Actual and insolation-weighted Northern Hemisphere snow cover and sea-ice between 1973–2002
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- Pielke, R.A., Liston, G.E., Chapman, W.L. et al. Climate Dynamics (2004) 22: 591. doi:10.1007/s00382-004-0401-5
Actual and insolation-weighted Northern Hemisphere snow cover and sea ice are binned by latitude bands for the years 1973–2002. Antarctic sea-ice is also analyzed for the years 1980–2002. The use of insolation weighting provides an improved estimate of the radiative feedbacks of snow cover and sea-ice into the atmosphere. One conclusion of our assessment is that while a decrease in both areal and insolation-weighted values have occurred, the data does not show a monotonic decrease of either Arctic sea-ice or Northern Hemisphere snow cover. If Arctic perennial sea-ice is decreasing since the total reduction in areal coverage is relatively small, a large portion of it is being replenished each year such that its radiative feedback to the atmosphere is muted. Antarctic sea-ice areal cover shows no significant long-term trend, while there is a slight decrease in the insolation-weighted values for the period 1980–2002. From the early 1990s to 2001, there was a slight increase in both values. The comparison of general circulation model simulations of changes over the last several decades to observed changes in insolation-weighted sea-ice and snow cover should be a priority research topic.
In Pielke et al. (2000), the effects of sea-ice and snow-cover trends on the surface energy budget are assessed by scaling with direct solar insolation. The influence of the radiative feedback to the atmosphere of sea-ice and snow cover is one of several important effects of the cryosphere on the climate system (NRC 2003). The current work extends that study by examining the interannual variability as a function of latitude bands, and by developing an integrated measure of the combined effect of sea-ice and snow cover. The monitoring of the trends in these quantities is an important climate assessment metric (Holloway and Sou 2002; Dye 2002). Comiso (2002), for example, reports on a significant decline in perennial sea-ice coverage in the Arctic. Vinnikov et al. (1999) also show climate change model simulations of Arctic sea-ice decline, which they illustrate in their Fig. 1 as about 0.7 million square km in the GFDL model and around 0.5 million square km in the Hadley Centre model for the period 1973–2002. These represent about 6% and 5% declines in sea-ice coverage, respectively, for the two model results over this time period. The tracking of whether these simulations are skillful predictions is a critical question.
Pielke et al. (2000) introduced a method to assess the influence of temporal changes in sea-ice and snow-cover distributions on solar radiation reflected back into space. This method weights the observed sea-ice and snow-cover distributions by a solar insolation factor that accounts for the seasonal variations in solar elevation angle and day length. While the method ignores cloud and surface albedo variations due to vegetation masking, and patchiness along the snowpack margins, as well as changes in sea-ice albedo during the melt season, it does provide a more useful scaling of atmosphere-cryosphere radiative feedback than areal coverage changes alone.
The advantage of an insolation-weighted assessment is that it more directly relates to the radiative feedback between the Earth’s surface and the atmosphere. For example, a decrease in snow cover for a Northern Hemisphere latitude band in February has a relatively small radiative effect because insolation is low at this time of the year. In contrast, the same snow-cover change in April would be more significant radiatively because insolation is higher.
The decrease in actual area of both Arctic sea-ice and snow cover since the beginning of the record in 1973 is about 5%, while the insolation-weighted value has decreased by 11%. The insolation-weighted value provides a more appropriate measure of the decrease with respect to the albedo radiative feedback to the atmosphere. The greater percentage decrease in the insolation-weighted value is a result of an earlier spring decrease in areal coverage.
The use of latitude bands and insolation effective areas provide a more detailed assessment of snow-cover and sea-ice trends than can be obtained using just total area by itself. One of the conclusions of our assessment is that while a decrease in both areal coverage and the insolation-weighted values have occurred, the data does not show a monotonic decrease of either Arctic sea-ice or Northern Hemisphere snow cover. If Arctic perennial sea-ice is decreasing, since the total areal reduction is relatively small, a large portion of it is being replenished each year such that its radiative feedback to the atmosphere is muted. Antarctic sea-ice shows no significant trend during the period of record analyzed although there is a small insolation-weighted decrease using a linear trend across the entire period of record.
The comparison of general circulation model simulations of changes over the last several decades in insolation-weighted sea-ice and snow cover should be a priority research topic in order to test the predictive skill of these models.
Our analysis approach should be generalized to include cloud cover and surface albedo effects on the insolation at the surface. The insolation received and reflection from snow cover and sea-ice are influenced by the cloud cover, which varies diurnally and monthly. The surface snow albedo is affected by vegetation masking, as well as its patchiness, while the albedo of sea-ice changes as it ages during the melt season.
Our analysis of snow cover and sea-ice uses a specific set of data. Other data sets, with different thresholds of areal coverage, should be applied to determine if our insolation-based assessment is robust. Our work, however, clearly demonstrates the value-added in climate studies of the cryosphere using this assessment approach.
The snow data was obtained from NOAA weekly snow maps, which are produced by a visual interpretation of visible satellite imagery. The information is standardized, converted to monthly values, and archived at Rutgers University (http://climate.rutgers.edu/snowcover ). The DMSP SSM/I sea-ice data were obtained from the National Snow and Ice Data Center. The Referees provided specific suggestions which significantly improved the study. Support for this work was provided by National Science Foundation Grants 0229973-001 and ATM-9905924, and by National Oceanic and Atmospheric Administration Grant NA06GPO566.