Environment Counts | Correlation between aerosols and temperature in Antarctica over the past 300,000 years
Author: Geoff Zeiss – Published At: 2012-10-31 12:20 – (990 Reads)
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Aerosols are small particles of various materials, typically sulphates and silicates, that either directly or indirectly by acting as cloud condensation nuclei can scatter solar radiation and cool the Earth’s climate. Sulphates typically originate from volcanic, anthropogenic, and marine sources. The long term impact of sulphate aerosols on the Earth’s energy balance is not well-understood. In this article the authors measure both sulphate salt (sodium sulphate and calcium sulphate) and sulphate/silicate (sulphate-adhered dust) aerosols as recorded over 300,000 years in the Dome Fuji ice cores. They determine that there is a negative correlation between sulphate salt flux and temperature, in other words higher sulphate salt fluxes correspond to lower temperatures. During the Last Glacial Maximum (26,500 to 19,000 years ago) the measured sulphate salt flux is twice what is measured for the present interglacial (since about 12000 years ago). Based on some assumptions they estimate that sulphate salt aerosols indirectly (acting as cloud condensation nuclei) could cool the Antarctica by 0.1 to 5 degrees Kelvin. In the Antarctic the change in temperature between the Last Glacial Maximum and current interglacial is estimated to be about ~ 8 °C. The results reported in this article clearly shows the large uncertainty in the climatic impact of indirect aerosol effects. Nature 490, 81–84(04 October 2012)
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Past analyses of sulphate chemistry in ice cores were unable to differentiate between H2SO4 and sulphate salt (Na2SO4 and CaSO4) fluxes. This article reports a complete record of the sulphate salt and sulphate-adhered dust fluxes into inland Antarctica over the past 300 ,000 years.
One of the interesting conclusions that were found is that aerosol particles in glacial inceptions and interglacial periods can be distinguished from those in glacial maxima by their sodium sulphate/calcium sulphate mass ratios and also by their salt/sodium sulphate mass ratios. During glacial maxima, sodium sulphate/calcium sulphate values are particularly low, whereas salt/sodium sulphate values are high.
a Temperature proxy (delta oxygen-18). Grey bars indicate glacial maxima.
b Sodium sulphate/calcium sulphate salt mass ratios. Vertical error bars are uncertainties.
c Salt/sodium sulphate salt mass ratios.
The authors find no correlation between the temperature proxy (delta oxygen-18) and sulphate-adhered dust. But they do find a significant inverse correlation between the sulphate salt flux and the oxygen-18 temperature proxy. The correlation suggests a coupling between temperature and particulate sulphur (through sulphate salts).
a Temperature proxy (delta oxygen-18). Open boxes (1, 5e and 7e) are interglacials, arrows (5d and 7d) are glacial inceptions and filled boxes (2, 4, 6b, 6d, 6f and 8d) are glacial maxima.
b Salt (NaCl) flux.
c Cumulative salt fluxes. Sodium sulphate )Na2So4), blue; Calcium sulphate (CaSO4), orange.
d Sulphate ion (SO42−) flux.
e Fractional contribution of sulphuric acid (H2SO4) and sulphate salt (Na2SO4 plus CaSO4) to the total sulphur flux.
For relatively warm interglacial periods, including our present epoch, sodium sulphate is generally controlled by the flux of sea salt from the ocean, which is relatively low. But in glacial maxima, the sea salt and dust fluxes increase, leading to greater particulate sulphur flux.
Observations for the three most recent interglacials indicate that sea salt rather than marine sulphur controls the sulphate salt flux. During glacial maxima, on the other hand, the greater sulphate salt flux is controlled by the marine sulphur flux.
The correlation between temperature and sulphate salts suggests that the sulphate salts have an indirect aerosol effect (via cloud condensation nuclei) on glacial–interglacial temperature changes. The authors have attempted to estimate the radiative forcing caused by the flux change of sulphate salt. They assume that the ratio of sulphate concentration during the Holocene to that during the Last Glacial Maximum is about the same as the ratio of pre-industrial anthropogenic sulphate emissions to present anthropogenic sulphate emissions. They also assume that the dust flux changes they observe in Antarctica are representative of global dust flux changes, for which there is some evidence.
With these assumptions, they estimate a net sulphate-induced radiative coolings in the Last Glacial Maximum of −1.85 Watts /square meter to −2.57 Watts /square meter. This corresponds to global climate coolings of 0.1 to 2.24  degrees Kelvin. By applying a polar amplification factor (1.3–2.3), the resulting cooling in Antarctica from the indirect aerosol effect ranges from 0.1 to 5 degrees  Kelvin.
In the Antarctic the change in temperature between the Last Glacial Maximum and current interglacial is estimated to be about ~ 8 °C. The results reported in this article clearly shows the large uncertainty in the climatic impact of indirect aerosol effects from sulphate salts.
Nature 490, 81–84(04 October 2012)
