Multifarious anchovy and sardine regimes in the Humboldt Current System during the last 150 years

The Humboldt Current System (HCS) has the highest production of forage fish in the world, although it is highly variable and the future of the primary component, anchovy, is uncertain in the context of global warming. Paradigms based on late 20th century observations suggest that large‐scale forcing controls decadal‐scale fluctuations of anchovy and sardine across different boundary currents of the Pacific. We develop records of anchovy and sardine fluctuations since 1860 AD using fish scales from multiple sites containing laminated sediments and compare them with Pacific basin‐scale and regional indices of ocean climate variability. Our records reveal two main anchovy and sardine phases with a timescale that is not consistent with previously proposed periodicities. Rather, the regime shifts in the HCS are related to 3D habitat changes driven by changes in upwelling intensity from both regional and large‐scale forcing. Moreover, we show that a long‐term increase in coastal upwelling translates via a bottom‐up mechanism to top predators suggesting that the warming climate, at least up to the start of the 21st century, was favorable for fishery productivity in the HCS.     

Millennial-scale climatic cycles in the Early Pliocene pollen record of Ptolemais, northern Greece

Early Pliocene lacustrine sediments from the Ptolemais Basin, northern Greece, exhibit a climatically induced cyclicity, which can be correlated with the orbital cycle of precession. Superimposed on precession-controlled cycles, palynological time-series data reveal a higher-order climatic cyclicity with periodicities of ∼10, ∼2.5, and ∼1.5 ka. These millennial-scale vegetation changes are likely to reflect changes of the soil-moisture gradient on mountain slopes, caused by fluctuations in orographic winter-precipitation. Such fluctuations corroborate the concept of a NAO-like North Atlantic climatic teleconnection during Early Pliocene times. The periodicities are similar to those of climate oscillations inferred from Quaternary records. The occurrence of millennial-scale cyclicity in a time interval when the Northern Hemisphere was essentially ice-free, implies an ultimate forcing mechanism that operated independently of changes in the thermohaline circulation in the Atlantic Ocean. The cycles are likely to be related to long-period variations in solar activity.     

Amazon Basin climate under global warming: the role of the sea surface temperature

The Hadley Centre coupled climate-carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change. When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5 degrees C warmer air temperature associated with a global mean SST warming.     

Influence of rainfall seasonality on African lowland vegetation during the Late Quaternary: pollen evidence from Lake Masoko, Tanzania

Aim  To determine if changes in the key bioclimatic parameters associated with rainfall seasonality can be quantified from fossil sequences and to distinguish them from changes in the total annual rainfall.
Location  Lake Masoko, southern Tanzania (9°20' S, 33°45' E, 840 m a.s.l.).
Methods  Fossil pollen was extracted from a long and well-dated sedimentary sequence spanning the period 45,000–4400 cal. yr bp . The modern habitat, geographical distribution and climatic range (mean annual rainfall and temperature, and length of the dry season) of selected East African plants were used to infer past rainfall attributes for the most characteristic fossil pollen taxa identified from the sediments of Lake Masoko.
Results  In the Masoko pollen sequence it has been possible to identify changes in the length/severity of the dry season during the last 45,000 cal. yr bp , which are interpreted to reflect shifts in the mean position of the Intertropical Convergence Zone (ITCZ) over the southern tropics. We suggest that this bioclimatic parameter has been the main driver of the vegetation dynamics in this area. The major inferred change occurred at the Younger Dryas–Holocene transition c . 11,800 cal. yr bp , when semi-deciduous forest disappeared, being replaced in the lake catchment by woodland, which persists to the present day in spite of locally high rainfall.
Main conclusions  In tropical regions under the influence of the ITCZ, the position and strength of which determine the distribution of rainfall through the year, more attention must be paid to the impact of the length and intensity of the dry season on the modern and past distribution and dynamics of the vegetation. This climatic parameter is as important as the total annual amount of rainfall, and probably one of the most relevant in lowland areas.     

The environmental context for the origins of modern human diversity: a synthesis of regional variability in African climate 150,000-30,000 years ago

We synthesize African paleoclimate from 150 to 30 ka (thousand years ago) using 85 diverse datasets at a regional scale, testing for coherence with North Atlantic glacial/interglacial phases and northern and southern hemisphere insolation cycles. Two major determinants of circum-African climate variability over this time period are supported by principal components analysis: North Atlantic sea surface temperature (SST) variations and local insolation maxima. North Atlantic SSTs correlated with the variability found in most circum-African SST records, whereas the variability of the majority of terrestrial temperature and precipitation records is explained by local insolation maxima, particularly at times when solar radiation was intense and highly variable (e.g., 150-75 ka). We demonstrate that climates varied with latitude, such that periods of relatively increased aridity or humidity were asynchronous across the northern, eastern, tropical and southern portions of Africa. Comparisons of the archaeological, fossil, or genetic records with generalized patterns of environmental change based solely on northern hemisphere glacial/interglacial cycles are therefore imprecise.We compare our refined climatic framework to a database of 64 radiometrically-dated paleoanthropological sites to test hypotheses of demographic response to climatic change among African hominin populations during the 150-30 ka interval. We argue that at a continental scale, population and climate changes were asynchronous and likely occurred under different regimes of climate forcing, creating alternating opportunities for migration into adjacent regions. Our results suggest little relation between large scale demographic and climate change in southern Africa during this time span, but strongly support the hypothesis of hominin occupation of the Sahara during discrete humid intervals ∼135-115 ka and 105-75 ka. Hominin populations in equatorial and eastern Africa may have been buffered from the extremes of climate change by locally steep altitudinal and rainfall gradients and the complex and variable effects of increased aridity on human habitat suitability in the tropics. Our data are consistent with hominin migrations out of Africa through varying exit points from ∼140-80 ka.     

Cyclic changes in Turonian to Coniacian planktic foraminiferal assemblages from the tropical Atlantic Ocean

Abundance patterns of planktic and benthic foraminifera from a tropical Atlantic drill site (Ocean Drilling Program Site 1259, Demerara Rise, Suriname margin) display a pronounced 400 kyr cyclicity, uninterrupted throughout our  87.8–92 Ma record, between two clearly distinguishable assemblages: (1) a pelagic foraminifer fauna, which represents a deep oxygen minimum zone, and (2) another assemblage representing a shallow oxygen minimum zone where the foraminifer fauna is dominated by a higher diversity population of mostly small clavate and biserial species common in epicontinental seas. The cyclic changes in the long eccentricity band (400 kyr) between these two assemblages are proposed to reflect changes in the mean latitudinal position of the Intertropical Convergence Zone (ITCZ). Associated fluctuations in precipitation and trade wind strength may have influenced the upwelling regime at Demerara Rise leading to the observed cyclicity of planktic foraminiferal assemblages. The severe Turonian to Coniacian paleoclimatic and paleoceanographic changes in the Atlantic Ocean (e.g., gateway opening, cooling, and glaciation), however, seem to have no influence on the composition of tropical planktic foraminiferal faunas. There is no apparent relationship between foraminifer abundances and a major deflection in the stable isotope record interpreted elsewhere as a sign of the growth and decay of a large polar ice sheet.     

Plio-Pleistocene facies environments from the KBS Member, Koobi Fora Formation: implications for climate controls on the development of lake-margin hominin habitats in the northeast Turkana Basin (northwest Kenya)

Climate change is hypothesized as a cause of major events of Plio-Pleistocene East African hominin evolution, but the vertically discontinuous and laterally confined nature of the relevant geological records has led to difficulties with assessing probable links between the two. High-resolution sedimentary sequences from lacustrine settings can provide comprehensive data of environmental changes and detailed correlations with well-established orbital and marine records of climate. Hominin-bearing deposits from Koobi Fora Ridge localities in the northeast Turkana Basin of Kenya are an archive of Plio-Pleistocene lake-margin sedimentation though significant developmental junctures of northern African climates, East African environments, and hominin evolution. This study examines alluvial channel and floodplain, nearshore lacustrine, and offshore lacustrine facies environments for the approximately 136-m-thick KBS Member (Koobi Fora Formation) exposed at the Koobi Fora Ridge. Aspects of the facies environments record information on the changing hydrosedimentary dynamics of the lake margin and give insights into potential climatic controls. Seasonal/yearly climate changes are represented by the varve-like laminations in offshore mudstones and the slickensides, dish-shaped fractures, and other paleosol features overprinted on floodplain strata. Vertical shifts between facies environments, however, are interpreted to indicate lake-level fluctuations deriving from longer-term, dry-wet periods in monsoonal rainfall. Recurrence periods for the inferred lake-level changes range from about 10,000 to 50,000 years, and several are consistent with the average estimated timescales of orbital precession ( approximately 20,000 years) and obliquity ( approximately 40,000 years). KBS Member facies environments from the Koobi Fora Ridge document the development of lake-margin hominin habitats in the northeast Turkana Basin. Environmental changes in these habitats may be a result of monsoonal rainfall variations that derive from orbital insolation and/or glacial forcing.     

Diatom productivity peak and increased circulation during latest Quaternary: Alboran Basin (Western Mediterranean)

Diatoms were studied quantitatively in six latest Quaternary (⋍70 kyr B.P. to Recent) piston cores from the westernmost Mediterranean, the Alboran Basin, and the Atlantic region immediately to the west of the Straits of Gibraltar.The Atlantic cores completely lack diatoms. In the Alboran Basin, diatoms are common from late Stage 3 (⋍27.5 kyr B.P.) to Termination Ib (9 kyr B.P.) and in Recent core tops, but are absent in the other latest Quaternary intervals. Maximum accumulation of diatoms and highest abundance of species normally in sediments associated with increased productivity occurred during the latest Quaternary deglaciation, in the first phase of Termination I (⋍14.8 kyr B.P.).In the modern Alboran Basin, a region of high biological productivity occurs immediately east of the Gibraltar Straits. This high productivity results from upwelling associated with the interaction between the Atlantic inflow and the bottom topography near the Spanish coast. The upwelled nutrient-rich waters are then advected to the east and southeast by the surficial anticyclonic gyral circulation. Late Quaternary variations in diatom abundance are considered to reflect changes in this upwelling intensity with highest diatom abundances inferred to result from increased upwelling associated with an intensification of the anticyclonic gyral circulation. Highest inferred upwelling rates occurred during the first phase of latest Quaternary deglaciation. It is possible that an intensification of circulation within the Mediterranean Basin as a whole occurred from late Stage 3 to mid Termination I because widespread hiatus formation has been reported at this time in the Straits of Sicily due to an increase in the formation of intermediate waters. Diatoms were not preserved in other latest Quaternary intervals due to insufficient productivity to counterbalance their dissolution.     

Expanded spatial extent of the Medieval Climate Anomaly revealed in lake‐sediment records across the boreal region in northwest Ontario

Multi‐decadal to centennial‐scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within‐lake calibration models developed using diatom assemblages collected from surface sediments across a water‐depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near‐shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate‐driven influences, and can provide a sensitive record of past drought. Our lake‐sediment records indicate two periods of synchronous signals, suggesting a common large‐scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900‐1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo‐lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models.     

Long‐term variability and rainfall control of savanna fire regimes in equatorial East Africa

Fires burning the vast grasslands and savannas of Africa significantly influence the global carbon cycle. Projecting the impacts of future climate change on fire‐mediated biogeochemical processes in these dry tropical ecosystems requires understanding of how various climate factors influence regional fire regimes. To examine climate–vegetation–fire linkages in dry savanna, we conducted macroscopic and microscopic charcoal analysis on the sediments of the past 25 000 years from Lake Challa, a deep crater lake in equatorial East Africa. The charcoal‐inferred shifts in local and regional fire regimes were compared with previously published reconstructions of temperature, rainfall, seasonal drought severity, and vegetation dynamics to evaluate millennial‐scale drivers of fire occurrence. Our charcoal data indicate that fire in the dry lowland savanna of southeastern Kenya was not fuel‐limited during the Last Glacial Maximum (LGM) and Late Glacial, in contrast to many other regions throughout the world. Fire activity remained high at Lake Challa probably because the relatively high mean‐annual temperature (~22 °C) allowed productive C₄ grasses with high water‐use efficiency to dominate the landscape. From the LGM through the middle Holocene, the relative importance of savanna burning in the region varied primarily in response to changes in rainfall and dry‐season length, which were controlled by orbital insolation forcing of tropical monsoon dynamics. The fuel limitation that characterizes the region's fire regime today appears to have begun around 5000–6000 years ago, when warmer interglacial conditions coincided with prolonged seasonal drought. Thus, insolation‐driven variation in the amount and seasonality of rainfall during the past 25 000 years altered the immediate controls on fire occurrence in the grass‐dominated savannas of eastern equatorial Africa. These results show that climatic impacts on dry‐savanna burning are heterogeneous through time, with important implications for efforts to anticipate future shifts in fire‐mediated ecosystem processes.     

Climate‐driven rampant speciation of the Cape flora

Aim To test whether the radiation of the extremely rich Cape flora is correlated with marine‐driven climate change. Location Middle to Late Miocene in the south‐east Atlantic and the Benguela Upwelling System (BUS) off the west coast of South Africa. Methods We studied the palynology of the thoroughly dated Middle to Late Miocene sediments of Ocean Drilling Program (ODP) Site 1085 retrieved from the Atlantic off the mouth of the Orange River. Both marine upwelling and terrestrial input are recorded at this site, which allows a direct correlation between changes in the terrestrial flora and the marine BUS in the south‐east Atlantic. Results Pollen types from plants of tropical affinity disappeared, and those from the Cape flora gradually increased, between 10 and 6 Ma. Our data corroborate the inferred dating of the diversification in Aizoaceae c. 8 Ma. Main conclusions Inferred vegetation changes for the Late Miocene south‐western African coast are the disappearance of Podocarpus‐dominated Afromontane forests, and a change in the vegetation of the coastal plain from tropical grassland and thicket to semi‐arid succulent vegetation. These changes are indicative of an increased summer drought, and are in step with the development of the southern BUS. They pre‐date the Pliocene uplift of the East African escarpment, suggesting that this did not play a role in stimulating vegetation change. Some Fynbos elements were present throughout the recorded period (from 11 Ma), suggesting that at least some elements of this vegetation were already in place during the onset of the BUS. This is consistent with a marine‐driven climate change in south‐western Africa triggering substantial radiation in the terrestrial flora, especially in the Aizoaceae.     

Climate‐growth analysis for a Mexican dry forest tree shows strong impact of sea surface temperatures and predicts future growth declines

Tropical forests will experience relatively large changes in temperature and rainfall towards the end of this century. Little is known about how tropical trees will respond to these changes. We used tree rings to establish climate‐growth relations of a pioneer tree, Mimosa acantholoba, occurring in tropical dry secondary forests in southern Mexico. The role of large‐scale climatic drivers in determining interannual growth variation was studied by correlating growth to sea surface temperature anomalies (SSTA) of the Atlantic and Pacific Oceans, including the El Niño‐Southern Oscillation (ENSO). Annual growth varied eightfold over 1970–2007, and was correlated with wet season rainfall (r=0.75). Temperature, cloud cover and solar variation did not affect growth, although these climate variables correlated with growth due to their relations with rainfall. Strong positive correlations between growth and SSTA occurred in the North tropical Atlantic during the first half of the year, and in the Pacific during the second half of the year. The Pacific influence corresponded closely to ENSO‐like influences with negative effects of high SSTA in the eastern Pacific Niño3.4 region on growth due to decreases in rainfall. During El Niño years growth was reduced by 37%. We estimated how growth would be affected by the predicted trend of decreasing rainfall in Central America towards the end of this century. Using rainfall predictions of two sets of climate models, we estimated that growth at the end of this century will be reduced by 12% under a medium (A1B) and 21% under a high (A2) emission scenario. These results suggest that climate change may have repercussions for the carbon sequestration capacity of tropical dry forests in the region.     

Surface water conditions in the Northern Benguela Region (SE Atlantic) during the last 450 ky reconstructed from assemblages of planktonic foraminifera

Planktonic foraminiferal records from Site 1083 (ODP Leg 175) were used to investigate changes in surface water conditions in the Northern Benguela Region over the past 450 ky. The assemblages of planktonic foraminifera are dominated by four species: sinistral coiling Neogloboquadrina pachyderma, dextral coiling N. pachyderma, Globigerina bulloides and Globorotalia inflata. Besides, tropical species deliver a small contribution to the assemblage.The most prominent temporal variations, displayed by N. pachyderma (s+d), represent changes in the coastal upwelling and the presence of cold, nutrient rich waters over the core site. Neogloboquadrina pachyderma (s+d) shows cyclic variability in the eccentricity and, to a less extent, precession frequencies. The changes indicate increased upwelling intensity in glacial maxima and precession maxima, and correlate well with the wind-strength record of Stuut et al. (2002). During glacial maxima, steep temperature gradients over the Southern Hemisphere caused strong SE trade winds and strong upwelling. Precession maxima cause a weak monsoonal circulation, more zonal SE trade winds, strong coastal upwelling, and nutrient-rich surface waters over ODP Site 1083.Advection of Angola Current (AC) surface water into the Walvis Basin, indicated by the tropical species, occurs when the Angola Benguela Front (ABF) is positioned southward. Occasionally, this happened during glacial maxima, as can be explained with the reconstructed and predicted meridional movements of the ABF. The amount of AC water was never sufficient to suppress the marine biological production at the core site.The contribution of Benguela Current (BC) water, reflected by Globorotalia inflata, is greatly determined by the upwelling. In periods of strong upwelling, the BC influence is suppressed.In several glacial substages, the temperature of the upwelling South Atlantic Central Water (SACW) may have been increased, as suggested by the dominance of Neogloboquadrina pachyderma (d) in the upwelling record. This phenomenon may be due to intensified subduction in the central South Atlantic that induces the formation of SACW, or to larger contributions of Eastern SACW to the upwelling water.Around 250–200 ky BP, a long-term shift to higher productivity occurred that is absent in the upwelling record. It was accompanied with a transition from a precession and obliquity variability to an eccentricity dominated variability in the Globorotalia inflata (BC) record. The shift was probably connected to a long-term southward shift of the circumpolar oceanic frontal systems south of the African continent.     

A study on the relationship between Atlantic sea surface temperature and Amazonian greenness

The growth of tropical rainforest in Amazon is critically vulnerable to the change in rainfall and radiation than in temperature, and that amount of rainfall and cloudiness in the northeast region of South American is strongly affected by the Atlantic sea surface temperature (SST). Results from recent model experiments for future climate projection have indicated a reduction of Amazonian greenness by a weakening of tropical vapor circulation system related with the change in SST. Therefore, the observational investigation of the relations between the Amazon greenness and Atlantic SST is fundamental to understand the response of Amazonian tropical forest to climate change. In this study, the effect of Atlantic SST on the spatial and temporal change of the Normalized Difference Vegetation Index (NDVI) in the Amazonian region is examined by using satellite remote sensing data for the period of 1981–2001. A strong correlation between NDVI and SST is found for certain regions in Amazon during the periods of 1980s and 1990s, respectively. In addition, strong correlations with NDVI lagging behind SST for two months and one year, respectively, are also identified from the interannual December-to-February (rain season) variations during 1981–2001. Despite these findings, the mechanisms behind the identified correlation remain unclear. Further analyses using observed precipitation and radiation data are required to understand the potential changes of Amazonian rainforest in the context of global warming.     

El Niño,volcanism, and global climate

The June 1991 eruption of Mt. Pinatubo in the Philippines produced one of the greatest volcanic aerosols in the last hundred years. The estimated net decrease of radiation may have peaked at 10% in the tropics. What was the impact of the Pinatubo aerosol on regional and global climate? Besides the expected net cooling of the average global surface temperature, correlation studies indicate that other types of climate anomalies may also be expected. These include the appearance of an El Niño event, decreased Indian monsoon rainfall, fewer tropical storms in the north Atlantic Ocean in 1991–1993, and normal to above normal winter rainfall in California in 1991/92, all of which were observed. A proposed physical mechanism for the almost-simultaneous occurrence of this constellation of climate anomalies is presented. The results of correlation studies between low-latitude volcanic aerosols and the El Niño/Southern Oscillation are presented in some detail as one example. The correlation between Indian monsoon rainfall and tropical storms in the north Atlantic Ocean is also shown and is updated for the most recent 5 years.     

The role of the North Atlantic Drift in the millennial timescale glacial climate fluctuations

The Greenland ice core records show that the overall cold climate of the last glacial period was repeatedly interrupted by short, rapid warmings. The events, known as Dansgaard-Oeschger (D-O) events, are strongly imprinted in the North Atlantic marine records suggesting that they were linked to North Atlantic circulation changes. However, the causes of the D-O events are poorly understood and they represent one of the most intriguing puzzles of the last glacial period. In order to investigate a possible mechanism we have studied variations in the distribution of benthic and planktonic foraminifera, oxygen isotopes and ice rafted debris during the last glacial period in eight cores from the North Atlantic and the Nordic Seas taken at mid depth from 853 to 1760 m. The parameters indicate, in agreement with previous studies, that the circulation system during the interstadials resembled the present system. Atlantic surface water flowed north into the Nordic Seas, where most of it sank through convection and, as cold deep water, flowed back into the North Atlantic. During the stadials, a halocline was established in the Nordic Seas and in the northernmost part of the North Atlantic and the outflow from the Nordic Seas stopped. However, below the cold, light surface layer, the relatively warm water of the North Atlantic Drift continued to flow across the North Atlantic and into the Nordic Seas, here warming up the deep-water masses. We suggest that the warm water gradually made the water column unstable. The pivotal abrupt climate warmings were caused by sudden upwelling of the deep, warm water masses causing overturning of the entire water column and onset of convection.     

Linking El Niño,local rainfall,and migration timing in a tropical migratory species

Current climate models project changes in both temperature and precipitation patterns across the globe in the coming years. Migratory species, which move to take advantage of seasonal climate patterns, are likely to be affected by these changes, and indeed, a number of studies have shown a relationship between changing climate and the migration timing of various species. However, these studies have almost exclusively focused on the effects of temperature change on species that inhabit temperate zones. Here, we explore the relationship between rainfall and migration timing in a tropical species, Gecarcoidea natalis (Christmas Island red crab). We find that the timing of the annual crab breeding migration is closely related to the amount of rain that falls during a ‘migration window’ period prior to potential egg release dates, which is in turn related to the Southern Oscillation Index, an atmospheric El Niño‐ Southern Oscillation Index. As reproduction in this species is conditional on successful migration, major changes in migration patterns could have detrimental consequences for the survival of the species. This study serves to broaden our understanding of the effects of climate change on migratory species and will hopefully inspire future work on rainfall and tropical migrations.     

ENSO, rainfall and temperature influences on extreme population declines among African savanna ungulates

Climatic variation associated with the North Atlantic Oscillation (NAO) and El Niño‐Southern Oscillation (ENSO) has a widespread influence on the population dynamics of many organisms worldwide. While previous analyses have related the dynamics of northern ungulates to the NAO, there has been no comparable assessment for the species rich assemblages of tropical and subtropical Africa. Census records for 11 ungulate species in South Africa's Kruger National Park over 1977–96 reveal severe population declines by seven species, which were inadequately explained by indices of ENSO or its effects on annual rainfall totals. An additional influence was an extreme reduction in dry season rainfall, concurrent with and perhaps related to a regional temperature rise, possibly a signal of global warming. Boundary fencing now restricts range shifts by such large mammals in response to climatic variation. Our models project near extirpation of three ungulate species from the park's fauna should these climatic conditions recur.     

Temperature and rainfall interact to control carbon cycling in tropical forests

Tropical forests dominate global terrestrial carbon (C) exchange, and recent droughts in the Amazon Basin have contributed to short‐term declines in terrestrial carbon dioxide uptake and storage. However, the effects of longer‐term climate variability on tropical forest carbon dynamics are still not well understood. We synthesised field data from more than 150 tropical forest sites to explore how climate regulates tropical forest aboveground net primary productivity (ANPP) and organic matter decomposition, and combined those data with two existing databases to explore climate – C relationships globally. While previous analyses have focused on the effects of either temperature or rainfall on ANPP, our results highlight the importance of interactions between temperature and rainfall on the C cycle. In cool forests (< 20 °C), high rainfall slowed rates of C cycling, but in warm tropical forests (> 20 °C) it consistently enhanced both ANPP and decomposition. At the global scale, our analysis showed an increase in ANPP with rainfall in relatively warm sites, inconsistent with declines in ANPP with rainfall reported previously. Overall, our results alter our understanding of climate – C cycle relationships, with high precipitation accelerating rates of C exchange with the atmosphere in the most productive biome on earth.     

Paleoceanography of the mid-Pleistocene

The fossil record of the Pleistocene calcareous nannoplankton indicates that during the mid-Pleistocene (0.93–1.25 my) occurred an episode of overwhelming dominance of smallGephyrocapsa. During this episode normally abundant, large size specimens of this genus (mainlyGephyrocapsa oceanica) were virtually excluded from the phytoplankton of tropical and subtropical oceans. The best modern analog of this dominantly smallGephyrocapsa assemblage is the subpolarEmiliania huxleyi assemblage, which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today. Increased equatorial upwelling in the oceans, on a scale much greater than today, may explain the above pattern.To achieve such broad equatorial upwelling there must be a source and a drive for cold, dense water. The Arctic Ocean, which was probably seasonally free of ice during this interval of the mid-Pleistocene, is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling. The energy balance of a predominantly ice-free Arctic Ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the North Atlantic to the Arctic Ocean, which dictates a corresponding or even greater increase in the volume of warm water entering the Arctic Ocean at the surface and cold dense water exiting at depth to the North Atlantic. Such enhanced dense water formation in the Arctic Ocean could drive the intensified equatorial upwelling implied by the smallGephyrocapsa dominance interval.If the above scenario is correct then the climate of the earth's northern hemisphere during the mid-Pleistocene may have been very different from the younger Pleistocene climate. One manifestation of this difference may be the mid-Pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky. Another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric CO₂ is thought to lead directly to melting of the Arctic Ocean ice cover and of the Greenland ice sheet. Thus, the “greenhouse” Arctic Ocean and its attendant ocean circulation would resemble the inferred mid-Pleistocene conditions.