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Summary

Comparison of sea-ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 1958--1976 to 1.8 m in the 1990s. The decrease is greater in the central and eastern Arctic than in the Beaufort and Chukchi seas. Preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s.

Introduction

There is substantial evidence that the Arctic climate is warming [Dickson, 1999]. In addition, the sea-ice cover shows signs of diminished extent and seasonal duration in (1) the Northern Hemisphere over the period 1978--1995 [Johannessen et al. 1999], (2) the eastern Arctic Ocean and Kara and Barents seas over the period 1979--1986 [Parkinson , 1992], and (3) the East Siberian and Laptev seas over the period 1979--1995 [ Maslanik et al., 1996]. In this paper we use submarine data to examine whether sea-ice thickness, or actually draft, in the Arctic Ocean is also changing.

Read the Paper

The text of the full paper will appear in the Dec. 1 issue of Geophysical Research Letters: http://www.agu.org/GRL/grlonli4.html . You can download the paper from there. Prepints of the paper are available here: Text in PDF Format.

Figures

The below figures are modified from the paper and summarize the results. Please see paper for a more detailed decription.

Figure 1: Submarine cruise tracks and comparison locations, indicated by location number. Tracks in the early cruises (1958--1976) are indicated by dotted red lines, and those in the 1990s by solid blue lines. The area from which SCICEX data could be released is the interior of the solid black polygon

Figure 2: Mean ice drafts at crossings of early cruises with cruises in the 1990s. Early data (1958--1976) are shown by open triangles and those from the 1990s by solid squares, both seasonally adjusted to September 15. The small dots show the original data before the seasonal adjustment. The crossings are grouped into six regions separated by the solid lines and named appropriately. (b) Changes in mean draft at cruise crossings (dots) from the early data to the 1990s. The change in the mean draft for all crossings in each region is shown by a large diamond. The abscissa gives the number of each crossing from Figure 1.

Figure 3: Changes in mean draft from the early period to the 1990s. The change at each crossing is shown numerically. The crossings within each regional group (Figure 3) are given the same shading equivalent to their group mean. Each square covers about 150 km, the typical sample size

Conclusions

In summary, ice draft in the 1990s is over a meter thinner than two to four decades earlier. The mean draft has decreased from over 3 m to under 2 m, and volume is down by some 40\%. The thinning is remarkable in that it has occurred in a major portion of the perennially ice-covered Arctic Ocean. This is not a case of thicker ice appearing in one region simultaneously with thinner ice appearing in another, induced perhaps by a change in surface winds and ice advection. Parkinson's [1992] study of the duration of the ice season, for instance, does show a pattern of compensating regional trends: a shortened season in eastern longitudes, focused in the Barents, Kara and Okhotsk seas, and a lengthened season in western longitudes, more diffusely spread over the Labrador, Beaufort, and Bering seas and Hudson Bay. The present analysis, by contrast, shows a widespread decrease in ice draft within the central Arctic Ocean, with the strongest decrease occurring in the eastern Arctic. Not only is the ice cover thinner in the 1990s than earlier, it appears to be continuing to decline in some regions through four years of SCICEX cruises at a rate of about 0.1 m/yr.

Whether ice volume has reached a minimum in a multi-decadal cycle or will continue the observed decline, this large thinning of the ice cover is a major climatic signal that needs to be accounted for in a successful theory of climate variability. There is not yet a good temporal record of ice draft over the Arctic Ocean; what we have constructed here is better thought of as two climatologies, one for 1958--1976 and another for 1993--1997. A larger pool of publicly archived data of ice draft profiles acquired by submarines over the last 40 years would be of immense help in refining this climatic signal.

Accounts of "Thinning of Arctic Ice" in the Press

Discovery Channel Online
BBC Online
NY-TIMES
CBS-News
NPR Online

Acknowledgements

We are grateful to the U.S. Navy for the extraordinary opportunity presented by the SCICEX program and to the staff of the Arctic Submarine Laboratory for their expertise in SCICEX cruise planning and data pre-processing. We gratefully acknowledge support from the National Science Foundation through Grant No. OPP-9617343 and from a joint ONR-NASA grant NAGW-5177. J. Zhang kindly supplied the seasonal cycle of modeled mean ice draft. We thank the reviewers for their constructive comments.

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