Holocene climatic optimum

Summary

The Holocene Climate Optimum (HCO) was a warm period that occurred in the interval roughly 9,500 to 5,500 years ago BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period. The warm period was followed by a gradual decline, of about 0.1 to 0.3 °C per millennium, until about two centuries ago. However, on a sub-millennial scale, there were regional warm periods superimposed on this decline.

For other temperature fluctuations, see temperature record.
For other past climate fluctuation, see paleoclimatology.
For the pollen zone and Blytt–Sernander period, associated with the climate optimum, see Atlantic (period).

Global effects A study in 2020 estimated that the average global temperature during the warmest 200 year period of the HCO, around 6,500 years ago, was around 0.7 °C war

Official source

https://en.wikipedia.org/wiki/Holocene_climatic_optimum

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The atmospheric circulation is a key area of uncertainty in climate model simulations of future climate change, especially in mid-latitude regions such as Europe where atmospheric dynamics have a significant role in climate variability. It has been proposed that the mid-Holocene was characterized in Europe by a stronger westerly circulation in winter comparable with a more positive AO/NAO, and a weaker westerly circulation in summer caused by anticyclonic blocking near Scandinavia. Model simulations indicate at best only a weakly positive AO/NAO, whilst changes in summer atmospheric circulation have not been widely investigated. Here we use a new pollen-based reconstruction of European mid-Holocene climate to investigate the role of atmospheric circulation in explaining the spatial pattern of seasonal temperature and precipitation anomalies. We find that the footprint of the anomalies is entirely consistent with those from modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive AO/NAO) and a weak westerly circulation in summer associated with anti-cyclonic blocking (positive SCAND). We find little agreement between the reconstructed anomalies and those from 14 GCMs that performed mid-Holocene experiments as part of the PMIP3/CMIP5 project, which show a much greater sensitivity to top-of-the-atmosphere changes in solar insolation. Our findings are consistent with data-model comparisons on contemporary timescales that indicate that models underestimate the role of atmospheric circulation in recent climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.

Copernicus Gesellschaft Mbh2014

The mid-Holocene vegetation of the Mediterranean region and southern Europe, and comparison with the present day

Pamela Collins, Basil Andrew Stansfield Davis, Jed Oliver Kaplan

Aim To contribute to the intense debate surrounding the relative influence of climate and humans on Mediterranean-region land cover over the past 6000 years, we assess the Holocene biogeography and vegetation history of southern Europe by means of an extensive pollen record dataset. Location The Mediterranean biogeographical zone and neighbouring parts of Iberia, the Alps and Anatolia, between 30 degrees N, 48 degrees N, 10 degrees W and 45 degrees E. Methods We compiled a southern European pollen record dataset using available pollen databases (124 sites) and other sources (74 sites), with improved spatial coverage and dating control compared with earlier studies. We used only those sites that had pollen data for both 0 ka and 6 ka. We reconstructed mid-Holocene and present-day biomes, arboreal pollen percentages and distribution and relative abundance of 11 key woody taxa, with anomaly maps. Results Northern temperate forest biomes extended further south at the mid-Holocene than at present, but not as far as earlier studies suggested. Sclerophyllous vegetation occurred along the Mediterranean coast throughout the region at 6 ka. Arboreal pollen percentages were up to 50% higher than at present. At 6 ka, Olea, Fagus and Juniperus had smaller distributions and/or abundances; Abies, Cedrus and both deciduous and evergreen Quercus had larger distributions and/or abundances; Phillyrea, Pistacia and Cistus showed minimal difference; and Pinus showed a cosmopolitan distribution with variable abundance. Main conclusions Temporal difference analysis is more meaningful when only sites containing samples for all time slices are analysed. During the mid-Holocene, southern Europe was more heavily forested with temperate vegetation than it is at present, but drought-tolerant xeric vegetation was still widespread along the southern margins of the region. Although human land use may have caused the degradation of land between the mid-Holocene and the present, the mere presence of xeric vegetation in the Mediterranean region does not require human impact. This challenges the commonly held belief that modern Mediterranean vegetation represents a degraded state.

Wiley-Blackwell2012

The temperature of Europe during the Holocene reconstructed from pollen data

Basil Andrew Stansfield Davis

We present the first area-average time series reconstructions of warmest month, coldest month and mean annual surface air temperatures across Europe during the last 12,000 years. These series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four-dimensional gridding procedure. This approach combines three-dimensional spatial gridding with a fourth dimension represented by time, allowing data from irregular time series to be ‘focussed’ onto a regular time step. We provide six regional reconstructed temperature time series as well as summary time series for the whole of Europe. The results suggest major spatial and seasonal differences in Holocene temperature trends within a remarkably balanced regional and annual energy budget. The traditional mid-Holocene thermal maximum is observed only over Northern Europe and principally during the summer. This warming was balanced by a mid-Holocene cooling over Southern Europe, whilst Central Europe occupied an intermediary position. Changes in annual mean temperatures for Europe as a whole suggest an almost linear increase in thermal budget up to 7800 BP, followed by stable conditions for the remainder of the Holocene. This early Holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west, which have continued to rise at a progressively decreasing rate up to the present day.

2003

The mid-Holocene vegetation of the Mediterranean region and southern Europe, and comparison with the present day

Pamela Collins, Basil Andrew Stansfield Davis, Jed Oliver Kaplan

Wiley-Blackwell2012