Past and future reorganizations in the climate system

doi:10.1016/S0277-3791(99)00067-0

Quaternary Science Reviews

Volume 19, Issues 1–5, 1 January 2000, Pages 301-319

https://doi.org/10.1016/S0277-3791(99)00067-0 Get rights and content

Abstract

High-resolution records of past climatic changes during the last glacial have revealed a number of abrupt changes on time scales of decades or less. Climate models suggest that the deep ocean circulation has the potential to act as a pacemaker of such changes. Based on results from ice cores from both polar regions, and the reference to a common time scale based on the methane record, it is suggested that the ocean is involved in the 24 Dansgaard-Oeschger events. For the longer events, northern and southern hemispheres are strongly coupled and exhibit climate changes of opposite sign. For the shorter events, the hemispheres are not coupled. The specific global response depends upon the forcing, and probably, the state of the ocean prior to the onset of these events. While such abrupt climate changes appear to be caused by a unique mechanism (changes in the sea surface freshwater balance), models suggest that the response of the ocean circulation depends on the amplitude and temporal evolution of the perturbation.

Section snippets

The ocean's role in the global heat budget

The ocean covers over 70% of the Earth's surface, the heat capacity of the seasonally active ocean layers exceeds that of the atmosphere by a factor of over 30, and the ocean is the biggest of the fast exchanging carbon reservoirs. The transport of water and energy is strongly influenced by the ocean and its currents. Continental run-off compensates the loss of water by net evaporation over the entire ocean and is estimated at about 1.15 Sv (1 Sv=1 Sverdrup=10⁶m³s⁻¹) (Webster, 1994). This implies

The thermohaline circulation

Temperature and salinity determine the density of sea water. Density changes (thermal and haline) caused by atmosphere-ocean fluxes and the circulation itself are the principal drivers of the thermo-haline circulation (henceforth THC) (Warren, 1981). Intensive cooling at high latitudes, as well as production of sea ice promote deep water formation. Deep waters derive from localised regions in the northern and southern high latitudes, mainly in the Greenland-Norwegian-Iceland, the Labrador and

Models and mechanisms of thermohaline changes

The American geologist T. C. Chamberlin, who has made significant contributions to the understanding of the ice ages, wrote already in 1906 (Chamberlin, 1906):

“In an endeavor to find some measure of the rate of the abysmal circulation, it became clear that the agencies which influence the deep-sea movements in opposite phases were very nearly balanced. From this sprang the suggestion that, if their relative values were changed to the extent implied by geological evidence, there might be a

Time scales and spatial distribution

The detection of abrupt climate changes on time scales of less than a few decades has only become possible with the exploitation of high-resolution palaeoclimatic archives which provide climate information that is nearly continuous in time. Palaeobotanical stratigraphic records were among the first to indicate that the last deglaciation was punctuated by climatic oscillations on time scales of a few thousand years (Mangerud et al., 1974; Alley and Clark, 1999). The most prominent of these

The perspective of palaeoclimate archives

The best evidence for interhemispheric connections comes from the ice core records from Greenland and Antarctica which are put on a common timescale by methane synchronisation (Blunier et al., 1998). The longest D/O events (number 8 at 36 kyr and number 12 at 45 kyr BP) in the Greenland ice cores have a correlative in the Antarctic ice cores (Fig. 5). These events are characterized by millennial warming in the south lasting for about 2–3 kyr and are interrupted roughly at the time when the north

The past as a window to the future

The combination of evidence from the paleoclimatic record and a wide range of model simulations suggests that the coupled atmosphere–ocean system has thresholds beyond which large-scale reorganisations of the deep ocean circulation can occur. The system is driven across these thresholds either by slow change of the background state or perturbations. During the glacial, such perturbations were most likely due to meltwater from the last great ice sheets on the American and European continents,

Conclusions

Abrupt climate change has been the rule rather than the exception during the last glacial period. The last abrupt event happened about 8200 years ago (Alley et al., 1997b) when still some ice melting on the American continent was underway (Licciardi et al., 1998). Ocean circulation, in particular, the thermohaline circulation of the Atlantic, and its changes have played a crucial role in amplifying these changes and transmitting them to other areas of the globe. A mechanism for interhemispheric

Acknowledgements

I am grateful to all scientists and students at the Division of Climate and Environmental Physics for their numerous contributions. This article is dedicated to the late Hans Oeschger whose ideas and vision of the Earth System were an inspiration. The support by the Swiss National Science Foundation is acknowledged. O. Marchal has contributed importantly to the model simulations. Comments by two anonymous reviewers, R. Alley, C. Appenzeller, T. Blunier, E. Boyle, W. Broecker, P. Clark, B.

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Past and future reorganizations in the climate system

Abstract

Section snippets

The ocean's role in the global heat budget

The thermohaline circulation

Models and mechanisms of thermohaline changes

Time scales and spatial distribution

The perspective of palaeoclimate archives

The past as a window to the future

Conclusions

Acknowledgements

Earth and Planetary Science Letters

Geochimica Cosmochimica Acta

Progress in Oceanography

Deep Sea Research

Quaternary Research

Quaternary Science Reviews

Progress in Oceanography

Deep-sea coral evidence for rapid change in ventilation of the deep North Atlantic 15,400 years ago

Science

The deglaciation of the northern hemispherea global perspective

Annual Review of Earth and Planetary Sciences

Holocene climatic instabilitya prominent, widespread event 8200 yr ago

Geology

Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event

Nature

Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge

Nature

Brief interstadial events in the Santa Barbara basin, NE Pacific, during the past 60 kyr

Nature

Synchronised terrestrial-atmospheric deglacial records around the North Atlantic

Science

Asynchrony of Antarctic and Greenland climate change during the last glacial period

Nature

Timing of temperature variations during the last deglaciation in Antarctica and the atmospheric CO2 increase with respect to the Younger Dryas event

Geophysical Research Letters

Correlations between climate records from North Atlantic sediments and Greenland ice

Nature

Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period

Nature

Iceberg discharges into the North Atlantic on millennial time scales during the last glaciation

Science

Deep-water formation and meridional overturning in a high-resolution model of the North Atlantic

Journal of Physical Oceanography

North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature

Nature

The biggest chill

Natural History

The great ocean conveyor

Oceanography

Origin of the northern Atlantic's Heinrich events

Climate Dynamics

High-latitude salinity effects and interhemispheric thermohaline circulations

Nature

On a possible reversal of deep-sea circulaton and its influence on geologic climates

Journal of Geology

Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP

Nature

Evidence from Southern Ocean sediments for the effect of North Atlantic deep-water flux on climate

Nature

Origin of the first global meltwater pulse

Paleoceanography

Sedimentary record of rapid climatic variability in the North Atlantic during the last glacial cycle

Paleoceanography

North Atlantic deep water cools the southern hemisphere

Paleoceanography

North Atlantic climatic oscillations revealed by deep Greenland ice cores

Evidence for general instability of past climate from a 250-kyr ice-core record

Nature

The abrupt termination of the Younger Dryas climate event

Nature

Younger Dryas age advance of Franz Josef glacier in the southern alps of New Zealand

Science

The production of North Atlantic Deep Watersources, rates, and pathways

Journal of Geophysical Research

Rapid climatic shifts during isotope stages 2–4 in the Polar North Atlantic

Geology

Palynological and oxygen isotope variations on late-glacial sediment cores from Switzerland

Boreas

Temporal-geographical meltwater influences on the North Atlantic conveyorimplications for the Younger Dryas

Timing of temperature variations during the last deglaciation in Antarctica and the atmospheric CO₂ increase with respect to the Younger Dryas event

Synchronous changes in atmospheric CH₄ and Greenland climate between 40 and 8 kyr BP