Yesterday, Dr. Meier commented on PIPS -VS- PIOMAS, here is Part 2
Here are some thoughts on three other sea ice issues addressed in recent posts: (1) concentration vs. extent, (2) the causes of the 2007 record minimum, and (3) whether it is possible for the Arctic to lose all its sea ice during summer. Again, I’m speaking only for myself and not as a representative of the National Snow and Ice Data Center or the University of Colorado at Boulder.
Concentration vs. Extent as a measure of the summer ice cover
Sea ice concentration fields are difficult to interpret during summer because of the significant melt that occurs. Areas of low concentration may include open water or ice with surface melt or some combination of both.
Atmospheric moisture, which is higher in the summer, can also affect the observed concentration (e.g., concentration values can change with passing storms). Extent is a more consistent and stable measure of the amount of surface covered by ice and is more legitimate in comparing the data from different years, which is why NSIDC uses extent. Because of the melt and atmospheric effects on concentration, it can be particularly misleading to Steve’s comparison of two single days of concentration data. If one wants to compare concentration, it is better to compare monthly averages, which smoothes out at least some of the ephemeral atmospheric and surface effects. Looking at anomalies for June from 1990 and 2010, there isn’t much difference in the middle of the Arctic (the 2010 anomaly is a bit lower) with lower anomalies in 2010 in coastal areas. Again though, in the central Arctic this may indicate more open water or simply more intensive surface melt. Comparisons with other years can be made at NSIDC’s Sea Ice Index.
Monthly concentration anomaly (in percent) for June 2010 (left) and June 1990 (right). Positive anomalies (higher than average concentration) are in red, negative anomalies (lower than average concentration) are in blue. Anomalies are relative to a 1979-2000 average.
Reasons behind the record low minimum 2007 ice extent
Ice motion has been discussed as a major reason for the record 2007 minimum. While ice motion was important, it was far from the only contributor. For example, Zhang et al. (2008) attribute ~30% of the ice volume loss to ice motion, with the remaining 70% due to pre-conditioning (i.e., thinner ice cover) and more solar heating. Kwok (2008) attributes 15% of the extent loss to motion of ice from the Pacific side across the pole toward the Atlantic. Ogi et al., (2008) calculated a 37% contribution of unusual winds (and resulting ice motion) to the September extent.
So, what else played a role? Kay et al. (2008) suggest that below normal cloud cover and enhanced solar energy played a role (though another study, Schweiger et al.  suggest the role may have been limited). Steele et al. (2008) found anomously high sea surface temperatures during 2007, which enhanced melt. Lindsay et al. (2009) showed that by 2007, the ice cover had thinned enough to reach a threshold where a dramatic loss in extent was possible under conditions experienced during the summer of 2007. Furthermore, the thinner ice cover allows the ice cover to be blown by winds more easily (Haas et al., 2008) – i.e., the winds contributed to the low extent, but the thin ice enhanced the effect of the winds.
In other words, the 2007 minimum was not simply the result of unusual ice motion. It was the result of ice motion, enhanced melt, warmer ocean temperatures, and a long-term thinning trend seen in a variety of observations (Maslanik et al., 2007; Nghiem et al., 2007; Kwok and Rothrock, 2009). The same atmospheric conditions would not have led to such a low extent in earlier years when the ice pack was thicker. As Ogi et al. (2008) say (with clarifying comments by me italicized in brackets): “… the precipitous decline in September SIE [sea ice extent] in recent years is mainly due to the cumulative loss of multi-year ice: summertime SLP [sea level pressure] anomalies [which control the strength and direction of the wind anomalies] have played an important role in setting the timing of record lows, but the long term trend is mainly due to preconditioning [the thinning of the ice cover].”
Can the Arctic really become sea ice-free during summer?
It has been suggested that the Arctic really can’t lose all its sea ice during summer because there isn’t enough energy to melt all of the ice in the short summer. There are a couple of reasons why this thinking is faulty.
First, we know the Arctic can potentially lose all its sea ice during summer because it has done so in the past. Examination of several proxy records (e.g., sediment cores) of sea ice indicate ice-free or near ice-free summer conditions for at least some time during the period of 15,000 to 5,000 years ago (Polyak et al., 2010) when Arctic temperatures were not much warmer than today.
Second, the primary evidence provided for the implausibility of ice-free summers is the plot of daily temperature for regions poleward of 80 degrees N from the Danish Meteorological Institute. It shows that temperatures rise only a couple degrees above freezing for a period of about 75 days throughout the entire record since 1958. So there is no warming trend of the surface air temperatures in the high Arctic. So how could one possibly melt ice near the pole with summer temperatures at most a couple degrees above freezing with no increasing trend?
North of 80 degrees, the Arctic has been continuously covered by ice, even during summer, throughout the entire record (except for a small area briefly during summer 2007). As a result, any heat energy in the vicinity will be used to melt ice and will not raise temperatures. Only after the ice melts can the ocean absorb the energy allowing the ocean surface and the air above it to warm significantly. So the summer near-freezing temperatures don’t say anything much about the energy available to melt ice, only that ice is melting. (I’ll note that it is possible to have higher air temperatures locally, for example due to a weather system bringing in warm air from the south, but the average over the entire region will stay near freezing).
However, there are still only ~75 days of melt, which isn’t much time. But one needs to think about the overall process of what happens in the Arctic, not simply the direct solar energy. As temperatures increase, summer extent decreases, which allows more absorption of solar energy. This melts more ice, decreasing the extent and thinning the ice. Heat absorbed in the ocean away from the ice edge will warm the ocean waters, which will delay freeze-up in the fall. This leads to less ice growth further thinning of the ice. With warmer temperatures, melt will begin earlier in the spring and freeze-up will start later in the fall (as has been observed, e.g., Markus et al. , Serreze et al. , Stroeve et al. ). This is a positive feedback (the sea ice-albedo feedback). Under this feedback, the ice will eventually become thin enough to melt completely most everywhere in the Arctic during a single summer.
There is little doubt in the sea ice community that during summer the Arctic can become ice-free and will become ice-free as temperatures continue to rise.
Haas , C., A. Pfaffling, S. Hendricks, L. Rabenstein, J.‐L. Etienne, and I. Rigor, 2008. Reduced ice thickness in Arctic Transpolar Drift favors rapid ice retreat, Geophys. Res. Lett., 35, L17501, doi:10.1029/2008GL034457.
Kay, J.E., T. L’Ecuyer, A. Gettelman, G. Stephens, and C. O’Dell, 2008. The contribution of cloud and radiation anomalies to the 2007 Arctic sea ice extent minimum, Geophys. Res. Lett., 35, L08503, doi:10.1029/2008GL033451.
Kwok, R., 2008. Summer sea ice motion from the 18 GHz channel of AMSR-E and the exchange of sea ice between the Pacific and Atlantic sectors. Geophys. Res. Lett., 35, L03504, doi:10.1029/2007GL032692.
Kwok , R. and D.A. Rothrock, 2009. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008, Geophys. Res. Lett., 36, L15501, doi:10.1029/2009GL039035.
Lindsay, R.W., J. Zhang, A. Schweiger, M. Steele, and H. Stern, 2009. Arctic sea ice retreat in 2007 follows thinning trend, J. Climate, 22, 165-176.
Markus , T., J. C. Stroeve, and J. Miller (2009), Recent changes in Arctic sea ice melt onset, freezeup, and melt season length, J. Geophys. Res., 114, C12024, doi:10.1029/2009JC005436.
Maslanik, J.A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi, and W. Emery, 2007. A younger, thinner Arctic ice cover: Increased potential for extensive sea-ice loss, Geophys. Res. Lett., 34, L24501, doi:10.1029/2007GL032043.
Meier, W.N., 2005. Comparison of passive microwave ice concentration algorithm retrievals with AVHRR data in Arctic peripheral seas, IEEE Trans. Geosci. Remote Sens., 43(6), 1324-1337.
Nghiem, S.V., I.G. Rigor, D.K. Perovich, P. Clemente-Colon, J.W. Weatherly, and G. Neumann, 2007. Rapid reduction of Arctic perennial sea ice, Geophys. Res. Lett., 34, L19504, doi:10.1029/2007GL031138.
Ogi , M., I.G. Rigor, M.G. McPhee, and J.M. Wallace, 2008. Summer retreat of Arctic sea ice: Role of summer winds, Geophys. Res. Lett., 35, L24701, doi:10.1029/2008GL035672.
Polyak, L., and 17 others, 2010. History of sea ice in the Arctic, Quaternary Science Rev., 29, 1757-1778, doi:10.1016/j.quascirev.2010.02.010.
Schweiger , A.J., J. Zhang, R.W. Lindsay, and M. Steele, 2008. Did unusually sunny skies help drive the record sea ice minimum of 2007?, Geophys. Res. Lett., 35, L10503, doi:10.1029/2008GL033463.
Serreze, M.C., A.P. Barrett, J.C. Stroeve, D.N. Kindig, and M.M. Holland. 2009. The emergence of surface-based Arctic amplification, The Cryosphere, 3, 11–19.
Steele, M., W. Ermold, and J. Zhang, 2008. Arctic Ocean surface warming trends over the past 100 years, Geophys. Res. Lett., 35, L02614, doi:10.1029/2007GL031651.
Stroeve, J., T. Markus, W.N. Meier, and J. Miller, 2006. Recent changes in the Arctic melt season, Ann. Glaciol., 44, 367-374.
Zhang, J., R. Lindsay, M. Steele, A. Schweiger, 2008. What drove the dramatic retreat of arctic sea ice during summer 2007?, Geophys. Res. Lett., 35, L11505, doi:10.1029/2008GL034005.