Saturday 26 December 2020

Understanding the climate of southeast Australia during the Eemian Interglacial.

The Eemian or peak of the Last Interglacial (Marine Isotope Stage 5e, roughly 129 000–116 000 years Before Present) is the most recent geologic period when global temperatures were similar to present, but in response to orbital forcing rather than greenhouse gas loading of the atmosphere. While this makes the Eemian an imperfect analogue for near-future climate due to anthropogenic global warming, the latitudinal temperature distribution was similar to the present. Eemian global mean temperature was 0-2°C warmer than present, mean global sea surface temperatures were indistinguishable from current sea surface temperatures, though sea level was around 6–9 m higher from meltwater inflows from the Greenland and Antarctic ice sheets. Therefore, understanding the climate of the Eemian may provide valuable insight to future climate and its variability.

While often thought of as a period of relative climate stability, climate variability during the Eemian was likely greater than in the Holocene. This has been attributed to meltwater outflows disturbing the Atlantic meridional overturning circulation and a general global cooling trend toward glacial inception. The resulting changes in sea surface salinity and temperature are believed to have driven regional changes in atmospheric circulation leading to periods of widespread aridity across Europe, and onset of abrupt cold periods as sea surface temperatures cooled by several degrees Celsius.

Our understanding of Southern Hemisphere Eemian climate is limited as proxy records are rare. The climate of Australia is globally relevant, as it responds to key global teleconnection forcing, including El Niño Southern Oscillation, Pacific Decadal Oscillation, and the Indian Ocean Dipole. It is also relevant as the severity of recent droughts and bushfires have been linked to increasing temperatures caused by anthropogenic greenhouse gas emissions, thereby stimulating interest in understanding past warm climate conditions such as during Marine Isotope Stage 5e.

Numerical modelling studies suggest reduced seasonal contrast in Marine Isotope Stage 5e temperatures across Australia with warmer winter temperatures, most notably in the north and northwest of the continent by +3°C to +5°C. Austral summer (December–February) temperatures are believed to have been cooler by −1°C to −2°C with modelling studies suggesting that the Australian monsoon was weaker and drier than present with January-April precipitation anomalies of more than − 3 mm per day. A weaker Marine Isotope Stage 5e monsoon aligns with evidence of a variable Lake Eyre hydrology with temporally isolated small inflow events These were likely caused by irregular southward penetration of the Intertropical Convergence Zone and decaying tropical cyclone(s) over the headwaters of the Lake Eyre Basin; conditions similar to those of today that deliver inflows to Lake Eyre.

Winter temperatures in southern Australia during Marine Isotope Stage 5e are believed to have been warmer than present, reflecting warmer sea surface temperatures in the Southern Ocean. This would have reduced the meridional temperature gradient between the sub-tropics and mid/high latitudes with less frequent precipitation bearing cold fronts and extratropical depressions at a time concurrent with strengthening of interannual variability of sea surface temperatures in the eastern tropical Pacific associated with El Niño Southern Oscillation. Slightly cooler Marine Isotope Stage 5e sea surface temperatures to the northwest of Australia relative to present may have contributed further to reduced rainfall over southeast Australia similar to the impact of present day positive Indian Ocean Dipole events. Combined with changes to Atlantic meridional overturning circulation and associated sea surface temperatures, which have been shown under present day conditions to teleconnect to the Pacific Ocean, the climate of Marine Isotope Stage 5e in Australia is likely to have been variable, but in general drier than present. In particular, in southeast Australia where under the current climate positive Indian Ocean Dipole and El Niño Southern Oscillation result in reduced rainfall. However, a dearth of high temporal resolution paleoclimate records from Marine Isotope Stage 5e have until present not been available to test this thesis and the possible links to climate state forcings such as teleconnections and solar variability.

In a paper published in the journal Scientific Reports on 22 October 2020, Hamish McGowan of the Atmospheric Observations Research Group at the University of Queensland, Micheline Campbell and John Nikolaus Callow of the School of Agriculture and Environment at the University of Western Australia, Andrew Lowry, also of the Atmospheric Observations Research Group at the University of Queensland, and Henri Wong of the Australian Nuclear Science and Technology Organisation, present a near annual resolution reconstruction of climate developed from a speleothem that spans the Eemian (Marine Isotope Stage 5e) from 117 500 to 123 500 years before present, the most recent period in the Earth’s history when temperatures were similar to those of today.

In Australia, speleothems offer the greatest potential to develop high temporal resolution terrestrial palaeoenvironmental records. Stalagmite GC001 was removed from a small chamber approximately 60 m into the Grotto Cave, Yarrangobilly Caves, New South Wales, Australia in 2012. The caves are located at the northern end of the Snowy Mountains, at an elevation where the water balance changes from an energy (demand) to supply (precipitation) limited system. The caves were formed through karstification of the Yarrangobilly Limestone, a massive Silurian limestone formation with an extent of about 1.4 km by 14 km and a maximum depth of roughly 450 m. Past work has concluded that reduced rainfall invokes prior calcite precipitation at Yarrangobilly and hence up (down) trend in magnesium/calcium ratios implies increased (decreased) drip water contact time under drier (wetter) conditions.

Location map of the Yarrangobilly Caves (a) and aerial oblique perspective of the entrance to the Grotto Cave (b) 300 mm long cross-section of stalagmite GC001 from which samples were extracted for uranium series dating and geochemical analysis (c). The small core perpendicular to the growth axis is the result of in-situ sampling of GC001 for age determination prior to removal from the Grotto Cave (c). McGowan et al. (2020).

The present day climate of the Yarrangobilly area is influenced by the northern extension of the mid-latitude westerly winds. It experiences cool to cold montane temperatures through winter, while in summer warm to hot and dry conditions dominate under the influence of the subtropical ridge. Major Southern Hemisphere ocean–atmosphere teleconnections including the El Niño Southern Oscillation, Southern Annular Mode, Pacific Decadal Oscillation, and Indian Ocean Dipole all affect the regional climate state. Rain bearing weather systems that affect Yarrangobilly Caves may originate from the tropics and the southern mid-latitudes. Mean annual precipitation is approximately 1147 mm, with a mean annual temperature of 12°C. The most effective precipitation occurs during the cool, wet winter and while snow may fall, the site is below the seasonal snowline and no snowpack remains through winter. Temperature logging from where GC001 was collected in the Grotto Cave found mean internal cave temperature was 8.8°C during the study (August 2014 to February 2015).

The magnesium time series for GC001 shows clear cycles of higher/lower concentrations with a marked increase occurring around 120 800 years before present. This period of elevated magnesium concentrations prevailed until around 118 500 years before present. It then remained mostly stable until a sharp decrease at approximately 117 850 years before present. This period of higher magnesium concentrations (drier) in GC001 aligns with a period of increased depletion of the proportion of oxygen¹⁸ in a speleothem from northern Borneo indicating wetter conditions there, indicative of a more northerly position of the Intertropical Convergence Zone. It also correlates with periods of increased dust deposition recorded at Dome C, Antarctica. Australia is a known source of interglacial dust to the Antarctic. Accordingly, McGowan et al. infer drier conditions in southeast Australia as indicated by higher magnesium concentrations from 120 800 to 118 500 years before present in GC001 sustained regional mega-drought(s) across this period with associated wind erosion contributing to increased dust flux at Dome C, Antarctica such as between 119 050 to 120 300 years before presnt. Slightly higher dust flux values from 117 500 to 117 800 years before presnt, may have also originated from southeast Australia but do not correlate with elevated magnesium concentrations in GC001, or any notable signal in the proportion of oxygen¹⁸ record from northern Borneo.

Time series of magnesium, strontium and barium concentrations from GC001 (a)–(c); proportion of oxygen¹⁸ from a speleothem collected at Whiterock Cave, northern Borneo (d); dust concentrations from Dome C, Antarctic (e), and temperature departure from the average of the past 1000 years Dome C, Antarctic (f). Pink shading indicates periods of intermediate magnesium concentrations and dust flux (a), (e); yellow shading highest magnesium concentrations and dust flux (a), (e) and blue shading (d) corresponding period of depleted proportion of oxygen¹⁸ indicating wetter conditions in Western Pacific. Vertical dashed blue lines constrain the period interpreted as a mega-drought(s). McGown et al. (2020).

The strontium time series displays some periods of higher/lower concentrations that are concurrent with variability in the magnesium record, such as around 121 000 years before present. However, unlike the magnesium record, the strontium time series exhibits an overall trend of decreasing concentrations with the period between approximately 120 800 years to 118 500 years before present, remaining relatively constant at approximately 25 parts per million, before decreasing more quickly. This is similar to the barium concentrations time series which is strongly correlated with strontium reflecting a reduced growth rate (less drip water) under the drier conditions known to have minimal impact on magnesium concentrations. Barium concentrations do display a more consistent decreasing trend that flattens between 120 800 and 119 100 years before present, before decreasing further after 118 500 years before present.The decreasing trends in the strontium and barium concentrations correlate with the Marine Isotope Stage 5e temperature anomaly reconstruction for Dome C, Antarctic and Marine Isotope Stage 5e sea surface temperatures reconstructions from northwest of Australia and east of New Zealand. Accordingly, the magnesium, strontium and barium concentrations in GC001 collectively record the onset of a prolonged drier period from around 120 750 years before present to 118 500 years before present that was associated with increased atmospheric dust and a cooling in atmospheric and ocean temperatures from Marine Isotope Stage 5e maximums.

Normalised Lomb-Scargle periodograms for magnesium, strontium and barium were calculated to test for the influence of teleconnection and/or solar cycle forcing on the Marine Isotope Stage 5e climate of southeast Australia. The periodograms show common peaks in spectral density around 200 years which align with the de Vries solar cycle (about 205 years). Spectral peaks are also found from 1147 years (magnesium) to 1268 years (strontium) and are within the range of the roughly 1000 year Eddy solar cycle. The strontium and barium records have peaks at about 66.8 and 88.6 years, which match with the Pacific Decadal Oscillation and Gleissberg solar cycle respectively and display coherent spectral peaks at 301.3 years, 454.8 years, 602.7 years and at 3443.8 years. The 301.3 years and 454.8 years cycles are close to the 300 year and 470 year cycles found in sediments from Jeju Island, South Korea. These have been attributed to solar forcing along with the 3443.8 year cycle, which is aligned with the 3300 year Holocene cycles in the proportion of oxygen¹⁸ record from Greenland Ice Sheet Project 2 core. The magnesium record displays a strong 709 year cycle that may be a harmonic of the 1400 year cycle found in glacial and interglacial periods in magnesium/calcium derived sea surface temperatures records from the South China Sea. Accordingly, the geochemistry of GC001 displays cyclic variability indicative of climate variability forced by both teleconnections and solar cycles recorded globally in geologic archives.

Periodograms of magnesium, strontium and barium with 0.9 (red dashed line) and 1.0 probability thresholds shown. Selected dominant periods (annotated) are rounded to the nearest decade. McGowan et al. (2020).

Wavelet transforms for magnesium, strontium and barium show regions of significant periodicity (95% confidence level). All three elements display significant 4 to 8 year cycles that McGowan et al. interpret as El Niño Southern Oscillation and around 20 to 30 years, and 50 to 65 years, which are indicative of the Pacific Decadal Oscillation. While caution is required in attributing shorter-duration cycles given laser ablation sampling resolution, the magnesium record shows clear gaps in El Niño Southern Oscillation (4–8 year) cycles from approximately 121 500 years before present to around 120 500 years before present. These coincide with a change in magnesium concentrations from about 140 to about 200 parts per million indicating a transition to drier conditions, while the period from 118 500 to 117 500 years before present overlaps with the return to magnesium concentrations, about 140 parts per million suggesting increased moisture availability. Pacific Decadal Oscillation like cycles are most common from around 119 800 years before present to 118 500 years before present.

Wavelet plots of magnesium (a), strontium (b), and barium (c). The thick black lines represent 95% confidence levels. The lightly shaded region toward the bottom is the Cone of Influence and values in the light region should be considered with caution. McGowan et al. (2020).

In the strontium wavelet transform diagram, the El Niño Southern Oscillation signals are most frequent from the start of the record to about 122 100 years before present and are concurrent with the occurrence of the multi-decadal Pacific Decadal Oscillation like cycle. This longer decadal cyclicity is also evident from 121 500 years before present to 118 500 years before present with evidence of a separate 20 to 30 year cycle and longer 55 to 75 year cycle. The barium wavelet plot shows also a dominant short duration El Niño Southern Oscillation signal in the early part of the record that progressively becomes less frequent. The Pacific Decadal Oscillation period cycle is not strong and only occurs above the 95% confidence level occasionally from 123 500 years before present to around 122 200 years before present and again from around 121 000 years before present to 119 500 years before present.

Understanding the causes and frequency of climate variability is essential to inform prediction of future climate. Proxy of climate variability dating from Marine Isotope Stage 5e, the most recent warm period with temperatures similar to the present, offer potential to develop this understanding. The magnesium Marine Isotope Stage 5e record from stalagmite GC001 shows multi-centennial periods of increased Mg concentration indicating drier climatic conditions in southeast Australia, notably from 118 500 to 120 750 years before present. This period coincides with increased dust flux recorded in Antarctic ice. The strontium and barium concentrations show evidence also of this climate variability with trends of decreasing concentrations correlated with cooling air temperatures and sea surface temperatures. Collectively, the magnesium, strontium and barium records from GC001 with supporting regional paleoclimate records suggest that southeast Australia during Marine Isotope Stage 5e experienced multi-centennial periods of reduced precipitation, temperature variability and increased atmospheric dust. These conditions such as from 118 900 to 120 400 years before present are indicative of mega-droughts. Superimposed on these multi-centennial periods of drier climatic conditions are higher frequency interannual to interdecadal teleconnections and solar forced cycles of further variability.

A drier climate during Marine Isotope Stage 5e such as from 118 900 to 120 400 years before present would align with results from numerical modelling which found a generally weaker summer monsoon and warmer winter conditions. A weaker monsoon aided by cooler sea surface temperatures northwest of Australia at this time would lessen moisture transport into southeast Australia via meridional conveyors such as northwest cloud bands, while warmer temperatures during winter would have increased evaporation, intensifying dry conditions that spanned centuries as evident in our record. Numerical modelling of future climate at +1.5°C indicates that the Australian monsoon may weaken, resulting in an increase in the frequency of dry days. Accordingly, the convergence of our paleoclimate record from stalagmite GC001 and published numerical modelling strongly suggest that the climate of southeast Australia will likely become drier throughout the twenty-first century with increased risk of multi-centennial duration dry periods widely referred to as mega-droughts.

The magnesium, strontium and barium records from stalagmite GC001 all display cycles with periodicity of well documented solar and teleconnection forcings of climate. Dominant solar cycles including the Gleissberg solar cycle (88 years), de Vries solar cycle (about 205 years) and possible Eddy solar cycle (about 1000 years) are evident in the geochemistry of GC001. Increases (and decreases) in total solar irradiance through such cycles result in direct heating (and cooling) of the troposphere and Earth’s surface. Indirectly, changes in total solar irradiance influence galactic cosmic ray flux affecting cloud microphysics and cloud cover. Variation in total solar irradiance also causes change in ultraviolet radiation flux, which for a 0.1% increase in total solar irradiance, increases by 4 to 8%. Concurrently this will increase ozone production in the mid and upper stratosphere through the photolysis of oxygen. Corresponding increases in ultraviolet absorption by stratospheric ozone leads to heating and change in the thermal stratification of the atmosphere.

Numerical modelling has shown that enhanced ultraviolet heating of the atmosphere corresponding to total solar irradiance maxima results in a stratospheric zonal wind anomaly that is most pronounced in the Southern Hemisphere during mid to late winter. Dynamical links between the stratosphere and troposphere caused by such ultraviolet forcing lead to change in atmospheric circulation, including jet-stream behaviour, and therefore tropospheric synoptic circulation and ocean dynamics. McGowan et al. therefore postulate that periods of increased total solar irradiance contribute to drier conditions (increased magnesium in GC001) over southeast Australia in response to increased zonal (westerly) flow and a corresponding more southern track of mid-latitude winter storm systems. This occurs in response to a stronger thermal wind component between the tropics/subtropics and the mid to high latitudes causing a response in atmospheric circulation similar to a more positive Southern Annular Mode. The associated decline in precipitation is recorded clearly in stalagmite GC001 by elevated magnesium concentrations and stable to slight decreases in strontium and barium concentrations due to reduced speleothem growth, i.e. reduced drip rate during Marine Isotope Stage 5e.

GC001 records 4 to 8 years El Niño Southern Oscillation-like cycles along with multi-decadal cyclicity indicating Pacific Decadal Oscillation type influences, two teleconnections which have been shown to have the greatest impact on the modern climate and in particular rainfall in southeast Australia. It is therefore reasonable to suggest that these Pacific Ocean teleconnections have been robust and long-lived drivers of interannual and multi-decadal hydroclimate variability in southeast Australia with warm/cool El Niño Southern Oscillation and Pacific Decadal Oscillation phases causing dry/wet periods.

Evidence of El Niño Southern Oscillation cycles in GC001 become no longer statistically significant from around 118 200 yrs before present. This is about 500 years before rapid decline in magnesium concentrations, which suggests reduction in drip water residence times and onset of a wetter climate. This change in magnesium and to a lesser amount in strontium and barium correspond to a rapid increase in sea level of around 5 to 6 m at 118 100 years before present caused by ice sheet melt. McGowan et al. suggest that this dramatic rise in sea level possibly disrupted El Niño Southern Oscillation and Pacific Decadal Oscillation teleconnections and their impact on the climate of southeast Australia at this time.

Collectively, the Marine Isotope Stage 5e climate record developed from GC001 indicates that southeast Australia will likely continue to experience interannual to interdecadal wet–dry cycles driven by teleconnections and solar variability at least until current global warming exceeds Eemian temperatures, possibly within the next decade. However, McGowan et al.'s record also shows that in a warm interglacial climate such as today or near future, there is risk of multi-centennial periods of less effective precipitation (mega-droughts), initiated by natural variability. Should such prolonged periods of drier conditions occur again, then they may be reinforced by anthropogenic global warming, thereby increasing their severity. As a result, McGowan et al. stress the need for further research into the Eemian climate of Australia and the Southern Hemisphere to resolve the causes of prolonged dry periods during Marine Isotope Stage 5e and to determine their spatial impacts. This will allow new insights to our future climate and the risks it may bring such as drought and associated bushfires.

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