VOLCANIC DUST, MELTING OF ICE CAPS, AND SEA LEVELS

VOLCANIC DUST, MELTING OF ICE CAPS, AND SEA LEVELS:

A REPLY

M. R. BLOCH

Negev Institute for Arid Zone Research, Beersheba (Israel)

(Received August 15, 1967)

223

In reply to the remarks of LAMB (1967) I am sorry that I cannot give details of Hester's experiments in the Andes, which I mentioned I had become acquainted with through private communication. But I hope he will publish the results him-

self. Similar and earlier experiments have now been reported by GORDIJENKO

(1967), and these have led to the practical application of ice discolouration by artificial means on a considerable scale through the work of I. Peschtschanski.

Gordijenko writes that artificial ice discolouration is being used to make "radiation channels" in compact ice: an airplane--containing colouring material and a special mechanism for dispersion--flies a straight course above a navigation channel in springtime. The dusting is done in April or May; by the end of June, when the snow has begun to melt, the dusted navigation channel is visible as a straight line. Along this line ships can reach port 15-25 days earlier than under normal conditions with an icebreaker. This method is used today also on inland shipping lanes. Gordijenko further notes that 1 million km 2 would need 50 million tons of colouring solution to produce a layer less than 0.01 cm thick.

Experiments concerning the effect of natural discolouration are much more difficult. Albedo measurements have been made on glaciers, but--with the excep- tion of those by DAVlTAYA (1965)--without quantitative reference to dust content. The albedo of pure snow and ice has not been measured for varying particle size, nor has the influence of adhering water on absorption of radiation near the infrared ( N 10,000 A) been investigated. Basic work by KUBELKA and MuNK (1931), and its continuation by KORTi3M and VOGEL (1958), concerning the re- flection spectra of powders, has not yet been related to snow and ice nor to mixtures with visibly coloured particles.

KORTiJM and VOGEL (1958) have conducted precise investigations into the well- known effect according to which powders lose their colour with increasing fineness of their grain. They showed that this effect is strong with weak absorption bands, such as those of Cu SO4.5H20. A similar effect can be expected for the weak absorption bands of HzO in the infrared (BLocI-I, 1966). Fine crystalline, dry, pure snow--as it occurs at low temperatures--reflects all the light near the in- frared, despite these absorption bands. As soon as the snow recrystallises to larger

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224 M.R. BLOCH

units, or when it gets wet, absorption of the ~ 10,000 A bands intensifies, with the albedo being reduced down to 45 ~o from 90-92~. This decisive crystallisation might be initiated by general warming up of the snow or by local heating when coloured particles are incorporated either during or after the formation of the snow crystals. Such coloured centres become centres of recrystallisation; then, warming up in the sunlight and increasing absorption develop as self-accelerating processes.

Increasing the path length of the infrared light, and consequently its ab- sorption, is not the only effect of the melting of snow; in addition, admixed coloured particles tend to sink to the solid ground of the melt and to form there a dense layer absorbing the visible light. Thus, the albedo might drop even further, to 15%.

The somewhat complicated conditions under which light is absorbed by snow of different crystal sizes and meltwater content is not the only reason why the quantity of colouring matter required to initiate and sustain the melting of ice under Antarctic conditions is difficult to assess. There are also difficulties connected with (a) the various geographic conditions of the ocean-based Arctic and the continental Antarctic, and (b) the different ways under which colouring matter was incorporated into the condensing, crystallising, and falling snow. According to KUMAI (1960), dust (volcanic, surface, or smoke-derived from incomplete com- bustion in industry or from bush fires) is most suitable for forming nuclei for snow crystallisation.

It seems reasonable to expect micron-size dust particles which reach the Antarctic but which do not move out in comparable amounts to have been very homogeneously mixed with nucleating snow coming down in the area. Considering the two mechanisms--(a) the self-increasing energy absorption of warming snow, and (b) the probable homogeneous mixture of dust and snow Lamb's estimate that a dust layer of 1 cm would be required for an effective melting mechanism is much too high. Greyness equivalent to 0.1 cm seems a very dense cover indeed, and 0.01 cm might be enough, under favourable conditions, to be catastrophic; even lesser amounts could make a measurable difference to certain climatic factors.

I agree with Lamb that the outbreaks registered in Crary's core (CRARY et al., 1962) have occurred in or near the Antarctic; but in the light of what we know of the albedo of dirty snow, I think the area affected was more extensive than 1 or 2 ~ of the Antarctic continent; my theory, however, does not assume a 1 cm layer, but that the equivalent of 0.I-0.01 cm was sufficient to affect the albedo in a significant fashion.

I am very grateful for the data concerning the Thompson Island explosion of 1893. This explains, in my view, why the rise of the Dead Sea was renewed about that time and continued steeply for some years more--even though the duts of the Krakatao explosion of 1883 should by then have been depleted from the atmosphere.

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VOLCANIC DUST, MELTING OF ICE CAPS, AND SEA LEVELS 225

In terms of its effect on the Antarctic, the Thompson Island explosion must have been stronger than the Krakatao one because, although the same quantity of dust was ejected, the former explosion was much closer. My expression "com- paratively weak" for the Krakatao explosion was certainly a misnomer, since I was really referring to its effect on the far-off Antarctic. A similar explosion in the Antarctic should have had a comparatively "strong" effect.

If our hypothesis is correct--and sea level changes in historic times are real and caused by albedo changes in the Antarctic--then temperature variations of the oceans are not easily deduced; it certainly cannot be said that a rise of the ocean means a rise of the ocean's temperature by 1.5 °C. Just the opposite--melt- water from the poles should have caused a fall in ocean temperature. This, by the way, should apply equally to the end of the last glaciation; just when the ice was melting (because of dust-induced albedo reduction), meltwater must have reduced the ocean temperatures (SHACKLETON, 1967).

Climatic changes connected with albedo changes in the Antarctic should be much less dramatic than sea level changes--and not very different whether the level changes are great or small or even unnoticeable: a reduction of the albedo would make the southern heat-sink more shallow than normal and simply intensify the consequences of its seasonal shallowness. It can be supposed that the normal southern migration of the horse latitudes was always intensified in years of reduced south polar albedo, with a simultaneous abnormal southern migration of certain cyclone tracks. Cyclone tracks crossing the Dead Sea abnormally to the south result in an abrupt change in this sea's level; such rises occurred not only in the cases of the Krakatao and Thompson Island eruptions, but also, most probably, after the eruptions which caused the ash deposit in Crary's ice core (CRARY et al., 1962); some smaller oscillations in the level of the Dead Sea might even be con- nected with catastrophic bush fires in Australia and Tasmania. It is hoped this hypothesis will be tested by further research about the history of level changes

in the Dead Sea. It is not easy to analyse the part played by dust clouds, whether their origin

is volcanic, terrestrial, or from great meteoric events. Such clouds certainly absorb part of the sunlight before it penetrates the atmosphere; this absorption changes the overall heat balance of the planet only if the cloud (because of its low heat capacity) gets warmer than the normally absorbing and radiating surface of the earth. Then a comparatively great amount of energy will be lost into space from the hot cloud, and finally the overall surface temperature of the globe will be reduced; but this is a long-term effect. The short-term effect will be that the dusty air, warmer than normal, descending over the poles, will add to the albedo effect of the dust on snow by making the polar heat-sinks still more shallow in their respective summers.

Ocean-level oscillation as an eustatic phenomenon can be made more tenable only by increasing the material of worldwide coincidence. Such material

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226 M.R. BLOCH

as I have managed to collect was gathered chiefly in connect ion with the fate of

the salt industry; bu t as more and more data accumulate (LAoD et al., 1967, and

SCnOFIELD and THOMPSON, 1964, are only two examples), these will have to be

considered together with climatic indications.

I agree wholeheartedly with L a m b that there is a wide field of experimental

and "his tor ical" research open.

REFERENCES

BLOCH, M. R., 1966. Historical evidence of sea-level change and its relation to polar albedo, ln: Syrup. Arctic Heat Budget and Atmospheric Circulation--Univ. Calif., Los Angeles, Calif., and Rand Corp., Lake Arrowhead, Calif., pp.179-196.

CRARY, A. P., ROBINSON, E. S., BENNETT, H. F. and BOYD, W. W., 1962. Glaciological regime of the Ross ice shelf. J. Geophys. Res., 67(7): 2791-2806.

DAVITAYA, F. F., 1965. On the possible influence of atmospheric dust content on the decrease in glaciers and on the warming of climate. Bull. Acad. Sci., U.S.S.R., Geograph. Ser., 2: 3-21.

GORDIJENKO, P., 1967. Die Polarforschung der Sowietunion. Econ Verlag, Dtisseldorf-Wien, 332 S.

KORTOM, G. und VOGEL, J., 1958. 1]ber regul~ire und diffuse Reflexion um Pulvern und ihre Abhangigkeit von der KorngriSsse. Z. Phys. Chem., 18: 230-241.

KUBELKA, P. and MLrNK, F., 1931. Ein Beitrag zur Optik der Farbenstriche. Z. Tech. Physik., 12: 593~601.

KUMAI, M., 1960. Electron-microscope study of snow crystal nuclei. J. MeteoroL, 8: 151-156. LADD, H. S., TRACEY, J. I. and GROSS, M. G., 1967. Drilling on Midway Atoll, Hawaii. Science,

156: 1088-1094. LAMB, H. H., 1968. Volcanic dust, melting of ice caps, and sea levels. Palaeogeography, Palaeo-

climatol., Palaeoecol., 4(3): 219-226. SCHOLFIELD, J. C. and THOMPSON, H. R., 1964. Postglacial sea levels and isostatic uplift. New

Zealand J. Geol. Geophys., 7: 359-370. SHACKELTON, N., 1967. Oxygen isotope analyses and Pleistocene temperatures reassessed. Nature,

215: 15-17.

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