January 18, 2007

Arctic Forecast: Nordic Sea Ice Expansion

Filed under: Arctic, Polar

What’s the first image that comes to mind when you hear the term “global warming”? The most common is that of melting ice. That image is then easily cultivated by climate change alarmists who would like you to translate it into a downward spiraling Arctic ecosystem and a sputtering global oceanic circulation. (The image that comes to our mind is that of Al Gore recently pretending to be a research professor on The Oprah Winfrey Show.) What we hear little about from the global warming crusade is research findings that suggest that a measure of the recent atmospheric warming is part of a natural cycle or that the impacts are far less than what is portrayed. Well, that’s what we at the World Climate Report are here for.

In an article recently appearing in the Journal of Geophysical Research, Dmitry Divine and Chad Dick (2006) of the Norwegian Polar Institute report on their use of historical observations of ice cover in the Nordic Seas region to construct time series of warm season ice edge position dating back to 1750. Their particular search for a climate change signal is important for three reasons. First, as an insulator between the ocean and the atmosphere, as an effective reflector of solar radiation, and as a modifier of atmospheric and oceanic circulation, sea ice cover has a significant impact on global climate. Second, the Nordic Seas region, which is inclusive of the Iceland, Greenland, Norwegian, and Barents seas, is a focal point for the global climate due to its primary role in driving the global oceanic thermohaline circulation. It is in this region that excess fresh water from melting ice in association with global warming is theorized to impact the thermohaline circulation enough to generate significant climate changes worldwide. (Alarmists hope that this evokes in your mind scenes from the movie Day After Tomorrow.) Third, Divine and Dick use data generated through direct observation of ice conditions, which allowed them to construct a historical record of a length that far surpasses those of sea ice studies that rely on satellite-derived ice cover products. Divine and Dick note that many studies “have revealed a dramatic retreat of ice extent over the past decades in almost all regions where sea ice exists as a seasonal or perennial phenomenon.” However, (and this is a big caveat) Divine and Dick point out that “debate continues over whether the recent shrinkage of ice cover gives direct evidence of global warming caused by human activity or is mainly a part of a secular cycle.” The 30-year record of satellite sensing of sea ice “has shed light on the spatial and temporal patterns of seasonal and interannual variability in ice extent and concentrations, but is still too short for resolving the multi-year variability in ice cover.”

Records of ice extent in the Nordic Seas region date to the mid-17th century, primarily in the form of ship logs and diaries early in the record and airborne and satellite observations later in the record. The data are sparse until the latter half of the 19th century when sealing and whaling in the region became popular. Data from 1850 to present are considered to be rather complete and the sea ice data for this period collectively represent one of the few data sets with direct measurements of climate conditions dating back more than a century. Divine and Dick note the importance of the period from a climate change perspective, as the 150+ years include well-known “Arctic cooling (1880s–1910s, 1960s–1970s) and warming (1920s–1930s, 1980s–1990s) events as well as the recent significant reduction of ice extent.”

The researchers used data from the Arctic Climate Systems Study Historical Ice Chart Archive to extend the historical record back to 1750. These data were complimented with Soviet aircraft reconnaissance ice charts for the period 1950 through 1965 to help fill a gap in data for the Barents Sea within the Ice Chart Archive. Data from two passive microwave sensors aboard satellites were used to also represent ice concentration within the region across the period 1978 through 2002. Divine and Dick simply define ice extent as a single line or “ice edge” separating “close pack ice from loose drift ice.” Data from the three sources were merged to produce a complete series of monthly ice charts representing the mean position of the ice edge. Based on oceanographic and geographical considerations the region was divided into seven sectors for analysis and data interpretation.

The researchers focused on the annual period from March through September, which represents the annual warm season that is conducive to hunting, and thereby associated with a much greater data density than the cold winter months. The period includes the typical timing of the annual ice cover maximum (April) and minimum (September), but in their analysis, Divine and Dick emphasized the months of April, June, and August, where June represents an approximate mid-point between ice maximum and minimum, and August is characterized by greater data density than during the time of minimum ice cover in September.

In agreement with the findings of previous work, Divine and Dick found a persistent ice retreat within the region since the second half of the 19th century. However, their analysis also indicates that the decreasing trend is being superimposed on multidecadal oscillations in ice edge position. Their work suggests the presence of a 60-80 year variability and two- to three-decadal oscillations in ice extent. Divine and Dick associate the multidecadal oscillations “with the so-called low-frequency oscillation found in Arctic climate and possibly associated with the North Atlantic thermohaline circulation variability.” The researchers further conclude that since the last Arctic cold period occurred in the late 1960s, it is likely that the Arctic ice pack is now at the height of its low frequency variability. This, the researchers say, “could explain the strong negative trend in ice extent during the last decades as a possible superposition of natural low frequency variability and greenhouse gas induced warming of the last decades.” So, the recent decreases in ice extent are likely to be only partially related to anthropogenic causes. And there is more. Divine and Dick also note that “a similar shrinkage of ice cover was observed in the 1920s–1930s, during the previous warm phase of the low-frequency oscillation, when any anthropogenic influence is believed to have still been negligible.” So, maybe greenhouse gas induced warming has a lot less to do with the recent retreat in ice extent than what has been portrayed.

Based on the findings of the work, Divine and Dick offer an interesting forecast for the Nordic seas region of the Arctic: “We suppose therefore that during decades to come, as the negative phase of the thermohaline circulation evolves, the retreat of ice cover may change to an expansion.” An expansion of Arctic sea ice! Now that’s a forecast that you probably haven’t heard anywhere else!

Reference:

Divine, D.V. and C. Dick. 2006. Historical variability of sea ice edge position in the Nordic Seas, Journal of Geophysical Research, 111, 10.1029/2004JC002851




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