Therefore, there is no evidence that the (significantly lower) sea levels and (larger) polar ice sheets of today as compared to the last interglacial are due to man's activity rather than the natural changes expected due to solar insolation changes similar today to the last interglacial.
Further, the authors find no significant changes in seasonality (temperature changes between summer and winter) of the last interglacial compared to the modern seasonality, and attribute such changes to solar insolation similar between the present and last interglacial ~118,000 years ago.
Tropical Atlantic temperature seasonality at the end of the last interglacial
-
- Nature Communications
- 6,
- Article number:
- 6159
- doi:10.1038/ncomms7159
- Received
- Accepted
- Published
The location of our coral site at Bonaire in the southern Caribbean Sea and surface ocean circulation patterns in the study area (Guyana Current, GC; Caribbean Current, CaC; North Equatorial Current, NEC) are indicated. Bonaire is situated off the continental shelf of South America in open-ocean waters. The inset shows the locations of our last interglacial (red circle, this study), Holocene23, 27 (orange circles) and modern23, 27 (white circles) coral sites at Bonaire.
Coral-based SST seasonality reconstruction
The 118-kyr-old Bonaire coral provides a monthly resolved snapshot of tropical Atlantic SST variability for a time window of 20 years at the end of the last interglacial. This is substantially longer than the only other seasonally resolved snapshot of tropical Atlantic SST for the last interglacial, an ~5-year record of a 127-kyr-old coral from Isla de Mona (67.9° W, 18.1° N) in the northern Caribbean Sea9. Our Bonaire monthly resolved coral Sr/Ca- and δ18O-SST reconstructions show clear annual cycles in both proxies (Fig. 2a,b), giving additional confidence that the analysed coral skeleton was not subject to diagenetic alteration. The Sr/Ca-SST reconstruction indicates a seasonality of 2.6±0.1 °C (±1 s.e.) at 118 kyr ago (Fig. 2a,e). Monthly resolved records of three modern Bonaire D. strigosa corals satisfactorily document the instrumental SST26 seasonality of 2.9±0.1 °C (±1 s.e.; 1910–2000), indicating a reconstructed modern Sr/Ca-SST seasonality that ranges from 2.4±0.3 °C (±1 s.e.) to 3.0±0.3 °C (±1 s.e.) for time intervals of the last century, resulting in a reconstructed modern mean seasonality of 2.8±0.4 °C (±1 s.d.; ref. 23; Fig. 2c,e). Taking into account these differences in the reconstructed SST seasonality among the three modern corals indicates that the reconstructed SST seasonality of 2.6±0.1 °C (±1 s.e.) at 118 kyr ago, at the end of the last interglacial, is not significantly different from today (Methods and Supplementary Note 1).
(a) Monthly Sr/Ca record of a fossil Bonaire Diploria strigosa coral that grew at 117.7±0.8 kyr ago for 20 years in southern Caribbean Sea surface waters. (b) The monthly coral δ18O record. (c) Monthly Sr/Ca record of a modern Bonaire D. strigosa coral that grew around AD 1912. (d) The monthly coral δ18O record. (e) Sr/Ca-based sea surface temperature (SST) seasonality from Bonaire D. strigosa corals for snapshots since 118 kyr ago, based on monthly records comprising a total of 315 years, and Bonaire instrumental SST seasonality (1910–2000, 2° × 2° gridbox centred at 12° N, 68° W, ERSST.v3b)26. The dark grey line represents the reconstructed modern mean SST seasonality based on three modern corals and the light grey bar the ±1 s.d. around this mean. (f) The coral δ18O-based SST seasonality. Deviations from Sr/Ca- and instrument-based estimates are due to seasonal seawater δ18O effects. Coral-based SST anomalies (corresponding mean value was subtracted) (a–d) and SST seasonalities (e,f) are derived from seasonal Sr/Ca-SST (−0.042 mmol mol−1 per °C) and δ18O-SST relationships (−0.196‰ per °C) for D. strigosa37. The uncertainty assigned to each SST seasonality estimate is the ±1 s.e. Holocene and modern coral data are from refs 23, 27.
The coral δ18O-SST reconstruction for 118 kyr ago indicates a seasonality of 2.4±0.1 °C (±1 s.e.), which is very similar to the Sr/Ca-based seasonality estimate of 2.6±0.1 °C (±1 s.e.; Fig. 2a,b,e,f). Thus, the coral δ18O seasonality at 118 kyr ago may be attributed mainly to the seasonality of SST. This is broadly in line with the modern situation27, where the mean SST seasonality reconstructed by coral δ18O of 2.3±0.3 °C (±1 s.d.) is slightly reduced (by ~0.5 °C, not correcting for seasonal seawater δ18O changes) relative to the Sr/Ca- and instrument-based estimates (Fig. 2e,f), most likely owing to hydrologic cycle effects such as the Bonaire winter rainfall regime24. The coral δ18O-SST reconstruction supports our major finding based on coral Sr/Ca, and both proxies indicate SST seasonality in the southern Caribbean Sea at the end of the last interglacial similar to today. Consequently, both proxies may also suggest a Bonaire hydrologic cycle similar to today at 118 kyr ago. Crucially, our results are robust towards the choice of the coral Sr/Ca-SST and δ18O-SST relationships, which affect mainly the absolute magnitude of reconstructed SST seasonality but have only minor effect on the relative seasonality estimates among corals, and we would have reached identical conclusions using other relationships (Supplementary Fig. 4).
Discussion
The annual SST cycle in the Caribbean Sea, with a minimum in boreal winter/spring and a maximum in boreal summer/fall, follows primarily the annual cycle of insolation28, 29. Bonaire monthly coral Sr/Ca records for snapshots since the mid-Holocene, comprising a total length of 295 years, suggest that the SST [sea surface temperature] annual cycle in the southern Caribbean Sea has not substantially changed, with the exception of a time interval at 2.35 kyr ago23 (Fig. 2e). Disregarding the 2.35 kyr coral, a trend towards lower SST seasonality during the time interval 6.22–1.84 kyr ago may be inferred from the coral Sr/Ca records, as well as a slightly but significantly higher SST seasonality than the present day at 6.22 kyr ago. Such an evolution through time would be consistent with an orbital insolation control on Holocene SST seasonality in the southern Caribbean Sea (Fig. 3a), which is supported by simulations with a coupled atmosphere–ocean general circulation model (Community Earth System Models, COSMOS; Methods and Fig. 3b). However, we note that the magnitude of the trend in the fossil coral data is minor, close to the ±1 s.d. range of the modern mean Sr/Ca-SST seasonality reconstructed from three modern corals (Fig. 2e), which may also reflect the relatively small magnitude of insolation-controlled SST seasonality changes at lower latitudes throughout the Holocene (Fig. 3a,b).
(a) Insolation seasonality5 at the latitude of Bonaire, calculated as difference of boreal summer (June–July–August, JJA) minus winter insolation (December–January–February, DJF). (b) Sea surface temperature (SST) seasonality at Bonaire simulated by the coupled atmosphere-ocean general circulation model COSMOS (1° × 1° gridbox centred at 12.5° N, 68° W), derived from the difference of simulated summer/autumn (September–October, SO) minus winter/spring (February–March, FM) SST. The SST seasonality evolution is very similar to that derived from the difference of warmest minus coolest SST (Supplementary Fig. 7). (c) Summer (JJA) and winter (DJF) insolation5 at the latitude of Bonaire. (d) Summer/autumn (SO) and winter/spring (FM) SST at Bonaire simulated by COSMOS. βold line (b,d) represents a 21-point running average, representing an average of 210 calendar years. Results of the freshwater hosing experiment are also shown (light blue). Dashed horizontal lines (a,b) indicate the modern value for insolation and simulated SST seasonality. Dashed vertical line indicates the Bonaire coral age (117.7±0.8 kyr).
Similarly, the coral δ18O-SST reconstruction27 reveals a trend towards lower seasonality during the time interval 6.22–1.84 kyr ago, which is more pronounced compared with the trend that may be inferred from coral Sr/Ca, as well as a substantially and significantly higher seasonality than the present day at 6.22 kyr ago (Fig. 2f). This evolution of coral δ18O seasonality through time is consistent with an insolation control on Holocene SST seasonality in the southern Caribbean Sea (Fig. 3a,b). Differences between the coral δ18O-SST and Sr/Ca-SST seasonality estimates (Fig. 2e,f) reflect primarily seasonal changes in seawater δ18O; however, we note that reconstructions of seawater δ18O seasonality can be sensitive towards the choice of the coral δ18O-SST and Sr/Ca-SST relationships (Supplementary Fig. 4). However, for 6.22 kyr ago, an anomalous seawater δ18O seasonality may be inferred from the coral records that could be explained by hydrologic cycle effects such as, among others, Bonaire summer rainfall27, which would be contrary to the present-day winter rainfall regime24 (Supplementary Fig. 4). This interpretation would be in line with reconstructions of increased summer rainfall over northernmost South America during the early to mid-Holocene, owing to a more northerly position of the boreal-summer ITCZ30 and possibly paired with a thermodynamic increase in rainfall because of strengthening local summer insolation31. We note that the subsequent southward migration of the boreal-summer ITCZ over the course of the Holocene that was controlled by orbital insolation changes30, 31 is also in line with the trend towards lower coral δ18O seasonality over this time interval (Fig. 2f).
The significantly increased SST seasonality at 2.35 kyr ago, indicated by coral Sr/Ca (Fig. 2e), may be related to internal climate variability and is interpreted to reflect a time interval of strengthened El Niño-Southern Oscillation (ENSO) teleconnections to the Caribbean region23, probably modulated by the North Atlantic Oscillation (NAO). This interpretation is broadly in line with the present-day modulation of southern Caribbean SST seasonality by ENSO teleconnections23, 32, which vary in strength on interdecadal timescales and are modulated by the NAO33. Indeed, pronounced interannual variability at a period of 5.7 years in the Sr/Ca record of the 2.35 kyr coral23, the most prominent period in the cospectrum of the instrumental indices of ENSO and NAO34, 35, may be indicative of pronounced ENSO–NAO interactions at that time23. Importantly, the strength of the ENSO phenomenon in the tropical Pacific did not change markedly around 2.3 kyr ago36. We note that the increased coral Sr/Ca-SST seasonality at 2.35 kyr ago is not accompanied by an increased coral δ18O-SST seasonality (Fig. 2e,f), which suggests an anomalous seawater δ18O seasonality that could be explained by hydrologic cycle effects such as, among others, increased Bonaire winter rainfall (Supplementary Fig. 4). This interpretation would be broadly in line with the present-day modulation of Bonaire climate by ENSO teleconnections, where La Niña events result in increased SST seasonality through anomalous winter cooling23 as well as in increased winter rainfall24.
Our coral-based finding of SST seasonality similar to today in the southern Caribbean Sea at 118 kyr ago (Fig. 2e) is consistent with an insolation seasonality at the latitude of Bonaire that was close to today’s value (Fig. 3a). This result could be interpreted in a way that southern Caribbean SST seasonality at that time was controlled mainly by orbital insolation changes. Simulations performed with a coupled atmosphere–ocean general circulation model (COSMOS) support this interpretation (Methods). The modelled changes in southern Caribbean SST seasonality at Bonaire throughout the last interglacial follow largely the variations in insolation forcing over the time interval 130–115 kyr ago (Fig. 3). Moreover, the modelled global surface air temperature anomaly indicates that temperature seasonality in the southern Caribbean at 118 kyr ago is part of a hemisphere-scale pattern that can be attributed largely to insolation forcing (Supplementary Fig. 5). Additional model simulations with freshwater forcing to mimic an abrupt ice-sheet collapse and weakening of the North Atlantic thermohaline circulation at 118 kyr ago or with reduced Greenland ice sheet and dynamic vegetation reveal very similar results (Methods), indicating no significant impact on southern Caribbean SST seasonality (Fig. 3 and Supplementary Figs 5 and6). Thus, our model-based results strongly suggest that SST seasonality in the tropical North Atlantic Ocean at the end of the last interglacial was controlled mainly by orbital insolation changes. Although the slightly lower modelled SST seasonality at 118 kyr ago relative to today (Fig. 3b) appears to be consistent with the coral Sr/Ca-SST seasonality estimate for the end of the last interglacial (Fig. 2e), we consider the latter as similar to today as a result of our uncertainty assignments that take into account the differences in the seasonality estimates among the three modern corals (Methods).
The relatively stable SST seasonality in the tropical North Atlantic Ocean at the end of the last interglacial and its inferred orbital control is remarkable as this period was characterized by large-scale perturbations of ocean circulation and climate resulting from instabilities of polar ice sheets11, 12, 13, 14, 15. Results from Western Australia suggest that, after a prolonged period of stable sea level at ~3–4 m above present sea level between 127 and 119 kyr ago, eustatic sea level rose rapidly to ~8 m above present at the end of the last interglacial, peaking at 118.1±1.4 kyr ago12, which is contemporaneous with the age of our southern Caribbean coral (117.7±0.8 kyr; Fig. 4b). It has been suggested that this substantial jump in sea level at the end of the last interglacial resulted from collapse of the Greenland and particularly Antarctic ice sheets, after a critical ice-sheet stability threshold was crossed12. Such an event may have had substantial impacts on global ocean circulation and climate. Interestingly, varved lake sediments in central Europe indicate an extreme 468-year arid and cold event at 118 kyr ago (Fig. 4d), which has been interpreted to result from a sudden southward shift of the warm North Atlantic drift15. Furthermore, western North Atlantic sediments indicate an abrupt ~400-year deep-water reorganization event at ~118 kyr ago associated with changes in the thermohaline circulation13 (Fig. 4e), which has been interpreted to mark the beginning of climate deterioration at the end of the last interglacial13. Recent evidence suggests even two events of substantial North Atlantic deep-water reduction at the end of the last interglacial, at ~119.5 and ~116.8 kyr ago14 (Fig. 4c).
Note in second graph from top, modern day sea level is zero and sea levels during the prior interglacial were up to 11 meters higher than today.
(a) LR04 stack of globally distributed benthic δ18O records, reflecting global ice volume changes66. (b) Relative sea level from Western Australian corals, indicating eustatic sea level rose to ~8 m above present at 118.1±1.4 kyr ago12. Open symbols indicate corals collected not in situ or affected by tectonic uplift12. (c) North Atlantic epibenthic foraminiferal δ13C record, indicating pronounced reductions in North Atlantic Deep Water production (bottom water δ13C reductions) at ~119.5 and ~116.8 kyr ago14. Bold line indicates a 3-point running average. (d) Eifel Laminated Sediment Archive greyscale stack from maar lakes in Germany, indicating a prominent cold and arid event at 118 kyr ago that was accompanied by high grass pollen abundance15. (e) Clay flux record from excess 230Th-measurements in North Atlantic sediments indicating a rapid increase in recirculation-derived clay supply (and the proportion of southern source water) at ~118 kyr ago, associated with a cessation in North Atlantic deep water flow13. The dark grey line indicates the Bonaire coral age (117.7 kyr) and the light grey shading the corresponding 2σ uncertainty (±0.8 kyr). Both Bonaire coral and sea-level jump12 were dated by the230Th/U-method, whereas the sediment records13, 14, 15, 66 were not absolutely dated. Age uncertainty is shown as reported in original publication, if available.
Our findings based on combining coral proxy records with climate model simulations indicate that northern tropical Atlantic SST seasonality at 118 kyr ago was similar to today and controlled mainly by orbital insolation changes, despite dramatic ocean circulation and climate perturbations resulting from instabilities of polar ice sheets that characterized the end of the last interglacial11,12, 13, 14, 15. Today, tropical Atlantic SST plays a major role in seasonal climate extremes, such as hurricanes, flashfloods and droughts16, 17, 18, 19, which cause severe socioeconomic damage on the adjacent continents. Our results indicate that SST seasonality in the tropical Atlantic did not substantially change during a period of abrupt high-latitude ice sheet, ocean and climate perturbations at the end of the last interglacial, and, thus, suggest that tropical SST seasonality is controlled mainly by orbital insolation changes during interglacials. However, more seasonally resolved proxy records of SST are needed to better constrain both the climate sensitivity of the tropical ocean in the past and the seasonal response in model-based scenarios of past and future climate change.
Thank you, MS, for posting science journal papers like this again. They've been missed in the last ~2 months. (o:
ReplyDeleteftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/jiang2015/jiang2015-md99-2275.txt
Mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes. However, the mechanisms for Sun-climate linkages remain a topic of debate. Here we present a high-resolution summer sea-surface temperature (SST) record covering the last 9300 yr from a site located at the present-day boundary between Polar and Atlantic surface-water masses. The record is age-constrained via the identification of 15 independently-dated tephra markers from terrestrial archives, circumventing marine reservoir age variability problems. Our results indicate a close link between solar activity and SSTs in the northern North Atlantic during the past 4000 years. They suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation. Furthermore, the high-resolution SST record indicates that climate in the North Atlantic regions follows solar activity variations on multi-decadal to centennial time scales.