Warm pool ocean heat content regulates ocean-continent moisture transport.

The Indo-Pacific Warm Pool (IPWP) exerts a dominant role in global climate by releasing huge amounts of water vapour and latent heat to the atmosphere and modulating upper ocean heat content (OHC), which has been implicated in modern climate change1. The long-term variations of IPWP OHC and their effect on monsoonal hydroclimate are, however, not fully explored. Here, by combining geochemical proxies and transient climate simulations, we show that changes of IPWP upper (0-200 m) OHC over the past 360,000 years exhibit dominant precession and weaker obliquity cycles and follow changes in meridional insolation gradients, and that only 30%-40% of the deglacial increases are related to changes in ice volume. On the precessional band, higher upper OHC correlates with oxygen isotope enrichments in IPWP surface water and concomitant depletion in East Asian precipitation as recorded in Chinese speleothems. Using an isotope-enabled air-sea coupled model, we suggest that on precessional timescales, variations in IPWP upper OHC, more than surface temperature, act to amplify the ocean-continent hydrological cycle via the convergence of moisture and latent heat. From an energetic viewpoint, the coupling of upper OHC and monsoon variations, both coordinated by insolation changes on orbital timescales, is critical for regulating the global hydroclimate.

Half-precessional cycle of thermocline temperature in the western equatorial Pacific and its bihemispheric dynamics.

The El Niño-Southern Oscillation (ENSO), which is tightly coupled to the equatorial thermocline in the Pacific, is the dominant source of interannual climate variability, but its long-term evolution in response to climate change remains highly uncertain. This study uses Mg/Ca in planktonic foraminiferal shells to reconstruct sea surface and thermocline water temperatures (SST and TWT) for the past 142 ky in a western equatorial Pacific (WEP) core MD01-2386. Unlike the dominant 100-ky glacial-interglacial cycle recorded by SST and δ18O, which echoes the pattern seen in other WEP sites, the upper ocean thermal gradient shows a clear half-precessional (9.4 ky or 12.7 ky) cycle as indicated by the reconstructed and simulated temperature (ΔT) and δ18O differences between the surface and thermocline waters. This phenomenon is attributed to the interplay of subtropical-to-tropical thermocline anomalies forced by the antiphased meridional insolation gradients in the two hemispheres at the precessional band. In particular, the TWT shows greater variability than SST, and dominates the ΔT changes which couple with the west-east SST difference in the equatorial Pacific at the half-precessional band, implying a decisive role of the tropical thermocline in orbital-scale climate change.

Model evidence for a seasonal bias in Antarctic ice cores.

Much of the global annual mean temperature change over Quaternary glacial cycles can be attributed to slow ice sheet and greenhouse gas feedbacks, but analysis of the short-term response to orbital forcings has the potential to reveal key relationships in the climate system. In particular, obliquity and precession both produce highly seasonal temperature responses at high latitudes. Here, idealized single-forcing model experiments are used to quantify Earth's response to obliquity, precession, CO2, and ice sheets, and a linear reconstruction methodology is used to compare these responses to long proxy records around the globe. This comparison reveals mismatches between the annual mean response to obliquity and precession in models versus the signals within Antarctic ice cores. Weighting the model-based reconstruction toward austral winter or spring reduces these discrepancies, providing evidence for a seasonal bias in ice cores.

155,000 years of West African monsoon and ocean thermal evolution.

A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial-interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.

Climate change and trace gases.

Palaeoclimate data show that the Earth's climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the 'albedo flip' property of ice/water, provides a powerful trigger mechanism. A climate forcing that 'flips' the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Inertia of ice sheet and ocean provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are also important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years. The most important of the non-CO2 forcings is methane (CH4), as it causes the second largest human-made GHG climate forcing and is the principal cause of increased tropospheric ozone (O3), which is the third largest GHG forcing. Nitrous oxide (N2O) should also be a focus of climate mitigation efforts. Black carbon ('black soot') has a high global warming potential (approx. 2000, 500 and 200 for 20, 100 and 500 years, respectively) and deserves greater attention. Some forcings are especially effective at high latitudes, so concerted efforts to reduce their emissions could preserve Arctic ice, while also having major benefits for human health, agricultural productivity and the global environment.

Global temperature change.

Global surface temperature has increased approximately 0.2 degrees C per decade in the past 30 years, similar to the warming rate predicted in the 1980s in initial global climate model simulations with transient greenhouse gas changes. Warming is larger in the Western Equatorial Pacific than in the Eastern Equatorial Pacific over the past century, and we suggest that the increased West-East temperature gradient may have increased the likelihood of strong El Niños, such as those of 1983 and 1998. Comparison of measured sea surface temperatures in the Western Pacific with paleoclimate data suggests that this critical ocean region, and probably the planet as a whole, is approximately as warm now as at the Holocene maximum and within approximately 1 degrees C of the maximum temperature of the past million years. We conclude that global warming of more than approximately 1 degrees C, relative to 2000, will constitute "dangerous" climate change as judged from likely effects on sea level and extermination of species.

The mid-Pleistocene transition in the tropical Pacific.

A sea surface temperature (SST) record based on planktonic foraminiferal magnesium/calcium ratios from a site in the western equatorial Pacific warm pool reveals that glacial-interglacial oscillations in SST shifted from a period of 41,000 to 100,000 years at the mid-Pleistocene transition, 950,000 years before the present. SST changes at both periodicities were synchronous with eastern Pacific cold-tongue SSTs but preceded changes in continental ice volume. The timing and nature of tropical Pacific SST changes over the mid-Pleistocene transition implicate a shift in the periodicity of radiative forcing by atmospheric carbon dioxide as the cause of the switch in climate periodicities at this time.

Middle Miocene Southern Ocean cooling and Antarctic cryosphere expansion.

Magnesium/calcium data from Southern Ocean planktonic foraminifera demonstrate that high-latitude (approximately 55 degrees S) southwest Pacific sea surface temperatures (SSTs) cooled 6 degrees to 7 degrees C during the middle Miocene climate transition (14.2 to 13.8 million years ago). Stepwise surface cooling is paced by eccentricity forcing and precedes Antarctic cryosphere expansion by approximately 60 thousand years, suggesting the involvement of additional feedbacks during this interval of inferred low-atmospheric partial pressure of CO2 (pCO2). Comparing SSTs and global carbon cycling proxies challenges the notion that episodic pCO2 drawdown drove this major Cenozoic climate transition. SST, salinity, and ice-volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat/vapor transport, triggering ice growth and global cooling.

Links between salinity variation in the Caribbean and North Atlantic thermohaline circulation.

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bølling/Allerød warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.

Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination.

A high-resolution western tropical Atlantic sea surface temperature (SST) record from the Cariaco Basin on the northern Venezuelan shelf, based on Mg/Ca values in surface-dwelling planktonic foraminifera, reveals that changes in SST over the last glacial termination are synchronous, within +/-30 to +/-90 years, with the Greenland Ice Sheet Project 2 air temperature proxy record and atmospheric methane record. The most prominent deglacial event in the Cariaco record occurred during the Younger Dryas time interval, when SSTs dropped by 3 degrees to 4 degrees C. A rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature, atmospheric methane, and high-latitude changes during the Younger Dryas.

The cause of carbon isotope minimum events on glacial terminations.

The occurrence of carbon isotope minima at the beginning of glacial terminations is a common feature of planktic foraminifera carbon isotopic records from the Indo-Pacific, sub-Antarctic, and South Atlantic. We use the delta13C record of a thermocline-dwelling foraminifera, Neogloboquadrina dutertrei, and surface temperature estimates from the eastern equatorial Pacific to demonstrate that the onset of delta13C minimum events and the initiation of Southern Ocean warming occurred simultaneously. Timing agreement between the marine record and the delta13C minimum in an Antarctic atmospheric record suggests that the deglacial events were a response to the breakdown of surface water stratification, renewed Circumpolar Deep Water upwelling, and advection of low delta13C waters to the convergence zone at the sub-Antarctic front. On the basis of age agreement between the absolute delta13C minimum in surface records and the shift from low to high delta13C in the deep South Atlantic, we suggest that the delta13C rise that marks the end of the carbon isotope minima was due to the resumption of North Atlantic Deep Water influence in the Southern Ocean.