Holocene climate changes
- This page is part of the topic The Holocene
Over the past ~11.7 ka there have been several abrupt changes in Antarctic climate. These are either related to, or superimposed on, the dynamic response of the ice sheet to past, longer term forcing. As a consequence the current configuration of the Antarctic ice sheet is the result of a multi-millennial scale lagged response to climate forcing. As an example, grounding lines in the marine based parts of the West Antarctic ice sheet at the head of the Ross Ice Shelf, started to retreat to their current position from close to the edge of the current Ross Ice Shelf 7-9 ka ago (Conway et al., 1999). Based on evidence developed from a synthesis of ice core isotopic records this massive retreat was preceded by an early Holocene climatic optimum between 11.5 and 9 ka ago (Masson et al., 2000) which was warmer than today and resulted in the rapid loss of some Antarctic Peninsula ice shelves (Hodgson et al., 2006b). These and other Antarctic data confirm that although the global climate of the Holocene superficially appears to have been relatively stable, there has been sufficient variability during the last ~9 ka to cause major changes to Antarctic ecosystems. This natural variability must be taken into account in understanding modern climate and the potential for future climate change.
A comparison of the behaviour of climate proxies measured in selected ice cores from GISP2 (Greenland) and Siple Dome (West Antarctic) (Figure 3.19a and Figure 3.19b) reveals periods of notable change in climate with some coincidence in both Northern and Southern Hemisphere polar regions during the Holocene. From this comparison several general conclusions can be drawn concerning the phasing and the magnitude of changes in atmospheric circulation and temperature between Northern and Southern Hemisphere polar latitudes. These conclusions are relevant to understanding not only the forcing of climate change over the polar regions, but also the implications of change over the polar regions for climate at the global scale.
With respect to changes in atmospheric circulation, Figure 3.19 shows that:
- North Atlantic climate (GISP2) displays more frequent and larger shifts in atmospheric circulation than does the Antarctic (Siple Dome). This difference is similar to that seen between Greenland and Antarctica in millennial scale events from glacial age ice core records (EPICA, 2006);
- North Atlantic atmospheric circulation (GISP2) generally displays more abrupt onset and decay of multi-centennial scale events than does the Antarctic (Siple Dome);
- GISP2 and Siple Dome ice core proxies for Northern and Southern Hemisphere westerlies show considerable similarity in event timing, frequency, and onset style suggestive of some common control; and
- the most dramatic changes in atmospheric circulation during the Holocene noted in the Antarctic are (i) the abrupt weakening of the Southern Hemisphere westerlies ~5.2 ka ago, and (ii) intensification of the westerlies and the Amundsen Sea Low starting ~1.2-1 ka ago.
With respect to temperature, Figure 3.19 shows that:
- the prominent temperature drop ~8,200 years ago over the North Atlantic, noted in the GISP2 isotope proxy for temperature, is missing in the Siple Dome isotope temperature proxy series, although it is suggested in a composite isotope record covering East Antarctica (Masson et al., 2000);
- Siple Dome isotopic temperature reconstructions reveal notable cooling ~6.4-6.2 ka ago followed by relatively milder temperatures over East Antarctica 6-3 ka ago (Masson et al., 2000), lasting until ~1,200 years ago in the Siple Dome area; and
- both Siple Dome and GISP2 proxies for temperature show a decline in temperature starting ~1.2-1 ka ago, followed by warming in the last few decades.
The abrupt climate change event commencing ~1,200-1,000 years ago is the most significant Antarctic climate event of the last ~5,000 years (Mayewski and Maasch, 2006). Its onset is characterized by strengthening of the Amundsen Sea Low and the Southern Hemisphere westerlies, with cooling both at Siple Dome (3.19) and in the East Antarctic composite isotope record (Masson et al., 2000). Consistent with this picture, a comparison of reconstructions of Southern Hemisphere temperature (Mann and Jones, 2003) and ice core proxies for atmospheric circulation covering the last 2,000 years suggests that in general temperature and circulation intensity are associated such that cooler temperatures coincide with more intense atmospheric circulation and warmer temperatures with milder circulation (Mayewski and Maasch, 2006).
Several interactive factors provide the forcing for decadal to centennial scale Holocene age climate events. These include variations in insolation, caused by variations in the Earth’s orbital properties and in solar output, and in the heating of the atmosphere by greenhouse gases or its cooling by volcanic aerosols. A number of key climate events and their possible forcing are displayed in Figure 3.19. The earliest is intensification of atmospheric circulation in the Northern Hemisphere (stronger Siberian High and westerlies, deeper Icelandic Low) and in the Southern Hemisphere (stronger westerlies and deeper Amundsen Sea Low) ~ 8,200 years ago. This is associated with the cooling of East Antarctica (Masson et al., 2000), and with a drop in CH4, a long-term decline in CO2, and an increase in solar output based on the 14C proxy for solar variability. The latter may have led to increased melting with consequent decrease in North Atlantic salinity and decrease in thermohaline circulation in the North Atlantic. Second, is intensification of the Southern Hemisphere westerlies ~6,400-5,200 years ago, associated with cooling ~6,400 years ago. This change is also associated with a crossover in the trend of insolation, a drop in CH4, a rise in CO2, a decrease in solar variability, and collapse of the Ross Sea ice sheet (Conway et al., 1999). These changes are coincident with climate change as far north as the Equator (Stager and Mayewski, 1997). Thirdly, is intensification of atmospheric circulation commencing ~1,200-1,000 years ago and lasting to the present, accompanied by relatively cooler conditions over East Antarctica (Masson et al., 2000) and West Antarctica (Siple Dome). This change is associated with a decrease in solar variability, a drop in CO2, and increased frequency of volcanic source sulfate aerosols over Antarctica. A satisfactory explanation for the forcing of these Holocene Antarctic climate changes remains elusive, though the link to variations in solar output is suggestive.
Evidence of a link to solar radiation comes from a more detailed examination of forcing over the last 2,000 years using the ice cores and other palaeoclimate records. This supports the close association in timing between changes in atmospheric circulation and solar energy output (Maasch et al., 2005). The impact of solar forcing (via UV induced changes in stratospheric ozone concentration) on the southern circumpolar westerlies at the edge of the polar vortex has been suggested through an association established between ice core climate proxies for the westerlies and solar variability (Mayewski and Maasch, 2006). This work reveals decadal-scale associations between the circumpolar westerlies, inferred from West Antarctic ice core Ca++, and 10Be, a proxy for solar variability in a South Pole ice-core (Raisbeck et al., 1990) over the last 600 years, and with annual-scale associations with solar variability inferred from the solar cycle since AD 1720. Increased solar irradiance is associated with increased zonal wind strength near the edge of the Antarctic polar vortex, and the winds decrease with decreasing irradiance. The association is particularly strong in the Indian and Pacific Oceans and may contribute to understanding the role of natural climate forcing on drought in Australia and other Southern Hemisphere climate events.
Over the last 700 years, evidence for abrupt climate change has been examined using ice core records from East Antarctica (Law Dome) and West Antarctica (Siple Dome). These selected studies reveal that these two regions have operated inversely with respect to temperature and to the strength of atmospheric circulation on multi-decadal to centennial scales (Figure 3.20) (Mayewski et al., 2004a). The exception is a climate change event commencing ~AD 1700 and ending by ~AD 1850, during which circulation and temperature acted synchronously. This cooling period is coincident with an increase in the frequency of penetration of El Niño events assessed using a South Pole ice core proxy (Meyerson et al., 2002) and with an increase in solar output. The close of this cooling event coincides with the onset of the modern rise in CO2, followed by the warmest temperatures of the last >700 years in West Antarctica based on the Siple Dome ice core record (Mayewski et al., 2005). The close of this event is coincident with a major transition from zonal to mixed flow in the North Pacific (Fisher et al., 2004), possibly suggesting a global scale association between Antarctic and North Pacific climate for this event.
Recent reductions in ice extent over the Arctic, around the Antarctic Peninsula, and for many mid-low latitude glaciers demonstrate some of the initial impacts of the rise in temperature measured over the last few decades (ACIA, 2005; IPCC, 2007). Reconstructions of past temperature indicate that this rise is anomalous relative to temperature variability over the last 2,000 years (e.g. Mann and Jones, 2003; Moberg et al., 2005). However, the association between change in temperature and change in atmospheric circulation under natural conditions has not been examined as vigorously. This association is perhaps most critical to investigate in the polar latitudes, where future warming is expected to be the greatest (Intergovernmental Panel on Climate Change, 2007). Synthesis of 50 well-dated, continuous palaeoclimate records covering the current interglacial (the last 11.7 ka, Holocene) reveals the occurrence of at least six periods of naturally forced abrupt climate change, of which several coincide with major disruptions in civilization (Mayewski et al., 2004b). It is within the last 2,000 years that annually resolved dating of stratigraphic records is most accurate and further this time period is characterized by boundary conditions (e.g., ice, ocean, atmosphere) most similar to those of today as well as two analogues for naturally warm (Northern Hemisphere Medieval Warm period) and cool (Little Ice Age) climates. Over the period of the last 2,000 years established, well-dated, proxy records of change in temperature are available (e.g. Mann and Jones, 2003). In addition, for this period, bipolar ice core records provide proxy reconstructions of past changes in regional scale atmospheric circulation, a major component of the climate system that has not received the same detailed attention as that given to past temperature, despite a strong association with temperature over a wide range of timescales (Mayewski et al., 1997; Thompson and Wallace, 2000; Bertler et al., 2004; Masson-Delmotte et al., 2005; Schneider et al., 2006). Comparison between ice core proxies for atmospheric circulation and multiple proxies for temperature reveals associations over the last few decades that are inconsistent with those of the past 2,000 years. Notably, patterns of middle to high latitude atmospheric circulation in both hemispheres are still within the range of variability of the last 6–10 centuries (Mayewski and Maasch, 2006, Figure 3.20), while, as demonstrated by Mann and Jones (2003), Northern Hemisphere temperatures over recent decades are the highest of the last 2,000 years. Further, recent temperature change in the Northern Hemisphere precedes change in middle to high latitude atmospheric circulation unlike the two most notable changes in climate of the past 2,000 years (Little Ice Age and Medieval Warm Period) during which change in atmospheric circulation preceded or coincided with change in temperature (Figure 3.21). In addition, the most prominent change in Southern Hemisphere temperature and atmospheric circulation of the past 2,000 years, and probably of the last 9,000 years, precedes change in temperature and atmospheric circulation in the Northern Hemisphere unlike the most recent change in Northern Hemisphere temperature that leads (Figure 3.21). These findings provide new verification that recent rise in temperature is inconsistent with natural climate forcing and is most likely related to anthropogenic activity in the form of enhanced greenhouse gases. Figure 3.21 also demonstrates that the delayed warming, relative to the Northern Hemisphere, over much of the Southern Hemisphere may be, in addition to other factors (ocean thermal capacity, Antarctic ice sheet and sea ice reflectivity and size) a consequence of underpinning by natural climate variability (Figure 3.21 and Figure 3.22).
Non-sea-salt (nss) Ca records from eight International Trans-Antarctic Science Expedition (ITASE) West Antarctic ice cores, covering the period AD 1400 – 2002, and the Siple Dome deep ice core, including the last 10,000 years, reveal that Southern Hemisphere westerlies intensification since ~1980 is unprecedented for at least the last 5,400 years, supporting the proposed role of human activity in this intensification (Dixon et al., In Review). This study also demonstrates the abrupt termination of the Southern Hemisphere westerlies intensification 5,400 years ago and abrupt terminations to earlier such intensifications of the westerlies. Based on this analogue (Dixon et al., In Review) propose that ozone hole recovery and continued greenhouse gas warming could trigger yet another abrupt weakening of the westerlies, leading to a hastening of Antarctic warming and accelerated changes in Antarctic and Southern Hemisphere climate.
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