Climate, volcanism and human impact on Iceland’s landscape during the last two millennia.

2020 ◽  
Author(s):  
Áslaug Geirsdóttir ◽  
David Harning ◽  
John Andrews ◽  
Gifford Miller ◽  
Yafang Zhong ◽  
...  
Keyword(s):  
Soil Erosion ◽  
Sea Ice ◽  
Human Impact ◽  
North Atlantic ◽  
Solar Irradiance ◽  
Little Ice Age ◽  
Regional Climate ◽  
Ice Age ◽  
Proxy Records ◽  
First Millennium

<p>Biogeochemical proxy records from Icelandic lake sediment reflect large-scale shifts in North Atlantic Holocene climate and highlight the impact that North Atlantic Ocean- and atmospheric circulation has on Iceland’s local climate. Following Early Holocene warmth, millennial-scale cooling has been modulated by centennial-scale climate change, culminating in the transition to the Little Ice Age (ca. 1300-1900 CE). Although the long-term cooling trend is presumably driven by variations in Earth’s orbit and the concomitant decline in Northern Hemisphere summer insolation, the centennial-scale variability has been linked to variations in solar irradiance, the strength of the Atlantic Meridional Overturning Circulation, volcanism coupled with sea ice/ocean related feedbacks and internal modes of atmospheric variability. One manifestation of these regional climate changes on Iceland is the intensification of soil erosion, resulting in the degradation of its eco-systems and landscape. In recent millennia, persistent and severe soil erosion has also been linked to human impact on the environment following the settlement ~874 CE, rapid population growth and the poorly consolidated nature of tephra dominated soils. However, against the argument that the onset of severe soil erosion coincided with human settlement are composite landscape stability proxies extracted from the high-resolution, precisely-dated lake sediment cores. These data suggest event-dominated landscape instability and soil erosion began in the Middle to Late Holocene with an intensification of landscape instability around ~500 CE, several centuries before the acknowledged settlement of Iceland, after which soil erosion continue to increase. In order to statistically identify abrupt and persistent changes within our landscape stability proxy records, we performed an analysis that targets mean regime shifts in individual time series. The first clear regime shift occured around ~500 CE, with a second large shift ~1200 CE. In order to provide a causal explanation for these regime shifts, we looked to a new 2 ka fully coupled climate transient simulation using CESM1, with forcing data from PMIP4, including insolation, volcanic aerosols, land-cover, and GHG. The CESM results show a ~0.5°C reduction in summer temperature in the first millennium CE, consistent with increased landscape instability and soil erosion in Iceland.  A second phase of persistent summer cooling in the model occurs after 1150 CE, with stronger cooling after 1450 CE, reaching a minimum shortly after 1850 CE, ~1°C lower than at the start of the experiment. Orbitally driven declines in summer insolation appear to be the dominant forcing early in the first millennium CE, with volcanism and solar irradiance reductions increasingly important after 500 CE and in the second millennium CE, but positive feedbacks from sea ice and the overturning circulation are necessary to explain the magnitude of peak LIA cooling when soil erosion is at its greatest in Iceland. Collectively, our initial results suggest that natural variations in regional climate and volcanism are likely responsible for soil erosion prior to human impact, with intensification of these processes following settlement particularly during the cooling associated with the Little Ice Age.</p>

scholarly journals A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age

2013 ◽  
Vol 9 (3) ◽  
pp. 1321-1330 ◽  
Author(s):  
C. F. Schleussner ◽  
G. Feulner
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Little Ice Age ◽  
Regime Shift ◽  
Regional Climate ◽  
Volcanic Eruptions ◽  
Meridional Overturning Circulation ◽  
Ice Age ◽  
Last Millennium ◽  
Subpolar Gyre

Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly succeeded by the Little Ice Age are of exceptional importance. The origin of these regional climate anomalies remains a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully coupled model of intermediate complexity Climber 3α forced with stochastically reconstructed wind-stress fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea ice–ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions in our model leads to a spin-up of the subpolar gyre and a weakened Atlantic meridional overturning circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for understanding the dynamics of past and future climate.

scholarly journals A volcanically triggered regime shift in the subpolar North Atlantic ocean as a possible origin of the Little Ice Age

2012 ◽  
Vol 8 (6) ◽  
pp. 6199-6219 ◽  
Author(s):  
C. F. Schleussner ◽  
G. Feulner
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Little Ice Age ◽  
Regime Shift ◽  
Regional Climate ◽  
Volcanic Eruptions ◽  
Meridional Overturning Circulation ◽  
Ice Age ◽  
Last Millennium ◽  
Subpolar Gyre

Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly (MCA), succeeded by the Little Ice Age (LIA) are of exceptional importance. The origin of these regional climate anomalies remains however a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully-coupled model Climber 3α forced with stochastically reconstructed wind fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea-ice – ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions leads to a spin-up of the subpolar gyre (SPG) and a weakened Atlantic Meridional Overturning Circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for past and future climate.

scholarly journals Amplified Inception of European Little Ice Age by Sea Ice–Ocean–Atmosphere Feedbacks

2013 ◽  
Vol 26 (19) ◽  
pp. 7586-7602 ◽  
Author(s):  
Flavio Lehner ◽  
Andreas Born ◽  
Christoph C. Raible ◽  
Thomas F. Stocker
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Little Ice Age ◽  
Barents Sea ◽  
Feedback Mechanism ◽  
Ice Age ◽  
The Arctic ◽  
Nordic Seas ◽  
Ice Growth ◽  
Ocean Atmosphere

Abstract The inception of the Little Ice Age (~1400–1700 AD) is believed to have been driven by an interplay of external forcing and climate system internal variability. While the hemispheric signal seems to have been dominated by solar irradiance and volcanic eruptions, the understanding of mechanisms shaping the climate on a continental scale is less robust. In an ensemble of transient model simulations and a new type of sensitivity experiments with artificial sea ice growth, the authors identify a sea ice–ocean–atmosphere feedback mechanism that amplifies the Little Ice Age cooling in the North Atlantic–European region and produces the temperature pattern suggested by paleoclimatic reconstructions. Initiated by increasing negative forcing, the Arctic sea ice substantially expands at the beginning of the Little Ice Age. The excess of sea ice is exported to the subpolar North Atlantic, where it melts, thereby weakening convection of the ocean. Consequently, northward ocean heat transport is reduced, reinforcing the expansion of the sea ice and the cooling of the Northern Hemisphere. In the Nordic Seas, sea surface height anomalies cause the oceanic recirculation to strengthen at the expense of the warm Barents Sea inflow, thereby further reinforcing sea ice growth. The absent ocean–atmosphere heat flux in the Barents Sea results in an amplified cooling over Northern Europe. The positive nature of this feedback mechanism enables sea ice to remain in an expanded state for decades up to a century, favoring sustained cold periods over Europe such as the Little Ice Age. Support for the feedback mechanism comes from recent proxy reconstructions around the Nordic Seas.

scholarly journals Initiation of the last ice age in Canada by extreme precipitation resulting from a cascade of oceanic salinity increases

2015 ◽  
Vol 3 (1) ◽  
pp. 237-250
Author(s):  
Robert G. Johnson
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Regional Climate ◽  
Ice Sheet ◽  
Northern Africa ◽  
Ice Age ◽  
Baffin Bay ◽  
The North ◽  
Last Ice Age ◽  
Polar Ocean

Numerical modeling has failed to confirm the classical Milankovitch hypothesis of initiation of the last ice age by Northern Hemisphere high latitude cooling due to decreasing summer insolation caused by orbital effects. The modeling failed to confirm ice sheet growth even with a widespread layer of glacial ice as an initial condition to embody positive feedback. The failure probably occurred because the initial conditions of the calculation did not include the actual effects of an altered climate in northeastern Canada that brought a sharp cooling to Europe and extreme amounts of precipitation to cloud-covered lands west of Greenland. In the conceptual model proposed here, diminishing orbital summer insolation in northern Africa is causally linked to this regional climate change by a cascade of oceanic salinity changes. The summer cooling in northern Africa weakened the monsoons, reduced the Nile River flow, and increased Mediterranean salinity and outflow at Gibraltar. The salt in the outflow contributes substantially to the salinities of the North Atlantic Drift and the Greenland Sea, to the formation rate of North Atlantic Deep Water (NADW) there, and to the northward flow of the Spitsbergen-Atlantic Current (SAC) that replaces the sinking NADW. When the increasing salt in the Mediterranean outflow made the SAC replacement flow sufficiently strong, the flow penetrated into the polar ocean along the north coast of Greenland. Denser and more saline Atlantic water then replaced the polar water flowing southward into Baffin Bay through the Nares Strait and Lancaster Sound, thus eliminating the stratification in the bay that enables freezing of winter sea ice. Without the southward flow of sea ice out of Baffin Bay, the Labrador Sea became much warmer. The warmer seas west of Greenland then triggered a persistent cyclonic circulation that caused large amounts of precipitation in eastern Canada and a much colder northern Europe. The resulting Canadian erosion yielded a 500yr-long deep-sea sediment record of the ice-free condition. Heavy snowfall started new ice sheet growth on Baffin Island, northern Quebec, Labrador, western Greenland, and the Barents Sea, causing world sea level to fall at a rate of 0.5cmyr-1. The modern increasing salinity of the Mediterranean Sea and extension of SAC flow into the polar ocean are now following the cascade steps toward an ice-free Baffin Bay and possible near term regional climate deterioration.

scholarly journals Did a bi-polar multi-level oceanic oscillation cause the Little Ice Age and other high latitude climate extremes?

2015 ◽  
Vol 3 (1) ◽  
pp. 228-236
Author(s):  
Robert G. Johnson
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
High Latitude ◽  
Little Ice Age ◽  
Climate Extremes ◽  
Atlantic Water ◽  
Sea Surface Salinity ◽  
Ice Age ◽  
Northern Coast ◽  
Greenland Sea

Variations of ice-rafted sand and sediment from deep-sea cores beneath the North Atlantic Drift imply 1500yr cycles of the Drift flow and its associated climate warmth in northern North Atlantic regions. The Drift cycle is part of a complex bi-polar oceanic oscillation. Central to the oscillation is the relatively higher sea surface salinity of the high-latitude Greenland Sea that enables the winter sinking of surface water to form North Atlantic Deep Water (NADW). This drives the oscillation, and the NADW is replaced by water from the Drift. The oscillation causes climate extremes in both northern and southern high latitudes and is framed here in a sinusoidal model. The model is consistent with and may explain the early medieval climate optimum, the subsequent Little Ice Age, the recent record maximum area of Antarctic winter sea ice, and the related record low rate of Antarctic Bottom Water (ABW) formation. The negative feedback of lower salinity ABW entering the northern North Atlantic tends to inhibit NADW formation and the northward Drift flow. The positive feedback of warmer and higher salinity NADW mixing into the Southern Ocean around Antarctica tends to reduce sea ice formation and enhance the rate of ABW formation. Because of these feedbacks, the rate of NADW formation oscillates over a range less than a maximum without feedback, the rate of ABW formation oscillates over a range greater than a minimum without feedback, and the phase of NADW oscillation in the model leads the ABW oscillation by 375 years. The model predicts another northern North Atlantic climate optimum about 2500 AD. However, increases in penetration of the polar ocean by flow of Atlantic water in the process of replacing the sinking NADW suggest that an interval of extreme warmth in the northeastern North Atlantic may occur within decades. This penetration could result in the loss of perennial sea ice along the northern coast of Greenland. Much of the inferred increase of northward winter flow of northeastern North Atlantic water into the Greenland Sea in recent decades may be due to stronger NADW formation caused by greater salt contributions to the Drift from the Mediterranean outflow.

scholarly journals Evidence for extreme export of Arctic sea ice leading the abrupt onset of the Little Ice Age

2020 ◽  
Vol 6 (38) ◽  
pp. eaba4320
Author(s):  
Martin W. Miles ◽  
Camilla S. Andresen ◽  
Christian V. Dylmer
Keyword(s):  
Sea Ice ◽  
Little Ice Age ◽  
Abrupt Onset ◽  
Arctic Sea Ice ◽  
Ice Age ◽  
The Arctic ◽  
Ocean Stratification ◽  
Proxy Records ◽  
Arctic Sea ◽  
Robust Evidence

Arctic sea ice affects climate on seasonal to decadal time scales, and models suggest that sea ice is essential for longer anomalies such as the Little Ice Age. However, empirical evidence is fragmentary. Here, we reconstruct sea ice exported from the Arctic Ocean over the past 1400 years, using a spatial network of proxy records. We find robust evidence for extreme export of sea ice commencing abruptly around 1300 CE and terminating in the late 1300s. The exceptional magnitude and duration of this “Great Sea-Ice Anomaly” was previously unknown. The pulse of ice along East Greenland resulted in downstream increases in polar waters and ocean stratification, culminating ~1400 CE and sustained during subsequent centuries. While consistent with external forcing theories, the onset and development are notably similar to modeled spontaneous abrupt cooling enhanced by sea-ice feedbacks. These results provide evidence that marked climate changes may not require an external trigger.

Ice-Core Pollen Record of Climatic Changes in the Central Andes during the last 400 yr

2005 ◽  
Vol 64 (2) ◽  
pp. 272-278 ◽  
Author(s):  
Kam-biu Liu ◽  
Carl A. Reese ◽  
Lonnie G. Thompson
Keyword(s):  
Little Ice Age ◽  
Ice Core ◽  
Climatic Changes ◽  
Ice Age ◽  
Striking Similarity ◽  
Pollen Record ◽  
Dry Period ◽  
Proxy Records ◽  
Ice Cap ◽  
Ice Accumulation

AbstractThis paper presents a high-resolution ice-core pollen record from the Sajama Ice Cap, Bolivia, that spans the last 400 yr. The pollen record corroborates the oxygen isotopic and ice accumulation records from the Quelccaya Ice Cap and supports the scenario that the Little Ice Age (LIA) consisted of two distinct phases�"a wet period from AD 1500 to 1700, and a dry period from AD 1700 to 1880. During the dry period xerophytic shrubs expanded to replace puna grasses on the Altiplano, as suggested by a dramatic drop in the Poaceae/Asteraceae (P/A) pollen ratio. The environment around Sajama was probably similar to the desert-like shrublands of the Southern Bolivian Highlands and western Andean slopes today. The striking similarity between the Sajama and Quelccaya proxy records suggests that climatic changes during the Little Ice Age occurred synchronously across the Altiplano.

scholarly journals Anomalous blocking over Greenland preceded the 2013 extreme early melt of local sea ice

2017 ◽  
Vol 59 (76pt2) ◽  
pp. 181-190 ◽  
Author(s):  
Thomas J. Ballinger ◽  
Edward Hanna ◽  
Richard J. Hall ◽  
Thomas E. Cropper ◽  
Jeffrey Miller ◽  
...  
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Regional Climate ◽  
Regional Climate Change ◽  
Circulation Pattern ◽  
The Arctic ◽  
Labrador Sea ◽  
First Year ◽  
Tropospheric Circulation ◽  
Blocking Index

ABSTRACTThe Arctic marine environment is undergoing a transition from thick multi-year to first-year sea-ice cover with coincident lengthening of the melt season. Such changes are evident in the Baffin Bay-Davis Strait-Labrador Sea (BDL) region where melt onset has occurred ~8 days decade−1 earlier from 1979 to 2015. A series of anomalously early events has occurred since the mid-1990s, overlapping a period of increased upper-air ridging across Greenland and the northwestern North Atlantic. We investigate an extreme early melt event observed in spring 2013. (~6σ below the 1981–2010 melt climatology), with respect to preceding sub-seasonal mid-tropospheric circulation conditions as described by a daily Greenland Blocking Index (GBI). The 40-days prior to the 2013 BDL melt onset are characterized by a persistent, strong 500 hPa anticyclone over the region (GBI >+1 on >75% of days). This circulation pattern advected warm air from northeastern Canada and the northwestern Atlantic poleward onto the thin, first-year sea ice and caused melt ~50 days earlier than normal. The episodic increase in the ridging atmospheric pattern near western Greenland as in 2013, exemplified by large positive GBI values, is an important recent process impacting the atmospheric circulation over a North Atlantic cryosphere undergoing accelerated regional climate change.

scholarly journals Climatic variability and human impact during the last 2000 years in western Mesoamerica: evidence of late Classic (AD 600–900) and Little Ice Age drought events

2015 ◽  
Vol 11 (9) ◽  
pp. 1239-1248 ◽  
Author(s):  
A. Rodríguez-Ramírez ◽  
M. Caballero ◽  
P. Roy ◽  
B. Ortega ◽  
G. Vázquez-Castro ◽  
...  
Keyword(s):  
Human Impact ◽  
Little Ice Age ◽  
Climatic Variability ◽  
Seasonal Migration ◽  
Ice Age ◽  
Cultural Tradition ◽  
Late Classic ◽  
High Pressure Cell ◽  
Western Mexico ◽  
The North

Abstract. We present results of analysis of biological (diatoms and ostracodes) and non-biological (Ti, Ca / Ti, total inorganic carbon, magnetic susceptibility) variables from an 8.8 m long, high-resolution (~ 20 yr sample−1) laminated sediment sequence from Lake Santa María del Oro (SMO), western Mexico. This lake lies at a sensitive location between the dry climates of northern Mexico, under the influence of the North Pacific subtropical high-pressure cell and the moister climates of central Mexico, under the influence of the seasonal migration of the intertropical convergence zone and the North American monsoon (NAM). The sequence covers the last 2000 years and provides evidence of two periods of human impact in the catchment, shown by increases in the diatom Achnanthidium minutissimum. The first from AD 100 to 400 (Early Classic) is related to the shaft and chamber tombs cultural tradition in western Mexico, and the second is related to Post-Classic occupation from AD 1100 to 1300. Both periods correspond to relatively wet conditions. Three dry intervals are identified from increased carbonate and the presence of ostracodes and aerophilous Eolimna minima. The first, from AD 500 to 1000 (most intense during the late Classic, from AD 600 to 800), correlates with the end of the shaft and chamber tradition in western Mexico after ca. AD 600. This late Classic dry period is the most important climatic signal in the Mesoamerican region during the last 2000 years, and has been recorded at several sites from Yucatan to the Pacific coast. In the Yucatan area, this dry interval has been related with the demise of the Maya culture at the end of the Classic (AD 850 to 950). The last two dry events (AD 1400 to 1550 and 1690 to 1770) correspond with the onset of, and the late, Little Ice Age, and follow largely the Spörer and Maunder minima in solar radiation. The first of these intervals (AD 1400 to 1550) shows the most intense signal over western Mexico; however this pattern is different at other sites. Dry/wet intervals in the SMO record are related with lower/higher intensity of the NAM over this region, respectively.

scholarly journals Bunker Cave stalagmites: an archive for central European Holocene climate variability

2012 ◽  
Vol 8 (3) ◽  
pp. 1687-1720 ◽  
Author(s):  
J. Fohlmeister ◽  
A. Schröder-Ritzrau ◽  
D. Scholz ◽  
C. Spötl ◽  
D. F. C. Riechelmann ◽  
...  
Keyword(s):  
Climate Variability ◽  
North Atlantic ◽  
Little Ice Age ◽  
Ice Age ◽  
Holocene Climate ◽  
Central European ◽  
The North ◽  
The North Atlantic ◽  
Meteoric Precipitation ◽  
Precipitation And Temperature

Abstract. Holocene climate was characterised by variability on multi-centennial to multi-decadal time scales. In central Europe, these fluctuations were most pronounced during winter. Here we present a new record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, Western Germany. Due to its central European location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high resolution records of δ18O, δ13C values and Mg/Ca ratios. We attribute changes in the Mg/Ca ratio to variations in the meteoric precipitation. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation and variations in the δ18O signal are interpreted as variations in meteoric precipitation and temperature. We found cold and dry periods between 9 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 to 0.2 ka. The proxy signals in our stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The prominent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker cave record. However, these events show a contrasting relationship between climate and δ18O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed to a pronounced negative phase of the North Atlantic Oscillation, the 8.2 ka event was triggered by cooler conditions in the North Atlantic due to a slowdown of the Thermohaline Circulation.

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