Using the mid-Holocene 'greening' of the Sahara to narrow acceptable ranges on climate model parameters

Hopcroft, Peter O.; Valdes, Paul J.; Ingram, William

doi:10.1002/essoar.10505398.1

Cited by 3 publications

(10 citation statements)

References 66 publications

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“…These include initiation and environmental mixing of convection, biophysical coupling to land surface, and coupling with the large-scale circulation. However, in the standard model version, these processes have potentially been misspecified, probably because present-day observations provide limited constraints for how these coupled processes operate during abrupt events ( 37 ).…”

Section: Discussionmentioning

confidence: 99%

“…In addition to the standard version (STD) evaluated before ( 40 ), we introduce changes to convection (CONV), as described in ref. 37 , and vegetation moisture stress (VMS), both described in more detail in SI Appendix . In CONV, convective entrainment/mixing detrainment is reduced at lower atmospheric levels and increased further aloft.…”

Section: Methodsmentioning

confidence: 99%

“…Alternatively, existing models may already include adequate representations of the key processes, but these are not currently optimized or tuned appropriately, meaning that feedbacks are too strong or too weak, and emergent properties are biased. The latter would imply that paleoclimate is uniquely useful for narrowing suitable parameter ranges in ESMs/GCMs ( 36 , 37 ).…”

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confidence: 99%

“…These configurations are based on the existing release model version (STD), but include paleoclimate-informed changes to atmospheric convection (+CONV) and dynamic vegetation (+VMS). +CONV increase the sensitivity of convection to summer insolation ( 37 ), and +VMS improves the simulation of vegetation cover in semiarid areas, which is required because many vegetation models have incorrect sensitivity ( 32 ). A fourth setup combines these (+CONV+VMS).…”

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confidence: 99%

“…A fourth setup combines these (+CONV+VMS). Conceptually, this approach is unique because the parameters were optimized against both present-day and mid-Holocene climate reconstructions ( 37 ). The +CONV configuration of HadCM3 produces nearly twice the precipitation increase over North Africa relative to STD.…”

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confidence: 99%

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Paleoclimate-conditioning reveals a North Africa land–atmosphere tipping point

Hopcroft

Valdes

2021

Proc. Natl. Acad. Sci. U.S.A.

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While paleoclimate records show that the Earth System is characterized by several different tipping points, their representation within Earth System models (ESMs) remains poorly constrained. This is because historical observations do not encompass variations large enough to provoke such regime changes, and paleoclimate conditions are rarely used to help develop and tune ESMs, which potentially ignores a rich source of information on abrupt climate change. A critical example is the early to mid-Holocene “greening” and subsequent rapid desertification of the Sahara, which most ESMs fail to reproduce, casting doubt on the representation of land–atmosphere coupling and monsoon dynamics. Here, we show that this greening and abrupt termination can be successfully simulated with one ESM after optimizing uncertain model components using both present-day observations and crucially mid-Holocene (6,000 y before present) reconstructions. The optimized model displays abrupt threshold behavior, which shows excellent agreement with long paleoclimate records that were not used in the original optimization. These results suggest that in order to realistically capture climate-system thresholds, ESMs first need to be conditioned with appropriate paleoclimate information.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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confidence: 99%

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Paleoclimate-conditioning reveals a North Africa land–atmosphere tipping point

Hopcroft

Valdes

2021

Proc. Natl. Acad. Sci. U.S.A.

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Northern Hemisphere vegetation change drives a Holocene thermal maximum

et al. 2022

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The Holocene thermal maximum, a period of global warmth evident in early to mid-Holocene proxy reconstructions, is controversial. Most model simulations of the Holocene have not reproduced this warming, leading to a disagreement known as the Holocene Temperature Conundrum. Pollen records document the expansion of vegetation in the early and mid-Holocene African Sahara and Northern Hemisphere mid- and high latitudes, which has been overlooked in previous modeling studies. Here, we use time slice simulations of the Community Earth System Model to assess the impact of Northern Hemisphere vegetation change on Holocene annual mean temperatures. Our simulations indicate that expansion of Northern Hemisphere vegetation 9000 and 6000 years ago warms Earth’s surface by ~0.8° and 0.7°C, respectively, producing a better match with proxy-based reconstructions. Our results suggest that vegetation change is critical for modeling Holocene temperature evolution and highlight its role in driving a mid-Holocene temperature maximum.

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North African Humid Periods over the past 800000 years – Timing, Amplitude and Forcing

Armstrong

Tallavaara

Hopcroft

et al. 2023

Preprint

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The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the position of the African monsoon system. However, most IPCC-class climate models cannot generate enough precipitation to reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we present an 800kyr climate dataset produced using a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800kyr which have good agreement with the timing and amplitude of NAHPs identified in proxy data. Our results confirm that precession determines their pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacials, cooling due to enhanced ice-sheet albedo suppresses the amplitude of the NAHPs during periods of precession minima. Our results highlight the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary.

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Using the mid-Holocene 'greening' of the Sahara to narrow acceptable ranges on climate model parameters

Cited by 3 publications

References 66 publications

Paleoclimate-conditioning reveals a North Africa land–atmosphere tipping point

Paleoclimate-conditioning reveals a North Africa land–atmosphere tipping point

Northern Hemisphere vegetation change drives a Holocene thermal maximum

North African Humid Periods over the past 800000 years – Timing, Amplitude and Forcing

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