New Open Access Paper published.

Here is the Abstract and Introduction. The paper is not paywalled.

Complex spatio-temporal structure of the Holocene Thermal Maximum

• Olivier Cartapanis, 
• Lukas Jonkers, 
• Paola Moffa-Sanchez, 
• Samuel L. Jaccard & 
• Anne de Vernal 

Nature Communications volume 13, Article number: 5662 (2022) Cite this article

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Abstract

Inconsistencies between Holocene climate reconstructions and numerical model simulations question the robustness of climate models and proxy temperature records.

Climate reconstructions suggest an early-middle Holocene Thermal Maximum (HTM) followed by gradual cooling, whereas climate models indicate continuous warming.

This discrepancy either implies seasonal biases in proxy-based climate reconstructions, or that the climate model sensitivity to forcings and feedbacks needs to be reevaluated. Here, we analyze a global database of Holocene paleotemperature records to investigate the spatiotemporal structure of the HTM.

Continental proxy records at mid and high latitudes of the Northern Hemisphere portray a “classic” HTM (8–4 ka). In contrast, marine proxy records from the same latitudes reveal an earlier HTM (11–7ka), while a clear temperature anomaly is missing in the tropics. The results indicate a heterogeneous response to climate forcing and highlight the lack of globally synchronous HTM.

Introduction

Natural climate variability results from multiple forcings and feedbacks with heterogenous spatiotemporal manifestations. Greenhouse gases, volcanic radiative forcing, and solar irradiance apply rather homogeneously across the Earth’s surface, while insolation varies both latitudinally and seasonally. In addition, the climate system response may be amplified or dampened by feedbacks inherent to changes in physiography, albedo, and by variations in oceanic and/or atmospheric circulation that (re)distribute heat across the Earth’s surface. Our understanding of climate processes is limited by the rather short temporal span and heterogenous spatial coverage of instrumental records. Evidence of past climate variability gleaned through the testimony of geological archives thus offers a unique opportunity to contextualize ongoing changes and to assess climate model performance on timescales going beyond the decadal climate variability recorded in the instrumental period.

The temperature at the Earth’s surface responds directly to global radiative forcing and thus provides fundamental insights into the state of the climate system. Over the past decades, quantitative indicators of past temperature (hereafter called “proxies”) based on different types of archives have been used to reconstruct climate variability over a range of timescales. The improvement of both spatial coverage and temporal resolution of temperature proxy records led to the development of regional and global temperature reconstructions, which have allowed the scientific community to highlight the unprecedented nature of anthropogenic climate change across the common era^1,2 and the Holocene^3,4,5,6. Global temperature reconstructions consistently depict a Holocene Thermal Maximum (HTM) typically ranging between 10 and 5 ka^4,5 with a maximal probability centered around 6.45 ka⁴. The HTM was followed by global cooling until the end of the nineteenth century CE, interrupted by rapid and sustained warming characterizing the industrial era towards the present. Yet, the cooling trend inferred from proxy records, often attributed to declining high northern latitude insolation, cannot be resolved in numerical simulations⁷. Indeed, in climate models, the simulated global mean temperature is predominantly driven by the ice-sheet extent and atmospheric greenhouse gas concentrations, which in synergy impose continuous warming over the course of the Holocene⁷.

This discrepancy between proxy data and model simulations, commonly referred to as “The Holocene Temperature Conundrum”⁷, casts doubt on the conceptual framework underlying temperature proxy interpretation and on climate model skill. For instance, it has been suggested that temperature reconstructions may be seasonally biased^7,8 and/or that the global mean value is skewed because of the overrepresentation of northern North Atlantic sea-surface temperature (SST) records^5,6,7. However, model-data inconsistencies may equally well result from geographically divergent trends due to sea-ice dynamics⁹, polar amplification¹⁰, insufficient model resolution¹¹, and boundary conditions used in numerical simulations¹². Although the HTM has been intensively studied from a global perspective^3,4,5,6,7, its spatio-temporal characteristics have received relatively little attention, even though the local and regional trends differ markedly from the globally averaged reconstructions^3,13.

In this study, we seek to document the spatiotemporal expression of the HTM in the marine and continental realms to shed light on the forcings and feedbacks underpinning the evolution of Holocene climate⁹.

Here is the press release from EurekAlert!

An international team of researchers from Germany, the United Kingdom, Switzerland, Canada and France reveal the complexity of temperature trends over the past 12,000 years.

Peer-Reviewed Publication

MARUM – CENTER FOR MARINE ENVIRONMENTAL SCIENCES, UNIVERSITY OF BREMEN

In this new study, scientists used the largest available database of past temperature reconstructions extending back 12,000 years to carefully investigate the geographic pattern of temperature change during the Holocene. Olivier Cartapanis and colleagues find that, contrary to previously thought, there is no globally synchronous warm period during the Holocene. Instead, the warmest temperatures are found at different times not only in different regions but also between the ocean and on land. This questions how meaningful comparisons of the global mean temperature between reconstructions and models actually are.

According to the lead author Olivier Cartapanis, “the results challenge the paradigm of a Holocene Thermal Maximum occurring at the same time worldwide”. And, while the warmest temperature was reached between 4,000 and 8,000 years ago in western Europe and northern America, the surface ocean temperature cooled since about 10,000 years ago at mid-high latitudes and remained stable in the tropics. The regional variability in the timing of maximum temperature suggests that high latitude insolation and ice extent played major roles in driving climate changes throughout the Holocene. 

Lukas Jonkers, co-author of the study and researcher at the MARUM – Center for Marine Environmental Sciences in Bremen, Germany, says “Because ecosystems and people do not experience the mean temperature of the Earth, but are affected by regional and local changes in climate, models need to get the spatial and temporal patterns of climate change right in order to guide policy makers”. Thus, the new work by Cartapanis and colleagues presents a clear target for climate models as the ability of climate model to reproduce Holocene climate variations in space and time, will increase confidence in their regional projections of future climate change.

MARUM produces fundamental scientific knowledge about the role of the ocean and the ocean floor in the total Earth system. The dynamics of the ocean and the ocean floor significantly impact the entire Earth system through the interaction of geological, physical, biological and chemical processes. These influence both the climate and the global carbon cycle, and create unique biological systems. MARUM is committed to fundamental and unbiased research in the interests of society and the marine environment, and in accordance with the Sustainable Development Goals of the United Nations. It publishes its quality-assured scientific data and makes it publicly available. MARUM informs the public about new discoveries in the marine environment and provides practical knowledge through its dialogue with society. MARUM cooperates with commercial and industrial partners in accordance with its goal of protecting the marine environment.

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JOURNAL

Nature Communications

DOI

10.1038/s41467-022-33362-1 

ARTICLE TITLE

Complex spatio-temporal structure of the Holocene Thermal Maximum

ARTICLE PUBLICATION DATE

3-Oct-2022

From EurekAlert!

via Watts Up With That?

October 5, 2022

The Last 12,000 Years Show a More Complex Climate History Than Previously Thought