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An updated new ice-age prediction

Robert G. Johnson

Department of Geology and Geophysics, University of Minnesota


Date: Tue, 12 Nov 2002 21:16:12 -0600
Organization: NASA - GISS
To: publisher@naturalscience.com
From: Robert Johnson bandbj@bitstream.net
Subject: An updated new ice-age prediction

The possible effects of the saline Mediterranean outflow at Gibraltar on oceanic circulation and climate have received increasing attention in the scientific community during recent years [1,2,3]. The observed increase in Mediterranean salinity [4] implies a stronger as well as a more saline outflow via the Strait of Gibraltar, because the outflow is driven by the density difference between Mediterranean and Atlantic water. The added salt reaching high latitude sea-surface waters from the Mediterranean would enhance "conveyor belt" circulation, increase the moisture supply to glacial nucleation areas in Canada, and favor rapid initial ice-sheet growth [5].

Larger amounts of Mediterranean salt reaching the Arctic Ocean have probably contributed to the thinning of the Arctic pack ice by 40% over the last 40 years, as measured by sonar during submarine voyages [6]. The suggestion has been made in a recently published book [7] that the incremental loss of the Arctic pack ice over the next few decades will lead to an eventual salinity increase in Baffin Bay, which could trigger new ice-sheet growth in Canada. It is of interest to examine how this might come about.

The Arctic Ocean pack ice (Figure 1) exists only because it is thermally isolated from warmer water beneath by near-surface density stratification. The stratification is caused by the annual discharge of about 3500 km3 of freshwater from rivers [8]. The ongoing reduction in pack ice may be attributed to greater summer melting of the top surface of the ice due to global warming, and smaller rates of winter freezing at the bottom surface owing to weaker stratification. The latter effect is probably caused by rising Arctic salinity associated with the more saline Norwegian Current inflow to the Arctic basin, an inflow fed by an increasingly saline Mediterranean outflow. The annual increment of net pack-ice loss adds a quantity of freshwater directly to the surface layer that enfolds the pack ice. This freshwater is equivalent to about 6% of the river discharges, and is an effective negative feedback factor that tends to maintain stratification and slow the pack ice loss.

Figure 1
Figure 1. SSM/I (Special Sensor Microwave/Imager)-derived map of polar ice in the Arctic. The left panel shows winter distribution, with ice covering the entire Arctic Ocean and nearly all of Baffin Bay. The right depicts summer ice. Source: NASA Goddard Space Flight Center, Remote Sensing Tutorial: http://rst.gsfc.nasa.gov/Sect8/Sect8_8.html .

If the present trend of pack-ice shrinkage continues, this annual increment of freshwater will diminish, and may become insignificant in another 30 years. The consequent increase in surface salinity plus the progressively higher upper-level salinity due to Norwegian Current inflow is expected to sharply increase the density of Arctic Ocean water entering the northern part of Baffin Bay through Lancaster Sound. That higher density will reduce stratification in the bay, and probably allow deep sinking of winter-cooled water to begin in the northern part of the bay a few decades from today. The resulting newly formed deepwater would flow southward out of the bay over the sill at Davis Strait, with saline replacement water entering the bay on the surface from the south. The abrupt development of a regional conveyor belt would then keep the bay free of winter sea ice, resulting in a major atmospheric circulation change. The large winter temperature contrasts between the open bay and nearby cold land surfaces would anchor winter storm systems in the Baffin/Labrador area. Snowfall could increase by an order of magnitude over Baffin Island, Northern Quebec and the coast of Greenland, thus initiating large-scale glaciation and an abrupt alteration of climate in northeastern North America.

This is the proposed mechanism that triggered the Wisconsinan ice age 120,500 years ago [7], and it is consistent with the records of sediment and marine fauna in deepsea cores. The strengthening of the critical Mediterranean outflow is doubtless occurring today owing to the measured increase of western Mediterranean salinity [4]. Although the outflow temperature is rising, the effect of the rise on density of the outflow is probably nullified by the same or a larger temperature increase of the inflow, so the enhanced outflow will be largely due to rising salinity. The ongoing salinity increase has two main causes: (1) the widespread modern use of river water for irrigation, and (2) global warming. All the major rivers flowing toward the Mediterranean and Black Seas now lose much of their water to irrigation of crops, as exemplified by the Nile. Since completion of the Aswan High Dam in 1968, only about 10% of the normal average Nile flow reaches the Mediterranean [9]. If evaporation losses are proportional to the sea water vapor pressure and if the summer sea-surface temperature is 20C, Mediterranean evaporation losses will be increasing at a rate of about 4% per C of temperature rise. In the last half-century losses may therefore have increased by a least 1% due to warming.

Although this undesirable new ice-sheet growth might be avoided by partially damming the Strait of Gibraltar [1], unchecked ice-sheet growth would have some short-term advantages. The new Canadian ice sheets would initiate regional summer cooling. This would tend to inhibit the melting of the Greenland Ice Sheet, which is now threatened by global warming, and thus could prevent a disastrous eventual sea level rise of as much as six meters. The withdrawal of sea water to form the new ice sheets could stop and probably reverse the present rising sea level, thus removing the threat of inundation of populous coastal areas that are now at or very near sea level. With Baffin Bay enclosed by growing ice sheets, regional temperatures would remain low, and the strong conveyor-belt circulation of the Bay would probably persist for some time despite global warming. The predicted major decrease of the entire North Atlantic thermohaline conveyor-belt circulation [10] due to general warming of the high latitudes might therefore be much delayed, thus maintaining European climate closer to that of present for a considerable time into the future. In the longer term, however, the persistence of new glaciation in Canada in the face of ever-increasing global temperature caused by greenhouse gases produced in the combustion of fossil fuels would remain an open question.

References

1. Johnson, R.G. 1997. Climate control requires a dam at the Strait of Gibraltar, EOS Trans. AGU 78:227-281.

2.Singer, S.F. 1998. A New European Ice Age Letter to naturalSCIENCE, March 17, 1998.

3. Mulder, T., P. Lecroart, M. Voisset, J. Schonfeld, E. Le Drezen, E. Gonthier, ,V. Hanquiez, R. Zahn, J.-C. Faugeres, F.J. Hernandez-Molina et al., Past Deep-ocean Circulation and the Paleoclimate Record--Gulf of Cadiz, EOS Trans. AGU 83, 481-487, 2002.

4. Williams, N. 1998. The Mediterranean beckons to Europe's oceanographers, Science 279:483-484.

5. Johnson, R.G. 1997. Ice age initiation by an ocean-atmospheric circulation change in the Labrador Sea. Earth and Planetary Science Letters 148:367-379.

6. Rothrock, D., Y. Yu, and G.A. Maykut. 1999. The thinning of Arctic sea-ice cover, Geophysical Research Letters 26:3469-3472.

7. Johnson, R.G. 2002. Secrets of the Ice Ages. The role of the Mediterranean Sea in climate change, Glenjay Publishing, Minnetonka, MN. http://www.secretsoftheiceages.com

8. Intergovernmental Panel on Climate Change, Climate Change. 2001. Impacts, Adaptation, and Vulnerability, Chapter 16: The Arctic: Sensitivity of Arctic Ocean to River Flow.

9. M. Gasser and F. El-Gamal. 1994. Aswan High Dam: Lessons learnt and on-going research, Water Power and Dam construction, p. 35-39, January, 1994.

10. Manabe, S. and R. Stouffer. 1993. Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system, Nature 364:215-218.

Related article

A New European Ice Age? naturalSCIENCE Cover Story, November 1997.

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