Holocene climates of the Iberian Peninsula: pollen-based reconstructions of changes in the west-east gradient of temperature and moisture

The Iberian Peninsula is characterised by a steep west-east moisture gradient today, reflecting the dominance of maritime influences along the Atlantic coast and more Mediterranean-type climate further east. Holocene pollen records from the Peninsula suggest that this gradient was less steep during the early to mid-Holocene, possibly reflecting the impact of orbital changes on circulation and thus regional patterns in climate. Here we use 7121 pollen samples from 117 sites covering part or all of the last 12,000 years to reconstruct changes in seasonal temperature and in moisture across the Iberian Peninsula quantitatively. We show that there is an increasing trend in winter temperature at a regional scale, consistent with known changes in winter insolation. However, summer temperatures do not show the decreasing trend through the Holocene that would be expected if they were a direct response to insolation forcing. We show that summer temperature is strongly correlated with plant-available moisture (α), as measured by the ratio of actual evapotranspiration to equilibrium evapotranspiration, which declines through the Holocene. The reconstructions also confirm that the west-east gradient in moisture was considerably less steep than today during the early to mid-Holocene, indicating that atmospheric circulation changes (possibly driven by orbital changes) have been important determinants of the Holocene climate of the region.

Variations in seasonal solar insolation are associated with a history of suicide attempts in bipolar I disorder

Background Bipolar disorder is associated with circadian disruption and a high risk of suicidal behavior. In a previous exploratory study of patients with bipolar I disorder, we found that a history of suicide attempts was associated with differences between winter and summer levels of solar insolation. The purpose of this study was to confirm this finding using international data from 42% more collection sites and 25% more countries. Methods Data analyzed were from 71 prior and new collection sites in 40 countries at a wide range of latitudes. The analysis included 4876 patients with bipolar I disorder, 45% more data than previously analyzed. Of the patients, 1496 (30.7%) had a history of suicide attempt. Solar insolation data, the amount of the sun’s electromagnetic energy striking the surface of the earth, was obtained for each onset location (479 locations in 64 countries). Results This analysis confirmed the results of the exploratory study with the same best model and slightly better statistical significance. There was a significant inverse association between a history of suicide attempts and the ratio of mean winter insolation to mean summer insolation (mean winter insolation/mean summer insolation). This ratio is largest near the equator which has little change in solar insolation over the year, and smallest near the poles where the winter insolation is very small compared to the summer insolation. Other variables in the model associated with an increased risk of suicide attempts were a history of alcohol or substance abuse, female gender, and younger birth cohort. The winter/summer insolation ratio was also replaced with the ratio of minimum mean monthly insolation to the maximum mean monthly insolation to accommodate insolation patterns in the tropics, and nearly identical results were found. All estimated coefficients were significant at p < 0.01. Conclusion A large change in solar insolation, both between winter and summer and between the minimum and maximum monthly values, may increase the risk of suicide attempts in bipolar I disorder. With frequent circadian rhythm dysfunction and suicidal behavior in bipolar disorder, greater understanding of the optimal roles of daylight and electric lighting in circadian entrainment is needed.

Milankovitch, the father of paleoclimate modeling

The history of the long-term variations in the astronomical elements used in paleoclimate research shows that, contrary to what might be thought, Milutin Milankovitch is not the father of the astronomical theory but he is definitely the father of paleoclimate modeling. He did not calculate these long-term variations himself but used them extensively for calculating the “secular march” of incoming solar radiation. He advanced our understanding of Quaternary climate variations by two important and original contributions fully described in his Canon of insolation. These are the definition and use of caloric seasons and the concept of the “mathematical climate”. How his mathematical model allowed him to give the caloric summer and winter insolation a climatological meaning is illustrated.

Milankovitch, the father of paleoclimate modelling

The origin of the long-term variations of the astronomical elements used by Milankovitch are first described, followed by the value of the astronomical periods. The detailed calculations by Milankovitch of the incoming solar radiation during the astronomical and caloric half-years are summarized, stressing the originality of the caloric ones. The second original contribution of Milankovitch to paleoclimate research was without any doubt his mathematical climate. How this model allowed him to give the caloric summer and winter insolation a climatological meaning is illustrated.

Moisture evolution in North Xinjiang (northwest China) during the last 8000 years linked to the westerlies’ winter half-year precipitation

Winter half-year precipitation dominates variations in hydroclimatic conditions in North Xinjiang, but few researchers have focused on this very important aspect of the Holocene climate. Here we report multiproxy evidence of Holocene hydroclimate changes from the sediments of Wulungu Lake in North Xinjiang. The site is a closed terminal lake fed mainly by meltwater from snow and ice, and today the area is climatically dominated by the westerlies. Grain-size end-member analysis implies an important mode of variation that indicates a gradually increasing moisture trend, with superimposed centennial-scale variations, since 8000 cal yr BP. From 8000 to 5350 cal yr BP, a permanent lake developed, and the lake level gradually rose. Between 5350 and 500 cal yr BP, the moisture status increased rapidly, with the wettest climate occurring between 3200 and 500 cal yr BP. After 500 cal yr BP, the lake level fell. The trend of increasing Holocene wetness indicates a rising winter precipitation in North Xinjiang during the Holocene. This was due to an increase in upwind vapor concentrations caused by increased evaporation and strength of the westerlies, which were determined by the increasing boreal winter insolation and its latitudinal gradient.

Pacific warm pool subsurface heat sequestration modulated Walker circulation and ENSO activity during the Holocene

Dynamics driving the El Niño–Southern Oscillation (ENSO) over longer-than-interannual time scales are poorly understood. Here, we compile thermocline temperature records of the Indo-Pacific warm pool over the past 25,000 years, which reveal a major warming in the Early Holocene and a secondary warming in the Middle Holocene. We suggest that the first thermocline warming corresponds to heat transport of southern Pacific shallow overturning circulation driven by June (austral winter) insolation maximum. The second thermocline warming follows equatorial September insolation maximum, which may have caused a steeper west-east upper-ocean thermal gradient and an intensified Walker circulation in the equatorial Pacific. We propose that the warm pool thermocline warming ultimately reduced the interannual ENSO activity in the Early to Middle Holocene. Thus, a substantially increased oceanic heat content of the warm pool, acting as a negative feedback for ENSO in the past, may play its role in the ongoing global warming.

High sensitivity of Bering Sea winter sea ice to winter insolation and carbon dioxide over the last 5500 years

Anomalously low winter sea ice extent and early retreat in CE 2018 and 2019 challenge previous notions that winter sea ice in the Bering Sea has been stable over the instrumental record, although long-term records remain limited. Here, we use a record of peat cellulose oxygen isotopes from St. Matthew Island along with isotope-enabled general circulation model (IsoGSM) simulations to generate a 5500-year record of Bering Sea winter sea ice extent. Results show that over the last 5500 years, sea ice in the Bering Sea decreased in response to increasing winter insolation and atmospheric CO2, suggesting that the North Pacific is highly sensitive to small changes in radiative forcing. We find that CE 2018 sea ice conditions were the lowest of the last 5500 years, and results suggest that sea ice loss may lag changes in CO2 concentrations by several decades.

High sensitivity of Bering Sea winter sea ice to winter insolation and carbon dioxide over the last 5,500 years

&lt;p&gt;Anomalously low winter sea-ice extent and early retreat in CE 2018 and 2019 challenges previous notions of relatively stable winter sea ice in the Bering Sea over the instrumental record, but long-term sea-ice records from sediment proxies remain limited.&amp;#160; Here we use a record of peat-cellulose oxygen isotopes from St. Matthew Island, along with isotope-enabled general circulation model (IsoGSM) simulations to generate a 5,500-year record of Bering Sea winter sea-ice extent.&amp;#160; Results show that over the instrumental period (CE 1979-2018), oxygen isotope variability is largest over the late winter to spring (February, March, April, May [FMAM]) and highly correlated (-0.77, p&lt;0.00001) with maximum winter sea-ice extent, months in which Bering sea ice reaches its winter maximum and then rapidly diminishes. We find that over the last 5,500 years, sea ice in the Bering Sea decreased in response to increasing winter insolation and atmospheric CO&lt;sub&gt;2&lt;/sub&gt;, and on shorter, centennial timescales, small (&lt;10 ppmv) &amp;#160;perturbations in atmospheric CO&lt;sub&gt;2&lt;/sub&gt;, suggesting that the North Pacific is highly sensitive to small (&lt;3 W m&lt;sup&gt;-2&lt;/sup&gt;) changes in radiative forcing. However, we find that reconstructed sea-ice loss lags CO&lt;sub&gt;2&lt;/sub&gt; concentrations by ~120 years, indicating that the extremely anomalous recent conditions are a legacy of the early 20&lt;sup&gt;th&lt;/sup&gt; century and that even with a complete cessation of greenhouse gas emissions today. As a consequence, the Bering Sea could lose all winter sea ice by mid-century, which it may not recover for millennia.&lt;/p&gt;

Holocene organic geochemical record from the Western Indus continental shelf (northern Arabian Sea)

Primary production on the Western Indus continental shelf has been linked to the large quantities of nutrients delivered to the shelf by the Indus River. Multiple geochemical tracers and biomarker records, including stable isotopes (δ13C and δ15N), total organic carbon (TOC), total nitrogen (TN), molar carbon/nitrogen (C/N) ratio, the branched and isoprenoid tetraether (BIT) index, and glycerol dialkyl glycerol tetraether (GDGT), have been analyzed from the Indus-23AP sediment core recovered from the northern Arabian Sea. Our records show evidence of a mixture of marine and terrestrially derived organic matter (OM) during the last 14,000 years, as indicated by the C/N ratio, δ13C, δ15N, and the BIT index. The three sterol biomarkers (brassicasterol, dinosterol, and cholesterol) show concurrent enrichments during the last 3 millennia reflecting increased phytoplankton abundance because of increased Indus river discharge of nutrients during the summer monsoon. GDGT crenarchaeol enrichment is related to the BIT index. The TEX86-derived sea surface temperature (SST) record is shifted toward the summer season because Crenarchaeota are more abundant and active during periods of high primary production. SSTs indicate a long-term warming trend during the Holocene related to increasing winter insolation in the low latitudes northern Hemisphere.

African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments

A climate/vegetation model simulates episodic wetter and drier periods at the 21,000-y precession period in eastern North Africa, the Arabian Peninsula, and the Levant over the past 140,000 y. Large orbitally forced wet/dry extremes occur during interglacial time, ∼130 to 80 ka, and conditions between these two extremes prevail during glacial time, ∼70 to 15 ka. Orbital precession causes high seasonality in Northern Hemisphere (NH) insolation at ∼125, 105, and 83 ka, with stronger and northward extended summer monsoon rains in North Africa and the Arabian Peninsula and increased winter rains in the Mediterranean Basin. The combined effects of these two seasonally distinct rainfall regimes increase vegetation and narrow the width of the Saharan–Arabian desert and semidesert zones. During the opposite phase of the precession cycle (∼115, 95, and 73 ka), NH seasonality is low, and decreased summer insolation and increased winter insolation cause monsoon and storm track rains to decrease and the width of the desert zone to increase. During glacial time (∼70 to 15 ka), forcing from large ice sheets and lowered greenhouse gas concentrations combine to increase winter Mediterranean storm track precipitation; the southward retreat of the northern limit of summer monsoon rains is relatively small, thereby limiting the expansion of deserts. The lowered greenhouse gas concentrations cause the near-equatorial zone to cool and reduce convection, causing drier climate with reduced forest cover. At most locations and times, the simulations agree with environmental observations. These changing regional patterns of climate/vegetation could have influenced the dispersal of early humans through expansions and contractions of well-watered corridors.