtropical gyre North Atlantic Current accelerates Stronger winter storms; more northerly track Labrador Current intensifies Stronger Icelandic low Stronger Azores high + + 70° W 40° W 10° W 100° W 20° E 70° N 50° N 30° N 10° N Jet stream 155 NATURE|Vol 458|12 March 2009 NEWS & VIEWS © 2009 Macmillan Publishers Limited. All rights reserved

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CARBON CYCLE

Fickle trends in the ocean Nicolas Gruber

A model analysis of the uptake of carbon dioxide in the North Atlantic carries with it a cautionary reminder about interpreting what may be short-term trends as signals of long-term climate change.

Several observational studies have suggested that the carbon sink in the North Atlantic Ocean has decreased in recent decades, possibly reflecting the impact of long-term climate change. Writing in Global Biogeochem-ical Cycles, Thomas et al.1 propose that this latter interpretation needs to be viewed with great caution. From their modelling work, they argue that the observed trends instead reflect fluctuations on a decadal timescale that are a response to climate variability in the North Atlantic region.

An underlying question in this line of research is whether the global carbon cycle has already been subject to fundamental alteration as a result of anthropogenic climate change. This question is often asked in the context of concern that the ocean’s carbon sinks, and those on land, will not continue to mitigate anthro-pogenic carbon dioxide emissions as they have in the past2. One modelling study3, for example, has suggested that the oceanic sink, which has removed about 30% of the global anthropo-genic emissions over the past 250 years or so4, might be stalling. Some of the best observa-tional evidence comes from the North Atlantic Ocean, where long-term measurements of the surface ocean’s partial pressure of CO2 (pCO2) indicate that its carbon uptake from the atmos-phere has decreased in recent decades, perhaps owing to climate change5,6. Thomas et al.1 challenge this interpretation.

The North Atlantic is the largest ocean sink for atmospheric CO2 in the Northern Hemisphere, with half of the flux in the North Atlantic being driven by the uptake of anthropogenic CO2 (ref. 7). The detection of long-term changes in this sink is challenging, however, because the sink varies substantially from year to year. That variation is largely associated with the North Atlantic Oscilla-tion (NAO)8, which is the dominant mode of climate variability in this region.

The NAO is a large-scale seesaw in atmos-pheric mass between a subtropical high-pressure system, typically near the Azores, and a subpolar low near Iceland. A positive phase of the NAO — that is, a stronger pressure gradi-ent between these two systems — is associated with more and stronger winter storms cross-ing the North Atlantic on a more northerly route, causing major anomalies in sea surface temperature, currents and convective activity throughout the North Atlantic (Fig. 1). The NAO is characterized by substantial decadal trends, starting with a period of negative values

in the 1960s, a long-term trend towards very positive values that culminated in the early 1990s, and a decreasing trend since.

Using a global model of the ocean carbon cycle, forced with observed atmospheric con-ditions, Thomas et al.1 investigated how these variations and trends in the NAO have affected the North Atlantic carbon sink since 1979. They find substantial year-to-year changes in the surface-ocean carbon cycle, with an overall tendency for increased uptake in the temperate and subpolar North Atlantic during positive phases of the NAO. This response is driven primarily by large-scale reorganizations of the North Atlantic circulation (Fig. 1).

Thomas and colleagues’ comparison of the model-simulated variations in pCO2 with observational data suggests that the model captures the main characteristics of the oceanic response to NAO variability. This permits them to put analyses of relatively short-term obser-vational trends5,6 into a longer-term context.

They point out that most of the observations were made after the early 1990s, a period dur-ing which the NAO changed from very positive to near normal values. Their model indicates that normal to negative phases of the NAO are associated with a lower oceanic uptake of CO2; so the trend towards lower NAO states since the early 1990s could explain the tendency of the North Atlantic to take up less CO2 than expected during this period. Thomas et al. also speculate that the North Atlantic carbon sink will probably rebound in the coming years, once the present NAO trend reverses.

The message that analyses of trends occur-ring over short periods need to be interpreted with great caution is clearly an important one — not least because scientists are often tempted to claim the detection of a climate-change signal in the absence of a full characterization of the variability. That said, are their conclu-sions robust? Or is it nevertheless possible that the carbon cycle in the North Atlantic has already undergone a fundamental shift?

It is clearly too early to draw final conclu-sions. But it is of interest that an extended analysis9 of long-term observations indicates that the carbon sink in several regions of the North Atlantic could have been decreasing for several decades already, implying that Thomas and colleagues’ analysis of NAO-driven variability is only part of the puzzle.

What about carbon sinks in other parts of the global ocean? With a few notable exceptions9,

Figure 1 | Impact of a positive phase of the North Atlantic Oscillation. In a positive phase, a stronger Azores high and stronger Icelandic low produce more and stronger winter storms on a more northerly track. As a consequence, the subtropical gyre extends northwards, and the North Atlantic Current accelerates, transporting increased amounts of warm, saline waters with low carbon concentrations northeastwards. This causes an intensified sink (minus sign) in the eastern subpolar North Atlantic, because these waters have the potential to take up a large amount of CO2 from the atmosphere when they are cooled along their northward journey. At the same time, the Labrador Current intensifies, bringing fresher, colder waters with high carbon concentrations from the Arctic into the subpolar gyre, creating a diminished sink (plus sign) near the coast of Canada. In the subtropical gyre, warm conditions and reduced convective activity also lead to reduced carbon uptake8. (Graphic based on the modelling results of Thomas et al.1.)

Northward extension

of subtropical gyre

North Atlantic

Current accelerates

Stronger winter storms;

more northerly track

Labrador Current

intensifies

Stronger

Icelandic low

Stronger

Azores high

–+

+

70° W

40° W

10° W

100° W20° E

70° N

50° N

30° N

10° N

Jet stream

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NATURE|Vol 458|12 March 2009 NEWS & VIEWS

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© 2009 Macmillan Publishers Limited. All rights reserved

NEUROSCIENCE

Up, down, flying aroundRuth Anne Eatock

The Johnston’s hearing organ of the fruitfly has newly discovered sensitivities to gravity and wind. As in our inner ear, different sensory signals from this organ travel in parallel to separate zones in the brain.

The hearing organ in the antennae of mosqui-toes and flies — first described by Johnston1 — detects sounds from nearby sources such as the vibrating wing of the courting male fruitfly. In this issue, Kamikouchi et al.2 and Yorozu et al.3 dramatically expand the number of known effective stimuli for Johnston’s organ to include gravity and wind. These results elevate Johnston’s organ from a near-field hearing sensor to a complex ear in which specialized clusters of neurons have distinct mechano sensory responsibilities.

Johnston’s organ is the largest of the chordo-tonal organs that are located beneath the epidermal layer of the insect skin. Chordotonal organs are assemblies of units each of which consists of a ciliated neuron and several asso-ciated support cells4; Johnston’s organ in the fruitfly contains hundreds of such units. Nearby sounds vibrate feathery structures (aristae) on the antennae, stretching cilia on Johnston’s organ neurons (Fig. 1) and thereby exciting them to make sensory signals. The molecu-lar mechanism of transduction is obscure, but sensory-neuron signals can be visualized by measuring increased levels of intracellular calcium ions, which universally accompany

observational records are generally too sparse to determine long-term changes in the oceanic carbon sink on a regional basis. One area that has received much attention recently, how-ever, is the Southern Ocean, because it is the strongest regional oceanic sink for anthrop-ogenic CO2 (ref. 7). Observations and model-ling studies3,9,10 suggest that the oceanic uptake there has slowed considerably relative to the expected growth of the sink in response to the rise in atmospheric CO2. But the observa-tional evidence is limited, and the modelling studies may be biased11, so it is not yet pos-sible to determine with sufficient confidence whether or not the Southern Ocean sink is changing in a fundamental manner. Nor is it possible to make such conclusions for any other large-scale oceanic region.

It is essential to resolve this issue. The first task is to ensure that appropriate observa-tional systems are put in place to permit accurate quantification of the oceanic carbon sinks and detect changes reliably. We also need to improve our understanding of the under lying processes so as to better assess how the ocean carbon cycle will behave in

the future. As international agreements to limit the further growth of atmospheric CO2 come closer to reality, accurate determi-nation of the fate of anthropogenic CO2 emissions is becoming a matter of ever-growing importance. ■

Nicolas Gruber is in the Environmental Physics

Group, Institute of Biogeochemistry and Pollutant

Dynamics, ETH Zurich, 8092 Zurich, Switzerland.

e-mail: [email protected]

1. Thomas, H. et al. Glob. Biogeochem. Cycles

doi:10.1029/2007GB003167 (2008).

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

3. Le Quéré, C. et al. Science 316, 1735–1738 (2007).

4. Sabine, C. L. et al. Science 305, 367–371 (2004).

5. Lefèvre, N. et al. Geophys. Res. Lett.

doi:10.1029/2003GL018957 (2004).

6. Schuster, U. & Watson, A. J. J. Geophys. Res.

doi:10.1029/2006JC003941 (2007).

7. Gruber, N. et al. Glob. Biogeochem. Cycles

doi:10.1029/2008GB003349 (2009).

8. Gruber, N., Keeling, C. D. & Bates, N. R. Science 298, 2374–2378 (2002).

9. Takahashi, T. et al. Deep Sea Res. II doi:10.1016/

j.dsr2.2008.12.009 (2009).

10. Lovenduski, N. S., Gruber, N. & Doney, S. C. Glob.

Biogeochem. Cycles doi:10.1029/2007GB003139

(2008).

11. Böning, C. W., Dispert, A., Visbeck, M., Rintoul, S. R.

& Schwarzkopf, F. U. Nature Geosci. 1, 864–869 (2008).

neuronal excitation. Kamikouchi et al. and Yorozu et al. therefore genetically engineered Johnston’s organ neurons to express molecules that fluoresce when calcium-ion concentra-tion rises. They then compared the calcium-ion signals evoked by sound, wind, gravity or movement of the aristae, looking at neurons either in Johnston’s organ1 or in the brain’s antennal mechanosensory centre2.

The need to know which way is up is con-sidered so pressing that it has been assumed that flies are sensitive to gravity. These insects lack an obvious statolith — crystal aggre-gates that weight gravity-sensing receptors in organisms as diverse as plants, octopuses and humans. In the absence of statoliths, chordo-tonal organs on the flies’ legs5 and Johnston’s organ6 were considered likely candidates for gravity sensing.

Kamikouchi et al.2 (page 165) show that it is Johnston’s organ that is responsible for gravity sensing in flies. They find that remov-ing aristae severely decreases the tendency of flies to walk upwards (the direction oppo-site to the gravity vector). Their test was not designed to rule out minor contributions from neurons outside Johnston’s organ to the

overall sensing of gravity. Nevertheless, it clearly shows that flies detect a change in their orientation relative to gravity through the effect of gravity on the position of the aristae. The authors’ calculations, taking into account an arista’s apparent mass and stiffness, suggest that gravitational changes maximally deflect an arista by about one micrometre — a move-ment 100 times greater than that evoked by sound at the threshold of hearing7.

A previous study8, and now that of Yorozu and colleagues3 (page 201), also used simple antennal manipulations to investigate the role of Johnston’s organ in sensitivity to wind. The earlier paper8 showed that gentle air currents such as those experienced in flight modify flight behaviour, an effect that is abolished by removal of the aristae. Yorozu et al. find that, in response to air currents moving at about 2 metres per second, fruitflies ‘freeze’ in place — a behaviour whimsically dubbed WISL, for wind-induced suppression of locomotion, and which also depends on the presence of the arista. The benefits of hunkering down in the wind may range from a short-term indi-vidual advantage of staying close to familiar terrain and food sources to a longer-term group benefit of controlling the dispersal of fly populations.

Whereas courtship song vibrates the aristae at frequencies of tens to hundreds of hertz, wind and gravity stimuli change more slowly and may be sustained. Both teams2,3 tested the importance of this difference for stimulus discrimination within Johnston’s organ by using probes to displace the arista at differ-ent frequencies or durations. They find that fast and slow stimuli differentially activate neuronal groups that originate from distinct zones in the Johnston’s organ array and target specific zones (A–E) in the antennal mech-anosensory centre. Sound-sensitive neurons of zones A and B responded briskly to arista vibrations or to the onset of position changes; when new positions were sustained, however, their response adapted by declining. Wind- and gravity-sensitive neurons of zones C and E, by contrast, responded poorly to vibra-tions and relatively slowly to position changes, but showed robust, sustained responses to long-lasting stimuli.

These observations indicate that the main difference between sound-sensitive and wind- or gravity-sensitive neurons might be in the speed at which their transduction cascades respond and adapt. This difference could reflect quantitative or qualitative differences in the transduction pathways of the neuronal groups involved. Indeed, Kamikouchi et al.2 identify a qualitative difference, in that only the sound-sensing neurons express a par-ticular stretch-activated ion channel called NompC (ref. 5).

But how do fruitflies distinguish wind from gravity? These two kinds of input are also likely to differ intrinsically from each other in duration and size — although the differences

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NATURE|Vol 458|12 March 2009NEWS & VIEWS

12.3 N&V.indd MH IFnew 15612.3 N&V.indd MH IFnew 156 6/3/09 17:25:246/3/09 17:25:24

© 2009 Macmillan Publishers Limited. All rights reserved