The Marine Isotope Stage 19c (MIS19c) interglaciation is regarded as the best orbital analog to the Holocene. The close of MIS19c (~777,000 years ago) thus serves as a proxy for a contemporary climate system unaffected by humans. Our global climate model simulation driven by orbital parameters and observed greenhouse gas concentrations at the end of MIS19c is 1.3 K colder than the reference pre-industrial climate of the late Holocene (year 1850). Much stronger cooling occurs in the Arctic, where sea ice and year-round snow cover expand considerably. Inferred regions of glaciation develop across northeastern Siberia, northwestern North America, and the Canadian Archipelago. These locations are consistent with evidence from past glacial inceptions and are favored by atmospheric circulation changes that reduce ablation of snow cover and increase accumulation of snowfall. Particularly large buildups of snow depth coincide with presumed glacial nucleation sites, including Baffin Island and the northeast Canadian Archipelago. These findings suggest that present-day climate would be susceptible to glacial inception if greenhouse gas concentrations were as low as they were at the end of MIS 19c.
The past 2,000 years provide a critical context for understanding twentieth century climate change. Due to the Past Global Changes 2k initiative, an extensive proxy database provides opportunities to assess regional climate change that have stronger ecological and societal implications than global‐mean temperature changes alone. However, the various sources of paleoclimate reconstruction poses serious challenges to scientifically and statistically sound inference within a unified framework. Here we show results from the first transient simulation for the past 2,000 years with the Community Earth System Model, called past2k, and use data‐model comparison to refine the interpretation of the proxy data. Our results indicate that the Atlantic Arctic was cooling at a faster rate than the Pacific Arctic over the past two millennia, in both proxy data and models. The model shows that this cooling pattern was dynamically supported by both a weakening of North Atlantic subpolar gyre and a stronger Aleutian Low.
more » « less- NSF-PAR ID:
- 10460008
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 45
- Issue:
- 22
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX86and UK′37proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasing
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Abstract The Fram Strait plays a crucial role in regulating the heat and sea‐ice dynamics in the Arctic. In response to the ongoing global warming, the marine biota of this Arctic gateway is experiencing significant changes with increasing advection of Atlantic species. The footprint of this ‘Atlantification’ has been identified in isolated observations across the plankton community, but a systematic, multi‐decadal perspective on how regional climate change facilitates the invasion of Atlantic species and affects the ecology of the resident species is lacking. Here we evaluate a series of 51 depth‐resolved plankton profiles collected in the Fram Strait during seven surveys between 1985 and 2015, using planktonic foraminifera as a proxy for changes in both the pelagic community composition and species vertical habitat depth. The time series reveals a progressive shift towards more Atlantic species, occurring independently of changes in local environmental conditions. We conclude that this trend is reflecting higher production of the Atlantic species in the Nordic Seas, from where they are advected into the Fram Strait. At the same time, we observe the ongoing extensive sea‐ice export from the Arctic and associated cooling‐induced decline in density and habitat shoaling of the subpolar
Turborotalita quinqueloba , whereas the residentNeogloboquadrina pachyderma persists. As a result, the planktonic foraminiferal community and vertical structure in the Fram Strait shift to a new state, driven by both remote forcing of the Atlantic invaders and local climatic changes acting on the resident species. The strong summer export of Arctic sea ice has so far buffered larger plankton transformation. We predict that if the sea‐ice export will decrease, the Arctic gateway will experience rapid restructuring of the pelagic community, even in the absence of further warming. Such a large change in the gateway region will likely propagate into the Arctic proper. -
Uncertainty about the influence of anthropogenic radiative forcing on the position and strength of convective rainfall in the Intertropical Convergence Zone (ITCZ) inhibits our ability to project future tropical hydroclimate change in a warmer world. Paleoclimatic and modeling data inform on the timescales and mechanisms of ITCZ variability; yet a comprehensive, long-term perspective remains elusive. Here, we quantify the evolution of neotropical hydroclimate over the preindustrial past millennium (850 to 1850 CE) using a synthesis of 48 paleo-records, accounting for uncertainties in paleo-archive age models. We show that an interhemispheric pattern of precipitation antiphasing occurred on multicentury timescales in response to changes in natural radiative forcing. The conventionally defined “Little Ice Age” (1450 to 1850 CE) was marked by a clear shift toward wetter conditions in the southern neotropics and a less distinct and spatiotemporally complex transition toward drier conditions in the northern neotropics. This pattern of hydroclimatic change is consistent with results from climate model simulations indicating that a relative cooling of the Northern Hemisphere caused a southward shift in the thermal equator across the Atlantic basin and a southerly displacement of the ITCZ in the tropical Americas, with volcanic forcing as the principal driver. These findings are at odds with proxy-based reconstructions of ITCZ behavior in the western Pacific basin, where changes in ITCZ width and intensity, rather than mean position, appear to have driven hydroclimate transitions over the last millennium. This reinforces the idea that ITCZ responses to external forcing are region specific, complicating projections of the tropical precipitation response to global warming.more » « less
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Abstract. Since the middle Miocene (15 Ma, million years ago), the Earth's climate has undergone a long-term cooling trend, characterised by a reduction in ocean temperatures of up to 7–8 ∘C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (δ13C) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE (“muffin” release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic δ13C patterns. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic δ13C patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 ‰ decline occurring in atmospheric (and ca. 1 ‰ ocean surface) δ13C that could be used to inform future δ13C-based proxy reconstructions.more » « less
