P ERSPECTIVES makes one worry about trivial artifacts, such about twice the number of coding genes. need to be to account for the length of the as contamination of PCR samples with This is satisfyingly similar to the conclu- conserved sequences. Other possibilities DNA from more closely related species, but sion of another multispecies comparison by for what they could do include acting as the authors took precautions that make this Thomas and colleagues, who used a very structural components of chromosomes, al- unlikely. Furthermore, most of the CNGs different strategy (6). They chose a 1.8- though it is equally difficult to imagine that could be found in the dog genome, which megabase region of human chromosome 7 these sequence elements require such a they did not need to obtain by PCR because that includes 10 genes. They determined the high degree of conservation. Uncovering its sequence was determined at The Institute sequences of the orthologous regions of 13 the part that CNGs play in the cell will cer- for Genomic Research (4). The degree and other vertebrate species: nine mammals, a tainly require experimentation, and that ac- length of conservation of the CNGs is espe- bird (chicken), and three fish. Using some- tivity is likely to occupy many people for cially surprising because this was not ob- what different criteria, they identified “mul- quite some time. served in a recent multispecies comparison tispecies conserved sequences” (MCSs). Early in the Human Genome Project, of the genomes of six yeast species (5). In Only about one-third of these conserved se- people argued about what to sequence. that study, highly conserved sequences, quences overlap with protein-coding re- Some advocated determining just the se- most of which are probably transcription gions, again revealing the large amount of quence of the protein-coding regions, be- factor binding sites, are appropriately noncoding DNA that is under functional se- cause the vast majority of the genome is short and are embedded in less conserved lection. Of course, the amount of conserva- “junk” DNA. This would, they argued, be sequences. The CNGs in mammalian tion varies depending on the species: cost effective because most of the impor- genomes are much more extensive and Nearly all of the coding MCSs are apparent tant information is in protein-coding DNA. more highly conserved than anything seen in both the chicken and fish genomes, Given what we’ve learned about the jewels in yeast, even though the evolutionary whereas only about 29% of the noncoding in the genome’s attic, aren’t we glad they time separating monotreme mammals MCSs are found in the chicken genome sequenced it all? from placental mammals is longer (by a with almost none in the fish genome. factor of at least 3) than that separating the What might the function of these CNGs References different yeast species. These findings im- be? The first thing that comes to mind is 1. R. H. Waterston et al., Nature 420, 520 (2002). 2. E.T. Dermitzakis et al., Science 302, 1033 (2003); pub- ply that the CNGs are subject to a very that they are regulatory regions controlling lished online 2 October 2003 (10.1126/science. strong and continual selective constraint, gene expression. However, many of them 1087047). enabling them to remain largely un- look too conserved to consist of protein 3. E. T. Dermitzakis et al., Nature 420, 578 (2002). changed for as long as 300 million years. binding sites (which are usually highly de- 4. E. F. Kirkness et al., Science 301, 1898 (2003). 5. P. Cliften et al., Science 301, 71 (2003); published on- Dermitzakis et al. estimate that there are generate), and it is hard to imagine the sites line 29 May 2003 (10.1126/science.1084337). about 60,000 CNGs in the human genome, being as densely packed as they would 6. J. W. Thomas et al., Nature 424, 788 (2003).

ATMOSPHERIC SCIENCE fall variability is the apparently different influence from different ocean basins on Drought in the Sahel interannual and interdecadal time scales (6, 8). By decomposing the modeled Sahel Ning Zeng rainfall into a high-frequency and a low- frequency component, Giannini et al. ince the late 1960s, the Sahel—a semi- cipitation and even less favorable condi- achieve high reproducibility compared to arid region in West Africa between the tions for vegetation (1–3). The second in- the observations on both interannual and SSahara desert and the Guinea coast vokes large-scale atmospheric circulation interdecadal time scales. Furthermore, they rainforest—has experienced a drought of changes triggered by multidecadal varia- identify the contributions from different unprecedented severity in recorded history. tions in global sea surface temperature ocean basins on different time scales. The drought has had a devastating impact (4–6) (see the figure). The Sahel is one of the most climatical- on this ecologically vulnerable region and On page 1027 of this issue, Giannini et ly sensitive zones in the world because of was a major impetus in the establishment al. (7) report the most comprehensive mod- the influence of a wide range of factors, of the United Nations Convention on eling study to date of the ocean temperature such as its unique geographic location. The Combating Desertification and Drought. scenario. The authors analyzed the results results reported by Giannini et al. are en- Two reports in this issue shed light on the from a state-of-the-art atmospheric general couraging signs that GCMs have improved likely causes of the drought and its conse- circulation model (GCM) developed at substantially in recent years, raising the quences for atmospheric dust transport. NASA Goddard Space Flight Center. When hope for better climate prediction on sea- Two main hypotheses have been pro- forced by observed global sea surface tem- sonal to interannual time scales. posed for the cause of the drought. The peratures from 1930 to 2000, the model re- Compared to the ocean scenario, the role first focuses on anthropogenic factors such produced much of the variability in the ob- of land use change has been more difficult as overgrazing and conversion of woodland served Sahel rainfall. The results provide to quantify. Given a large change in land to agriculture. Both of these processes tend strong evidence that global sea surface tem- surface properties that could conceivably be to increase surface albedo (less sunlight is perature is a major forcing in this region. caused by human activities, atmospheric absorbed) and reduce moisture supply to Using a statistical tool called principal com- models can simulate a substantial reduction the atmosphere. They thus lead to less pre- ponents analysis, Giannini et al. show that in rainfall. However, the question more rel- Sahel rainfall is most closely related to a evant to the recent Sahel drought is whether largely tropical sea surface temperature such large anthropogenic disturbance has The author is in the Department of Meteorology and anomaly pattern that spans the Pacific, actually taken place. A recent study of pop- Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA. Atlantic, and Indian oceans. ulation dynamics and land use history sug- E-mail: [email protected] Another characteristic of the Sahel rain- gests a modest land use change over the last

www.sciencemag.org SCIENCE VOL 302 7 NOVEMBER 2003 999 P ERSPECTIVES Changes in SST Monsoon 35 to 40 years that is not nearly enough to dust sources account for about ation weaker explain the observed drought (9). half of the global total today, the This is not to say that sea surface tem- drought in recent decades may water input Drier, peratures are the whole story. Similar to have increased the total global less Cooler Higher vegetation many earlier model studies, Giannini et al. dust loading by one-third. albedo were able to obtain only 25 to 35% of the Dust in the air reflects sun- 30¡N observed rainfall change. What is missing shine and changes cloud proper- may well be land-atmosphere feedbacks ties, thus modifying the energy that positively enhance the drying tendency balance in the atmosphere and at ITCZ shifts initiated by the changes in global sea sur- the surface. The dust minerals al- south face temperature. One such feedback in- so serve as a nutrient for marine volves natural vegetation (10), which phytoplankton in iron-limited re- 0 Monsoon circulation would be reduced in response to an initial gions. A substantial change in weakens decrease in rainfall. The subsequent feed- dust supply may modify the glob- backs through increased albedo and re- al carbon cycle and climate, as duced evaporation are similar to those pro- might have happened during the posed in the land use change mechanism. ice ages (12). Thus, regardless of Ð30¡S It is very likely that sea surface temper- the relative importance of natural Warmer ature change, natural vegetation processes, versus anthropogenic causes, the and land use change have acted synergisti- drought in the Sahel may have an 45¡W 0 45¡E cally to produce the unusual drought in the unexpected influence on global Complex feedbacks. The recent Sahel drought was likely ini- Sahel (see the figure). This speaks for the climate. tiated by a change in worldwide ocean temperatures, which need to monitor the global oceans even for The quantification of natural reduced the strength of the African monsoon, and was exac- regional climate prediction. The importance and anthropogenic causes is im- erbated by land-atmosphere feedbacks through natural vege- of atmosphere, land, and ocean processes portant. If humans are mostly re- tation and land cover change. Land use changes by humans are emphasized in field experiments such sponsible for the drought, further may have also played an important role. SST, sea surface tem- as the upcoming African Monsoon Multi- land degradation may lead to a perature; ITCZ, intertropical convergence zone. disciplinary Analysis (AMMA). catastrophic reduction in the A much less studied consequence of ecosystem’s carrying capacity. If, instead, References drought in semiarid regions is atmospheric natural variability is the main player, we can 1. J. Otterman, Science 186, 531 (1974). dust. On page 1024 of this issue, Prospero take some comfort in knowing that we 2. J. G. Charney, Q. J. R. Meteorol. Soc. 101, 193 (1975). and Lamb (11) report observational evi- might be on the other side of the swing soon. 3. Y. Xue, J. Shukla, J. Clim. 5, 2232 (1993). dence of dust transported across the However, even in the latter case, the sys- 4. P. J. Lamb, Tellus 30, 240 (1978). Atlantic from Africa to the Caribbean island tem may not be able to recover completely 5. C. K. Folland,T. N. Palmer, D. E. Parker, Nature 320, 602 (1986). of Barbados. The dust concentrations at given that the Sahelian population is doubling 6. D. P. Rowell, C. K. Folland, K. Maskell, M. N. Ward, Q. J. Barbados are anticorrelated with concur- every 20 years. Furthermore, if the trend in R. Meteorol. Soc. 121A, 669 (1995). rent rainfall amount in the Sahel. The anti- global sea surface temperature is related to 7. A. Giannini, R. Saravanan, P. Chang, Science 302, 1027 correlation is even higher with rainfall from anthropogenic global warming, as some stud- (2003); published online 9 October 2003 (10.1126/ science.1089357). the previous year, indicating a delayed re- ies suggest (13), we may again be on a one- 8. M. N. Ward, J. Clim. 11, 3167 (1998). sponse. No data are available before the late way road. In this case, the Sahel may be a har- 9. C. M. Taylor et al., J. Clim. 15, 3615 (2002). 1960s, but extrapolation of the interannual binger of global changes. A combination of 10. N. Zeng, J. D. Neelin, W. K.-M. Lau, C. J. Tucker, Science correlation suggests that there was substan- improved climate prediction, sensible land 286, 1537 (1999). 11. J. M. Prospero, P. J. Lamb, Science 302, 1024 (2003). tially less dust transport during the wet pe- use practice, and CO2 emission reductions 12. J. H. Martin, Paleoceanography 5,1 (1990). riod before the drought. Because African will be the key to the future of the Sahel. 13. N. E. Graham, Science 267, 666 (1995).

ATMOSPHERIC SCIENCE In recent years, the nucleation and growth of nanometer-scale atmospheric aerosol particles have been observed with How Particles Nucleate and Grow state-of-the-art technology in many differ- Markku Kulmala ent atmospheric environments, including the lower stratosphere, the free troposphere, erosol particles are ubiquitous in ticles nucleate and grow in the atmosphere. the continental boundary layer just above Earth’s atmosphere. They influence Particle nucleation in the atmosphere has Earth’s surface, and coastal environments Athe quality of life in many ways: for exercised the minds of scientists since John (5). Some continental sampling sites can example, through their climatic and health Aitken built the first apparatus to measure serve as natural laboratories. For example, effects and by affecting visibility. Better un- the number of dust and fog particles in the at the SMEAR I station in Lapland, clean derstanding of these effects, especially their late 19th century. Today, size distributions Arctic air is frequently replaced by polluted role in climate change, requires knowl- of nanometer-scale particles and concentra- plumes from the Kola Peninsula, allowing edge of the mechanism by which new par- tions of gases participating in particle nu- different conditions for particle nucleation cleation can be measured directly in the at- and growth to be studied in one location. mosphere (1). However, although several Particles typically form during the late The author is in the Department of Physical Sciences, mechanisms for particle nucleation have morning and then grow throughout the day, Division of Atmospheric Sciences, University of −1 Helsinki, FIN-00014 Helsinki, Finland. E-mail: kulmala@ been proposed (2–4), it remains unclear reaching growth rates of 1 to 20 nm hour .

pcu.helsinki.fi which of them dominates in the atmosphere. In the boundary layer, measured nucleation CREDIT: N. ZENG, J. MEYERSON

1000 7 NOVEMBER 2003 VOL 302 SCIENCE www.sciencemag.org