The tropical lapse rate steepened during the Last Glacial Maximum

Loomis, S. E.; Russell, James M.; Verschuren, Dirk; Morrill, C.; Cort, Gijs De; Damsté, Jaap S. Sinninghe; Olago, Daniel; Eggermont, Hilde; Street-Perrott, F. Alayne; Kelly, M. A.

doi:10.1126/sciadv.1600815

Cited by 105 publications

(135 citation statements)

References 58 publications

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“…During the Late Glacial (pre‐LGM and LGM) a downward shift of the montane forest, characterized by the development a sub‐alpine vegetation associated with components of afro‐alpine grassland at our site is consistent with a colder climate than generally recorded in tropical East Africa today (Gasse, Chalié, Vincens, Williams, & Williamson, ; Loomis et al., ). On the other hand, hydrological changes reveal a more complex pattern characterized by generally dry conditions interrupted by more humid periods prior to the LGM (Tierney, Russell, & Huang, ).…”

Section: Discussionsupporting

confidence: 80%

Late Quaternary ecotone change between sub‐alpine and montane forest zone on the leeward northern slope of Mt. Kilimanjaro

Montade¹,

Schüler²,

Hemp

et al. 2018

J Vegetation Science

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Questions How did the montane forest of the leeward northern slopes on Mt. Kilimanjaro respond to climate fluctuation during the late Quaternary? Location Mt. Kilimanjaro, Tanzania. Methods Pollen, phytolith and macro‐charcoal analyses from a palaeosol. Results Palaeoecological analyses indicate that vegetation was mainly characterized by Ericaceae with Poaceae dominated by cold‐adapted C3 highland grasses during the pre‐LGM and the LGM. From the last deglaciation onwards, the previous assemblage was progressively replaced by Podocarpus forest associated with Clematis and later the expansion of ferns, then Araliaceae, Hagenia abyssinica, Ilex mitis and Olea started in the early Holocene. Conclusion Vegetation dynamics reveal that open sub‐alpine vegetation associated with some components of afro‐alpine grassland was located at a lower elevation than today, related to cold and dry conditions during the Late Glacial. In contrast to Mt. Kenya, our results show an absence of C4 grass expansion on the northern slope of Mt. Kilimanjaro during the LGM. From the last deglaciation to the early Holocene, under temperature and precipitation increase, the vegetation zones moved upward, as indicated by the development of upper montane forest at the expense of the sub‐alpine vegetation. Although we confirm the altitudinal movements of vegetation zones generally observed from the Late Glacial to the Holocene in East African mountains, our results also reveal the importance of local environmental conditions in mountain vegetation response.

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Section: Discussionsupporting

confidence: 80%

Late Quaternary ecotone change between sub‐alpine and montane forest zone on the leeward northern slope of Mt. Kilimanjaro

Montade¹,

Schüler²,

Hemp

et al. 2018

J Vegetation Science

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“…LGM simulations typically feature a global annual cooling of 4 to 6 • C relative to pre-industrial; e.g., Braconnot et al, 2007) -Loomis et al (2017 showed that the tropical atmospheric lapse rate may have increased from about 5.8 • C km −1 in the modern climate, to 6.7 • C km −1 at the LGM. The elevation difference in the interior of the Laurentide ice sheet is around 200 m between the T85 and T21 cases (Fig.…”

Section: Discussionmentioning

confidence: 99%

The influence of atmospheric grid resolution in a climate model-forced ice sheet simulation

Löfverström

Liakka

2018

The Cryosphere

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Abstract. Coupled climate-ice sheet simulations have been growing in popularity in recent years. Experiments of this type are however challenging as ice sheets evolve over multimillennial timescales, which is beyond the practical integration limit of most Earth system models. A common method to increase model throughput is to trade resolution for computational efficiency (compromise accuracy for speed). Here we analyze how the resolution of an atmospheric general circulation model (AGCM) influences the simulation quality in a stand-alone ice sheet model. Four identical AGCM simulations of the Last Glacial Maximum (LGM) were run at different horizontal resolutions: T85 (1.4 • ), T42 (2.8 • ), T31 (3.8 • ), and T21 (5.6 • ). These simulations were subsequently used as forcing of an ice sheet model. While the T85 climate forcing reproduces the LGM ice sheets to a high accuracy, the intermediate resolution cases (T42 and T31) fail to build the Eurasian ice sheet. The T21 case fails in both Eurasia and North America. Sensitivity experiments using different surface mass balance parameterizations improve the simulations of the Eurasian ice sheet in the T42 case, but the compromise is a substantial ice buildup in Siberia. The T31 and T21 cases do not improve in the same way in Eurasia, though the latter simulates the continent-wide Laurentide ice sheet in North America. The difficulty to reproduce the LGM ice sheets in the T21 case is in broad agreement with previous studies using low-resolution atmospheric models, and is caused by a substantial deterioration of the model climate between the T31 and T21 resolutions. It is speculated that this deficiency may demonstrate a fundamental problem with using low-resolution atmospheric models in these types of experiments.

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“…If a direct transfer function relating temperature to GDGTs is available for a given site or region, it can be specified as the parameter β in function lake_sensor_gdgt.py. For the purposes of demonstration, a number of potential calibration values for β are listed based on values used by Powers et al () and Loomis et al ().The temperature‐ T E X 86 relationship is inverted to yield a forward estimate of the proxy measurement:

TE X_{86} = β * LST,

TE X_{86} = false(LST + 14.0 false) false/ 55.0, false[Powers et al ., 2011 false]

TE X_{86} = false(LST + 10.92 false) false/ 54.88, false[Loomis et al ., 2017 false]

where L S T is lake surface temperature, and β is the proxy/temperature transfer function or calibration set by the user.Recent calibration studies have developed calibrations for brGDGTs in lakes with mean annual temperatures based upon a version of the MBT index which use only 5‐methyl isomers (

MB T_{5ME}^{'}

). These compounds in particular have been shown to correlate with MAAT (e.g., De Jonge et al, ; Russell et al, ).…”

Section: Proxy System Modeling For Lacustrine Archives Of Past Climatementioning

confidence: 99%

Section: Proxy System Modeling For Lacustrine Archives Of Past Climatementioning

confidence: 99%

“…However, until recently, very few temperature reconstructions from tropical continents were available to assess model performance (Bartlein et al, ; Bonnefille et al, ; Chalie, ; Mark et al, ; Waelbroeck et al, ). The recent application of GDGT‐based analyses in tropical and subtropical lacustrine archives has changed that, adding much‐needed data from the tropics to global databases of temperature changes from LGM to present (Loomis et al, , ; Morrissey et al, ; Powers et al, ; Tierney et al, ). Complicating these data‐model comparisons, however, is the fact that GDGT‐based temperature proxies respond to lake (and not air) temperatures, creating uncertainty in the drivers of observed temperature changes and our methodology for assessing records alongside GCMs.…”

Section: Applications: Model Validation and Lgm Temperatures In Africmentioning

confidence: 99%

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PRYSM v2.0: A Proxy System Model for Lacustrine Archives

Dee

Russell

Morrill

et al. 2018

Paleoceanog and Paleoclimatol

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Reconstructions of temperature and hydrology from lake sedimentary archives have made fundamental contributions to our understanding of past, present, and future climate and help evaluate general circulation models (GCMs). However, because paleoclimate observations are an indirect (proxy) constraint on climatic variables, confounding effects of proxy processes complicate interpretations of these archives. To circumvent these uncertainties inherent to paleoclimate data‐model comparison, proxy system models (PSMs) provide transfer functions between climate variables and the proxy. We here present a new PSM for lacustrine sedimentary archives. The model simulates lake energy and water balance, sensors including leaf wax δD and carbonate δ18O, bioturbation, and compaction of sediment to lend insight toward how these processes affect and potentially obfuscate the original climate signal. The final product integrates existing and new models to yield a comprehensive, modular, adaptable, and publicly available PSM for lake systems. Highlighting applications of the PSM, we forward model lake variables with GCM simulations of the last glacial maximum and the modern. The simulations are evaluated with a focus on sensitivity of lake surface temperature and mixing to climate forcing, using Lakes Tanganyika and Malawi as case studies. The PSM highlights the importance of mixing on interpretations of air temperature reconstructions from lake archives and demonstrates how changes in mixing depth alone may induce nonstationarity between in situ lake and air temperatures. By placing GCM output in the same reference frame as lake paleoclimate archives, we aim to improve interpretations of past changes in terrestrial temperatures and water cycling.

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The tropical lapse rate steepened during the Last Glacial Maximum

Abstract: A new temperature record from East Africa demonstrates that the tropical lapse rate steepened during the last ice age.

Cited by 105 publications

References 58 publications

Late Quaternary ecotone change between sub‐alpine and montane forest zone on the leeward northern slope of Mt. Kilimanjaro

Late Quaternary ecotone change between sub‐alpine and montane forest zone on the leeward northern slope of Mt. Kilimanjaro

The influence of atmospheric grid resolution in a climate model-forced ice sheet simulation

PRYSM v2.0: A Proxy System Model for Lacustrine Archives

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