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The Levant, a transition zone between the temperate Mediterranean domain and subtropical deserts, is a key area to study the latitudinal migrations of zonal climatic belts in response to glacial-interglacial conditions. The region underwent large climatic shifts during the Late Quaternary, with dramatic impacts on water resources and human populations. This paper presents the unique long-term hydro-climatic record (ca. 400 ka) from Northern Levant, derived from a sediment core taken from a small tectonic basin of northern Lebanon, the currently wettest area of the Levant. Combined biotic (pollen, biologic aquatic remains) and abiotic proxies (sediment properties, carbonate oxygen isotopes) reveal relatively high water availability during interglacials and generally drier glacials periods during the past four climate cycles. These general trends are in line with pollen records from southeastern and near east Mediterranean areas, but differ from some paleohydrological records from the southern Levant. Southward migrations of the rain-bearing mid-latitude westerlies during glacial periods and regional land topography are likely to explain the observed regional climate signatures. In addition, distinctive climate signatures of the successive wetter/drier intervals reflect interactions between global forcing (insolation, atmospheric greenhouse gas concentration), eastern Mediterranean Sea surface conditions intimately linked to North Atlantic climate and to ice-sheet and sea-ice extent at high northern latitudes, land topography, and local hydrogeological processes. Our record brings new benchmarks for understanding the spatial heterogeneity of eastern Mediterranean responses to global climate changes.

Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxygen isotope records partly because the factors controlling the terrestrial δ18O signals are not fully understood in this region characterized by sharp climatic gradients. Here, we address this purpose by presenting the first 14C-dated isotope record from the northern Levant. The record is based on oxygen isotopes from ostracod shells from lacustrine-palustrine deposits accumulated in a small karstic, hydrologically open basin (Yammouneh), located on the eastern flank of Mount Lebanon. We have first constructed a composite isotopic record obtained from three different ostracod taxa. This yields an oxygen isotope record of ostracods (δost) related to the most widespread species (Ilyocypris inermis), and converted to δ18O values coeval with calcite precipitated in equilibrium (δC) with the lake water (δL). As with other records from the Mediterranean region, the Yammouneh profile shows maximum and minimum δC values during the LGM and the early Holocene, respectively, and a slight late Holocene increase. In order to discuss the potential causes of the observed δ18O values fluctuations (e.g., changes in the isotopic composition of the moisture source, temperature, precipitation minus evaporation (P-E) balance, or atmospheric circulation), we tentatively reconstruct the lake water isotope composition by correcting the δC values for lake water temperature using regional paleotemperature estimates. Inferred δL values were then corrected for the isotopic composition of the Eastern Mediterranean sea surface water (the moisture source) derived from planktonic foraminifera δ18O values corrected for alkenone-based sea surface temperature. Our study suggests that Holocene δL fluctuations are primarily linked to changes in the sea surface water composition (source effect) amplified by enhanced inland rainfall during the early Holocene. During the LGM, low δL values at Yammouneh are likely due to the ground-temperature effect on the rainfall isotope composition, possibly associated with a steeper altitudinal thermal gradient inland, and with changes in air mass trajectories over the Mediterranean Sea.

We present the first synchronously coupled transient simulation of the evolution of the northern Africa climate-ecosystem for the last 6500 years in a global general circulation ocean–atmosphere–terrestrial ecosystem model. The model simulated the major abrupt vegetation collapse in the southern Sahara at about 5 ka, consistent with the proxy records. Local precipitation, however, shows a much more gradual decline with time, implying a lack of strong positive vegetation feedback on annual rainfall during the collapse. The vegetation change in northern Africa is driven by local precipitation decline and strong precipitation variability. In contrast, the change of precipitation is dominated by internal climate variability and a gradual monsoonal climate response to orbital forcing. In addition, some minor vegetation changes are also simulated in different regions across northern Africa The model also simulated a gradual annual mean surface cooling in the subtropical North Atlantic towards the latest Holocene, as well as a reduced seasonal cycle of SST. The SST response is caused largely by the insolation forcing, while the annual mean cooling is also reinforced by the increased coastal upwelling near the east boundary. The increased upwelling results from a southward retreat of the North Africa monsoon system, and, in turn, an increased northeasterly trade wind. The simulated changes of SST and upwelling are also largely consistent with marine proxy records, albeit with a weaker magnitude in the model. The mismatch between the collapse of vegetation and gradual transition of rainfall suggests that the vegetation collapse is not caused by a strong positive vegetation feedback. Instead, it is suggested that the Mid-Holocene collapse of North African vegetation is caused mainly by a nonlinear response of the vegetation to a precipitation threshold in the presence of strong climate variability. The implication to the modeling and observations is also discussed.

Thirteen Holocene palaeolakes in the western Sahara and Sahel have provided diatom records, with carbonate oxygen isotope profiles available from eight of them. Most of these palaeolakes were groundwater-fed. Lake water chemistry is reconstructed using diatom transfer functions. Lake water salinity and 18 O records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15,000 yr. Data are in general agreement with climate simulations using coupled atmosphere–ocean–vegetation models which show a mid-Holocene wetting over the whole of northwest Africa, and a rapid drying by 6–4 ka. The lake record also shows that at many sites the major lake infilling lags the end of the Younger Dryas by 1–2 ka. Regional differences also appear in the timing of the lake hydrological optimum: ca. 10.5–8.5 and 7.5–4.5 ka in the northern Sahara, 10–8.5 ka in the Air–Tenere, 10–5.7 or 4.5 in the Sahel, and 7.5 ka in Lake Chad. The whole of the Holocene is punctuated by short-term drying events. Changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity. During the wet Holocene period however, the very low δ values in the southern Sahara also imply changes in the moisture transport pattern or rainfall mechanisms. Data suggest an apparent decrease in 18 O content of precipitation along the monsoon flow, in contrast with modern patterns. Changes in water availability and quality have driven population migrations in and out of the Sahara-Sahel, but relationships between climate and cultures are complex. Short-term dry events might have driven inventive adaptations. In the Sahara, drying at 5–4.5 ka coincides with both the collapse of the classical Neolithic civilization and the settlement of new cultures.

Paleohydrological data from the African tropics and subtropics, including lake, groundwater and speleothem records, are reviewed to show how environments and climates from both hemispheres are inter-related. Although orbitally induced changes in the monsoon strength account for a large part of long-term climatic changes in tropical Africa, the Late Pleistocene–Holocene hydrological fluctuations rather appear to have been a series of abrupt events that reflect complex interactions between orbital forcing, atmosphere, ocean and land surface conditions. During the Last Glacial Maximum (23–18 ka BP), most records indicate that generally dry conditions have prevailed in both hemispheres, associated with lower tropical land- and sea-surface temperatures. This agrees with simulations using coupled ocean–atmosphere models, which predict cooling and reduced summer precipitation in tropical Africa; the global hydrological cycle was weaker than today when the extent of large polar ice-sheets and sea-ice was a prominent forcing factor of the Earth's climate. Glacial-interglacial climatic changes started early: a first wetting/warming phase at ca. 17–16 ka BP took place during a period of rapid temperature increase in Antarctica. Next, two drastic arid-humid transitions in equatorial and northern Africa occurred around 15–14.5 ka BP and 11.5–11 ka BP. Both are thought to match the major Greenland warming events, in concert with the switching of the oceanic thermohaline circulation to modern mode. However, part of the climatic signal after 15 ka BP also seems related to the Antarctica climate. During the Holocene, Africa has also experienced rapid hydrological fluctuations of dramatic magnitude compared to the climatic changes at high latitudes. In particular, major dry spells occurred around 8.4–8 ka and 4.2–4 ka BP in the northern monsoon domain. Comparison with other parts of the world indicates that these events have a worldwide distribution but different regional expressions. In the absence of large polar ice sheets, changes in the continental hydrological cycles in the tropics may have a significant impact on the global climate system. Climate information gathered here allows to identify geographical and methodological gaps, and raise some scientific questions that remain to be solved to better understand how the tropics respond to changes in major climate-forcing factors, and how they influence climate globally.

Five lakes in the Qinghai–Tibet Plateau and North Xinjiang have provided oxygen isotope records covering the past 17–11 cal. kyr BP. The modern lakes are first presented within the major atmospheric circulation and climate patterns of China, with the climate-related isotope composition of input water. Equations used to extract hydrological and climatic information from δ18O in carbonates are then given. After a discussion on potential causes for δ18O changes, each of the five isotope profiles is interpreted in the light of its specific environmental data. The most significant event observed in all records occurred at 12–11 cal. kyr BP: an abrupt intensification of summer rainfall associated with an extremely rapid 1000–1500 km westward migration of the monsoon fronts. The monsoon regime weakened in stages after 8–7 cal. kyr BP. Holocene maximal aridity is recorded at all sites between about 4.5 and 3.5 cal. kyr BP. Long-term changes in monsoon strength as inferred from our isotopic interpretation are in good agreement with climate model reconstructions in the Tibet–Qinghai Plateau, while observed hydrological changes in North Xinjiang are not simulated by the models. Also, the non-linearity of the Holocene monsoon variations cannot be accounted for by orbital forcing.

Lakes in arid and semi-arid regions respond to climatic change through shifts in lake water volume and ionic concentration. Because diatom distribution is highly correlated with lake hydrochemistry, diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates. Here we critically examine the use of diatom-inferred salinity as a climate proxy, with examples taken from both modern waterbodies and sedimentary profiles. Sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems. Dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages. Furthermore threshold effects, salinity regulation via groundwater seepage, antecedent conditions, and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing. These complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages, as well as evaluation of potential non-linearities in the salinity/climate relationship through comparison of diatom-inferred salinity with other proxy records of hydrochemical change, historical documents where they exist. and with other regional lake systems.

Hydrologic and climatic changes in western Tibet since 13,000 BP are inferred from mineralogical and isotopic analyses of lacustrine sediments from the Sumxi-Longmu Co basin. The interpretation is based on isotope data from the modern system. Carbon isotope chemistry is complex because of the presence of old limestones on the watershed. Sediments contain detrital and authigenic calcites, the proportions of which are quantitatively estimated by X-ray diffractometry. Stable isotope contents of the authigenic calcite are reconstructed from mass-balance equations. The high 13C contents (up to +6‰) are attributed to an enhancement of calcite precipitation from water surface during freezing periods. The 18O content is related to residence time and evaporation. The validity of 14C dates obtained from different types of material is discussed. Our record, in good agreement with paleobiological data from the same material, shows the sudden establishment of wet/warm conditions at ≈10,000 BP which culminated at ≈7500-6000 BP after a brief reversal event at ≈7900 BP. Maximum aridity for the whole Holocene is recorded at ≈4300 BP.