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CL IMATOLOGY 2015 The Authors, some rights reserved;exclusive licensee American Association forthe Advancement of Science. Distributedunder a Creative Commons AttributionNonCommercial License 4.0 (CC BY-NC).10.1126/sciadv.1500682
Past and future rainfall in the Horn of AfricaJessica E. Tierney,1,2* Caroline C. Ummenhofer,2 Peter B. deMenocal3
The recent decline in Horn of Africa rainfall during the MarchMay long rains season has fomented drought andfamine, threatening food security in an already vulnerable region. Some attribute this decline to anthropogenicforcing, whereas others maintain that it is a feature of internal climate variability. We show that the rate of dryingin the Horn of Africa during the 20th century is unusual in the context of the last 2000 years, is synchronous withrecent global and regional warming, and therefore may have an anthropogenic component. In contrast to 20thcentury drying, climate models predict that the Horn of Africa will become wetter as global temperatures rise.The projected increase in rainfall mainly occurs during the SeptemberNovember short rains season, in responseto large-scale weakening of the Walker circulation. Most of the models overestimate short rains precipitation whileunderestimating long rains precipitation, causing the Walker circulation response to unrealistically dominate theannual mean. Our results highlight the need for accurate simulation of the seasonal cycle and an improvedunderstanding of the dynamics of the long rains season to predict future rainfall in the Horn of Africa.
INTRODUCTIONDuring the last 30 years, the Horn of Africa has experienced a per-sistent decline in rainfall during the March-April-May (MAM) longrains season, the primary rainy season for the region (13). This hashad major consequences for regional food security, where agriculturelargely depends on rainfall and is thus highly vulnerable to climaticchange (1, 4). It is unclear whether this decline is caused by internalmultidecadal variability associated with changes in the tropical Pacific(35) or anthropogenically driven warming in the Indian Ocean orwestern Pacific region (1, 2, 68). Yet, thus far, the decline has only beenanalyzed within the context of sparse and short regional climatologicaldata. Establishing whether this drying trend is unusual, and thereforepossibly anthropogenically caused, requires past climate informationthat extends beyond the instrumental record. Here, we present a de-tailed record of changes in regional temperature and aridity in theeastern Horn of Africa region (Somalia, Djibouti, and eastern Ethiopia)spanning the last 2000 years, which places the recent drying in a his-torical context. We then juxtapose the paleoclimate information withclimate simulations conducted as part of the recent Coupled ModelIntercomparison Project 5 (CMIP5, http://cmip-pcmdi.llnl.gov/cmip5/)and discuss the implications for future prediction of regional aridity.
Natural archives of climate change in the eastern Horn of Africa re-gion are limitedthere are no permanent lake basins, few trees, andno high-altitude ice cores. However, the nearby Gulf of Aden containsorganic-rich sediments that accumulate sufficiently quickly to allowfor recent paleoclimate reconstruction (9). For this study, we gener-ated a high-resolution paleoclimate record based on data derived fromtwo overlapping marine cores (Fig. 1A). Relatively high accumulationrates at our core site (0.05 to 0.1 cm/year, see the Supplementary Ma-terials) allow for a decadal resolution time series of both marine andterrestrial climate. To reconstruct past changes in sea surface tempera-ture (SST), we used the TEX86 (tetraether index of 86 carbons) proxy,which is based on the relative cyclization of marine archaeal glyceroldialkyl glycerol tetraethers, and a Bayesian calibration that improvesthe accuracy of regional predictions (see the Supplementary Materials)
(10). To reconstruct aridity, we measured the hydrogen isotopic com-position of leaf waxes (dDwax). dDwax is a faithful indicator for pastchanges in aridity in Africa (11), with more positive (negative) valuescorresponding to arid (wet) conditions (9, 12, 13). Leaf waxes are pri-marily transported from the eastern Horn of Africa to the Gulf ofAden by aeolian processes, providing a comprehensive view of averagearidity in the eastern Horn and Afar regions (see the SupplementaryMaterials) (9).
Figure 1B presents the proxy data from the Gulf of Aden. Before the20th century, SSTs in the region varied within 1.5C, with the first mil-lennium (1501050 AD) on average slightly (0.2C) warmer than thesecond millennium (10501850 AD). TEX86 indicates that regional SSTsrose by ca. 0.8C between the late 1800s and 2001, in good agreementwith observational estimates (fig. S5). Given the calibration errors as-sociated with the TEX86 proxy, we cannot assess whether the currentSSTs are unprecedented in the last 2000 years, but the rapid 20th cen-tury rise is unique in the context of past variability (Fig. 1B). Overall,the TEX86-based SST record gives a clear picture of how temperaturesnear the Horn of Africa have changed in the recent past. Notably, wefind that there have been only minimal changes in surface temperaturesbefore the most recent warming. There is no indication of a warm pe-riod associated with the Northern Hemisphere Medieval Warm Period(ca. 10001400 AD) or a cool period associated with the Little Ice Age(LIA, ca. 14501800).
The dDwax data indicate that regional aridity has changed more sub-stantially than temperature in the past few millennia (Fig. 1C). Unlikethe TEX86 data (Fig. 1B), the dDwax data follow the trajectory of North-ern Hemisphere temperatures with warm temperatures correspondingto drier conditions, and cold temperatures, corresponding to wetterconditions (Fig. 1C). Given the known influence of western IndianOcean SSTs on the East African short rains (8, 14), the fact that thedDwax record follows Northern Hemisphere temperatures, rather thanour local TEX86-based SST record, may indicate that changes in longrains season precipitation (during which time the relationship to westernIndian Ocean SSTs is weak) dominate the recent paleoclimate record. Al-ternatively, the differences between the TEX86 record and Northern
1Department of Geosciences, The University of Arizona, Tucson, AZ 85721, USA. 2WoodsHole Oceanographic Institution, Woods Hole, MA 02540, USA. 3Lamont Doherty EarthObservatory, Palisades, NY 10964, USA.*Corresponding author. E-mail: email@example.com
R E S EARCH ART I C L E
Tierney, Ummenhofer, deMenocal Sci. Adv. 2015;1:e1500682 9 October 2015 1 of 8
on October 13, 2015
Hemisphere temperatures may simply be due to local variability in theGulf of Aden that has little influence on eastern Horn of Africa rainfall.
The wettest period of the last 2000 years occurred during the LIA.This Little Ice Age Pluvial is seen in other proxy records from eastern-most equatorial Africa (15, 16), whereas locations farther to the westnear the East African Rift Lakes were dry during the LIA (17, 18). Thesedata, together with our new record from the eastern Horn region, rein-force the view that the response of East African hydroclimate to LIAcooling was the development of a coastal-interior, wet-dry dipole (19).
Following this wet period, dDwax rose sharply, indicating increas-ingly arid conditions toward the present day (Fig. 1C). Althoughtransport and sedimentation processes, especially fluvial processes,can in some cases substantially delay the delivery of leaf waxes to ma-rine sediments (20), carbon isotope measurements on the Gulf of Adenleaf waxes show a pronounced decline in the 20th centurya clearevidence of the Suess effect (Fig. 2 and see the Supplementary Mate-
rials), indicating that the aeolian-dominated transport in this region issufficiently rapid to preserve 20th century trends. The rise in dDwax istherefore contemporary, and although the values of dDwax achieved bythe endof the20th century arenot unprecedented in the context of the last2000 years, the rate of change is unusual (fig. S6). The coeval rise inglobal temperatures and dDwax during the last 150 years is a continuationof the close association between easternHorn of Africa aridity and globaltemperatures that has persisted over the past 2000 years; clearly, aridityin this region evolves with the state of the global climate. Because themost recent rise in global temperature can be attributed to greenhouse gasemissions (21), and aridity rapidly increases during this time,we infer thatthere is an anthropogenic component to the 20th century drying.
Although the relative paucity of instrumental data across the easternHorn region limits the analysis of historical 20th century trends, avail-able observations support the proxy record in suggesting an overalldrying of the region (Fig. 3A). On a mean annual basis, parts of the
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Fig. 1. Temperature and aridity proxy data from marine core site P178-15 in the Gulf of Aden. (A) L