NEWS & VIEWS RESEARCH mass of dust in the circumgalactic medium of foreground galaxies, concluding that, amazingly, more of these carbon- and silicon-rich molecules reside, not in the galaxies, but outside them. The combined results of these two studies point to a tantalizing conclusion: most of the elements constituting life are not found in galaxies, where they can be incorporated into future stars and planetary systems; instead, they are predominantly distributed thousands to millions of light years away from galaxies. Although it has been known, or strongly suspected, for decades that galaxies do not contain most of the metals that they have produced, it has been only in recent years that astronomers have been able to systematically locate, quantify and characterize this material outside galaxies. Both studies, however, highlight how little astronomers know — and thus how much remains to be discovered — about the detailed

physical conditions and hence the fates of the Universe’s metals. Shull and co-workers have produced an estimate of the inter­galactic density of metals that is for a time when the Universe is much more mature than the earlier epochs at which similar previous measurements6,7 were made. However, it is not a trivial task to connect these newly detected metals to the individual galaxies in which they were born. Although previous studies8 have indicated that there is a significant reservoir of intergalactic dust, Peek and colleagues’ analysis exquisitely shows how a galaxy’s dusty environment depends on the galaxy itself. Large surveys will be required to obtain similar measurements at earlier cosmic epochs, to decipher when galaxies expelled this dust. It is astounding that most of the potential building blocks of life are found so far from their birthplaces, and that so much of it can survive the trip out of galaxies as complex molecules. As our understanding of intergalactic

PAL A EO CLIMATE

Monsoon matters A simplified global climate model that keeps track of water as it moves through Earth’s water cycle throws fresh light on how the Asian summer monsoon has varied during the past 150,000 years. BRONWEN KONECKY

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he Asian summer monsoon (ASM) is the predominant source of rainyseason precipitation in tropical Asia, and is known to vary in response to global climate changes1. However, different geological archives from South and East Asia, such as cave deposits and marine sediments, provide conflicting accounts of how the ASM responds to basic climate forcings (Fig. 1) — for example, varying concentrations of greenhouse gases, the growth and collapse of ice sheets and changes in Earth’s orbit around the Sun. Writing in Nature Communications, Caley et al.2 use a simplified model to simulate Earth’s climate during the past 150,000 years, and find that this conflict may originate from our interpretation of climate signals recorded in stalagmites from Chinese caves. On timescales of thousands to millions of years, changes in the tilt of Earth relative to the plane of its orbit, the shape of the orbit and its slow wobble about its axis (or ‘precession’) regulate and distribute incoming solar radiation (insolation) that fuels the ASM. The monsoon is thought to intensify during minima in precession, approximately every 23,000 years, when Earth is closest to the Sun during Northern Hemisphere summer (unlike today, when this occurs in winter). Further strengthening

of the ASM occurs in part as a result of the growth and collapse of ice sheets covering northern Eurasia, which alter key wind patterns and shift the amount and location of East Asian rainfall. Greenhouse gases also influence the ASM by increasing the build-up

ASIA Tianmen

Sanbao

Hulu

Dongge

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Figure 1 | The Indo-Asian monsoon system.  The shaded branched arrow denotes prevailing monsoonal wind flow. Red dots represent locations of cave stalagmites, which provide records of past climate. Caley et al.2 question whether these stalagmites truly reflect variations in the Asian summer monsoon, in contrast to previous interpretations.

space in the local Universe increases, so does what this vast expanse of rarefied gas tells us about galaxies — and about just how rare and precious our own place in the cosmos is. ■ Molly S. Peeples is at the Space Telescope Science Institute, Baltimore, Maryland 21218, USA. e-mail: [email protected] 1. Shull, J. M. et al. Astrophys. J. 796, 49 (2014). 2. Peek, J. E. G., Ménard, B. & Corrales, L. Preprint at http://arxiv.org/abs/1411.3333 (2014). 3. Burbidge, E. M., Burbidge, G. R., Fowler, W. A. & Hoyle, F. Rev. Mod. Phys. 29, 547–650 (1957). 4. Danforth, C. W. et al. Preprint at http://arxiv.org/ abs/1402.2655 (2014). 5. Abazajian, K. N. et al. Astrophys. J. Suppl. 182, 543–558 (2009). 6. Cooksey, K. L. et al. Astrophys. J. 708, 868–878 (2010). 7. Cooksey, K. L. et al. Astrophys J. 762, 37–52 (2013). 8. Ménard, B. et al. Mon. Not. R. Astron. Soc. 405, 1205–1039 (2010).

of moisture in the atmosphere that is available to feed monsoonal precipitation. But how can we tell what caused ASM variations in the geological past? When decoding Earth’s climate history, timing is everything. The timing of semi-regular changes in rainfall relative to that of insolation, ice sheets and greenhouse-gas concentrations tells us which mechanisms played what part, and when. In many geological archives, ASM maxima seem to result from a combination of three factors: orbitally driven heating of the Asian landmass; release of energy in the southern Indian Ocean, where monsoon moisture originates; and minima in global ice volume. By contrast, stalagmite archives from South and East Asian caves — arguably the best-dated, highestresolution, most continuous records of Asian palaeoclimate recovered so far — suggest that local insolation is the dominant factor, with changes in global ice volume being secondary. This apparent contradiction in timing has long challenged scientists researching the under­ lying controls on the ASM. Like many rainfall proxies from the tropics, cave stalagmite records are based on the ratios of stable oxygen and hydrogen isotopes in water. The abundances of heavy but rare 18O and 2H relative to their lighter, more common counterparts (expressed as δ18O and δ2H, respectively) carry an imprint of that water’s history as it evaporated from the ocean, moved through the water cycle and eventually fell as rain or snow. When water gets incorporated into the calcite of cave stalagmites, it carries with it the isotopic signature that it had when it fell as precipitation. Precipitation δ18O is a proven tracer of hydrological and atmospheric circulation processes3. However, studies4,5 have shown that interpreting precipitation δ18O is not always straightforward, because it integrates many

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RESEARCH NEWS & VIEWS processes across space and time. ‘Isotopeenabled’ climate models6 are quickly gaining prominence as tools for deciphering climate signals contained in δ18O. But even with stateof-the-art supercomputers, running a climate model over many thousands of years is expensive, and adding in the extra code to trace water isotopes only steepens that cost. Caley et al. solve this dilemma by using an ‘intermediate-complexity’ climate model and by accelerating climate forcings by a factor of ten to reduce the amount of time needed to run the simulation. The model, called iLOVECLIM, involves a simple, threelayer atmosphere, together with basic isotope physics to describe the fractionation (separation) of O and H isotopes during precipitation and evaporation. These isotopic signatures are then transported through the atmosphere, ocean and land. The model’s coarse resolution blurs Earth’s topography and neglects complex dynamics of clouds and convection, but its simplicity makes it computationally affordable, enabling the authors to perform multiple 150,000-year experiments. Despite its simplicity, iLOVECLIM reproduces long-term variations in Chinese-cave δ18O very well, albeit with a few telling exceptions. Although Caley et al. find that precessional and ice-sheet forcing can explain most cave δ18O variations, iLOVECLIM does not quite capture the observed timing offsets between precipitation, cave δ18O and precession minima. This suggests that relevant climate forcings or feedbacks are missing, including ‘known unknowns’ such as ice–ocean feedbacks that were deliberately left out of the model, but possibly other processes as well. Importantly, the authors argue that annual δ 18 O variations in iLOVECLIM match Chinese-cave records better than summer δ18O variations — contradicting the common interpretation of cave records as an ASM proxy. Modern data support the idea that annual rainfall δ18O in China reflects both monsoonal and non-monsoonal processes7. Non-monsoonal sources of moisture, along with their transport paths and the types of precipitation they produce, account for a large proportion of δ18O variability5,8. Infiltration of water through limestone en route to a cave further integrates rainwater δ18O signals over many seasons9. Hence, ASM rainfall drives much of the yearto-year variability in annual δ18O, but it is not the only contributor. The complexity of precipitation δ 18O highlights the need to model precipitation and isotopic fractionation in monsoonal Asia realistically. Such modelling requires more-sophisticated atmospheric physics and dynamics than a simplified model can provide. iLOVECLIM demonstrates this point by poorly simulating young (thousands of years old) cave δ18O deposits in South and East Asia, mainly because the model oversimplifies precipitation δ18O. Hence, offsets between

Chinese-cave δ18O and modelled-cave δ18O — both summer and annual — may arise from real-world, season-specific precipitation and isotopic phenomena that iLOVECLIM does not capture. The proportion of long-term δ18O variability that can be attributed to the ASM, then, remains difficult to determine. By explicitly modelling water isotopes, Caley et al. have taken a crucial first step towards understanding how global climate forcings drive the ASM. Further deciphering δ18O and resolving apparent contrasts in proxy data will require a systems approach: modelling the climate signals themselves, as well as the archives that preserve them10. Once the controls on precipitation δ18O have been identified, the underlying causes of ASM variations will come into focus. ■ Bronwen Konecky is at the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, USA, and at the College of

Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis. e-mail: [email protected] 1. Christensen, J. H. et al. in Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker, T. F. et al.) Ch. 14, 1217–1308 (Cambridge Univ. Press, 2013). 2. Caley, T., Roche, D. M. & Renssen, H. Nature Commun. 5, 5731; http://dx.doi.org/10.1038/ ncomms6371 (2014). 3. Dansgaard, W. Tellus 16, 436–468 (1964). 4. Aggarwal, P. K., Frölich, K., Kulkarni, K. M. & Gourcy, L. L. Geophys. Res. Lett. 31, L08203 (2004). 5. Dayem, K. E., Molnar, P., Battisti, D. S. & Roe, G. H. Earth Planet. Sci. Lett. 295, 219–230 (2010). 6. Noone, D. & Sturm, C. in Isoscapes: Understanding Movement, Pattern, and Process on Earth Through Isotope Mapping (eds West, J. B., Bowen, G. J., Dawson, T. E. & Tu, K. P.) 195–219 (Springer, 2010). 7. Clemens, S. C., Prell, W. L. & Sun, Y. Paleoceanography 25, PA4207 (2010). 8. Lee, J.-E. & Fung, I. Hydrol. Process. 22, 1–8 (2008). 9. Moerman, J. W. et al. Geophys. Res. Lett. 41, 7907–7915 (2014). 10. Evans, M. N., Tolwinski-Ward, S. E., Thompson, D. M. & Anchukaitis, K. J. Quat. Sci. Rev. 76, 16–28 (2013).

C EL L BI O LO GY

On the endocytosis rollercoaster Endocytosis is a process by which molecules gain access to a cell. An unusual mode of endocytosis has now been shown to regulate cell signalling, and to be highjacked by bacterial toxins. See Article p.460 & Letter p.493 VOLKER HAUCKE

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he plasma membrane that surrounds cells forms a barrier that gates access to the cell, permitting entry to nutrients and extracellular messenger molecules, but locking out hazardous compounds and deadly viruses1. Clathrin protein coats some regions of this membrane, and controls cellular entry of beneficial molecules through a process called clathrin-mediated endocytosis1. Cells are also thought to use clathrin-independent modes of endocytosis2, but these have proved difficult to pinpoint. In this issue, Boucrot et al.3 (page 460) describe a fast, clathrin-independent pathway for the import of activated receptor proteins. In a separate study, Renard et al.4 (page 493) show that this pathway is hijacked by toxins from bacteria of the genera Shigella or Vibrio (which causes cholera), to allow them to gain entry to the cell. During clathrin-mediated endocytosis (CME), nutrients, hormones, or other ligands that bind to receptor proteins on clathrincoated membrane regions, are shuttled into cells in small vesicles, which form from invaginations in the membrane. CME not only

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controls uptake of receptor-bound molecules, but also serves a general function in regulating the turnover of many membrane-bound proteins. In addition to CME, the cell uses several atypical, clathrin-independent routes of endocytosis. Molecules thought to be imported through these pathways include bacterial toxins such as Shiga toxin B (ref. 5), from Shigella, and certain membrane-bound receptor proteins2,6 that, when on the cell surface, mediate transmission of signals from extracellular signalling factors to the cell nucleus. Boucrot and colleagues set out to investigate the molecular nature of clathrin-independent endocytosis by studying a membrane-deforming protein, endophilin. Previous genetic analysis of mice lacking all three isoforms7 of endophilin shows that one function of this protein is to promote clathrin-coat shedding from vesicles during the late stages of CME. However, there is evidence that endophilin can also interact with membrane-bound receptors — either directly, as with the β1-adrenergic receptor8, or indirectly through an intermediate adaptor protein, as with certain growth-factor receptors9,10. The authors confirmed that endophilin

Palaeoclimate: Monsoon matters.

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