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
- Article number:
Abstract: The end of the last interglacial period, ~118 kyr ago, was characterized by substantial ocean circulation and climate perturbations resulting from instabilities of polar ice sheets. These perturbations are crucial for a better understanding of future climate change. The seasonal temperature changes of the tropical ocean, however, which play an important role in seasonal climate extremes such as hurricanes, floods and droughts at the present day, are not well known for this period that led into the last glacial. Here we present a monthly resolved snapshot of reconstructed sea surface temperature in the tropical North Atlantic Ocean for 117.7±0.8 kyr ago, using coral Sr/Ca and δ18O records. We find that temperature seasonality was similar to today, which is consistent with the orbital insolation forcing. Our coral and climate model results suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.
The last interglacial, although not a direct analogue for future climate, has received much attention in the climate-modelling community1, 2, 3 and has been suggested as a test bed for models developed for future climate prediction2, 4. This period (~127–117 kyr ago) was characterized by strong orbital insolation forcing5, relative warmth6 and high sea level7. In the Northern Hemisphere, changes in the Earth’s orbit around the sun led to a stronger seasonality of insolation compared to today5, which resulted in increased temperature seasonality at the Earth’s surface as inferred from proxy records8, 9, 10 that commonly represent the time interval of maximum seasonal insolation forcing5 between ~127 and ~124 kyr ago. In contrast, the temperature seasonality at the end of the last interglacial (~118 kyr ago), when Northern Hemisphere insolation seasonality was close to today’s value5, is not well known. This period that led into the last glacial is particularly interesting as it was characterized by catastrophic collapse of polar ice sheets and substantial sea-level rise11, 12, abrupt changes in ocean circulation13, 14and large-scale climate perturbations15. It has been suggested that the end of the last interglacial may provide clues to a better understanding of the potential for rapid ice-sheet collapse and sea-level rise and, consequently, for abrupt perturbations of the ocean–atmosphere system, under future climate change11, 12, 14. At the present day, the seasonal temperature changes of the tropical ocean play an important role in seasonal climate extremes such as hurricanes, floods and droughts16, 17, 18, 19. A better understanding of the temperature seasonality ~118 kyr ago is, thus, essential to establish a baseline to evaluate the seasonal response in climate model simulations, for both the end of the last interglacial and for projections of future climate change.
Here we investigate the monthly resolved Sr/Ca and δ18O environmental proxy signals in a precisely dated shallow-water fossil coral recovered from the southern Caribbean and reconstruct the temperature seasonality in the surface waters of the tropical North Atlantic Ocean at the end of the last interglacial. Sr/Ca variations in aragonitic coral skeletons are a proxy for temperature variability20, which has previously been successfully applied to last interglacial fossil corals9, 10,21. Coral δ18O, a proxy that reflects both temperature and seawater δ18O variations, is used to support our reconstruction. The 230Th/U method allows precise dating of corals that grew during the last interglacial period22. Our findings indicate that temperature seasonality in the southern Caribbean Sea at 118 kyr ago was similar to today. Our coral records and simulations with a coupled atmosphere–ocean general circulation model indicate an orbital control on temperature seasonality in the tropical North Atlantic at the end of the last interglacial, despite the large-scale perturbations of ocean circulation and climate during this period, and suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.
Coral preservation and age
The fossil shallow-water coral (Diploria strigosa) was recovered at Bonaire, an open-ocean island in the southern Caribbean Sea, located ~100 km north of South America and ~300 km northwest of the Cariaco Basin (Fig. 1). Bonaire is situated off the South American continental shelf in the northwestward-flowing Caribbean Current, an extension of the Guyana Current that transports equatorial Atlantic surface waters along northeastern South America towards the Caribbean Sea. Thus, sea surface temperature (SST) at Bonaire is representative for a large area of the tropical North Atlantic Ocean23. Bonaire is influenced by the easterly trade winds, and its present-day climate is semi-arid with an annual precipitation of ~550 mm and the main rainy season during boreal winter24. Bonaire is not influenced by the seasonally migrating Intertropical Convergence Zone (ITCZ) because the northernmost ITCZ position that is reached during boreal summer is located south of Bonaire, over northern South America and the Cariaco Basin25. The fossil coral colony (BON-5-D) was drilled in growth position on top of an elevated reef terrace at the eastern coast of Bonaire (Washikemba). The coral site (68° 11.765′ W, 12° 8.246′ N) is at ~1.5 to ~2.0 m above present sea level, in a distance of ~50 m from the present-day sea cliff. Nearby D. strigosacolonies in growth position (<6 community="" coral="" distance="" fossil="" i="" is="" m="" nbsp="" preserved="" suggest="" that="" this="">in situ6>
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).
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).
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).
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).
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:ReplyDelete
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.