A Possible Influence of Equatorial Winds on the September 2002 Southern Hemisphere Sudden Warming Event

The stratospheric sudden warming in the Southern Hemisphere (SH) in September 2002 was unexpected for two reasons. First, planetary wave activity in the Southern Hemisphere is very weak, and midwinter warmings have never been observed, at least not since observations of the upper stratosphere became regularly available. Second, the warming occurred in a west phase of the quasi-biennial oscillation (QBO) in the lower stratosphere. This is unexpected because warmings are usually considered to be more likely in the east phase of the QBO, when a zero wind line is present in the winter subtropics and hence confines planetary wave propagation to higher latitudes closer to the polar vortex. At first, this evidence suggests that the sudden warming must therefore be simply a result of anomalously strong planetary wave forcing from the troposphere. However, recent model studies have suggested that the midwinter polar vortex may also be sensitive to the equatorial winds in the upper stratosphere, the region dominated by the semiannual oscillation. In this paper, the time series of equatorial zonal winds from two different data sources, the 40-yr ECMWF Re-Analysis (ERA) and the Met Office assimilated dataset, are reviewed. Both suggest that the equatorial winds in the upper stratosphere above 10 hPa were anomalously easterly in 2002. Idealized model experiments are described in which the modeled equatorial winds were relaxed toward these observations for various years to examine whether the anomalous easterlies in 2002 could influence the timing of a warming event. It is found that the 2002 equatorial winds speed up the evolution of a warming event in the model. Therefore, this study suggests that the anomalous easterlies in the 1–10-hPa region may have been a contributory factor in the development of the observed SH warming. However, it is concluded that it is unlikely that the anomalous equatorial winds alone can explain the 2002 warming event.

Analysis of an ancient global warming event, the Paleocene-Eocene thermal maximum, under examination from ODP Holes 113-689B and 113-690B

Spatiotemporal patterns of carbonate dissolution provide a critical constraint on carbon input during an ancient (~55.5 Ma) global warming event known as the Paleocene-Eocene thermal maximum (PETM), yet the magnitude of lysocline shoaling in the Southern Ocean is poorly constrained due to limited spatial coverage in the circum-Antarctic region. This shortcoming is partially addressed by comparing patterns of carbonate sedimentation at the Site 690 PETM reference section to those herein reconstructed for nearby Site 689. Biochemostratigraphic correlation of the two records reveals that the first ~36 ka of the carbon isotope excursion (CIE) signaling PETM conditions is captured by the Site 689 section, while the remainder of the CIE interval and nearly all of the CIE recovery are missing due to a coring gap. A relatively expanded stratigraphy and higher carbonate content at mid-bathyal Site 689 indicate that dissolution was less severe than at Site 690. Thus, the bathymetric transect delimited by these two PETM records indicates that the lysocline shoaled above Site 689 (~1,100 m) while the calcite compensation depth remained below Site 690 (~1,900 m) in the Weddell Sea region. The ensuing recovery of carbonate sedimentation conforms to a bathymetric trend best explained by gradual lysocline deepening as negative feedback mechanisms neutralized ocean acidification. Further, biochemostratigraphic evidence indicates the tail end of the CIE recovery interval at both sites has been truncated by a hiatus most likely related to vigorous production and advection of intermediate waters.

Climate change, coral bleaching and the future of the world's coral reefs

Sea temperatures in many tropical regions have increased by almost 1 degrees C over the past 100 years, and are currently increasing at similar to 1-2 degrees C per century. Coral bleaching occurs when the thermal tolerance of corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Mass coral bleaching has occurred in association with episodes of elevated sea temperatures over the past 20 years and involves the loss of the zooxanthellae following chronic photoinhibition. Mass bleaching has resulted in significant losses of live coral in many parts of the world. This paper considers the biochemical, physiological and ecological perspectives of coral bleaching. It also uses the outputs of four runs from three models of global climate change which simulate changes in sea temperature and hence how the frequency and intensity of bleaching events will change over the next 100 years. The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years. Most information suggests that the capacity for acclimation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world's reefs. The rapidity of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.

The effect of abrupt climatic warming on biogeochemical cycling and N2O emissions in a terrestrial ecosystem

The large, rapid increase in atmospheric N2O concentrations that occurred concurrent with the abrupt warming at the end of the Last Glacial period might have been the result of a reorganization in global biogeochemical cycles. To explore the sensitivity of nitrogen cycling in terrestrial ecosystems to abrupt warming, we combined a scenario of climate and vegetation composition change based on multiproxy data for the Oldest Dryas–Bølling abrupt warming event at Gerzensee, Switzerland, with a biogeochemical model that simulates terrestrial N uptake and release, including N2O emissions. As for many central European sites, the pollen record at the Gerzensee is remarkable for the abundant presence of the symbiotic nitrogen fixer Hippophaë rhamnoides (L.) during the abrupt warming that also marks the beginning of primary succession on immature glacial soils. Here we show that without additional nitrogen fixation, climate change results in a significant increase of N2O emissions of approximately factor 3.4 (from 6.4 ± 1.9 to 21.6 ± 5.9 mg N2O–N m− 2 yr− 1). Each additional 1000 mg m− 2 yr− 1 of nitrogen added to the ecosystem through N-fixation results in additional N2O emissions of 1.6 mg N2O–N m− 2 yr− 1 for the time with maximum H. rhamnoides coverage. Our results suggest that local reactions of emissions to abrupt climate change could have been considerably faster than the overall atmospheric concentration changes observed in polar ice. Nitrogen enrichment of soils due to the presence of symbiotic N-fixers during early primary succession not only facilitates the establishment of vegetation on soils in their initial stage of development, but can also have considerable influence on biogeochemical cycles and the release of reactive nitrogen trace gases to the atmosphere.

(Table 1) Normalized difference vegetation index in control and warming plots, Abisco Research Station

Extreme winter warming events in the sub-Arctic have caused considerable vegetation damage due to rapid changes in temperature and loss of snow cover. The frequency of extreme weather is expected to increase due to climate change thereby increasing the potential for recurring vegetation damage in Arctic regions. Here we present data on vegetation recovery from one such natural event and multiple experimental simulations in the sub-Arctic using remote sensing, handheld passive proximal sensors and ground surveys. Normalized difference vegetation index (NDVI) recovered fast (2 years), from the 26% decline following one natural extreme winter warming event. Recovery was associated with declines in dead Empetrum nigrum (dominant dwarf shrub) from ground surveys. However, E. nigrum healthy leaf NDVI was also reduced (16%) following this winter warming event in experimental plots (both control and treatments), suggesting that non-obvious plant damage (i.e., physiological stress) had occurred in addition to the dead E. nigrum shoots that was considered responsible for the regional 26% NDVI decline. Plot and leaf level NDVI provided useful additional information that could not be obtained from vegetation surveys and regional remote sensing (MODIS) alone. The major damage of an extreme winter warming event appears to be relatively transitory. However, potential knock-on effects on higher trophic levels (e.g., rodents, reindeer, and bear) could be unpredictable and large. Repeated warming events year after year, which can be expected under winter climate warming, could result in damage that may take much longer to recover.

Galápagos coral reef persistence after ENSO warming across an acidification gradient

Anthropogenic CO2 is causing warming and ocean acidification. Coral reefs are being severely impacted, yet confusion lingers regarding how reefs will respond to these stressors over this century. Since the 1982-1983 El Niño-Southern Oscillation warming event, the persistence of reefs around the Galápagos Islands has differed across an acidification gradient. Reefs disappeared where pH<8.0 and aragonite saturation state (Omega arag)<=3 and have not recovered, whereas one reef has persisted where pH>8.0 and Omega arag>3. Where upwelling is greatest, calcification by massive Porites is higher than predicted by a published relationship with temperature despite high CO2, possibly due to elevated nutrients. However, skeletal P/Ca, a proxy for phosphate exposure, negatively correlates with density (R=-0.822, p<0.0001). We propose that elevated nutrients have the potential to exacerbate acidification by depressing coral skeletal densities and further increasing bioerosion already accelerated by low pH.

Sedimentology, geochemistry and mineralogy of the Paleocene-Eocene thermal maximum (PETM) : sediment records from Egypt, India and Spain

A gradual increase in Earth's surface temperatures marking the transition from the late Paleocene to early Eocene (55.8±0.2Ma), represents an extraordinary warming event known as Paleocene-Eocene Thermal Maximum (PETM). Both marine and continental sedimentary records during this period reveal evidences for the massive injection of isotopically light carbon. The carbon dioxide injection from multiple potential sources may have triggered the global warming. The importance of the PETM studies is due to the fact that the PETM bears some striking resemblances to the human-caused climate change unfolding today. Most notably, the culprit behind it was a massive injection of heat-trapping greenhouse gases into the atmosphere and oceans, comparable in volume to what our persistent burning of fossil fuels could deliver in coming centuries. The exact knowledge of what went on during the PETM could help us to foresee the future climate change. The response of the oceanic and continental environments to the PETM is different. Many factors might control the response of the environments to the PETM such as paleogeography, paleotopography, paleoenvironment, and paleodepth. To better understand the mechanisms triggering PETM events, two different environments were studied: 1) shallow marine to inner shelf environment (Wadi Nukhul, Sinai; and the Dababiya GSSP, Luxor, Egypt), and 2) terrestrial environments (northwestern India lignite mines) representing wetland, and fluvial environments (Esplugafreda, Spain) both highlighting the climatic changes observed in continental conditions. In the marine realm, the PETM is characterized by negative ö13Ccar and ô13Corg excursions and shifts in Ô15N to ~0%o values above the P/E boundary and persisting along the interval suggesting a bloom and high production of atmospheric N2-fixers. Decrease in carbonate contents could be due to dissolution and/or dilution by increasing detrital input. High Ti, K and Zr and decreased Si contents at the P/E boundary indicate high weathering index (CIA), which coincides with significant kaolinite input and suggests intense chemical weathering under humid conditions at the beginning of the PETM. Two anoxic intervals are observed along the PETM. The lower one may be linked to methane released from the continental shelf with no change in the redox proxies, where the upper anoxic to euxinic conditions are revealed by increasing U, Mo, V, Fe and the presence of small size pyrite framboids (2-5fim). Productivity sensitive elements (Cu, Ni, and Cd) show their maximum concentrated within the upper anoxic interval suggesting high productivity in surface water. The obtained data highlight that intense weathering and subsequent nutrient inputs are crucial parameters in the chain of the PETM events, triggering productivity during the recovery phase. In the terrestrial environments, the establishment of wetland conditions and consequence continental climatic shift towards more humid conditions led to migration of modern mammals northward following the extension of the tropical belts. Relative ages of this mammal event based on bio-chemo- and paleomagnetic stratigraphy support a migration path originating from Asia into Europe and North America, followed by later migration from Asia into India and suggests a barrier to migration that is likely linked to the timing of the India-Asia collision. In contrast, at Esplugafereda, northeastern Spain, the terrestrial environment reacted differently. Two significant S13C shifts with the lower one linked to the PETM and the upper corresponding to the Early Eocene Thermal Maximum (ETM2); 180/160 paleothermometry performed on two different soil carbonate nodule reveal a temperature increase of around 8°C during the PETM. The prominent increase in kaolinite content within the PETM is linked to increased runoff and/or weathering of adjacent and coeval soils. These results demonstrate that the PETM coincides globally with extreme climatic fluctuations and that terrestrial environments are very likely to record such climatic changes. - La transition Paléocène-Eocène (55,8±0,2 Ma) est marquée par un réchauffement extraordinaire communément appelé « Paleocene-Eocene Thermal Maximum » (PETM). Les données géochimiques caractérisant les sédiments marins et continentaux de cette période indiquent que ce réchauffement a été déclenché par une augmentation massive de CO2 lié à la déstabilisation des hydrates de méthane stockés le long des marges océaniques. L'étude des événements PETM constitue donc un bon analogue avec le réchauffement actuel. Le volume de CO2 émis durant le PETM est comparable avec le CO2 lié à l'activité actuelle humaine. La compréhension des causes du réchauffement du PETM peut être cruciale pour prévoir et évaluer les conséquences du réchauffement anthropogénique, en particulier les répercussions d'un tel réchauffement sur les domaines continentaux et océaniques. De nombreux facteurs entrent en ligne de compte dans le cas du PETM, tels que la paléogéographie, la paléotopographie et les paléoenvironnement. Pour mieux comprendre les réponses environnementales aux événements du PETM, 2 types d'environnements ont été choisis : (1) le domaine marin ouvert mais relativement peu profond (Wadi Nukhul. Sinai, Dababiya, Luxor, Egypte), (2) le milieu continental marécageux humide (mines de lignite, Inde) et fluviatile, semi-aride (Esplugafreda, Pyrénées espagnoles). Dans le domaine marin, le PETM est caractérisé par des excursions négatives du ô13Ccar et ô13Corg et un shift persistant des valeurs de 815N à ~ 0 %o indiquant une forte activité des organismes (bactéries) fixant l'azote. La diminution des carbonates observée durant le PETM peut-être due à des phénomènes de dissolution ou une augmentation des apports terrigènes. Des taux élevés en Ti, K et Zr et une diminution des montants de Si, reflétés par des valeurs des indices d'altération (CIA) qui coïncident avec une augmentation significative des apports de kaolinite impliquent une altération chimique accrue, du fait de conditions plus humides au début du PETM. Deux événements anoxiques globaux ont été mis en évidence durant le PETM. Le premier, situé dans la partie inférieur du PETM, serait lié à la libération des hydrates de méthane stockés le long des talus continentaux et ne correspond pas à des variations significatives des éléments sensibles aux changements de conditions redox. Le second est caractérisé par une augmentation des éléments U, Mo, V et Fe et la présence de petit framboids de pyrite dont la taille varie entre 2 et 5pm. Le second épisode anoxique est caractérisé par une forte augmentation des éléments sensibles aux changements de la productivité (Cu, Ni et Co), indiquant une augmentation de la productivité dans les eaux de surface. Les données obtenues mettent en évidence le rôle crucial joué par l'altération et les apports en nutriments qui en découlent. Ces paramètres sont cruciaux pour la succession des événements qui ont conduit au PETM, et plus particulièrement l'augmentation de la productivité dans la phase de récupération. Durant le PETM, le milieu continental est caractérisé par l'établissement de conditions humides qui ont facilité voir provoqué la migration des mammifères modernes qui ont suivi le déplacement de ces ceintures climatiques. L'âge de cette migration est basé sur des arguments chimiostratigraphiques (isotopes stables), biostratigraphiques et paléomagnétiques. Les données bibliographiques ainsi que celles que nous avons récoltées en Inde, montrent que les mammifères modernes ont d'abord migré depuis l'Asie vers l'Europe, puis dans le continent Nord américain. Ces derniers ne sont arrivés en Inde que plus tardivement, suggérant que le temps de leur migration est lié à la collision Inde-Asie. Dans le Nord-Est de l'Espagne (Esplugafreda), la réponse du milieu continental aux événements PETM est assez différente. Comme en Inde, deux excursions signicatives en ô13C ont été observées. La première correspond au PETM et la seconde est corrélée avec l'optimum thermique de l'Eocène précoce (ETM2). Les isotopes stables de l'oxygène mesurés 2 différents types de nodules calcaires provenant de paléosols suggère une augmentation de 10°C pendant le PETM. Une augmentation simultanée des taux de kaolinite indique une intensification de l'altération chimique et/ou de l'érosion de sols adjacents. Ces résultats démontrent que le PETM coïncide globalement avec des variations climatiques extrêmes qui sont très aisément reconnaissables dans les dépôts continentaux.

Predictable climate impacts of the decadal changes in the ocean in the 1990s

During the 1990s there was a major change in the state of the world's oceans. In particular, the North Atlantic underwent a rapid warming, with sea surface temperatures (SSTs) in the subpolar gyre region increasing by 1°C in just a few years. Associated with the changes in SST patterns were changes in the surface climate, in particular, a tendency for warm and dry conditions over areas of North America in all seasons, and warm springs and wet summers over areas of Europe. Here, the extent to which a climate prediction system initialized using observations of the ocean state is able to capture the observed changes in seasonal mean surface climate is investigated. Rather than examining predictions of the mid-1990s North Atlantic warming event itself, this study compares hindcasts started before and after the warming, relative to hindcasts that do not assimilate information. It is demonstrated that the hindcasts capture many aspects of the observed changes in seasonal mean surface climate, especially in North, South, and Central America and in Europe. Furthermore, the prediction system retains skill beyond the first year. Finally, it is shown that, in addition to memory of Atlantic SSTs, successfully predicting Pacific SSTs was likely important for the hindcasts to predict surface climate over North America.

Carbon and oxygen isotopic compositions of Cibicidoides spp. and fine fraction separates (<10 µm) from ODP Holes 113-689B, 119-738B, and 120-748B

A prominent middle Eocene warming event is identified in Southern Ocean deep-sea cores, indicating that long-term cooling through the middle and late Eocene was not monotonic. At sites on Maud Rise and the Kerguelen Plateau, a distinct negative shift in d18O values (~1.0 per mil) is observed ca. 41.5 Ma. This excursion is interpreted as primarily a temperature signal, with a transient warming of 4°C over 600 k.y. affecting both surface and middle-bathyal deep waters in the Indian-Atlantic region of the Southern Ocean. This isotopic event is designated as the middle Eocene climatic optimum, and is interpreted to represent a significant climatic reversal in the midst of middle to late Eocene deep-sea cooling. The lack of a significant negative carbon isotope excursion, as observed during the Paleocene-Eocene thermal maximum, and the gradual rate of high-latitude warming suggest that this event was not triggered by methane hydrate dissociation. Rather, a transient rise in pCO2 levels is suspected, possibly as a result of metamorphic decarbonation in the Himalayan orogen or increased ridge/arc volcanism during the late middle Eocene.

Eocene to Early Miocene palaeoclimate of Saxony, Germany (Table 4)

In the present study, proxy data concerning changes in atmospheric CO2 and climatic conditions from the Late Eocene to the Early Miocene were acquired by applying palaeobotanical methods. Fossil floras from 10 well-documented locations in Saxony, Germany, were investigated with respect to (1) stomatal density/index of fossil leaves from three different taxa (Eotrigonobalanus furcinervis, Laurophyllum pseudoprinceps and Laurophyllum acutimontanum), (2) the coexistence approach (CA) based on nearest living relatives (NLR) and (3) leaf margin analysis (LMA). Whereas the results of approach (1) indicate changes in atmospheric CO2 concentration, approaches (2) and (3) provide climate data. The results of the analysis of stomatal parameters indicate that the atmospheric CO2 concentration was higher during the Late Eocene than during the Early Oligocene and increased towards the Late Oligocene. A lower atmospheric pCO2 level after the Late Eocene is also suggested by an increase in marine palaeoproductivity at this time. From the Late Oligocene onwards, no changes in atmospheric CO2 concentration can be detected with the present data. For the considered sites, the results of the coexistence approach and of the leaf margin analysis document a significant cooling event from the Late Eocene to the Early Oligocene. The pCO2 decrease from the Late Eocene to the Early Oligocene indicated by the stomatal data raised in this study was thus coupled to a temperature decrease which is reflected by the present datasets. From the Early Oligocene onwards, however, no further fundamental climate change can be inferred for the considered locations. The pCO2 increase from the Early Oligocene to the Late Oligocene, which is indicated by the present data, is thus not accompanied by a climate change at the considered sites. A warming event during the Late Oligocene is, however, recorded by marine climate archives. According to the present data, no change in pCO2 occurred during the cooling event at the Oligocene/Miocene boundary, which is also indicated by marine data. The quality and validity of stomatal parameters as sensors for atmospheric CO2 concentration are discussed.

Siliceous microfossils in middle Eocene sediments of ODP Holes 120-748B and 120-749B

The Middle Eocene Climatic Optimum (MECO) is a major transient warming event that occurred at ~ 40 Ma and reversed a long-term cooling trend through the early and middle Eocene. We report the results of a high-resolution, quantitative study of siliceous microfossils at Ocean Drilling Program Sites 748 and 749 (Southern Kerguelen Plateau, Southern Ocean, ~ 58°S) across a ~ 1.4 myr interval spanning the MECO event. At both sites, a significant increase in biosiliceous sedimentation is associated with the MECO event. Rich siliceous planktonic microfossil assemblages in this interval are unusual in that they are dominated by ebridians, with radiolarians as a secondary major component. Silicoflagellates and diatoms comprise only a minor fraction of the assemblage, in contrast to siliceous microfossil assemblages that characterize modern Southern Ocean sediments. Based on our new siliceous microfossil records, we interpret two ~ 300 kyr periods of elevated nutrient availability in Southern Ocean surface waters which span the peak warming interval of the MECO and the post-MECO cooling interval. A diverse assemblage of large silicoflagellates belonging to the Dictyocha grandis plexus is linked to the rapid rise in sea-surface temperatures immediately prior to peak warmth, and a pronounced turnover is observed in both ebridian and silicoflagellate assemblages at the onset of peak warming. The interval of peak warmth is also characterized by high abundance of cosmopolitan ebridians (e.g., Ammodochium spp.) and silicoflagellates (e.g., Naviculopsis spp.), and increased abundance of tropical and subtropical diatom genera (e.g., Asterolampra and Azpeitia). These observations confirm the relative pattern of temperature change interpreted from geochemical proxy data at multiple Southern Ocean sites. Furthermore, rapid assemblage changes in both autotrophic and heterotrophic siliceous microfossil groups indicate a reorganization of Southern Ocean plankton communities in response to greenhouse warming during the MECO event.

Geochemistry of Late Paleocene thermal maximum sediments

The late Paleocene thermal maximum (LPTM) was a dramatic, short-term global warming event that occurred ~55 Ma. Warming of high-latitude surface waters and global deep waters during the LPTM has been well documented; however, current data suggest that subtropical and tropical sea surface temperatures (SSTs) did not change during the event. Conventional paradigms of global climate change, such as CO2-induced greenhouse warming, predict greater warming in the high latitudes than in the tropics or subtropics but, nonetheless, cannot account for the stable tropical/subtropical SSTs. We measured the stable isotope values of well-preserved late Paleocene to early Eocene planktonic foraminifera from South Atlantic Deep Sea Drilling Project (DSDP) Site 527 to evaluate the subtropical response to the climatic and environmental changes of the LPTM. Planktonic foraminiferal d18O values at Site 527 decrease by ~0.94 per mil from pre-LPTM to excursion values, providing the first evidence for subtropical warming during the LPTM. We estimate that subtropical South Atlantic SSTs warmed by at least ~1°-4°C, on the basis of possible changes in evaporation and precipitation. The new evidence for subtropical SST warming supports a greenhouse mechanism for global warming involving elevated atmospheric CO2 levels.