The influence of vegetation-atmosphere-ocean interaction on climate during the mid-holocene

Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean leads to a warming of the Southern Hemisphere.     

Catastrophic drought in the Afro-Asian monsoon region during Heinrich event 1

Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.     

Millennial-scale instability of the antarctic ice sheet during the last glaciation

Records of ice-rafted detritus (IRD) concentration in deep-sea cores from the southeast Atlantic Ocean reveal millennial-scale pulses of IRD delivery between 20,000 and 74,000 years ago. Prominent IRD layers correlate across the Polar Frontal Zone, suggesting episodes of Antarctic Ice Sheet instability. Carbon isotopes (delta(13)C) of benthic foraminifers, a proxy of deepwater circulation, reveal that South Atlantic IRD events coincided with strong increases in North Atlantic Deep Water (NADW) production and inferred warming (interstadials) in the high-latitude North Atlantic. Sea level rise or increased NADW production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the Weddell Sea region of West Antarctica.     

A mid-brunhes climatic event: long-term changes in global atmosphere and ocean circulation

A long-term climatic change 4.0 x 10(5) to 3.0 x 10(5) years ago is recorded in deep-sea sediments of the Angola and Canary basins in the eastern Atlantic Ocean. In the Angola Basin (Southern Hemisphere) the climatic signal shows a transition to more humid ("interglacial") conditions in equatorial Africa, and in the Canary Basin (Northern Hemisphere) to more "glacial" oceanic conditions. This trend is confirmed by comparison with all well-documented marine and continental records from various latitudes available; in the Northern Hemisphere, in the Atlantic north of 20 degrees N, climate merged into more "glacial" conditions and in equatorial regions and in the Southern Hemisphere to more "interglacial" conditions. The data point to a more northern position of early Brunhes oceanic fronts and to an intensified atmosphere and ocean surface circulation in the Southern Hemisphere during that time, probably accompanied by a more zonal circulation in the Northern Hemisphere. The mid-Brunhes climatic change may have been forced by the orbital eccentricity cycle of 4.13 x 10(5) years.     

Forcing of Atlantic Equatorial and Subpolar Millennial Cycles by Precession

Abundance cycles of the marine alga Florisphaera profunda centered on a period of 7600 carbon-14 years (8400 calendar years) are present in high-resolution records from the equatorial Atlantic spanning 0 to 45,000 years ago. These cycles correlate with Heinrich events 1 through 5, which document rapid changes in continental ice melting around the subpolar North Atlantic. These variations in F. profunda are a direct response to modulation in zonal wind-driven divergence produced by a precessional component of orbital variation during a time of reduced eccentricity modulation.     

Temporal relationships of carbon cycling and ocean circulation at glacial boundaries

Evidence from high-sedimentation-rate South Atlantic deep-sea cores indicates that global and Southern Ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice-sheet growth and retreat, respectively. No consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility that ocean circulation triggered some millenial climate changes. At the major glacial-interglacial transitions, the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets.     

Instability of glacial climate in a model of the ocean- atmosphere-cryosphere system

In contrast to the relatively stable climate of the past 10,000 years, during glacial times the North Atlantic region experienced large-amplitude transitions between cold (stadial) and warm (interstadial) states. In this modeling study, we demonstrate that hydrological interactions between the Atlantic thermohaline circulation (THC) and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode. These interactions have the potential to destabilize the THC, which is already more sensitive for glacial conditions than for the present-day climate, thus providing an explanation for the increased variability of glacial climate.     

Modeling the ice-age climate

Using the boundary conditions of seasurface temperature, ice sheet topography, and surface albedo assembled by CLIMAP for 18,000 B.P., the global ice-age July climate has been simulated with a two-level dynamical atmospheric model. Compared with the simulation for present July climate, the ice age is substantially cooler and drier over the unglaciated continental areas, with the maximum zonal westerlies in the Northern Hemisphere displaced southward in the vicinity of the ice sheets. The simulated changes of surface air temperature agree reasonably well with the estimates available from the analysis of fossil pollen and periglacial data, and are consistent with the simulated changes of other climatic variables. These results are generally supported by independent investigations with simpler models. In spite of this qualified success, further analysis of both simulated and verification data is needed to establish the details of ice-age climate, especially the precipitation regimes, and to document the role of eddy fluxes in maintaining the heat, momentum, and moisture balances of the ice-age general circulation. New paleoclimatic data bases for both July and January of 18,000 B.P. are being assembled by CLIMAP and will be used in new simulations of the seasonal ice-age climate.     

On Pleistocene Surface Temperatures of the North Atlantic and Arctic Oceans

Two additional interpretations are given for the important data of D. B. Ericson on the correlation of coiling directions of Globigerina pachyderma in late Pleistocene North Atlantic sediments with ocean surface temperatures. One interpretation relates the distribution of this species to the distribution and circulation of ocean water masses. On the basis of our ice-age theory, our second interpretation uses the data and correlations of Ericson to establish temperature limits of a thermal node, a line on which glacial and interglacial temperatures were equal, for the North Atlantic Ocean. This line crosses the strait between Greenland and Scandinavia. Further, Ericson's interpretation of the 7.2 degrees C isotherm implies that the glacial-stage surface waters of the Arctic Ocean were between 0 degrees and 3.5 degrees C.     

Arctic paleo-oceanography in late cenozoic time

Sediment cores from the Arctic Ocean yield significant faunal and lithologic evidence of alternating cold and milder periods for the last 6 million years. Although high-latitude continental glaciation commenced prior to 6 million years ago, the Arctic Ocean remained free of permanent pack ice up to approximately 0.7 million years ago, after which successive ice-covered and ice-free conditions existed.     

The surface of the ice-age Earth

In the Northern Hemisphere the 18,000 B.P. world differed strikingly from the present in the huge land-based ice sheets, reaching approximately 3 km in thickness, and in a dramatic increase in the extent of pack ice and marine-based ice sheets. In the Southern Hemisphere the most striking contrast was the greater extent of sea ice. On land, grasslands, steppes, and deserts spread at the expense of forests. This change in vegetation, together with extensive areas of permanent ice and sandy outwash plains, caused an increase in global surface albedo over modern values. Sea level was lower by at least 85 m. The 18,000 B.P. oceans were characterized by: (i) marked steepening of thermal gradients along polar frontal systems, particularly in the North Atlantic and Antarctic; (ii) an equatorward displacement of polar frontal systems; (iii) general cooling of most surface waters, with a global average of -2.3 degrees C; (iv) increased cooling and up-welling along equatorial divergences in the Pacific and Atlantic; (v) low temperatures extending equatorward along the western coast of Africa, Australia, and South America, indicating increased upwelling and advection of cool waters; and (vi) nearly stable positions and temperatures of the central gyres in the subtropical Atlantic, Pacific, and Indian oceans.     

Upwelling intensification as part of the Pliocene-Pleistocene climate transition

A deep-sea sediment core underlying the Benguela upwelling system off southwest Africa provides a continuous time series of sea surface temperature (SST) for the past 4.5 million years. Our results indicate that temperatures in the region have declined by about 10 degrees C since 3.2 million years ago. Records of paleoproductivity suggest that this cooling was associated with an increase in wind-driven upwelling tied to a shift from relatively stable global warmth during the mid-Pliocene to the high-amplitude glacial-interglacial cycles of the late Quaternary. These observations imply that Atlantic Ocean surface water circulation was radically different during the mid-Pliocene.     

Oceanic mechanisms for amplification of the 23,000-year ice-volume cycle

Situated adjacent to the largest Northern Hemispher ice sheets of the ice ages, the mid-latitude North Atlantic Ocean has an important role in the earth's climate history. It provides a significant local source of moisture for the atmosphere and adjacent continents, forms a corridor that guides moisture-bearing storms northward from low latitudes, and at times makes direct contact along its shorelines with continental ice masses. Evidence of major ice-ocean-air interactions involving the North Atlantic during the last 250,000 years is summarized. Outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures, summer heat storage, winter sea-ice extent, and global sea level. These oceanic responses in turn influence the winter moisture flux back to the ice sheets, as well as ablation of land ice by calving. Spectral data indicate that the oceanic moisture and sea-level feedbacks, in part controlled by glacial melt products, amplify Milankovitch (insolation) forcing of the volumetrically dominant mid-latitude ice sheets at the 23,000-year precessional cycle.     

Zonal temperature-anomaly maps of Indian ocean surface waters: modern and ice-age patterns

Maps of sea surface temperature anomalies in the Indian Ocean in modern and ice-age times reveal striking changes in its surface circulation. During the last glacial maximum (18,000 years before the present), the Indian Ocean had colder average zonal surface temperatures, a cooler and less extensive Agulhas Current, a distinct eastern boundary current, and decreased upwelling and a weaker southwest monsoon in its northwestern region.     

Abrupt climate events 500,000 to 340,000 years ago: evidence from subpolar north atlantic sediments

Subpolar North Atlantic proxy records document millennial-scale climate variations 500,000 to 340,000 years ago. The cycles have an approximately constant pacing that is similar to that documented for the last glacial cycle. These findings suggest that such climate variations are inherent to the late Pleistocene, regardless of glacial state. Sea surface temperature during the warm peak of Marine Isotope Stage 11 (MIS 11) varied by 0.5 degrees to 1 degrees C, less than the 4 degrees to 4.5 degrees C estimated during times of ice growth and the 3 degrees C estimated for glacial maxima. Coherent deep ocean circulation changes were associated with glacial oscillations in sea surface temperature.     

Enhanced open ocean storage of CO2 from shelf sea pumping

Seasonal field observations show that the North Sea, a Northern European shelf sea, is highly efficient in pumping carbon dioxide from the atmosphere to the North Atlantic Ocean. The bottom topography-controlled stratification separates production and respiration processes in the North Sea, causing a carbon dioxide increase in the subsurface layer that is ultimately exported to the North Atlantic Ocean. Globally extrapolated, the net uptake of carbon dioxide by coastal and marginal seas is about 20% of the world ocean's uptake of anthropogenic carbon dioxide, thus enhancing substantially the open ocean carbon dioxide storage.     

Decline of subpolar North Atlantic circulation during the 1990s

Observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid- to high-latitude North Atlantic Ocean. TOPEX/Poseidon altimeter data show that subpolar sea surface height increased during the 1990s, and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation. Combining the data from earlier satellites, we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s. Direct current-meter observations in the boundary current of the Labrador Sea support the weakening circulation trend of the 1990s and, together with hydrographic data, show that the mid- to late 1990s decline extends deep in the water column. Analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations: A weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation.     

Atlantic water flow pathways revealed by lead contamination in Arctic basin sediments

Contaminant lead in sediments underlying boundary currents in the Arctic Ocean provides an image of current organization and stability during the past 50 years. The sediment distributions of lead, stable lead isotope ratios, and lead-210 in the major Arctic Ocean basins reveal close coupling of the Eurasian Basin with the North Atlantic during the 20th century. They indicate that the Atlantic water boundary current in the Eurasian Basin has been a prominent pathway, that contaminant lead from the Laptev Sea supplies surface water in the transpolar drift, and that the Canadian and Eurasian basins have been historically decoupled.     

Influence of the Atlantic subpolar gyre on the thermohaline circulation

During the past decade, record-high salinities have been observed in the Atlantic Inflow to the Nordic Seas and the Arctic Ocean, which feeds the North Atlantic thermohaline circulation (THC). This may counteract the observed long-term increase in freshwater supply to the area and tend to stabilize the North Atlantic THC. Here we show that the salinity of the Atlantic Inflow is tightly linked to the dynamics of the North Atlantic subpolar gyre circulation. Therefore, when assessing the future of the North Atlantic THC, it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account.     

Synchronous climate changes in antarctica and the north atlantic

Central Greenland ice cores provide evidence of abrupt changes in climate over the past 100,000 years. Many of these changes have also been identified in sedimentary and geochemical signatures in deep-sea sediment cores from the North Atlantic, confirming the link between millennial-scale climate variability and ocean thermohaline circulation. It is shown here that two of the most prominent North Atlantic events-the rapid warming that marks the end of the last glacial period and the Bolling/Allerod-Younger Dryas oscillation-are also recorded in an ice core from Taylor Dome, in the western Ross Sea sector of Antarctica. This result contrasts with evidence from ice cores in other regions of Antarctica, which show an asynchronous response between the Northern and Southern Hemispheres.